Information

Lack of yolk in mammalian oocytes as compared to other vertebrates?


Why do mammalian oocytes have little to no yolk? How does it compare to other vertebrates such as frogs, fish, and birds?


In oviparous animals (those that lay eggs which hatch outside the body), the eggs need to be provided nutrition for embryonic growth, which is the main function of yolk.

In placental mammals, the embryo is provided nutrition directly from the mother via the placenta.

Therefore, most mammalian oocytes do not need to develop large quantities of yolk to support the embryo in the way the eggs of most birds, reptiles, amphibians and fish do.


In mammals, the first part of oogenesis starts in the germinal epithelium, which gives rise to the development of ovarian follicles, the functional unit of the ovary.

Oogenesis consists of several sub-processes: oocytogenesis, ootidogenesis, and finally maturation to form an ovum (oogenesis proper). Folliculogenesis is a separate sub-process that accompanies and supports all three oogenetic sub-processes.

Cell type ploidy/chromosomes chromatids Process Time of completion
Oogonium diploid/46(2N) 2C Oocytogenesis (mitosis) Third trimester
primary oocyte diploid/46(2N) 4C Ootidogenesis (meiosis I) (Folliculogenesis) Dictyate in prophase I for up to 50 years
secondary oocyte haploid/23(1N) 2C Ootidogenesis (meiosis II) Halted in metaphase II until fertilization
Ootid haploid/23(1N) 1C Ootidogenesis (meiosis II) Minutes after fertilization
Ovum haploid/23(1N) 1C

Oogonium —(Oocytogenesis)—> Primary Oocyte —(Meiosis I)—> First Polar body (Discarded afterward) + Secondary oocyte —(Meiosis II)—> Second Polar Body (Discarded afterward) + Ovum

Oocyte meiosis, important to all animal life cycles yet unlike all other instances of animal cell division, occurs completely without the aid of spindle-coordinating centrosomes. [3] [4]

The creation of oogonia Edit

The creation of oogonia traditionally doesn't belong to oogenesis proper, but, instead, to the common process of gametogenesis, which, in the female human, begins with the processes of folliculogenesis, oocytogenesis, and ootidogenesis. Oogonia enter meiosis during embryonic development, becoming oocytes. Meiosis begins with DNA replication and meiotic crossing over. It then stops in early prophase.

Maintenance of meiotic arrest Edit

Mammalian oocytes are maintained in meiotic prophase arrest for a very long time -- months in mice, years in humans. Initially the arrest is due to lack of sufficient cell cycle proteins to allow meiotic progression. However, as the oocyte grows, these proteins are synthesized, and meiotic arrest becomes dependent on cyclic AMP. [5] The cyclic AMP is generated by the oocyte by adenylyl cyclase in the oocyte membrane. The adenylyl cyclase is kept active by a constitutively active G-protein-coupled receptor known as GPR3 and a G-protein, Gs, also present in the oocyte membrane. [6]

Maintenance of meiotic arrest also depends on the presence of a multilayered complex of cells, known as a follicle, that surrounds the oocyte. Removal of the oocyte from the follicle causes meiosis to progress in the oocyte. [7] The cells that comprise the follicle, known as granulosa cells, are connected to each other by proteins known as gap junctions, that allow small molecules to pass between the cells. The granulosa cells produce a small molecule, cyclic GMP, that diffuses into the oocyte through the gap junctions. In the oocyte, cyclic GMP prevents the breakdown of cyclic AMP by the phosphodiesterase PDE3, and thus maintains meiotic arrest. [8] The cyclic GMP is produced by the guanylyl cyclase NPR2. [9]

Reinitiation of meiosis and stimulation of ovulation by luteinizing hormone Edit

As follicles grow, they acquire receptors for luteinizing hormone, a pituitary hormone that reinitiates meiosis in the oocyte and causes ovulation of a fertilizable egg. Luteinizing hormone acts on receptors in the outer layers of granulosa cells of the follicle, causing a decrease in cyclic GMP in the granulosa cells. [10] Because the granulosa cells and oocyte are connected by gap junctions, cyclic GMP also decreases in the oocyte, causing meiosis to resume. [11] Meiosis then proceeds to second metaphase, where it pauses again until fertilization. Luteinizing hormone also stimulates gene expression leading to ovulation. [12]

Oogenesis Edit

Oogenesis starts with the process of developing primary oocytes, which occurs via the transformation of oogonia into primary oocytes, a process called oocytogenesis. [13] Oocytogenesis is complete either before or shortly after birth.

Number of primary oocytes Edit

It is commonly believed that, when oocytogenesis is complete, no additional primary oocytes are created, in contrast to the male process of spermatogenesis, where gametocytes are continuously created. In other words, primary oocytes reach their maximum development at

20 [14] weeks of gestational age, when approximately seven million primary oocytes have been created however, at birth, this number has already been reduced to approximately 1-2 million.

Two publications have challenged the belief that a finite number of oocytes are set around the time of birth. [15] [16] The renewal of ovarian follicles from germline stem cells (originating from bone marrow and peripheral blood) has been reported in the postnatal mouse ovary. In contrast, DNA clock measurements do not indicate ongoing oogenesis during human females' lifetimes. [17] Thus, further experiments are required to determine the true dynamics of small follicle formation.

Ootidogenesis Edit

The succeeding phase of ootidogenesis occurs when the primary oocyte develops into an ootid. This is achieved by the process of meiosis. In fact, a primary oocyte is, by its biological definition, a cell whose primary function is to divide by the process of meiosis. [18]

However, although this process begins at prenatal age, it stops at prophase I. In late fetal life, all oocytes, still primary oocytes, have halted at this stage of development, called the dictyate. After menarche, these cells then continue to develop, although only a few do so every menstrual cycle.

Meiosis I Edit

Meiosis I of ootidogenesis begins during embryonic development, but halts in the diplotene stage of prophase I until puberty. The mouse oocyte in the dictyate (prolonged diplotene) stage actively repairs DNA damage, whereas DNA repair is not detectable in the pre-dictyate (leptotene, zygotene and pachytene) stages of meiosis. [19] For those primary oocytes that continue to develop in each menstrual cycle, however, synapsis occurs and tetrads form, enabling chromosomal crossover to occur. As a result of meiosis I, the primary oocyte has now developed into the secondary oocyte and the first polar body.

Meiosis II Edit

Immediately after meiosis I, the haploid secondary oocyte initiates meiosis II. However, this process is also halted at the metaphase II stage until fertilization, if such should ever occur. If the egg is not fertilized, it is disintegrated and released (menstruation) and the secondary oocyte does not complete meiosis II (and doesn't become an ovum). When meiosis II has completed, an ootid and another polar body have now been created. The polar body is small in size.

Folliculogenesis Edit

Synchronously with ootidogenesis, the ovarian follicle surrounding the ootid has developed from a primordial follicle to a preovulatory one.

Maturation into ovum Edit

Both polar bodies disintegrate at the end of Meiosis II, leaving only the ootid, which then eventually undergoes maturation into a mature ovum.

The function of forming polar bodies is to discard the extra haploid sets of chromosomes that have resulted as a consequence of meiosis.

In vitro maturation Edit

In vitro maturation (IVM) is the technique of letting ovarian follicles mature in vitro. It can potentially be performed before an IVF. In such cases, ovarian hyperstimulation isn't essential. Rather, oocytes can mature outside the body prior to IVF. Hence, no (or at least a lower dose of) gonadotropins have to be injected in the body. [20] Immature eggs have been grown until maturation in vitro at a 10% survival rate, but the technique is not yet clinically available. [21] With this technique, cryopreserved ovarian tissue could possibly be used to make oocytes that can directly undergo in vitro fertilization. [21]

In vitro oogenesis Edit

By definition it means, to recapitulate mammalian oogenesis and producing fertilizable oocytes in vitro.it is a complex process involving several different cell types, precise follicular cell-oocyte reciprocal interactions, a variety of nutrients and combinations of cytokines, and precise growth factors and hormones depending on the developmental stage. [22] In 2016, two papers published by Morohaku et al. and Hikabe et al. reported in vitro procedures that appear to reproduce efficiently these conditions allowing for the production, completely in a dish, of a relatively large number of oocytes that are fertilizable and capable of giving rise to viable offspring in the mouse. This technique can be mainly benefited in cancer patients where in today's condition their ovarian tissue is cryopreserved for preservation of fertility. Alternatively to the autologous transplantation, the development of culture systems that support oocyte development from the primordial follicle stage represent a valid strategy to restore fertility. Over time, many studies have been conducted with the aim to optimize the characteristics of ovarian tissue culture systems and to better support the three main phases: 1) activation of primordial follicles 2) isolation and culture of growing preantral follicles 3) removal from the follicle environment and maturation of oocyte cumulus complexes. While complete oocyte in vitro development has been achieved in mouse, with the production of live offspring, the goal of obtaining oocytes of sufficient quality to support embryo development has not been completely reached into higher mammals despite decades of effort. [23]

BRCA1 and ATM proteins are employed in repair of DNA double-strand break during meiosis. These proteins appear to have a critical role in resisting ovarian aging. [24] However, homologous recombinational repair of DNA double-strand breaks mediated by BRCA1 and ATM weakens with age in oocytes of humans and other species. [24] Women with BRCA1 mutations have lower ovarian reserves and experience earlier menopause than women without these mutations. Even in woman without specific BRCA1 mutations, ovarian aging is associated with depletion of ovarian reserves leading to menopause, but at a slower rate than in those with such mutations. Since older premenopausal women ordinarily have normal progeny, their capability for meiotic recombinational repair appears to be sufficient to prevent deterioration of their germline despite the reduction in ovarian reserve. DNA damages may arise in the germline during the decades long period in humans between early oocytogenesis and the stage of meiosis in which homologous chromosomes are effectively paired (dictyate stage). It has been suggested that such DNA damages may be removed, in large part, by mechanisms dependent on chromosome pairing, such as homologous recombination. [25]

Some algae and the oomycetes produce eggs in oogonia. In the brown alga Fucus, all four egg cells survive oogenesis, which is an exception to the rule that generally only one product of female meiosis survives to maturity.

In plants, oogenesis occurs inside the female gametophyte via mitosis. In many plants such as bryophytes, ferns, and gymnosperms, egg cells are formed in archegonia. In flowering plants, the female gametophyte has been reduced to an eight-celled embryo sac within the ovule inside the ovary of the flower. Oogenesis occurs within the embryo sac and leads to the formation of a single egg cell per ovule.

In ascaris, the oocyte does not even begin meiosis until the sperm touches it, in contrast to mammals, where meiosis is completed in the estrus cycle.

In female Drosophila flies, genetic recombination occurs during meiosis. This recombination is associated with formation of DNA double-strand breaks and the repair of these breaks. [26] The repair process leads to crossover recombinants as well as at least three times as many noncrossover recombinants (e.g. arising by gene conversion without crossover). [26]

Cho WK, Stern S, Biggers JD. 1974. Inhibitory effect of dibutyryl cAMP on mouse oocyte maturation in vitro. J Exp Zool.187:383-386


Extra-Embryonic Membranes in Chick

During the development of chick and other vertebrates, certain specialized embry­onic tissues or structures are produced that temporarily or permanently do not enter into the formation of the embryo themselves. These are external and devoted in one way or another to the care and maintenance of the developing embryo.

These structures are collectively termed as extra-embryonic mem­branes or foetal membranes or extra-embry­onic sacs. These are not precursors of any of the organs of the adult or the larva but serve to satisfy the requirements of the embryo in connection with nutrition, gas exchange, removal or storage of waste materials and protection.

The extra-embryonic membranes have developed to make the eggs capable of deve­loping on dry land. The eggs of reptiles, birds and prototherian mammals have a protective shell around it. In some reptiles and eutherian mammals the shell has given way to uterine development, but the basic form and function of the extra-embryonic membranes has remained the same.

Extra-embryonic membranes are those membranes formed of embryonic tissues, which extend out and beyond the strict con­fines of the embryonic body and are adapted to fulfill the care and maintenance of the developing embryo.

Kinds of Extra-Embryonic Membranes:

Four sets of extra-embryonic membranes are common to the embryos of all terrestrial vertebrates including chick.

These four extra=embryonic membranes in chick are dis­cussed below:

The amnion is a thin mem­brane which eventually encloses the entire developing embryo in a fluid-filled sac. Reptiles, birds and mammals possess­ing this amnion are often called amniotes, while fishes and amphibians, lacking it, are collectively called anamniotes.

ii. Yolk Sac:

It is the most primitive struc­ture containing network of blood vessels and encloses the yolk of the egg. A yolk sac is also present in those fishes which have megalecithal eggs. Despite the lack of stored yolk in mammalian eggs (except in prototherians), the yolk sac has been preserved, as it serves many impor­tant secondary functions.

Allantois serves as an excre­tory and respiratory structure. It is a large sac like structure in reptiles and birds, while its role in mammals varies with the efficiency of the interchange that takes place at the foetal-maternal inter­face.

In pig embryo, the allantois rivals that of the bird’s in both size and func­tional importance’s, while the allantois in human has been reduced to a mere ves­tige which contributes only as a well-developed vascular network to the high­ly efficient placenta.

Chorion is a very thin membrane and it covers the embryo and other extra-embryonic membranes. It is formed by the fusion of the amniotic folds over the embryo. All these extra-embryonic membranes are composite structures as they involve two germ layers.

The amnion and chorion are made up of extra-embryonic ectoderm and somatic layer of mesoderm, while the yolk sac and allantois are composed of extra-embryonic endoderm and splanchnic layer of mesoderm.

Development of Extra-Embryonic Membranes:

During neurulation (neural tube forma­tion) of chick, the lateral plate mesoderm splits into an outer somatic layer of meso­derm lying at the inner side of the ectoderm and an inner splanchnic layer of mesoderm lying outer to the endodermal layer. Both these mesodermal layers enclose a coelomic space between them.

The somatic layer of mesoderm and ectoderm are collectively known as somatopleure, while the splanch­nic layer of mesoderm along with endoderm forms the splanchnopleure. At the time of development of the avian blastoderm, the somatopleure and splanchnopleure gradu­ally spread peripherally over the yolk mass, far beyond the area where the body of the embryo is taking form.

Shortly, the embryo proper begins to be undercut by a series of body folds that serve to delimit the embryo­nic regions from the more peripheral extra-­embryonic somatopleure and splanchno­pleure (Fig. 5.47). After the formation of the body folds, the somatopleure and splanchno­pleure of chick develop into the four extra-embryonic membranes.

i. Development of Yolk Sac:

The yolk sac is the first extra-embryonic membrane to make its appearance. As the early blastoderm expands, the extra-embryonic splanchno­pleure continues to spread over the yolk mass and eventually encloses the yolk com­pletely to form the yolk sac (Fig. 5.48).

Coincidentally, the intra-embryonic splan­chnopleure is subjected to superficial body folds, which serve to establish a walled digestive tract or gut, in the body of the embryo. The middle of the gut (mid gut) remains connected with the yolk sac by a nar­row yolk stalk, where the walls of the gut is continuous with the walls of the yolk sac.

Although the yolk sac is connected with the digestive tract by the yolk stalk, the yolky food reserves are not transmitted to the embryo by this route. Rather, the digestion of the yolk is done by the endodermal lining of the yolk sac through the mediation of appro­priate enzymes.

In chick, on the 2nd and 3rd day of incu­bation, networks of blood vessels develop on the inner part of the area opaca, which becomes the area vasculosa (Fig. 5.49). The outer part of area opaca is called the area vitellina.

All the blood vessels of area vascu­losa communicate with each other and are joined together on the periphery by the ter­minal sinus, which incidentally forms the boundary between area vasculosa and area vitellina.

The network of the area vasculosa becomes prolonged into the area pellucida and eventually establishes connection with the embryo proper.

Connections with the blood system of the embryo is formed at two points:

(a) With the Venus system by means of the right and left vitelline (omphalome­senteric) veins. These join with the unpaired ductus venosus, which in turn enter the sinus venosus of the heart,

(b) With the arte­rial system by means of right and left vitelline (omphalomesenteric) arteries, which branch off from the dorsal aorta.

At about the middle of the 2nd day of incubation, the heart of the embryo begins to beat. Between the 38th and 40th hours of incubation the blood starts circulating through the network of the yolk sac. The blood vessels in the area vasculosa pene­trate deep into the yolk. The endodermal surface of the yolk sac is thrown into folds that penetrate the yolk mass.

Then, through the action of appropriate digestive enzymes secreted by the endodermal cells, the yolk is digested or made soluble and is ultimately absorbed by the endodermal lining of the yolk sac. It is then passed on through the endothelial lining of the vitelline blood ves­sels to the circulating blood, from where it is carried to all parts of the growing embryo.

According to Young et al. (1980), the endo­derm of the yolk sac in addition to its absorptive function, is also the sole site of synthesis of the serum proteins like transfer­ring, alpha globulins and pre-albumin.

Also during the growth of the allantois, the albu­men is forced towards the distal end and gets surrounded by an extension of the yolk sac. It is absorbed along with yolk and trans­ferred by way of the extra-embryonic circula­tion to the embryo.

The entire yolk is not completely absorbed during embryonic life. On the 19th day when the period of incubation is nearing its end, the remains of the yolk sac are enclosed within the body walls of the embryo.

During the first 6 days after hatch­ing, the resorption of the remaining part of the yolk sac and yolk gets completed. This remaining yolk reserves are vital to the newly hatched chick while it is adapting to a free-living existence and is developing its feeding behaviour.

ii. Development of Amnion and Chorion:

The amnion and chorion are deve­loped simultaneously and both are derived from the extra-embryonic somatopleure. At about the 30th hour of incubation, the head of the embryo sinks somewhat into the yolk and at the same time the extra-embryonic somatopleure is elevated over the embryo by a folding process consisting essentially of a doubling of the somatopleure upon itself.

The initial elevation is over the head end of the embryo, producing a double somatopleuric hood, called the cephalic amniotic fold or head fold of the amnion (Fig 5.48A). From a dorsal aspect, the margin of this fold is crescentic in shape, with its concavity directed towards the head of the embryo. As the embryo increases in length, its head grows forward into the amniotic fold.

As the cephalic amniotic fold gradually extends backward, towards the tail region, its caudally extending side limbs called lateral amnio­tic folds arch over the embryo from each side to be joined finally by a similar fold or eleva­tion from the tail region called the caudal amniotic fold or tail fold.

All these amniotic folds finally converge at the midline, so as to encase the embryo by two sheets of somato­pleure from all the sides except from the region of the yolk stalk. The place where all the amniotic folds meet is called the seroamniotic connection or the amniotic raphe, which is a scar like thickening. The seroamniotic connection opens before the hatching of the embryo and admits albumen into the amniotic cavity.

The fusion of the amniotic folds results in the formation of two sac-like membranes and two cavities. The inner somatopleuric membrane becomes the amnion and the outer one, the chorion or serosa (Fig. 5.48B). The cavity between the amnion and the embryo is called the amniotic cavity and is lined by ectoderm.

Muscle fibres differenti­ate within the mesoderm of the amnion and the amniotic cavity gets filled with fluid called amniotic fluid. The cavity lying between the amnion and chorion is called the chorionic cavity and is lined by the mesoderm.

This chorionic cavity is actually the extra-embryonic coelomic cavity, which is continuous with the coelomic cavity in the embryo proper. The chorion is lined on the outside by the extra embryonic ectoderm. According to Balinsky (1970) the formation of the amniotic cavity has a somewhat negative effect as it removes the embryo from the source from where it could obtain oxygen.

iii. Development of Allantois:

The allantois first appears late in the 3rd day of incubation. It bulges out as a ventral out­growth of the endodermal hindgut and cor­responds exactly in nature to the urinary bladder of the amphibians. The outgrowth consists of an inner layer of endoderm and an outer layer of splanchnic mesoderm.

The allantois enlarges very rapidly from the fourth day to the tenth day of incubation. It penetrates into the extra-embryonic coelom, into the space between the yolk sac, the amnion and the chorion (Fig. 5.48A and B).

The base of the allantois remains connected with the hindgut of the embryo by means of a narrow allantoic stalk. When the body of the embryo contracts separating the embryo from the extra-embryonic parts, the allantoic stalk and the stalk of the yolk sac remain enclosed together forming an umbilical cord.

The distal part of allantois penetrates between the amnion and yolk sac on one side and the chorion on the other side. By the 4th to 10th day of incubation period, the allantois spreads rapidly and completely covers the coelomic space.

Soon, the mesodermal layer of the allantois becomes fused with the adja­cent mesodermal layer of the chorion to form a single mesodermal layer called chorioal­lantoic membrane (Fig. 5.48C). In the mean-time, the expanding chorioallantois bursts through the vitelline membrane of egg and pushes outward towards the shell mem­brane.

As it does so it progressively envelops the albumen and becomes a sac filled with albumen, called the albumen sac, that helps in the absorption of water and albumen. The chick embryo, through the chorioallantoic membrane and the shell, takes up about 5 liters of oxygen and gives off about 4 liters of carbon dioxide during its 21 days period before hatching.

On the external surface of the allantois, a network of blood vessels develop and this network is in communication with the embryo proper by means of blood vessels running along the stalk of the allantois and through the umbilical cord.

Blood flows to the allantois through the right and left umbi­lical arteries, that leaves the dorsal aorta at a point which is much more caudal than the starting point of the vitelline arteries. The returning blood flows to the heart through a pair of umbilical veins that originally enter the right and left ducts of Cuvier.

Soon, the right umbilical artery and the right umbilical vein disappear and the left umbilical vein develops a new connection. It joins the left hepatic vein and the connection with the duct of Cuvier gets closed.

The allantoic circulation functions till the hatching of the chick and when it starts breathing the surrounding air. The umbilical vessels then close. The allantois dries up and separates from the body of the young chick.

Functions of the Extra-Embryonic Membranes:

Development of extra-embryonic mem­branes are important for those vertebrates that lay their eggs on land. Eggs developing in water, encounter minimum external inter­ference and water provides the egg with various favourable environmental condi­tions. However, none of the favourable features are provided on dry land where eggs are subjected to desication and sudden changes in temperature.

The extra-embryonic mem­branes have thus developed to serve the following functions:

i. Functions of Yolk Sac:

(a) The yolk sac which spreads over the large amount of yolk, serves as the digestive and absorptive organ by which the yolk is made available for the growing embryo.

(b) It functions as the first respiratory organ.

(c) It acts as a haemopoetic organ like the liver.

(d) The yolk sac also serves as the place of origin of blood cells, at later stages of development.

ii. Functions of Amnion and Chorion:

(a) The amniotic cavity contains a salty fluid surrounding the embryo. Thus, the embryo can accomplish its development in a fluid medium although it is “on dry land”. Therefore, the amnion serves as a protective organ where the embryo is saved from the danger of desiccation.

(b) The amniotic fluid acts as an efficient shock absorber and thus, protects the soft, collapsible and almost skeletons early chick embryo from mechanical shocks.

(c) As the amnion isolates the embryo from the egg shell, it thus protects it from adhesion to the shell or from friction against it.

(d) The mesoderm of amnion, during later developmental stages, form muscle cells which contract rhythmically, thus rocking the embryo within the amniotic fluid. This rock­ing prevents the adhesion of amnion to the different embryonic membranes. It also helps in preventing the stagnation of blood in the vessels, a condition that might tend to occur on account of pressure from growing organs.

(e) The chorion at later developmental stage joins with the allantois to serve as a nutritional and respiratory organ.

iii. Functions of Allantois:

(a) Allantois acts as a reservoir for the secretions (excretory wastes) coming from the developing excretory organs. During early stages of development the chick excretes mostly urea, but later it becomes chiefly uric acid. This change is significant as urea is a relatively soluble substance and would require large amount of water to keep it at nontoxic level. Uric acid is relatively insoluble and can be stored without any ill effects.

(b) The chorioallantoic membrane acts as a respiratory surface for the embryo. Thus, the yolk sac, amnion, chorion and allantois can be regarded as an adaptation for the egg and embryo to carry on its develop­ment on dry land.


Lack of yolk in mammalian oocytes as compared to other vertebrates? - Biology

Trends in organ systems: reproduction in birds and mammals

As amniotes, birds and mammals have amniotic eggs with 4 extra embryonic membranes: yolk sac, allantois, amnion, chorion

adaptation for terrestrial life

Both usually produce few young at one time, and ensure their survival by caring for them.

Unlike mammals, birds lay eggs and initial stages of development occur outside the mother’s body.

Reproductive system of birds

The gonads develop from two sources during embryonic development. When the nephric ridges are well established, genital ridges appear on their surface.

This tissue forms sex cords which will develop into testes if the bird becomes a male or into the ovary if the bird becomes a female.

Several thousand cells near the base of the yolk sac become very large, migrate individually into the gut mesoderm, up the mesenteries and invade the genital ridges. These are the primitive sex cells that will form eggs and sperm.

Each embryo starts out with two sets of ducts - Wolffian and Mullerian ducts .

If bird becomes male, Wolffian ducts develop into vas deferens or sperm duct and Mullerian ducts atrophy. The reverse occurs in the developing female.

However, only the left Mullerian duct (and usually only the left ovary) develops.

Male reproductive system is straightforward.

Testes - composed of seminiferous tubules which produce large numbers of spermatozoa

Vas deferens - spermatozoa leave testes and accumulate in the vas deferens where they are held until copulation.

Sperm is transferred to female by a pseudo penis in some birds like ostriches and ducks.

In more derived birds, copulation involves simple juxtaposition of male and female cloacas.

Female reproductive system

Ovary is located high in abdominal cavity.

Only the left ovary develops in most birds.

Ovary enlarges during breeding season and shrinks for the rest of the year.

similar pattern for testes

weight reducing adaptation

A few oocytes begin to accumulate yolk.

As the yolk increases, the ova move towards the surface of the ovary and finally protrude from it.

Final growth of the ovum takes place here - 4 to 7 days.

Each protruding mature ovum and its thin covering of ovarian tissue is called an ovarian follicle .

Ovulation occurs when the follicle ruptures and releases the ovum into the body cavity.

Fertilization occurs in the body cavity or just after the ovum enters the oviduct. Normally only one ovum is released at a time.

The oviduct can be divided into 5 functional regions:

Contractile folds envelop the ovum as it breaks out of the ovarian follicle.

Ovum remains here for short period (20 minutes) then moves on to next section.

2nd and largest section of oviduct, lined with glandular cells that secrete albumen around the ovum.

Albumen composed of protein and water stored by use by embryo

Egg remains for 3 hours and passes on.

Narrow part of duct is isthmus.

Egg receives the shell membranes, fibrous keratin membranes.

large muscular uterus or shell gland

Calcareous shell is secreted by uterus, and is composed of a light protein framework like collagen and a heavy deposition of inorganic minerals, mainly calcium carbonate.

Minerals are arranged as vertical crystals separated by minute pores through which oxygen and carbon dioxide pass.

terminal section of oviduct

Mucus glands and muscular wall aid in laying egg.

Birds lay relatively few eggs compared to other vertebrates.

Eggs contain a considerable store of yolk (telolecithal, megalecithal eggs)

Incubate eggs - ensure rapid development and early hatching

Usually both parents involved to ensure high survival

Mammalian reproduction is very different but also produces few young with high survival.

Reproduction among all mammals is similar, in that all have internal fertilization and females nourish their young on secretions of mammary glands.

Differences in reproduction do occur and living mammals are divided into subclasses and infraclasses primarily on their method of reproduction.

Platypus and echidna retain their basic reptilian heritage as egg layers.

The ovaries are large and eggs are much larger than other mammals with large amounts of yolk.

Ova released into coelom and picked up by the infundibulum -> oviduct, where fertilization takes place -> uterus -> coated with leathery mineralized shell -> urogenital sinus -> cloaca (urogenital sinus and digestive tract meet) -> vented through a single opening

Egg laid into a nest, temporarily develop a simple pouch, incubate egg for 10 days. Mother may suckle young for 7 months.

Platypus lays 2 eggs, no pouch, lactate young once they hatch.

1 living order - Marsupialia (opossum, kangaroo)

Some structural features shared with monotremes and eutherians, but not an intermediate group, just a different reproductive strategy

Ova release -> oviduct, site of fertilization -> ova has more yolk than Eutheria but a lot less than Monotremes -> zygote moves into uterus where blastocyst implants into wall of uterus

Uterus is made of a thick muscular wall with a special lining which enlarges during the reproductive cycle in preparation for implantation.

Thickened wall of uterus is called endometrium - highly vascularized

The embryo is retained in the uterus for a relatively short period (12 days in opossum, a few weeks in the kangaroo)

Young are born at very early stage and climb outside belly of mother to pouch.

Embryo at birth in kangaroo is only 2 - 3 cm long.

At birth, the fetus passes down the vaginal canal to the urogenital sinus. The young then climbs to the marsupium for final development. Young attaches to nipple in pouch. Nipple swells to firmly attach young -> complex development

Paired ovaries where ova and female sex hormones are produced, ova released -> oviduct site of fertilization

Very little yolk embryo nourished by secretions and fusion from wall of uterus

Zygote moves down to uterus where endometrial wall has thickened and blastocyst implants very deeply very close association with fetal and maternal blood supply.

extended gestation period

Female reproduction tract has quite a bit of variation re # uteri, # cervices

Major achievement of mammals is refinement of viviparity, made possible by the evolution of the placenta in therians.

Placenta is not unique to mammals. Certain fish and reptiles have placenta-like connections allowing diffusion of materials between vascularized oviduct and embryo.

Mammalian placenta is much more effective in transferring materials.

Connection between embryo and uterus is necessary if young develop in uterus without large amount of yolk supplied as in dogfish.

Connecting structure is the placenta, consisting of embryonic and maternal tissues, and allowing nutritional, respiratory and excretory exchange of material by diffusion between embryonic and maternal circulatory systems. The marsupials have a yolk sac placenta (the initial stage in the development of the placenta in placental mammals. In placental mammals, the chorion and the allantois together form the fetal side of the mature placenta.

This is a chorioallantoic placenta.

Yolk sac is relatively small

Chorion forms part of fetal tissue, allantois becomes greatly enlarged and highly vascularized.

Chorioallantoic placenta is designed to provide high quantities of nutrients for long periods.

Blastocyst adheres to uterus, sinks into endometrium

As implantation proceeds, extensions from fetal side - chorionic villi - grow rapidly and push deep into endometrium.

Breakdown of uterine tissue next to the blastocyst produces nutritive substances for the embryo until the villi are functional.

Uterus becomes more highly vascularized at the site of implantation in response to the blastocyst.

When the placenta is fully formed, the highly vascularized villi provide a large surface area, for rapid exchange of materials between maternal and fetal circulation.

human - total length of villi = 50 km

Eutherians vary in the degree to which maternal and fetal blood streams are separated in the placenta.

Variation in number of layers of tissue between fetal and maternal blood, with loss of some of the tissue layers during placental development.

In nondeciduous placenta , 6 layers of tissue separate maternal and fetal circulation. (lemurs, cetaceans, some ungulates)

Oxygen and nutrients pass through the walls of the uterine blood vessels, layers of connective tissue epithelium, through walls of fetal blood vessels to fetal blood.

In deciduous placenta , destruction of placental tissue takes place to reduce the number of layers.

In humans, only two layers remain.

In rabbits and some rodents, only the endothelial linings of the fetal blood vessels in the villi separate fetal blood from surrounding maternal blood sinuses.

In all mammals, there is no mixing of fetal and maternal blood.

At birth ( parturition ), fetal part of placenta are expelled along with the fetus. In mammals with fewer layers, the wall of the endometrium is also lost, because of extensive erosion and intermingling of uterine and fetal tissues.

In marsupials, the young are born after a very short gestation period.

At birth, the hind limbs, eyes and other regions are poorly developed.

Small, underdeveloped young - 12 newborn opossums could

Forelimbs are well developed and capable of grasping.

Eutherians are born at later stage of development.

Some species’ young are able to move on their own very soon after birth ( precocial ), some are more dependent ( altricial ).

After parturition or hatching, all mammals feed on milk secreted by mammary glands

Unique feature of mammals

Monotremata - two mammary gland regions in abdomen

Secrete milk and young lap up the milk

In Eutheria, glands are concentrated and young suckle on nipples or teats (nipples most common)

Nipples have numerous ducts that release milk during suckling.

Teats have one common duct - cows, deer, etc

During pregnancy, hormonal and physiological changes (increases in estrogen and progesterone) stimulate development of mammary tissue in preparation for lactation.

Suckling of young stimulates nerves in nipples which cause oxytocin release, which returns to mammary gland and releases milk from alveoli of mammary glands

Colostrum - first fluid produced by mammary glands - is rich in antibodies, too large to cross the placental barrier.

Early stages of evolution unknown thought to have evolve from sebaceous glands associated with hair.

An abdominal incubation area probably evolved in the endothermic therapsids or early mammals.

Glands may have secreted recognition hormones, moistened the brood area to prevent desiccation of eggs and young

Some nutrients may have been in secretions, increasing survival of young. Strong selection would have favored increased nutrient content of secretions.

Text discusses the origins of lactation, suggesting that occlusion and diphyodonty would have required the previous evolution of lactation. With early milk, jaw could grow without teeth and larger teeth could erupt in a closer to adult jaw. Otherwise, functional dentition is required for baby to process its own food.

Milk provides young with continuous supply of food, while allowing female to forage at optimal times.

Mammal young often helpless and female provides nutrients and parental care. Young do not need to run risks attendant with foraging themselves.

3 major adaptations allowed mammals to retain their embryos in their reproductive tract and become viviparous.

2. specialization of female reproductive system (uterus), with secretions from uterus nourishing embryo

3. modification of extra-embryonic membranes to form the placenta, providing nutrients and disposing of wastes through the circulatory system of the mother.

Young protected within body of mother reduced predation and other risks

Keys to success of viviparity has been lactation, which provides for extended post-natal period of growth and development.


Reptiles and Birds

The amniotes&mdashreptiles, birds, and mammals&mdashare distinguished from amphibians by their terrestrially adapted (shelled) egg and an embryo protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes could develop within an aquatic environment inside the egg. This led to less dependence on a water environment for development and allowed the amniotes to invade drier areas. This was a significant evolutionary change that distinguished them from amphibians, which were restricted to moist environments due to their shell-less eggs. Although the shells of various amniotic species vary significantly, they all allow retention of water. The membranes of the amniotic egg also allowed gas exchange and sequestering of wastes within the enclosure of an eggshell. The shells of bird eggs are composed of calcium carbonate and are hard and brittle, but possess pores for gas and water exchange. The shells of reptile eggs are more leathery and pliable. Most mammals do not lay eggs however, even with internal gestation, amniotic membranes are still present.

In the past, the most common division of amniotes has been into classes Mammalia, Reptilia, and Aves. Birds are descended, however, from dinosaurs, so this classical scheme results in groups that are not true clades. We will discuss birds as a group distinct from reptiles with the understanding that this does not reflect evolutionary history.

Reptiles

Reptiles are tetrapods. Limbless reptiles&mdashsnakes&mdashmay have vestigial limbs and, like caecilians, are classified as tetrapods because they are descended from four-limbed ancestors. Reptiles lay shelled eggs on land. Even aquatic reptiles, like sea turtles, return to the land to lay eggs. They usually reproduce sexually with internal fertilization. Some species display ovoviviparity, with the eggs remaining in the mother&rsquos body until they are ready to hatch. Other species are viviparous, with the offspring born alive.

One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin, containing the protein keratin and waxy lipids, which prevented water loss from the skin. This occlusive skin means that reptiles cannot use their skin for respiration, like amphibians, and thus all must breathe with lungs. In addition, reptiles conserve valuable body water by excreting nitrogen in the form of uric acid paste. These characteristics, along with the shelled, amniotic egg, were the major reasons why reptiles became so successful in colonizing a variety of terrestrial habitats far from water.

Reptiles are ectotherms, that is, animals whose main source of body heat comes from the environment. Behavioral maneuvers, like basking to heat themselves, or seeking shade or burrows to cool off, help them regulate their body temperature,

Class Reptilia includes diverse species classified into four living clades. These are the Crocodilia, Sphenodontia, Squamata, and Testudines.

The Crocodilia (&ldquosmall lizard&rdquo) arose approximately 84 million years ago, and living species include alligators, crocodiles, and caimans. Crocodilians (Figure 15.6.8a) live throughout the tropics of Africa, South America, the southeastern United States, Asia, and Australia. They are found in freshwater habitats, such as rivers and lakes, and spend most of their time in water. Some species are able to move on land due to their semi-erect posture.

Figure 15.6.8: (a) Crocodilians, such as this Siamese crocodile, provide parental care for their offspring. (b) This Jackson&rsquos chameleon blends in with its surroundings. (c) The garter snake belongs to the genus Thamnophis, the most widely distributed reptile genus in North America. (d) The African spurred tortoise lives at the southern edge of the Sahara Desert. It is the third largest tortoise in the world. (credit a: modification of work by Keshav Mukund Kandhadai credit c: modification of work by Steve Jurvetson credit d: modification of work by Jim Bowen)

The Sphenodontia (&ldquowedge tooth&rdquo) arose in the Mesozoic Era and includes only one living genus, Tuatara, with two species that are found in New Zealand. There are many fossil species extending back to the Triassic period (250&ndash200 million years ago). Although the tuataras resemble lizards, they are anatomically distinct and share characteristics that are found in birds and turtles.

Squamata (&ldquoscaly&rdquo) arose in the late Permian living species include lizards and snakes, which are the largest extant clade of reptiles (Figure 15.6.8b). Lizards differ from snakes by having four limbs, eyelids, and external ears, which are lacking in snakes. Lizard species range in size from chameleons and geckos that are a few centimeters in length to the Komodo dragon, which is about 3 meters in length.

Snakes are thought to have descended from either burrowing lizards or aquatic lizards over 100 million years ago (Figure 15.6.8c). Snakes comprise about 3,000 species and are found on every continent except Antarctica. They range in size from 10 centimeter-long thread snakes to 7.5 meter-long pythons and anacondas. All snakes are carnivorous and eat small animals, birds, eggs, fish, and insects.

Turtles are members of the clade Testudines (&ldquohaving a shell&rdquo) (Figure 15.6.8d). Turtles are characterized by a bony or cartilaginous shell, made up of the carapace on the back and the plastron on the ventral surface, which develops from the ribs. Turtles arose approximately 200 million years ago, predating crocodiles, lizards, and snakes. Turtles lay eggs on land, although many species live in or near water. Turtles range in size from the speckled padloper tortoise at 8 centimeters (3.1 inches) to the leatherback sea turtle at 200 centimeters (over 6 feet). The term &ldquoturtle&rdquo is sometimes used to describe only those species of Testudines that live in the sea, with the terms &ldquotortoise&rdquo and &ldquoterrapin&rdquo used to refer to species that live on land and in fresh water, respectively.

Birds

Data now suggest that birds belong within the reptile clade, but they display a number of unique adaptations that set them apart. Unlike the reptiles, birds are endothermic, meaning they generate their own body heat through metabolic processes. The most distinctive characteristic of birds is their feathers, which are modified reptilian scales. Birds have several different types of feathers that are specialized for specific functions, like contour feathers that streamline the bird&rsquos exterior and loosely structured down feathers that insulate (Figure 15.6.9a).

Feathers not only permitted the earliest birds to glide, and ultimately engage in flapping flight, but they insulated the bird&rsquos body, assisting the maintenance of endothermy, even in cooler temperatures. Powering a flying animal requires economizing on the amount of weight carried. As body weight increases, the muscle output and energetic cost required for flying increase. Birds have made several modifications to reduce body weight, including hollow or pneumaticbones (Figure 15.6.9b) with air spaces that may be connected to air sacs and cross-linked struts within their bones to provide structural reinforcement. Parts of the vertebral skeleton and braincase are fused to increase its strength while lightening its weight. Most species of bird only possess one ovary rather than two, and no living birds have teeth in their jaw, further reducing body mass.

Figure 15.6.9: (a) Primary feathers are located at the wing tip and provide thrust secondary feathers are located close to the body and provide lift. (b) Many birds have hollow pneumatic bones, which make flight easier.

Birds possess a system of air sacs branching from their primary airway that divert the path of air so that it passes unidirectionally through the lung, during both inspiration and expiration. Unlike mammalian lungs in which air flows in two directions as it is breathed in and out, air flows continuously through the bird&rsquos lung to provide a more efficient system of gas exchange.


Discussion

In egg-laying animals, the principal nutrient source for embryonic development is yolk proteins, which accumulate in oocytes during oogenesis. Yolk precursors in most organisms are not synthesized by the oocyte but are produced by somatic cells and are incorporated into the oocyte through receptor-mediated endocytosis [41]. In Drosophila, yolk proteins, Yp1–3, are predominantly synthesized in fat bodies and ovarian follicle cells [42–44]. Yp1–3 are secreted from these tissues and are selectively taken up in the developing oocyte by the yolk receptor, Yl, which binds yolk proteins on the oocyte surface and mediates their internalization by clathrin-mediated endocytosis. It has been well known that Yl-mediated endocytosis is essential for incorporation and storage of nutrient yolk proteins. We now show that the process is also crucial for the maintenance of polarized microtubule arrays that promote osk mRNA localization, and long Osk-mediated actin remodeling that leads to the anchoring of germ plasm components to the oocyte cortex (S6 Fig).

In addition to Yp1–3 proteins, we have identified processed Apolpp fragments (ApoLI and ApoLII) as Yl-ligand proteins (Fig 1). Yolk proteins can be classified into 2 distinct groups [45]. The major group is vitellogenins, which are found in most oviparous animals such as fish, frog, chicken, and nematode. The other group is called yolk proteins and is found specifically in dipteran insects including Drosophila. Despite the similarity in their physiological roles, both groups are heterogeneous in their primary sequences. Given that Apolpp is a vitellogenin group protein, it is conceivable that dipteran insects also utilize vitellogenins as yolk proteins. In somatic cells, Drosophila Apolpp has vital functions in morphogenesis it forms complexes with lipid-linked signaling molecules, such as Hedgehog and Wingless, and helps to establish their proper morphogen gradient in somatic tissues such as wing discs [35,46]. Interestingly, RNAi-mediated apolpp knockdown in the fat body of the adult female causes degeneration of egg chambers during previtellogenic stages of oogenesis [47]. Thus, Apolpp appears to have, in addition to functions as yolk proteins, vital roles in oogenesis independently of Yl.

Yolk-depleted eggs from yp1–3 −/− apolpp −/+ females frequently completed embryogenesis and developed into adults (Fig 1O and S1 Data). These findings indicate that enormous amounts of yolk proteins are not necessarily critical for successful embryonic development. Similarly, in Caenorhabditis elegans eggs, yolk is largely dispensable for embryogenesis [48,49], but its titer impacts on postembryonic phenotypes such as developmental speed, starvation resistance, and fecundity [50]. In addition, we found that the yolk depletion caused reduction in the number of pole cells in stage-5 embryos (Fig 2G and S1 Data). Thus, there might be a mechanism for adjusting fecundity to food availability.

Yl was associated with long Osk presumably on endocytic vesicles (Fig 1). Yl is a single-pass transmembrane protein with a large extracellular domain that binds to yolk proteins [28] (S2A Fig). The cytoplasmic tail of Yl contains 2 endocytic sorting signals: the noncanonical FXNPXA sorting sequence and the atypical dileucine sequence, which are recognized by Ced-6 and the AP-2 complex, respectively [27]. Ced-6 and the AP-2 complex act as clathrin adaptors and redundantly promote endocytosis of Yl [27]. Given that long Osk is produced in the cytoplasm, it would associate with the cytoplasmic tail of Yl. It is an interesting future issue to examine whether the endocytic sorting signals of Yl and/or their binding adaptors are involved in association and function of long Osk.

We showed that Yl-mediated endocytosis was required for actin remodeling to form long actin projections that are proposed to anchor germ plasm components to the oocyte cortex (Fig 4). In oocytes defective in Yl-mediated endocytosis, cortical actin at the posterior region was disorganized, and ectopic Osk at the anterior pole induced aberrant F-actin aggregates. Similar defects have been observed in endocytosis-defective rab5 or rbsn-5 oocytes [11,15]. Notably, in yl −/− or Yl ligand-depleted oocytes, endocytosis can be activated in response to Osk (Fig 4). Thus, it is likely that aberrant actin remodeling in these mutant oocytes is caused specifically by the absence of long Osk-Yl association, rather than by a general decrease in endocytic activity. Our findings further suggest that an amount of Yl on endosomes might be critical for the proper actin remodeling to anchor germ plasm components to the oocyte cortex.

We previously identified several actin regulators (Mon2, Capu, and Spir) that are involved in Osk-mediated actin remodeling [15]. These actin regulators appear to be localized on membranous structures, such as endosomes. Mon2 is localized on Rab7- or Rab11-positive vesicles as well as Golgi in Drosophila and mammalian cells [15,51]. In addition, in mouse oocytes, proteins homologous to Spir (Spire1 and Spire2) and Capu (Formin-2) are colocalized on Rab11a-positive vesicles [52]. Depletion of Rab11a-positive vesicles in mouse oocytes leads to the release of these actin nucleators from vesicles, resulting in disorganization of cytosolic actin network [53]. We propose that the surface of long Osk and Yl-coated endocytic vesicles acts as platforms where the actin regulators promote the formation of long F-actin projections to anchor germ plasm components.

The yolk uptake-defective oocyte failed to maintain the localization of Kin-βgal, a microtubule plus-end marker, to the posterior cortex, resulting in the diffused localization of osk mRNA (Fig 3). In contrast, microtubule-dependent localization of Nod-βgal and bcd mRNA at the anterior or grk mRNA at the anterior-dorsal corner of the oocyte were all intact even in the absence of Yl-mediated endocytosis (Fig 3). These data indicate that effects caused by the malfunctioning of yolk uptake are limited in the posterior region. The posterior microtubule organization appears to be maintained by Osk-dependent recruitment of further microtubule plus ends [54]. Thus, the Yl-ligand axis may contribute to this Osk-dependent process.

Alternatively, the yolk uptake-dependent maintenance of microtubule organization at the oocyte posterior may be independent of Osk function. The posterior microtubule organization requires the plus-end-directed microtubule motor, Kinesin. It transports not only osk mRNA but also Dynactin to the posterior pole of the oocyte, which stabilizes the microtubule plus ends [55]. In addition, Kinesin drives bulk movement of the ooplasm during stages 10b to 12, known as ooplasmic streaming [56,57]. While the ooplasmic streaming is essential for the posterior accumulation of germ plasm components at late stages of oogenesis, its premature onset disrupts polarized microtubule arrays along the anterior-posterior axis, resulting in mislocalization of osk mRNA. The timing of the ooplasmic streaming is regulated by Capu and Spir [58]. Capu and Spir promote assembly of the ooplasmic actin mesh that is proposed to negatively regulate Kinesin [59,60]. Given the Capu- and Spir-mediated cortical actin remodeling, the ooplasmic actin mesh might also be disorganized in yolk uptake-defective oocytes, resulting in misregulation of the Kinesin-dependent processes to maintain posterior microtubule organization.

Intracellular localization of specific mRNAs in developing oocytes often occurs concurrently with vitellogenesis in oviparous vertebrates. In Xenopus, when the oocyte proceeds to the vitellogenic phase, several mRNAs including Vg1 and Dead end, which are initially distributed throughout the cytoplasm in previtellogenic stage oocytes, start to be localized to the vegetal cortex [61,62]. Similarly, in zebrafish oocytes, several mRNAs show specific localization patterns, including localization to the animal pole (e.g., zorba and c-mos) [63,64] or to the vegetal pole (e.g., mago nashi) [65] during vitellogenic stages. The transport and anchoring of Xenopus Vg1 mRNA to the vegetal cortex are microtubule- and actin-dependent processes [66]. Interestingly, the Xenopus Vg1 mRNA localization is disrupted by the depletion of vitellogenin [67]. Thus, the yolk uptake process appears to be linked to the cytoskeletal organization and mRNA localization in the vitellogenic-stage oocyte of diverse species.


Answers-1, BIO 3220, Early Development

2. Compare microlecithal, mesolecithal, and macrolecithal eggs. What does lecithal mean?
Microlecithal – small amount of yolk found in amphioxus and eutherians
Mesolecithal – medium amount of yolk found in lampreys, dipnoans, chondosteans and amphibians
Macrolecithal – large amount of yolk found in most fish, reptiles, birds, monotremes
Lecithal – yolk

3. Compare isolecithal and telolecithal eggs.
Isolecithal – even yolk distribution
Telolecithal – uneven yolk distribution

4. Define animal and vegetal pole.
Animal pole – the region of the egg with the highest metabolic activity
Vegetal pole – the region of the egg with the highest concentration of yolk

5. Describe the various membranes/enclosures around different eggs.
Vitelline membrane – a thin non-cellular membrane secreted by the oocyte and follicular cells that surround an oocyte and that will form the fertilization membrane if sperm penetration occurs
Jelly – protective covering of eggs for example, amphibian eggs
Capsule – outermost encapsulating structure of the egg, consisting of one or more membranes, the protective shell
Shell – hard protective covering of eggs
Albumin – the egg white of bird eggs formed in the oviduct before the addition of the shell
Corona Radiata – a layer of protective follicle cells derived from the cumulus oophorus surrounding the zona pellucida of an oocyte after ovulation
Zona Pellucida – a translucent non-cellular membrane surrounding a mammalian egg through which sperm must penetrate in order for fertilization to occur

6. Distinguish and give and example of oviparous, viviparous, and ovoviviparous animals.
Oviparous – producing eggs that develop and hatch outside the body of the female, as in many fish, birds, and many amphibians
Viviparous – bringing forth living young, rather than laying eggs producing live young from within the body of the parent female, as in most mammals
Ovoviviparous – retaining the eggs within the body of the female in a brood chamber in which the development of the embryo takes place, as in Squalus

7. Define zygote.
Zygote – Cell formed from the union of an egg and a sperm fertilized egg joined gametes

8. Compare cleavage, blastula formation, and gastrulation in microlecithal, mesolecithal, and macrolecithal eggs.
Microlecithal eggs – cleavage is equal blastula is a hollow ball of cells with a blastocoel gastrulation process is involution
Mesolecithal eggs – cleavage is unequal blastocoel is displaced into animal pole yolk impedes mitotic process more mitosis at animal pole gastrulation process is epiboly
Macrolecithal eggs –only mitotic division at animal pole blastoderm forms at animal pole gastrulation process is delamination

9. Name the 3 germ layers and list tissues derived from each of the layers.
Endoderm – first forms as outgrowth of inner cell mass in blastocyst in mammals grows down to surround the blastocoel which then becomes the yolk sack gives rise to digestive, liver, lungs, pancreas, thyroid gland, other glands
Ectoderm – cells of inner cell mass thicken and begin to multiply forming first the neural tube gives rise to CNS, sense organs, mammary glands, sweat glands, skin, hair, hooves
Mesoderm – separate from the ectoderm and fills space between endoderm and ectoderm gives rise to circulatory, skeletal, muscle, reproductive tracts, kidneys, urinary ducts

10. Explain the process of neurulation and organogenesis.
Neurulation begins with the formation of a neural plate, a thickening of the ectoderm caused when cuboidal epithelial cells become columnar. Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube. The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis. These cells become the neural crest cells.
During organogenesis considerable cell interactions and rearrangements occur which produces the tissues and organs of the body. Many cells undergo migrations from their place of origin during organogenesis and many organs contain cells from more than one germ layer.

11. Define the following terms –
Involution – enfolding tucking in
Epiboly – streaming in ex. cells move into middle of cell in gastrulation
Chordomesoderm – cells that form notochord during gastrulation
Epimere = dorsal mesoderm – the dorsal portion of the mesoderm of a chordate embryo that gives rise to the dermatone, myotome, and sclerotome
Somite – block of dorsal mesodermal cells adjacent to the notochord during vertebrate organogenesis. These transient structures define the segmental pattern of the embryo, and subsequently give rise to vertebrae and ribs, dermis of the back, and skeletal muscles of the back, body wall and limbs
Mesomere = intermediate mesoderm – layer of the mesoderm that gives rise to kidney tubules and associated ducts
Hypomere = lateral plate mesoderm – layer of the mesoderm that gives rise to the somatic, splanchnic, somatopleure, splanchnnopleure, and coelom layers
Somatic mesoderm – layer of the lateral plate mesoderm (hypomere) that contributes to bone of the girdles and limbs, muscles of body wall
Splanchnic mesoderm – layer of the lateral plate mesoderm (hypomere) that contributes to muscles of the heart, blood vessels and other visceral structures
Somatopleure – layer of the lateral plate mesoderm (hypomere) that forms the muscles and connective tissue of body wall
Splanchnnopleure – layer of the lateral plate mesoderm (hypomere) that forms the muscles and connective tissue of the gut tube
Coelom – layer of the lateral plate mesoderm (hypomere) that forms the body cavity
Primitive streak – thickening of the epiblast cell layer caused my movement of mesodermal cells into the blastocoel this structure is characteristic of avian, reptilian and mammalian gastrulation
Mesenchyme – mesodermal tissue that forms connective tissue and blood and smooth muscles mesoderm “on the move,” detaches and moves to another location
Notochord – a flexible rodlike structure in embryos of higher vertebrates, from which the spinal column develops
Blastoderm – the layer of cells formed by the cleavage of a fertilized mammalian egg, which later divides into the three germ layers from which the embryo develops
Delamination – the process by which the blastoderm forms 2 layers, the epiblast and the hypoblast the epiblast becomes the ectoderm, the hypoblast becomes the endoderm
Archenteron – the central cavity of the gastrula, which ultimately becomes the intestinal or digestive cavity
Dermatome – the part of a mesodermal somite from which the dermis develops
Myotome – the segment of a somite in a vertebrate embryo that differentiates into skeletal muscle
Sclerotome – the portion of a somite that proliferates mesenchyme which migrates about the notochord to form the axial skeleton and ribs

12. List the amniote classes.
Reptiles, birds and mammals

13. Define body stalk and umbilical cord.
Body stalk – a band of mesoderm that connects the yolk and the embryo
Umbilical cord – a cord that connects the fetus to the maternal placenta in viviparous mammals

14. Name the main extraembryonic membranes and discuss their functions.
Body stalk – connects the yolk and the embryo
Yolk sac – in birds, to nourish embryo in mammals atrophies, but source of blood cells and primordial germ cells
Amnion – non-vascular, fluid-filled protective cushion
Chorion – outermost membrane of embryo attachment to mother
Allantois – for gas exchange and waste receptacle
Placenta – allows for better waste removal and nutrient uptake primarily a yolk sac in marsupials


Regulators of Lipid Metabolism During Oocyte Maturation

It is well known that lipogenesis and lipolysis are important processes during oocyte maturation and embryo development, thus, the regulators of lipid metabolism had been thoroughly studied. Melatonin (N-acetyl-5-methoxytryptamine) which is synthesized during night time from the pineal gland of mammals, has antioxidant properties and regulates various physiological processes such as lipid profile and metabolic syndrome (Kozirog et al., 2011 Stehle et al., 2011 Kitagawa et al., 2012 Calvo et al., 2013 Reiter et al., 2016). Beneficial effects of melatonin on oocyte development have been documented in various mammalian species including sheep, cows, mice, cattle and pigs (Wang et al., 2013, 2014). Recently, the effects of different melatonin concentrations (10 3 , 10 5 , 10 7 , and 10 9 M) on lipid metabolism of the porcine oocytes during in vitro maturation have been investigated and a significant increase in the rate of blastocyst formation has been observed with 10 9 M concentration of melatonin compared to other experimental groups. Moreover, the upregulated expression of lipid metabolism-associated genes such as ACACA, FASN, PPARγ, and SREBF1 were noted in melatonin treated groups. Subsequently, the role of melatonin in lipolysis has also been evaluated and a greater uptake of FA has been observed in treated groups. Expression of fatty acid oxidation-related genes (CPT1a and b and CPT2 II) was noted to be higher in the melatonin group (Jin et al., 2017). These observations demonstrated the importance of melatonin in lipid metabolism for the acquisition of oocyte developmental competence.

Supplementation of antioxidants during in vitro maturation of bovine and other mammalian oocytes is necessary to decrease the generation of reactive oxygen species (ROS) as well as to neutralize the adverse effects on oocyte and embryo development (Jeong et al., 2006 Cicek et al., 2012). Ascorbic acid (AC) and α-tocopherol are well-known antioxidants that are generally used for ROS scavenging both in vivo and in vitro (Hossein et al., 2007). The addition of these molecules to culture media protects the oocytes and embryos from oxidative damage as well as improves the blastocyst formation. However, AC is sensitive to high temperature and humidity, and thus it should be encapsulated in methyl−β𢄬yclodextrin (CD) to form an inclusion complex that helps to increase the bioavailability of AC for the developing embryos (Hu et al., 2012). The effects of AC-cyclodextrin complex on the in vitro maturation efficiency and lipid metabolism of bovine oocytes has been extensively investigated. Interestingly, no obvious differences had been found in the nuclear maturation of the control and AC-cyclodextrin treated groups, however, AC-cyclodextrin treated oocytes and cumulus cells displayed differential expression of apoptosis and lipid metabolism associated genes. The expression of apoptosis related genes (BAX and BMP15) were downregulated in AC-cyclodextrin group while lipid metabolism associated gene (CYP51A1) expression was upregulated. Though neither blastocyst formation rate nor cleavage rate displayed any significant difference, the increased expression of CYP51A1 in CCs of AC-cyclodextrin group indicated that AC regulates the cholesterol synthesis during in vitro maturation of oocytes (Torres et al., 2019). Overall, these observations might lay the foundation for future improvement of in vitro oocyte culture by modifying the metabolism of lipids.


  • Access everything in the JPASS collection
  • Read the full-text of every article
  • Download up to 120 article PDFs to save and keep

Oocytes obtained freely from coeloma of breeding Salmo gairdneri and embryos from cleavage to early somite stages (aged between 0 and 8 days at 10 C, stages 2—11) were incubated for 2, 4 or 15 hours with radioactive pregnenolone, progesterone, dehydroepiandrosterone or estradiol-17β. The accumulation of radioactivity in embryos increased during incubation to over 50 % of the dose (about over 0.08 nmol/embryo. The yolk and chorion did not affect this accumulation, as shown by comparison of intact to yolk-free embryos. In all incubations the radioactivity was retained by oocytes more effectively than by embryos. Thin layer chromatographic analysis from embryos and medium showed minor conversion to metabolites in all cases. In embryos pregnenolone was slowly converted to progesterone. Progesterone was further reduced to 5α- and 5β-pregnanedione by oocytes as well as by embryos. In contrast to earlier results obtained with other vertebrates, rainbow trout embryos appeared to lack Δ4-steroid hydroxylases.

Annales Zoologici Fennici is an established, international, peer-reviewed journal, open to all scientists, appearing in six-number yearly volumes (until 2002 quarterly). It was founded in 1964 by Societas Biologica Fennica Vanamo replacing its predecessor which was published between 1932 and 1963. Between 1978 and 1994 it was published by the Finnish Zoological Publishing Board, and since 1994 by the Finnish Zoological and Botanical Publishing Board. Annales Zoologici Fennici publishes original research reports, in-depth reviews, short communications and commentaries on: ecology, paleoecology and ecometrics, paleontology (tertiary and quaternary) and evolution, conservation biology and wildlife management, animal behaviour and interactions, bioenergetics, genetics and phylogenetics.

This item is part of a JSTOR Collection.
For terms and use, please refer to our Terms and Conditions
Annales Zoologici Fennici © 1984 Finnish Zoological and Botanical Publishing Board
Request Permissions


14.9 Review Questions

Spermatogenesis is the process in which an animal produces ________ .

Oogenesis is the process in which an animal produces ________.

The gametes fuse to form _________, which develop via multiple successive mitoses and differentiation into new individuals.

In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves.

If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a ________ if the first pore becomes the anus then it is a ________ .

In triplobastic animals, the three tissue (germ) layers of the gastrula are the


Amphibians reproduce sexually with either external or internal fertilization. They attract mates in a variety of ways. For example, the loud croaking of frogs is their mating call. Each frog species has its own distinctive call that other members of the species recognize as their own. Most salamanders use their sense of smell to find a mate. The males produce a chemical odor that attracts females of the species.

Amphibian Eggs

Unlike other tetrapod vertebrates (reptiles, birds, and mammals), amphibians do not produce amniotic eggs. Therefore, they must lay their eggs in water so they won&rsquot dry out. Their eggs are usually covered in a jelly-like substance, like the frog eggs shown in Figure below. The &ldquojelly&rdquo helps keep the eggs moist and offers some protection from predators.

Frog Eggs. Frog eggs are surrounded by &ldquojelly.&rdquo What is its function?

Amphibians generally lay large number of eggs. Often, many adults lay eggs in the same place at the same time. This helps to ensure that eggs will be fertilized and at least some of the embryos will survive. Once eggs have been laid, most amphibians are done with their parenting.

Amphibian Larvae

The majority of amphibian species go through a larval stage that is very different from the adult form, as you can see from the frog in Figure below. The early larval, or tadpole, stage resembles a fish. It lacks legs and has a long tail, which it uses to swim. The tadpole also has gills to absorb oxygen from water. As the larva undergoes metamorphosis, it grows legs, loses its tail, and develops lungs. These changes prepare it for life on land as an adult frog.

Frog Development: From Tadpole to Adult. A frog larva (tadpole) goes through many changes by adulthood. Notice the visible changes that occur at each stage. How do these changes prepare it for life as an adult frog?


Watch the video: Egg Types in Embryology. A, Micro, Meso, Telo, Centrolecithal Eggs (January 2022).