What is this part of blastula called and how is endoderm formed?

Could you tell me what the yellow mass within the blastocoel is called ? I know that blastocoel is a fluid filled cavity, so are the yellow coloured cells the inner part of blastoderm or are they some other cells ?

If they are the part of blastoderm itself, why is it not in the same colour of light-orange as the blastoderm, or is it the mesoderm, or the inner cell mass ?

And is it the blastoderm that forms ectoderm , and endoderm pinches off the ectoderm during gastrula phase? And could you also explain how the orange mesoderm formed ?

The blastocoel, as you say yourself, is a fluid-filled cavity (wikipedia) so it has no cells. Fig. 1 shows cellular and non-cellular regions of the blastocyst.

Your questions on the formation of the three germ layers during germination are nicely shown in the illustration in Fig. 2, right two panels.

Fig. 1. Blastocyst. source: wikipedia

Fig. 2. Formation of the germ layers. source: Class Connection

What is this part of blastula called and how is endoderm formed? - Biology

In fertilization, the sperm binds to the egg, allowing their membranes to fuse and the sperm to transfer its nucleus into the egg.

Learning Objectives

Describe the process of fertilization

Key Takeaways

Key Points

  • A mammalian egg is covered by a layer of glycoproteins called the zona pellucida, which the sperm must penetrate in order to fertilize the egg.
  • Upon binding with the egg, the sperm initiates the acrosome reaction, in which it releases digestive enzymes that degrade the zona pellucida, allowing the plasma membrane of the sperm to fuse with that of the egg.
  • Upon fusion of the two plasma membranes, the sperm’s nucleus enters the egg and fuses with the nucleus of the egg.
  • Both the sperm and the egg each contain one half the normal number of chromosomes, so when they fuse the resulting zygote is a diploid organism with a complete set of chromosomes.
  • When the egg is successfully fertilized, it releases proteins that prevent it from being fertilized by another sperm, a condition known as polyspermy.

Key Terms

  • fertilization: the act of fecundating or impregnating animal or vegetable gametes
  • zona pellucida: a glycoprotein membrane surrounding the plasma membrane of an oocyte
  • acrosome: a structure forming the end of the head of a spermatozoon
  • polyspermy: the penetration of an ovum by more than one sperm


Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm are haploid, which means they each contain one set of chromosomes upon fertilization, they will combine their genetic material to form a zygote that is diploid, having two sets of chromosomes. A zygote that has more than two sets of chromosomes will not be viable therefore, to ensure that the offspring has only two sets of chromosomes, only one sperm must fuse with one egg.

In mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reaction, take place. In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane. The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. Fusion between the oocyte plasma membrane and sperm follows and allows the sperm nucleus, centriole, and flagellum, but not the mitochondria, to enter the oocyte. The nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome. This process ultimately leads to the formation of a diploid cell called a zygote. The zygote divides to form a blastocyst and, upon entering the uterus, implants in the endometrium, beginning pregnancy.

Process of fertilization: (a) Fertilization is the process in which sperm and egg fuse to form a zygote. (b) Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus.

To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. If this mechanism fails, multiple sperm can fuse with the egg, resulting in polyspermy. The resulting embryo is not genetically viable and dies within a few days.

Endoderm Function

The endoderm will become the digestive tract (or gut), as well as a number of associated organs and glands. It will give rise to the lungs, liver, and pancreas, as well as the thymus, thyroid, and parathyroid glands. In addition, endoderm cells will form the lining of many of the body’s organ systems including the respiratory system, the digestive system, the urinary system, and the reproductive system.

This figure depicts the organs and glands that develop from the endoderm. These include the digestive and respiratory systems, and the thyroid, parathyroid, and thymus glands.

1. What structure is not formed by endoderm cells?
A. thymus gland
B. pituitary gland
C. thyroid gland
D. parathyroid gland

2. What structure will go on to form the gut?
A. archenteron
B. blastocoel
C. blastopore
D. yolk cells

3. Where are the primitive endoderm cells found?
A. animal region
B. equatorial band
C. marginal zone
D. vegetal region

Early Development of Epidermis and Neural Tissue

Epidermal Development as a Result of BMP Signaling

Dissociation of blastula ectoderm leads to the suppression of epidermal markers, which can be overcome by exogenous BMP4. Similarly, blocking BMP signaling with a dominant negative BMP receptor inhibits epidermal differentiation and activated neural tissue markers in animal cap explants ( Wilson and Hemmati-Brivanlou, 1997 ). Moreover, BMP2, 4 and 7 are expressed in the early Xenopus embryo and promote the epidermal fate ( De Robertis and Kuroda, 2004 Harland, 2000 Wilson and Hemmati-Brivanlou, 1997 ). These findings support the notion that BMP signaling is responsible for epidermal development.

Mutagenesis screens in zebrafish implicated a large number of BMP signaling mediators in the specification of ventral cell fates ( De Robertis and Kuroda, 2004 ). In Xenopus embryos, knockdown of BMP2, 4 and 7 resulted in the significant expansion of neural tissue, but epidermis still developed ( Reversade and De Robertis, 2005 ). The same study also described anti-dorsalizing morphogenetic protein (ADMP), a member of the TGFβ superfamily with BMP4-like activity. Interestingly, a combined knockdown of ADMP, BMP2, BMP4 and BMP7 with specific MOs converts the entire epidermis into neural tissue ( Reversade and De Robertis, 2005 ). Thus, multiple, redundant, BMP proteins direct epidermal development ( Figure 11.2 ).

Endoderm Function

The endoderm will become the digestive tract (or gut), as well as a number of associated organs and glands. It will give rise to the lungs, liver, and pancreas, as well as the thymus, thyroid, and parathyroid glands. In addition, endoderm cells will form the lining of many of the body’s organ systems including the respiratory system, the digestive system, the urinary system, and the reproductive system.

The gut is formed during gastrulation when the endoderm and mesoderm move inside the embryo in a process called invagination. As the cells move into the interior of the embryo the dorsal endoderm forms a line of cells along the mesoderm, and a gap forms between the dorsal endoderm and the vegetal endoderm cells. This gap is the archenteron which is the precursor of the gut cavity.

What is this part of blastula called and how is endoderm formed? - Biology

The early stages of embryonic development, such as fertilization, cleavage, blastula formation, gastrulation, and neurulation, are crucial for ensuring the fitness of the organism.

Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. The egg and sperm each contain one set of chromosomes. To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. The acrosomal reaction causes an egg to prevent additional sperm from penetrating. As the egg completes meiosis II, sperm and egg nuclei fuse.

The formed zygote undergoes rapid cell division to form the blastula. The rapid, multiple rounds of cell division are termed cleavage that produces over 100 cells in the embryo. This process is called the blastula formation . During cleavage, the cells divide without an increase in mass that is, one large single-celled zygote divides into multiple smaller cells. As the blastula forms the blastocyst in the next stage of development, the cells in the blastula arrange themselves into the inner cell mass, and an outer layer.

The typical blastula is a ball of cells. The next stage in embryonic development is the first cell movement and the formation of the primary germ layers. The cells in the blastula rearrange themselves spatially to form three layers of cells. This process is called gastrulation . These three germ layers are the endoderm, the ectoderm , and the mesoderm . The ectoderm gives rise to skin and the nervous system the endoderm to the intestinal organs and the mesoderm to the rest of the organs.

Following gastrulation, the neurulation process develops the neural tube in the ectoderm, above the notochord of the mesoderm. The ectoderm gives rise to the nervous system by folding into a neural tube.

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• Upon fusion of the two plasma membranes, the sperm’s nucleus enters the egg and fuses with the nucleus of the egg.

• Both the sperm and the egg each contain one half the normal number of chromosomes, so when they fuse, the resulting zygote is a diploid organism with a complete set of chromosomes.

• Gastrulation takes place after cleavage and the formation of the blastula.

• The ectoderm gives rise to skin and the nervous system the endoderm to the intestinal organs and the mesoderm to the rest of the organs.

• Neurulation is the formation of the neural tube from the ectoderm, which forms into a neural tube.

blastula : a 6-32-celled hollow structure that is formed after a zygote undergoes cell division

inner cell mass : a mass of cells within a primordial embryo that will eventually develop into the distinct form of a fetus in most eutherian mammals

gastrulation : the stage of embryo development at which a gastrula is formed from the blastula by the inward migration of cells

neurulation : The process that forms the vertebrate nervous system in embryos.

notochord : Composed of cells derived from the mesoderm, this provides signals to the surrounding tissue during development.

Developmental Biology. 6th edition.

Cleavage in most frog and salamander embryos is radially symmetrical and holoblastic, just like echinoderm cleavage. The amphibian egg, however, contains much more yolk. This yolk, which is concentrated in the vegetal hemisphere, is an impediment to cleavage. Thus, the first division begins at the animal pole and slowly extends down into the vegetal region (Figure 10.1 see also Figures 2.2D and 8.4). In the axolotl salamander, the cleavage furrow extends through the animal hemisphere at a rate close to 1 mm per minute. The cleavage furrow bisects the gray crescent and then slows down to a mere 0.02𠄰.03 mm per minute as it approaches the vegetal pole (Hara 1977).

Figure 10.1

Cleavage of a frog egg. Cleavage furrows, designated by Roman numerals, are numbered in order of appearance. (A, B) Because the vegetal yolk impedes cleavage, the second division begins in the animal region of the egg before the first division has divided (more. )

Figure 10.2A is a scanning electron micrograph showing the first cleavage in a frog egg. One can see the difference in the furrow between the animal and the vegetal hemispheres. Figure 10.2B shows that while the first cleavage furrow is still cleaving the yolky cytoplasm of the vegetal hemisphere, the second cleavage has already started near the animal pole. This cleavage is at right angles to the first one and is also meridional. The third cleavage, as expected, is equatorial. However, because of the vegetally placed yolk, this cleavage furrow in amphibian eggs is not actually at the equator, but is displaced toward the animal pole. It divides the frog embryo into four small animal blastomeres (micromeres) and four large blastomeres (macromeres) in the vegetal region. This unequal holoblastic cleavage establishes two major embryonic regions: a rapidly dividing region of micromeres near the animal pole and a more slowly dividing vegetal macromere area (Figure 10.2C Figure 2.2E). As cleavage progresses, the animal region becomes packed with numerous small cells, while the vegetal region contains only a relatively small number of large, yolk-laden macromeres.

Figure 10.2

Scanning electron micrographs of the cleavage of a frog egg. (A) First cleavage. (B) Second cleavage (4 cells). (C) Fourth cleavage (16 cells), showing the size discrepancy between the animal and vegetal cells after the third division. (A from Beams and (more. )

An amphibian embryo containing 16 to 64 cells is commonly called a morula (plural: morulae from the Latin, “mulberry,” whose shape it vaguely resembles). At the 128-cell stage, the blastocoel becomes apparent, and the embryo is considered a blastula. Actually, the formation of the blastocoel has been traced back to the very first cleavage furrow. Kalt (1971) demonstrated that in the frog Xenopus laevis, the first cleavage furrow widens in the animal hemisphere to create a small intercellular cavity that is sealed off from the outside by tight intercellular junctions (Figure 10.3). This cavity expands during subsequent cleavages to become the blastocoel.

Figure 10.3

Formation of the blastocoel in a frog egg. (A) First cleavage furrow, showing a small cleft, which later develops into the blastocoel. (B) 8-cell embryo showing a small blastocoel (arrow) at the junction of the three cleavagefurrows. (From Kalt 1971 (more. )

The blastocoel probably serves two major functions in frog embryos: (1) it permits cell migration during gastrulation, and (2) it prevents the cells beneath it from interacting prematurely with the cells above it. When Nieuwkoop (1973) took embryonic newt cells from the roof of the blastocoel, in the animal hemisphere, and placed them next to the yolky vegetal cells from the base of the blastocoel, these animal cells differentiated into mesodermal tissue instead of ectoderm. Because mesodermal tissue is normally formed from those animal cells that are adjacent to the vegetal endoderm precursors, it seems plausible that the vegetal cells influence adjacent cells to differentiate into mesodermal tissues. Thus, the blastocoel appears to prevent the contact of the vegetal cells destined to become endoderm with those cells fated to give rise to the skin and nerves.

While these cells are dividing, numerous cell adhesion molecules keep the blastomeres together. One of the most important of these molecules is EP-cadherin. The mRNA for this protein is supplied in the oocyte cytoplasm. If this message is destroyed (by injecting antisense oligonucleotides complementary to this mRNA into the oocyte), the EP-cadherin is not made, and the adhesion between the blastomeres is dramatically reduced (Heasman et al. 1994a,b), resulting in the obliteration of the blastocoel (Figure 10.4).

Figure 10.4

Depletion of EP-cadherin mRNA in the Xenopus oocyte results in the loss of adhesion between blastomeres and the obliteration of the blastocoel. (A) control embryo (B) EP-cadherin-depleted embryo. (From Heasman et al. 1994b photographs courtesy of J. (more. )


The blastula stage of early embryo development begins with the appearance of the blastocoel. The origin of the blastocoel in Xenopus has been shown to be from the first cleavage furrow, which is widened and sealed with tight junctions to create a cavity. [11]

In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore exclusively from the mother. [12] [13]

Midblastula transition Edit

In many organisms including Xenopus and Drosophila, the midblastula transition usually occurs after a particular number of cell divisions for a given species, and is defined by the ending of the synchronous cell division cycles of the early blastula development, and the lengthening of the cell cycles by the addition of the G1 and G2 phases. Prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle. [13] The addition of the two growth phases into the cell cycle allows for the cells to increase in size, as up to this point the blastomeres undergo reductive divisions in which the overall size of the embryo does not increase, but more cells are created. This transition begins the growth in size of the organism. [3]

The mid-blastula transition is also characterized by a marked increase in transcription of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point, either by proteins such as SMAUG in Drosophila [14] or by microRNA. [15] These two processes shift the control of the embryo from the maternal mRNA to the nuclei.

A blastula is a sphere of cells surrounding a blastocoel. The blastocoel is a fluid filled cavity which contains amino acids, proteins, growth factors, sugars, ions and other components which are necessary for cellular differentiation. The blastocoel also allows blastomeres to move during the process of gastrulation. [16]

In Xenopus embryos, the blastula is composed of three different regions. The animal cap forms the roof of the blastocoel and goes on primarily to form ectodermal derivatives. The equatorial or marginal zone, which compose the walls of the blastocoel differentiate primarily into mesodermal tissue. The vegetal mass is composed of the blastocoel floor and primarily develops into endodermal tissue. [7]

In the mammalian blastocyst (term for mammalian blastula) there are three lineages that give rise to later tissue development. The epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac. [6]

In mouse embryo, blastocoel formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient which is the result of Na + /K + ATPases that produce a high Na + gradient on the basolateral side of the trophectoderm. This movement of water is facilitated by aquaporins. A seal is created by tight junctions of the epithelial cells that line the blastocoel. [6]

Cellular adhesion Edit

Tight junctions are very important in embryo development. In the blastula, these cadherin mediated cell interactions are essential to development of epithelium which are most important to paracellular transport, maintenance of cell polarity and the creation of a permeability seal to regulate blastocoel formation. These tight junctions arise after the polarity of epithelial cells is established which sets the foundation for further development and specification. Within the blastula, inner blastomeres are generally non-polar while epithelial cells demonstrate polarity. [16]

Mammalian embryos undergo compaction around the 8-cell stage where E-cadherins as well as alpha and beta catenins are expressed. This process makes a ball of embryonic cells which are capable of interacting, rather than a group of diffuse and undifferentiated cells. E-cadherin adhesion defines the apico-basal axis in the developing embryo and turns the embryo from an indistinct ball of to a more polarized phenotype which sets the stage for further development into a fully formed blastocyst. [16]

Xenopus membrane polarity is established with the first cell cleavage. Amphibian EP-cadherin and XB/U cadherin perform a similar role as E-cadherin in mammals establishing blastomere polarity and solidifying cell-cell interactions which are crucial for further development. [16]

Fertilization technologies Edit

Experiments with implantation in mice show that hormonal induction, superovulation and artificial insemination successfully produce preimplantation mouse embryos. In the mice, ninety percent of the females were induced by mechanical stimulation to undergo pregnancy and implant at least one embryo. [17] These results prove to be encouraging because they provide a basis for potential implantation in other mammalian species, such as humans.

Stem cells Edit

Blastula-stage cells can behave as pluripotent stem cells in many species. Pluripotent stem cells are the starting point to produce organ specific cells that can potentially aid in repair and prevention of injury and degeneration. Combining the expression of transcription factors and locational positioning of the blastula cells can lead to the development of induced functional organs and tissues. Pluripotent Xenopus cells, when used in an in vivo strategy, were able to form into functional retinas. By transplanting them to the eye field on the neural plate, and by inducing several mis-expressions of transcription factors, the cells were committed to the retinal lineage and could guide vision based behavior in the Xenopus. [18]

Formation and Fate of Three Germ Layers

Transformation of blastula or blastocyst into gastrula is called gastrulation. During gastrulation the cells of the inner cell mass of blastocyst or blastula move in small mass to their new final location.

Such movement of cells is called morphogenetic movements Gastrulation results in the formation of three germ layers: ectoderm, mesoderm and endoderm.

Each germ layer gives rise to specific tissues, organs and organ-systems.

The fate of the germ layers is the same in all triploblastic animals.

(i) Formation of Endoderm (Fig. 3(B).13):

The blastocyst grows in size by obtaining nutrition from the uterus. Some cells separate from the inner cell mass (embryonic knob) to form endoderm in blastocoel. The endoderm differentiates into the primitive gut a part of it gives rise to alimentary canal and the other portion forms yolk sac. After the formation of endoderm, the remaining mass of cells of the inner cell mass forms embryonic disc. It has three parts: cephalic margin, embryonic disc proper and caudal margin.

(ii) Formation of Mesoderm (Fig. 3(B). 14):

Mesoderm is formed from the caudal margin of the embryonic disc. Prior to this the existing cells undergo rapid division and a mass of cells detach from the embryonic disc to form mesoderm.

(iii) Formation of Ectoderm (Fig. 3(B). 14):

After the separation of mesoderm, the remaining cells of the embryonic disc form the ectoderm layer. In this manner the three germ layers such as ectoderm, mesoderm and endoderm are formed.

Fate of Three Germ Layers (Fig. 3(B). 15):

Each germ layer forms specific tissues, organs and organ-systems.

The three germ layers produce tissues, organs and organ-system in following manner:

1. Ectoderm:

(i) Epidermis of skin, epidermal derivatives like epidermal glands, hair, nail etc.

(iii) Medulla of adrenal gland, posterior and intermediate lobes of pituitary gland, pineal gland,

(iv) Eye (conjunctiva, cornea, lens, retina, iris and ciliary muscles),

(vi) Nasal and olfactory epithelia,

(viii) Epithelium of fore gut and hind gut

(ix) Some glands—sweat glands, oil glands, mammary glands, salivary glands and lacrimal glands.

2. Mesoderm:

(ii) Muscles except iris and ciliary muscles,

(vii) Heart, blood and lymph vessels,

(viii) Urinary and reproductive ducts,

(xi) Pericardium and pleura,

(xiii) Cortex of adrenal gland,

(xv) Sclera and choroid of eyes,

(xvi) Wall of the gut except its lining.

3. Endoderm:

(i) Lining of gut except for gut and hind gut,

(ii) Some glands—pancreas, liver, gastric glands, intestinal glands, thyroid, parathyroid, thymus and larger part of prostate,

(iii) Inner layer of tympanic membrane,

(v) Trachea, bronchi and lungs,

Plant Embryonic Development

Similar to animals, plant embryogenesis occurs as the result of sexual reproduction via the fertilization of the ovule via pollination. The resulting diploid zygote is accompanied by an endosperm, which together form the seed (shown below). The endosperm consists of dense nutrients which will supply the growing embryo.

After undergoing asymmetrical cell division, a small apical cell, which forms the embryo, and a larger basal cell that supplies nutrients to the embryo via the endosperm are formed. At the eight-cell stage, the embryo begins to flatten, forming the axis of the structures which will eventually form the shoot meristem, cotyledons, hypocotyl, roots, and root meristem. In plants, the embryonic stage ends with germination, when the plant begins to grow out of the seed. At this stage, the plant is termed a plantling. The specific processes involved in plant embryogenesis differ depending on the species of plant.

Watch the video: Embryology. Ectoderm (January 2022).