How long do human egg cell grow up?

I read article about alcohol harm where written that child may bear with birth defect cause mother used to drink alcohol 10 years ago so egg cell stay damages that long.

I don't believe that human egg cell wait for such long time until activated.

How long human egg cell grow up until sperm fuse it?

How long human egg cell grow up until sperm fuse it?

Ovules are made while the female is still in the uterus of her mother. The ovules (in the form of ovarian follicles) are kept in the ovaries for a long time, and just a month before leaving the ovary (ovulation), the ovarian follicle (and the future ovule) gain in size. In other words, the ovules are slightly older than the age (counting from birth/labour) than the person carrying them. You'll find more information on wikipedia > ovule > ovule Development, oogenesis, and ovarian follicle.

Does it address your interest about alcohol consumption?

Now, I don't think this question/answer address your interest about alcohol consumption. Whether or not the ovule is create by a division that happen just a day before ovulation or many years earlier doesn't change anything to the fact that this ovule has experienced alcohol consumption (either directly or in the lineage) before.

Where to ask the question that I suspect interest you the most

If you want to question the article you read about alcohol consumption, the best you could do is to go on Skeptics.SE, link the article, quote something from the article and ask "is it true?"

Foetus in the womb

An egg from the mother is fertilised by a sperm from the father and turns into an embryo inside the mother’s womb. At first this creation looks like a bundle of cells. By about eight weeks this bundle of cells gradually turns into the shape of the human body. This is called the foetus . The foetus totally depends on its mother as it cannot breathe, drink or eat by itself.

Example of a foetus in the womb

After nine months in the mother’s womb, the baby is born. Babies from birth to 1 year are also known as infants . Newborn babies can breathe, suck, swallow and cry when they feel hungry, cold and hot temperatures or any uncomfortable situation. This is how they communicate as they still cannot talk. Babies are usually fed on mother’s milk.

A baby
birth – 1 year

A Fertility First: Human Egg Cells Grow Up in Lab

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For the first time, scientists have managed to grow mature human eggs from immature cells in the lab, a technique that may eventually help save the fertility of female cancer patients who aren't eligible for traditional egg harvest.

Researchers from Northwestern University took immature egg cells, encased in a protective sac called a follicle, from 14 women who wanted to preserve their fertility before undergoing chemotherapy. By placing the cells in a unique three-dimensional growing environment for 30 days, the scientists coaxed the cells into becoming what appear to be healthy, functional human eggs.

"It is a major first," said infertility expert Sherman Silber of St. Luke's Hospital in St. Louis, who was not involved in the research. "No one has yet tested the eggs by in-vitro fertilization and pregnancy, but they look quite normal and we are all excited about it."

The traditional way to preserve a female cancer patient's fertility is to surgically remove fully mature eggs from her ovary, fertilize the eggs immediately in the lab, and freeze the resulting embryos. But since only one follicle matures each month for ovulation, that method requires two to six weeks of hormone therapy to generate enough mature eggs for harvest.

"The cancer patient usually does not have this much time to waste," Silber said. "And to give her any kind of assurance, you would have to put her through three to six or more such cycles to get enough eggs to be comfortable. With ovary tissue freezing, we will get hundreds of thousands of eggs, and the patient can get a much greater measure of security for her future fertility."

In addition, giving high doses of hormones is dangerous for patients with certain types of cancer, such as breast or ovarian tumors, and the traditional method won't work for girls who haven't gone through puberty. If physicians could take immature ovarian follicles and grow them into eggs outside the body, they could skip the hormone step altogether.

Until now, however, no one has been able to grow human ovarian follicles in the lab. Most previous attempts had involved trying to culture the cells in a two-dimensional environment, but it turns out that any kind of pressure on the egg cells inhibits their growth.

"Scientists had been putting them on a flat piece of plastic for years," said fertility researcher Teresa Woodruff of Northwestern University, a co-author on the paper published Monday in Human Reproduction. "When you do that, the cells around the egg begin to move away, and the connection between the egg and its supporting cells is lost."

The supporting cells are critical, Woodruff said, because they provide the hormones and nutrients that the egg needs to grow. To create the ideal growing environment for a follicle, the researchers collaborated with biomedical engineers who specialize in biomaterials.

"Our breakthrough was to use a hydrogel called alginate, which doesn't touch or contact the follicle cells, but just supports them," Woodruff said. It turns out that the rigidity of the gel is crucial to follicle function: If the gel is too stiff, the follicles start looking sick and making the wrong kind of hormones. "We kind of lucked out in the very beginning in that we used a very soft gel," she said.

After incubating the follicles for 30 days in the three-dimensional matrix, the researchers discovered that they had grown to the size of mature eggs and were producing all the right hormones, such as estrogen and progesterone, in the right quantities.

The true test of the lab-grown eggs will be to see if they can undergo a final stage of cell division to be ready for fertilization.

"We did it in a mouse and got it all the way to fertilization and live birth," Woodruff said. But regulations from the National Institutes of Health won't let scientists fertilize human eggs for research — the final step of the process must be done by physicians in an infertility clinic for a patient who's ready to have a baby.

"The proof would be if they could fertilize the eggs in vitro and get babies out of it," Silber said. "But that's an extra stage, and it's amazing they've got this far."

"Ten years ago it was considered such an impossible task that we didn't think it would happen for another 50 years," he said. "The amazing thing is that it turned out to be much simpler than we ever dreamed."

Image: An in vitro matured human egg. Image captured by Susan Barrett on a Leica SP5 confocal with resonance scanner.

Getting a Grasp

Study of Arms and Hands, a sketch by Leonardo da Vinci popularly considered to be a preliminary study for the painting "Lady with an Ermine." - Wikimedia

Wolpert believes it is important to distinguish growth from symmetry. Symmetry means similarity between two things, like cutting an apple down its center and looking at two sides that are almost the mirror-image of one another. It is harder to tell why paired body parts like arms and legs grow at the same pace and stay “in-sync” with each other.

Look at your arms. If your right arm was shorter than your left, it would be more difficult to do some things. Now imagine having a small left hand and a really big right hand. You wouldn’t be as good at playing video games, now would you?

As you grow up, your cells grow with you. Human bodies grow because their cells are in a constant cycle of proliferation. That means that new cells are created to replace old and dying cells. Hormones and organs in the endocrine system are also important because they signal and guide these new cells to tell them what kind of tissue to be, and what their role is in the body. Ever wonder why girls seem to be taller than boys during elementary and middle school? Estrogen, a female-specific hormone, helps bones in the legs, arms, and spine grow at a faster rate. Girls have more estrogen than boys do, so they are taller than boys during early adolescence.

Wolpert believes that bones grow because there is “some sort of signal system,” as part of a “growth plate,” in our bones that controls how long they grow. The growth plates produce cells that help bones to grow, but eventually the cells run out. Scientists like Wolpert are stumped as to how growth plates between matching arms or legs manage to grow the paired bones at exactly the same rate.

Diseases & conditions

Most ovarian problems are caused by cysts. Ovarian cysts, growths on the ovaries, are common and most women will get them at least once, according to the Mayo Clinic. Most women don't even know when they have one because typically they are not painful or anything to worry about.

Polycystic ovary syndrome (PCOS) is an ailment defined by multiple cysts growing on the outer edge of the ovaries due to a lack of hormones that allow an egg to be released from the follicle. This disorder can lead to infertility and other serious complications such as heart disease, diabetes or stroke.

Sometimes a cyst will become cancerous. One in 75 women will develop ovarian cancer, according to the American Cancer Society. There are current tests that can help detect a woman's likelihood of developing ovarian cancer. In some cases, women choose to remove their ovaries as a precautionary measure.

"If you have your ovaries removed due to certain hereditary cancer screening results such as BRCA, then we also remove your fallopian tubes because you can also get cancer from your fallopian tubes," said Dr. Sarah Yamaguchi, an OB/GYN at Good Samaritan Hospital in Los Angeles, California. "However, even with that done, you can still get primary peritoneal cancer which is very similar to ovarian cancer."

Adults in Herds

Females will usually live their entire lives in the same herd, mating with the dominant male each year. However, sometimes the herd may be broken up during fights between rival males.

Young males will often stay with the herd and challenge the dominant male for breeding rights. Sometimes males will leave the herd and look for another male to challenge.

Sloppy science or groundbreaking idea? Theory for how cells organize contents divides biologists

For 7 years as president of the Howard Hughes Medical Institute, Robert Tjian helped steer hundreds of millions of dollars to scientists probing provocative ideas that might transform biology and biomedicine. So the biochemist was intrigued a couple of years ago when his graduate student David McSwiggen uncovered data likely to fuel excitement about a process called phase separation, already one of the hottest concepts in cell biology.

Phase separation advocates hold that proteins and other molecules self-organize into denser structures inside cells, like oil drops forming in water. That spontaneous sorting, proponents assert, serves as a previously unrecognized mechanism for arranging the cell’s contents and mustering the molecules necessary to trigger key cellular events. McSwiggen had found hints that phase separation helps herpesviruses replicate inside infected cells, adding to claims that the process plays a role in functions as diverse as switching on genes, anchoring the cytoskeleton, and repairing damaged DNA. “It’s pretty clear this process is at play throughout the cell,” says biophysicist Clifford Brangwynne of Princeton University.

The pharmaceutical industry is as excited as some academic researchers, given studies linking phase separation to cancer, amyotrophic lateral sclerosis (ALS), diabetes, and other diseases. Dewpoint Therapeutics, a startup pursuing medical treatments targeting cellular droplets, recently signed development deals worth more than $400 million with pharma giants Merck and Bayer. And three other companies looking to exploit the process opened their doors late last year. Reflecting that enthusiasm, Science picked phase separation as a runner-up in its 2018 Breakthrough of the Year issue.

Tjian says he was agnostic at first about the importance of the process. But after McSwiggen’s findings inspired him and colleagues to look more closely at the range of claims, the researchers began to have doubts. Tjian and a camp of similarly skeptical biologists now argue that the evidence that liquidlike condensates arise naturally in cells is largely qualitative and obtained with techniques that yield equivocal results—in short, they believe much of the research is shoddy.

Moreover, the contention that those intracellular droplets perform important roles “has gone from hypothetical to established dogma with no data,” says Tjian, who stepped down as president of Howard Hughes in 2016 and now co-directs a lab at the University of California (UC), Berkeley. “That to me is so perverse and destructive to the scientific discovery process.”

Although proponents of phase separation bridle at some of those criticisms, many scientists agree that the research requires a jolt of rigor. “I don’t think the whole field is bunk,” says biophysicist Stephanie Weber of McGill University. “But we do need to be more careful” in identifying instances of phase separation in cells and ascribing functions to them.

The process may be less important than many scientists now assert, adds quantitative cell biologist Amy Gladfelter of the University of North Carolina, Chapel Hill. Some researchers, she says, have tried to make it “the answer to everything.”

Phase separation could answer a fundamental question that has nagged biologists for more than 100 years: How do cells arrange their contents so that the molecules necessary to carry out a particular job are in the right place at the right time? One obvious way is with internal membranes, such as those fencing off the Golgi bodies and mitochondria. Yet many other well-known cellular structures, including the nucleolus—an organelle within the nucleus—and the RNA-processing Cajal bodies, lack membranes.

Phase separation is an appealing answer. Many proteins sport sticky patches that attract other proteins of the same or a different type. Test tube studies have shown that under certain conditions, such as when protein concentration climbs above a certain level, the molecules may begin to huddle, forming dropletlike condensates. Researchers understand the mechanics best for proteins, but nucleic acids such as RNA could also aggregate with proteins. If the process happens in the cell, it could generate and maintain organelles and permit unique functions. “It’s a principle that could explain how many things in the cell and nucleus are organized,” says biophysicist Mustafa Mir of the University of Pennsylvania, who as a postdoc once worked with Tjian.

Although biologists mooted a role for intracellular droplets as far back as the 1890s, evidence that they are vital began to coalesce a little over 10 years ago. Brangwynne, then a postdoc with cell biologist Anthony Hyman at the Max Planck Institute of Molecular Cell Biology and Genetics, was tracing P granules, flecks of protein and RNA that, in nematode embryos, mark the cells that go on to produce sperm and eggs. To observe the granules’ movements, Brangwynne squeezed worm gonads that harbor the structures between two microscope cover slips. Under pressure, P granules responded not like solids but like liquids, flowing along the surface of the nucleus and dripping off, Brangwynne, Hyman and colleagues reported in Science in 2009. The granules’ watery behavior “was mind-blowing. It was so different than anything in cells,” says Weber, a former postdoc of Brangwynne’s.

P granules (green), pockets of protein and RNA in early worm embryos that mark where sperm or egg cells will arise, have become the classic example of phase-separated regions in cytoplasm.

In 2012, Brangwynne and colleagues saw similar fluid features in the nucleolus, a dense mix of proteins, RNA, and DNA that manufactures ribosomes, the cell’s protein factories. The same year, biophysicist Michael Rosen of the University of Texas Southwestern Medical Center and colleagues showed that three proteins that collaborate to organize part of the cytoskeleton form liquid droplets in a test tube solution. They found that the process speeds the assembly of one type of skeletal fiber in vitro—and might do the same in the cell. Scientists have since reported dozens of examples of cellular structures that are round, prone to fuse, and tend to bead on or flow across surfaces—hallmarks of droplets formed by phase separation.

To confirm that a molecular gathering in a cell is a liquid and not something more solid, scientists often deploy a technique called fluorescence recovery after photobleaching (FRAP). Using a cell that contains fluorescent proteins, researchers zap the region in question with a laser to darken the molecules and then trace how long the fluorescence takes to diffuse back in from other parts of the cell. A liquid, which the fluorescent proteins easily penetrate, should light up more quickly than a solid. Another test involves applying 1,6-hexanediol, a compound that fractures some of the molecular interactions that hold droplets together, to determine whether the structure dissolves.

Rosen notes that three papers published last year in Cell offer some of the strongest evidence for phase separation in cells. One, from Brangwynne’s lab, showed a particular protein had to reach a threshold concentration in cells to allow formation of stress granules—organelles that pop up during hard times and have been attributed to phase separation. The other two studies also identified threshold conditions for phase separation. Because a threshold is an attribute of the process, the studies provide “good but not perfect data that these structures are going through phase separation,” Rosen says.

Imagine that human life cycles resembled those of the earliest plants. If you think about this analogy, you may begin to realize that many plants, which appear so inert to our roving eyes and active minds, actually lead secret lives of surprising variety.

You know that humans develop, or gradually change, from infants to quite different, sexually mature adults. You also know that meiosis in your own ovaries or testes produces haploid eggs or sperm, which must join in fertilization to become a new individual. Each of us, of course, began as that single cell made when a sperm united with an egg. Now, through mitosis and the miracle of development, we are made of trillions of cells organized into tissues, organs, and organ systems, which make us complex, amazing, active, individual beings. None of us would doubt that we have changed significantly since we began as single cells. Each of us has a unique identity that we keep throughout our entire lives, until death marks our end. We may give birth to other individuals by producing eggs or sperm, but only if they join with other sperm or eggs to produce new, separate lives.

If, however, we lived like some plants, your father would not have produced the sperm cell destined to provide half of your genes, although there would be such a sperm cell. Your mother would not have produced the egg cell destined to produce the other half. In fact, your parents, and you, would not be distinguishable as male or female. Instead, both parents (or maybe just one parent) would have released thousands of haploid spore cells, each of which would grow, by mitosis, into a new individual being, entirely different in form and habitat from its parents - and you. Small spores would become males, and large spores females, but as if sperm and egg had decided to postpone their &ldquomarriage&rdquo and grow up on their own, these beings would live very different, &ldquonon-human&rdquo lives.

Who are these beings? You are certainly not one of them, because you begin only when egg meets sperm. Their differences from you would be far greater than the differences between tadpole and frog, or caterpillar and butterfly, because every individual butterfly or frog could (theoretically) identify exactly which individual caterpillar or tadpole it used to be. Not so with these haploid creatures.

At some time during their relatively long lives, the male and female beings would produce sperm cells and egg cells by mitosis. Fertilization would not involve mating, of course. Depending on which kind of plant we chose as our model, sperm might swim on their own (with two or more flagella) from male to female being, or they might be blown by the wind, or carried by an animal. After sperm and egg join, you would begin your life as a single cell, and grow into an &ldquoadult,&rdquo eventually producing your own haploid spores. But you would never be able to identify your parents &ndash if indeed you had two &ndash nor would you know your children, because entire haploid lives would separate you. Why do plants lead such complex, multiple lives?

Most of the plants you are probably familiar with produce flowers. However, plants existed for hundreds of millions of years before they evolved flowers. In fact, the earliest plants were different from most modern plants in several important ways. They not only lacked flowers, but also lacked leaves, roots, and stems. You might not even recognize them as plants. So why are the earliest plants placed in the plant kingdom? What traits define a plant?

What are Plants?

Plants are multicellular eukaryotic organisms with cell walls made of cellulose. Plant cells also have chloroplasts. In addition, plants have specialized reproductive organs. These are structures that produce reproductive cells. Male reproductive organs produce sperm, and female reproductive organs produce eggs. Male and female reproductive organs may be on the same or different plants.

How Do Plants Obtain Food?

Almost all plants make food by photosynthesis. Only about 1 percent of the estimated 300,000 species of plants have lost the ability to photosynthesize. These other species are consumers, many of them predators. How do plants prey on other organisms? The Venus fly trap in Figure below shows one way this occurs.

Venus fly trap plants use their flowers to trap insects. The flowers secrete enzymes that digest the insects, and then they absorb the resulting nutrient molecules.

What Do Plants Need?

Plants need temperatures above freezing while they are actively growing and photosynthesizing. They also need sunlight, carbon dioxide, and water for photosynthesis. Like most other organisms, plants need oxygen for cellular respiration and minerals to buildproteins and other organic molecules. Most plants support themselves above the ground with stiff stems in order to get light, carbon dioxide, and oxygen. Most plants also grow roots down into the soil to absorb water and minerals. And, of course, we need the energy stored in plants through photosynthesis to survive. Life as we know it would not be possible without plants.

Female Reproductive System

Reproduction is the process by which organisms make more organisms like themselves. But even though the reproductive system is essential to keeping a species alive, unlike other body systems, it's not essential to keeping an individual alive.

In the human reproductive process, two kinds of sex cells, or gametes (GAH-meetz), are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system. When sperm fertilizes (meets) an egg, this fertilized egg is called a zygote (ZYE-goat). The zygote goes through a process of becoming an embryo and developing into a fetus.

The male reproductive system and the female reproductive system both are needed for reproduction.

Humans, like other organisms, pass some characteristics of themselves to the next generation. We do this through our genes, the special carriers of human traits. The genes that parents pass along are what make their children similar to others in their family, but also what make each child unique. These genes come from the male's sperm and the female's egg.

What Is the Female Reproductive System?

The external part of the female reproductive organs is called the vulva, which means covering. Located between the legs, the vulva covers the opening to the vagina and other reproductive organs inside the body.

The fleshy area located just above the top of the vaginal opening is called the mons pubis. Two pairs of skin flaps called the labia (which means lips) surround the vaginal opening. The clitoris, a small sensory organ, is located toward the front of the vulva where the folds of the labia join. Between the labia are openings to the urethra (the canal that carries pee from the bladder to the outside of the body) and vagina. When girls become sexually mature, the outer labia and the mons pubis are covered by pubic hair.

A female's internal reproductive organs are the vagina, uterus, fallopian tubes, and ovaries.

The vagina is a muscular, hollow tube that extends from the vaginal opening to the uterus. Because it has muscular walls, the vagina can expand and contract. This ability to become wider or narrower allows the vagina to accommodate something as slim as a tampon and as wide as a baby. The vagina's muscular walls are lined with mucous membranes, which keep it protected and moist.

The vagina serves three purposes:

  1. It's where the penis is inserted during sexual intercourse.
  2. It's the pathway (the birth canal) through which a baby leaves a woman's body during childbirth.
  3. It's the route through which menstrual blood leaves the body during periods.

A very thin piece of skin-like tissue called the hymen partly covers the opening of the vagina. Hymens are often different from female to female. Most women find their hymens have stretched or torn after their first sexual experience, and the hymen may bleed a little (this usually causes little, if any, pain). Some women who have had sex don't have much of a change in their hymens, though. And some women's hymens have already stretched even before they have sex.

The vagina connects with the uterus, or womb, at the cervix (which means neck). The cervix has strong, thick walls. The opening of the cervix is very small (no wider than a straw), which is why a tampon can never get lost inside a girl's body. During childbirth, the cervix can expand to allow a baby to pass.

The uterus is shaped like an upside-down pear, with a thick lining and muscular walls — in fact, the uterus contains some of the strongest muscles in the female body. These muscles are able to expand and contract to accommodate a growing fetus and then help push the baby out during labor. When a woman isn't pregnant, the uterus is only about 3 inches (7.5 centimeters) long and 2 inches (5 centimeters) wide.

At the upper corners of the uterus, the fallopian tubes connect the uterus to the ovaries. The ovaries are two oval-shaped organs that lie to the upper right and left of the uterus. They produce, store, and release eggs into the fallopian tubes in the process called ovulation (av-yoo-LAY-shun).

There are two fallopian (fuh-LO-pee-un) tubes, each attached to a side of the uterus. Within each tube is a tiny passageway no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a funnel. This fringed area wraps around the ovary but doesn't completely attach to it. When an egg pops out of an ovary, it enters the fallopian tube. Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow passageway toward the uterus.

The ovaries (OH-vuh-reez) are also part of the endocrine system because they produce female sex such as estrogen (ESS-truh-jun) and progesterone (pro-JESS-tuh-rone).

How Does the Female Reproductive System Work?

The female reproductive system enables a woman to:

  • produce eggs (ova)
  • have sexual intercourse
  • protect and nourish a fertilized egg until it is fully developed
  • give birth

Sexual reproduction couldn't happen without the sexual organs called the gonads. Most people think of the gonads as the male testicles. But both sexes have gonads: In females the gonads are the ovaries, which make female gametes (eggs). The male gonads make male gametes (sperm).

When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which stay inactive until puberty begins. At puberty, the (in the central part of the brain) starts making hormones that stimulate the ovaries to make female sex hormones, including estrogen. The secretion of these hormones causes a girl to develop into a sexually mature woman.

Toward the end of puberty, girls begin to release eggs as part of a monthly period called the menstrual cycle. About once a month, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes.

Unless the egg is fertilized by a sperm while in the fallopian tube, the egg leaves the body about 2 weeks later through the uterus — this is menstruation. Blood and tissues from the inner lining of the uterus combine to form the menstrual flow, which in most girls lasts from 3 to 5 days. A girl's first period is called menarche (MEH-nar-kee).

It's common for women and girls to have some discomfort in the days leading to their periods. Premenstrual syndrome (PMS) includes both physical and emotional symptoms that many girls and women get right before their periods, such as:

  • acne
  • bloating
  • tiredness
  • backaches
  • sore breasts
  • food cravings
  • irritability
  • trouble concentrating or handling stress

PMS is usually at its worst during the 7 days before a girl's period starts and disappears after it begins.

Many girls also have belly cramps during the first few days of their periods caused by prostaglandins, chemicals in the body that make the smooth muscle in the uterus contract. These involuntary contractions can be dull or sharp and intense.

It can take up to 2 years from menarche for a girl's body to develop a regular menstrual cycle. During that time, her body is adjusting to the hormones puberty brings. On average, the monthly cycle for an adult woman is 28 days, but the range is from 23 to 35 days.

What Happens If an Egg Is Fertilized?

If a female and male have sex within several days of the female's ovulation, fertilization can happen. When the male ejaculates (when semen leaves the penis), a small amount of semen is deposited into the vagina. Millions of sperm are in this small amount of semen, and they "swim" up from the vagina through the cervix and uterus to meet the egg in the fallopian tube. It takes only one sperm to fertilize the egg.

About 5 to 6 days after the sperm fertilizes the egg, the fertilized egg (zygote) has become a multicelled blastocyst. A blastocyst (BLAS-tuh-sist) is about the size of a pinhead, and it's a hollow ball of cells with fluid inside. The blastocyst burrows itself into the lining of the uterus, called the endometrium. The hormone estrogen causes the endometrium (en-doh-MEE-tree-um) to become thick and rich with blood. Progesterone, another hormone released by the ovaries, keeps the endometrium thick with blood so that the blastocyst can attach to the uterus and absorb nutrients from it. This process is called implantation.

As cells from the blastocyst take in nourishment, another stage of development begins. In the embryonic stage, the inner cells form a flattened circular shape called the embryonic disk, which will develop into a baby. The outer cells become thin membranes that form around the baby. The cells multiply thousands of times and move to new positions to eventually become the embryo (EM-bree-oh).

After about 8 weeks, the embryo is about the size of a raspberry, but almost all of its parts — the brain and nerves, the heart and blood, the stomach and intestines, and the muscles and skin — have formed.

During the fetal stage, which lasts from 9 weeks after fertilization to birth, development continues as cells multiply, move, and change. The fetus (FEE-tis) floats in amniotic (am-nee-AH-tik) fluid inside the amniotic sac. It gets oxygen and nourishment from the mother's blood via the placenta (pluh-SEN-tuh). This disk-like structure sticks to the inner lining of the uterus and connects to the fetus via the umbilical (um-BIL-ih-kul) cord. The amniotic fluid and membrane cushion the fetus against bumps and jolts to the mother's body.

Pregnancy lasts an average of 280 days — about 9 months. When the baby is ready for birth, its head presses on the cervix, which begins to relax and widen to get ready for the baby to pass into and through the vagina. Mucus has formed a plug in the cervix, which now loosesn. It and amniotic fluid come out through the vagina when the mother's water breaks.

When the contractions of labor begin, the walls of the uterus contract as they are stimulated by the pituitary hormone oxytocin (ahk-see-TOE-sin). The contractions cause the cervix to widen and begin to open. After several hours of this widening, the cervix is dilated (opened) enough for the baby to come through. The baby is pushed out of the uterus, through the cervix, and along the birth canal. The baby's head usually comes first. The umbilical cord comes out with the baby. It's clamped and cut close to the navel after the baby is delivered.

The last stage of the birth process involves the delivery of the placenta, which at that point is called the afterbirth. After it has separated from the inner lining of the uterus, contractions of the uterus push it out, along with its membranes and fluids.

Molecules on the surface of the cell match those on its neighbours. It is a bit like having a postcode. The code makes it very difficult for the cell to move to the wrong place. But if the cell does find itself in a place where its postcode is different from its neighbours, it dies.

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