What forms the human amniotic sac?

What forms the human amniotic sac?

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I am trying to understand the formation of embryonic disc for human and chick so the following question is related to here about formation of embryonic disc.

I had thought that the amniotic sac forms only from embryonic disc in humans, particularly of lateral mesoderm and ectoderm of embryonic disc. However, my thought seems to be wrong as my professor says that amnion form only for birds from embryonic disc, but from many different parts besides of embryonic disc for human.

What, besides the embryonic disc, forms the human amniotic sac?

In humans, the amnion (amniotic sac) persists from the primitive amniotic cavity1. One side of this is formed from the cytoblast (a prismatic epithelium) and the plasmodioblast. Together these two layers are the ectoplacenta or chorion. They are also referred to as Rauber's layer. These replace the lining epithelium of the uterus, whereupon internal cells undergo atrophy to create the amniotic sac.2

The other side of the amniotic sac is formed of the epiblast/ectoderm (internally) and the hypoblast (externally). Within the epiblast the other layer is comprised of prismatic cells whereas the inner layer are flattened (the hypoblast/entoderm). This double layer forms the bilaminar blastodermic membrane.2

I'd really reccomend that you borrow the referenced books from your university library as I found the diagrams much easier to understand than the text.

1 Gray, Henry. "Embryology: Formation of Membranes." Ed. Robert Howden. Anatomy Descriptive and Surgical. Ed. T. P. Pick. 15th ed. London: Chancellor, 1994. 90. Print. Colloquially Gray's Anatomy

2 Gray, Henry. "Embryology: The Ovum." Anatomy Descriptive and Surgical. Ed. T. P. Pick and Robert Howden. 15th ed. London: Chancellor, 1994. 82-83. Print. Colloquially Gray's Anatomy

The amnion is a defining characteristic of amniotes, a group of animals that includes reptiles, birds, and mammals. Amniotes are believed to have separated from non-amniotic tetrapods about 300 – 350 million years ago. Amniotes are tetrapods that have evolved adaptations to live on land vertebrate embryos require an aquatic environment in order to develop, and the amniotic egg provides this environment. Amniotes have also developed a number of other adaptations that allowed them to move away from the water and exploit a larger terrestrial environment.

The amnion is an extraembryonic membrane that surrounds an amniote embryo. The membrane is not part of the embryo itself, but derives from tissues that emerged from the embryo. The amnion is made from two germ layers: the mesoderm and the ectoderm. The ectoderm forms the inner portion of the amnion, and a thin mesoderm layer connects the amnion to the chorion.

Recreating the earliest stages of life

In their effort to understand the very earliest stages of life and how they can go wrong, scientists are confronted with ethical issues surrounding the use of human embryos. The use of animal embryos is also subject to restrictions rooted in ethical considerations. To overcome these limitations, scientists have been trying to recreate early embryos using stem cells.

One of the challenges in creating these so-called synthetic embryos is to generate all the cell types normally found in a young embryo before it implants into the wall of the uterus. Some of these cells eventually give rise to the placenta. Others become the amniotic sac in which the fetus grows. Both the placenta and the amniotic sac are crucial for the survival of the fetus, and defects in these embryo components are major causes of early pregnancy loss.

A group of scientists from Gladstone Institutes, the Center for iPS Cell Research and Application (CiRA) from Kyoto University, and the RIKEN Center for Biosystems Dynamics Research in Kobe, Japan, has now demonstrated the presence of precursors of the placenta and the amniotic sac in synthetic embryos they created from mouse stem cells.

"Our findings provide strong evidence that our system is a good model for studying the early, pre-implantation stages of embryo development," says Kiichiro Tomoda, PhD, research investigator at the recently opened iPS Cell Research Center at Gladstone and first author of the study published in the journal Stem Cell Reports. "Using this model, we will be able to dissect the molecular events that take place during these early stages, and the signals that the different embryonic cells send to each other."

Ultimately, this knowledge might help scientists develop strategies to decrease infertility due to early embryonic development gone awry.

The new findings could also shed light on a defining property of the earliest embryo cells that has been difficult to capture in the lab: their ability to produce all the cell types found in the embryo and, ultimately, the whole body. Scientists refer to this property as "totipotency."

"Totipotency is a very unique and short-lived property of early embryonic cells," says Cody Kime, PhD, an investigator at the RIKEN Center for Biosystems Dynamics Research and the study's senior author.

"It has been much harder to harness in the lab than pluripotency," he adds, referring to the ability of some cells to give rise to several -- but not all -- cell types. "A very exciting prospect of our work is the ability to understand how we can reprogram cells in the lab to achieve totipotency."

Growing the Fundamental Components of Early Embryos in the Lab

To generate synthetic embryos, the scientists started from mouse pluripotent stem cells that normally give rise to the fetus only -- not the placenta or amniotic sac. They can grow these cells, called epiblast stem cells, and multiply them indefinitely in the lab.

In previous work, the team had discovered a combination of nutrients and chemicals that could make epiblast stem cells assemble into small cell structures that closely resemble pre-implantation embryos. In fact, the structures could even reach the implantation stage when transferred into female mice, though they degenerated shortly thereafter.

"This meant that we might successfully reprogram the epiblast cells to revert to an earlier stage, when embryonic cells are totipotent, and provided a clue to how we might generate both the fetus and the tissues that support its implantation," explains Tomoda, who is also a program-specific research center associate professor at CiRA.

To build on that work and better understand the reprogramming process, the scientists needed molecular resolution. In their new study, they turned to single-cell RNA sequencing, a technique that allows scientists to study individual cells based on the genes they turn on or off.

After analyzing thousands of individual cells reprogrammed from epiblast stem cells, and sifting the data through computer-powered analyses, they confirmed that, after 5 days of reprogramming, some cells closely resembled all three precursors of the fetus, the placenta, and the amniotic sac.

Moreover, as they were grown in the lab for a few more days, the three cell types displayed more distinct molecular profiles with striking similarity to real embryonic model cells. This is the same as would be expected during the growth of a normal embryo, when the three tissues acquire distinct physical properties and biological functions.

"Our single-cell RNA-sequencing analysis confirms the emergence in our synthetic embryo system of the cell types that lead to the three fundamental components of an early mammalian embryo," says Kime. "In addition, it unveils in amazing detail the genes and biological pathways involved in the development of these precursors and their maturation into specific tissues."

This knowledge provides a comprehensive backdrop against which to understand the mechanisms of early embryo development and the possible causes of its failure.

For now, the scientists plan to work on ways to increase the efficiency of their reprogramming process, so as to reliably produce large amounts of pre-implantation-like synthetic embryos for further studies. This would allow them to carry out experiments that were up to now unthinkable, such as large-scale screens for gene mutations that disrupt early embryos. And it may shed light on the causes of pregnancy loss due to early embryo failure.

They also want to better understand the molecular steps involved in reprogramming. In particular, they plan to look earlier than 5 days into the reprogramming process, with the hope of pinpointing truly totipotent cells at the origin of their synthetic embryos.

"The discovery that we could reprogram cells to adopt earlier, more pluripotent states revolutionized developmental biology 15 years ago," says Tomoda, referring to the discovery of induced pluripotent stem cells by his and Kime's mentor, Nobel Laureate Shinya Yamanaka.

"In the last few years, the field of synthetic embryology utilizing stem cells has seen a true explosion," he says. "Our method of generating synthetic embryos is simpler than others, and quite efficient. We think it will be a great resource for many labs."

Change history

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Amniotic sac rupture

Premature rupture of membranes is a rupture (breaking open) of the amniotic sac before labor begins. If premature rupture of membranes occurs before 37 weeks of pregnancy, it is called preterm premature rupture of membranes (PPROM).

Premature rupture of membranes occurs in about 8 to 10 percent of all pregnancies. Preterm premature rupture of membranes (PPROM) (before 37 weeks) accounts for one fourth to one third of all preterm births.

What causes premature rupture of amniotic sac?

Rupture of the amniotic sac near the end of pregnancy (term) may be caused by a natural weakening of the membranes or from the force of contractions. Before term, preterm premature rupture of membranes (PPROM) is often due to an infection in the uterus. Other factors that may be linked to premature rupture of the amniotic sac include the following:

  • Low socioeconomic conditions (as women in lower socioeconomic conditions are less likely to receive proper prenatal care)
  • Sexually transmitted infections (STIs), such as chlamydia and gonorrhea
  • Previous preterm birth
  • Vaginal bleeding
  • Cigarette smoking during pregnancy
  • Unknown causes

Prevention of premature rupture of the amniotic sac

Unfortunately, there is no way to actively prevent premature rupture of the amniotic sac. However, this condition does have a strong link with cigarette smoking and mothers should stop smoking as soon as possible.

Why is premature rupture of the amniotic sac a concern?

Premature rupture of the amniotic sac is a complicating factor in as many as one third of premature births. A significant risk of preterm premature rupture of membranes (PPROM) (before 37 weeks) is that the baby is very likely to be born within a few days of the membrane rupture. Another major risk of premature rupture of the amniotic sac is development of a serious infection of the placental tissues called chorioamnionitis, which can be very dangerous for mother and baby. Other complications that may occur with premature rupture of the amniotic sac include placental abruption (early detachment of the placenta from the uterus), compression of the umbilical cord, cesarean birth, and postpartum (after delivery) infection.

What are the symptoms of premature rupture of the amniotic sac?

The following are the most common symptoms of premature rupture of the amniotic sac. However, each woman may experience symptoms differently. Symptoms may include:

If you notice any symptoms of premature rupture of the amniotic sac, be sure to call your doctor as soon as possible. The symptoms of premature rupture of the amniotic sac may resemble other medical conditions. Consult your doctor for a diagnosis.

How is premature rupture of the amniotic sac diagnosed?

In addition to a complete medical history and physical examination, Premature rupture of the amniotic sac may be diagnosed in several ways, including the following:

  • An examination of the cervix (may show fluid leaking from the cervical opening)
  • Testing of the pH (acid or alkaline) of the fluid
  • Looking at the dried fluid under a microscope (may show a characteristic fern-like pattern)
  • Ultrasound. A diagnostic imaging technique that uses high-frequency sound waves and a computer to create images of blood vessels, tissues, and organs. Ultrasounds are used to view internal organs as they function, and to assess how much fluid is around the baby.

Treatment for premature rupture of the amniotic sac

Specific treatment for premature rupture of the amniotic sac will be determined by your doctor based on:

  • Your pregnancy, overall health, and medical history
  • Extent of the condition
  • Your tolerance for specific medications, procedures, or therapies
  • Expectations for the course of the condition
  • Your opinion or preference

Treatment for premature rupture of the amniotic sac may include:

Amniote Egg

The amniotic egg was an evolutionary invention that allowed the first reptiles to colonize dry land more than 300 million years ago. Fishes and amphibians must lay their eggs in water and therefore cannot live far from water. But thanks to the amniotic egg, reptiles can lay their eggs nearly anywhere on dry land.

The amniotic egg of reptiles and birds is surrounded by a tough outer shell that protects the egg from predators, pathogens , damage, and drying. Oxygen passes through tiny pores in the shell, so the embryo doesn't suffocate. Inside the shell are four sacs. The first sac inside the shell is the chorion, which carries oxygen from the shell to the embryo and waste carbon dioxide from the embryo to the shell. Within the chorion is the amnion, the membrane for which the amniotic egg is named. The amnion keeps the embryo from drying out, so it's critical to living on land. A third sac, the allantois, stores wastes from the embryo and also fuses with the chorion to form the chorioallantoic membrane, which carries oxygen and carbon dioxide to and from the embryo, just like a lung. A fourth membrane, the yolk sac, holds and digests nutritious yolk for the developing embryo.

Together, the shell and membranes create a safe watery environment in which an embryo can develop from a few cells to an animal with eyes and ears, brain, and heart. Because reptiles, birds, and mammals all have amniotic eggs, they are called amniotes.

The duck-billed platypus and some other mammals also lay eggs. But most mammals have evolved amniotic eggs that develop inside the mother's womb, or uterus, and so lack a shell. In humans and other mammals, the chorion fuses with the lining of the mother's uterus to form an organ called the placenta. The placenta transports oxygen and carbon dioxide to and from the embryo and delivers nutrients from the mother's blood.


Chorioamnionitis is inflammation of the amniotic sac ( chorio- + amnion + -itis ), usually because of infection. It is a risk factor for neonatal sepsis.

During labor, the amniotic sac must break so that the child can be born. This is known as rupture of membranes (ROM). Normally, it occurs spontaneously at full term either during or at the beginning of labor. A premature rupture of membranes (PROM) is a rupture of the amnion that occurs prior to the onset of labor. An artificial rupture of membranes (AROM), also known as an amniotomy, may be clinically performed using an amnihook or amnicot in order to induce or to accelerate labour.

The amniotic sac has to be punctured to perform amniocentesis. [10] [11] This is fairly routine procedure, but can lead to infection of the amniotic sac in a very small number of cases. [12] Infection more commonly arises vaginally. [12] [13]

Labor and Birth

Labor and birth are divided into three stages: the dilation of the cervix, the delivery of the baby, and the expulsion of the placenta.

Learning Objectives

Describe the process of labor and birth in humans

Key Takeaways

Key Points

  • At the end of gestation, estrogen receptors on the uterine wall bind oxytocin, which causes the uterine muscles to contract as the muscles contract, they signal for the release of more oxytocin in a positive feedback loop.
  • During the first stage of labor, the cervix thins and dilates to allow passage of the baby into the birth canal typically over the course of several hours, the cervix will dilate to its full width of 10 centimeters.
  • During the second stage of labor, contractions become very strong and, aided by the pushing of the mother, the baby is expelled from the uterus.
  • During the third stage of labor, the placenta, amniotic sac, and the remainder of the umbilical cord are expelled from the uterus, usually within a few minutes after birth.
  • When the baby begins suckling at the breast after delivery, prolactin signals the release of milk from the mammary glands, providing nutrition and immunity against disease to the infant.

Key Terms

  • prolactin: a peptide gonadotrophic hormone secreted by the pituitary gland it stimulates growth of the mammary glands and lactation in females
  • parturition: the act of giving birth childbirth
  • oxytocin: a hormone that stimulates contractions during labor, and then the production of milk

Labor and Birth

Labor is the physical effort of expulsion of the fetus and the placenta from the uterus during birth (parturition). The total gestation period from fertilization to birth is about 38 weeks (birth usually occurring 40 weeks after the last menstrual period). Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head engaging the cervix (uterine opening). This causes the cervix to stretch, sending nerve impulses to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force.

There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm, a process that may take many hours, especially in a woman bearing her first child. At some point, the amniotic sac bursts and the amniotic fluid escapes. During stage two, the baby is expelled from the uterus with the umbilical cord still attached. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall, usually within a few minutes. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.

Cervix dilation: During the first stage of labor, the cervix, which is normally closed, must open and widen to accommodate the passage of the baby. A cervix is considered fully dilated at 10 centimeters.

The mother’s mammary glands go through changes during the third trimester to prepare for lactation and breastfeeding. When the baby begins suckling at the breast, signals are sent to the hypothalamus causing the release of prolactin from the anterior pituitary, which signals the mammary glands to produce milk. Oxytocin is also released, promoting the release of the milk. The milk contains nutrients for the baby’s development and growth as well as immunoglobulins to protect the child from bacterial and viral infections.


After implantation, the inner cell mass subdivides into a bilaminar disc consisting of the hypoblast and epiblast.


Hypoblast cells migrate along the inner surface of the cytotrophoblast and will form the primary yolk sac. The primary yolk sac becomes reduced in size and is known as the secondary yolk sac. In humans the yolk sac contains no yolk but is important for the transfer of nutrients between the fetus and mother.


Epiblast cells cavitate to form the amnion, an extra-embryonic epithelial membrane covering the embryo and amniotic cavity. Cells from the epiblast will also eventually form the body of the embryo.

Extra-embryonic mesoderm

Extra-embryonic mesoderm cells migrate between the cytotrophoblast and yolk sac and amnion. Extraembryonic somatic mesoderm lines the cytotrophoblast and covers the amnion is. Extraembryonic somatic mesoderm also forms the connecting stalk that is the primordium of the umbilical cord. Extraembryonic visceral mesoderm covers the yolk sac.

At the end of the second week it is possible to distinguish the dorsal (amniotic cavity) from the ventral (yolk sac) side of the embryo.

Figure 2 - Day 14 blastocyst showing structure of the placenta

Development of the Blastocyst

About 6 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or 10.

The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick. The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta. The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously. The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother.

Some of the cells from the placenta develop into an outer layer of membranes (chorion) around the developing blastocyst. Other cells develop into an inner layer of membranes (amnion), which form the amniotic sac. When the sac is formed (by about day 10 to 12), the blastocyst is considered an embryo. The amniotic sac fills with a clear liquid (amniotic fluid) and expands to envelop the developing embryo, which floats within it.

Brave New World is being reinvented with synthetic embryos—and the right reasons

In his 1932 novel Brave New World, Aldous Huxley walks you into a terrifying lab where human embryos and fetuses are being grown in glass containers and genetically engineered to fit into a certain rank of society. It continues to be nightmare fuel for college students everywhere.

Now that we live in an era where science fiction is morphing into science, conceiving artificial embryos sounds like an incarnation of the book—but couldn’t be further from it. Scientists have proven it is possible to synthetically create embryos from stem cells. This is a viable and ethical alternative to studying human or animal embryos, and a new frontier in finding out more about how preimplantation embryos may mutate or fail.

More biology

Instead of trying to build a societal hierarchy from human beings born in vitro, researchers Cody Kime, Kiichiro Tomoda and their team from the RIKEN Center for Biosystems Dynamics Research are developing new systems to find out what can go wrong with embryos in their earliest phases and cause early pregnancy loss. They recently published a study in Stem Cell Reports and are optimistic that the glitches of nature can someday be prevented.

"As you can imagine, there is tremendous power, inevitable risk, and serious ethical responsibility, although using cultured cells we can greatly reduce animal experiments. Perhaps one of the best applications is screening genetic mutations that impede fertility and reproduction," Kime told SYFY WIRE. "if those mutations are tolerated in our starting stem cell population, we can initiate reprogramming, and see how those mutations affect the synthetic embryo system. From there, we can get a better picture of how those genes may affect human fertility and improve on treatments."

If organoids (even the brain) can be grown for further research, so can embryoids. The team, whose in vitro synthetic embryo systems (SESs) came from mouse stem cells, was trying to successfully recreate totipotency, meaning that the cells would have everything they needed to develop into a whole organism. Totipotency does not last long in early embryonic cells. Pluripotency, which is the ability to produce some types of necessary cells, but not all, is much easier to achieve with reprogramming. This is still a positive. Dr. Kime wonders if the pluripotent stem cells, might possibly reprogram to be totipotent and form complete embryo.

An early-phase human embryo in the amniotic sac. Credit: Art Images/Getty Images

There are three types of cells that emerge from totipotency: the cells of the actual embryo, the placenta and the amniotic sac. This is such a fleeting state in mammals because the cells in the embryo multiply and polarize fast to turn into one of those three things. Kime and Tomoda didn’t try to do everything at once. The team started by growing pluripotent mouse cells, or epiblast stem cells, responsible for only the fetus. Pluripotent epiblast stem cells are capable of arranging themselves to represent a post-implantation embryo. Hitting the rewind button on those cells with reprogramming might revert them back to the totipotent phase, before they specialized to just the fetus.

"We have seen evidence that something like totipotency may be happening in our reprogramming system, and it arises by taking a later stage embryonic stem cell and treating it with specific natural molecules and nutrients," Kime said. "In a way the cell is ‘tricked’ to reprogram and gain the ability to form the other embryonic lineages."

Reprogramming meant that the scientists would need to be able to tell which genes each cell turned on or off. They used a process called RNA sequencing, which sees how much RNA is and how many sequences of that RNA is in a sample. RNA (ribonucleic acid) tells DNA how to put together different proteins. Sequencing reveals its transcriptome, or everything that makes up RNA, and allows scientists to better understand cells up close. They observed the gene expression in thousands of cells, which told them which cells could be potentially reprogrammed to become totipotent. Hi-res regulation of gene expression could even show what ways cells were changing. In Kime's system, after 5 days of reprogramming, some of the reprogrammed epiblast stem cells (EPISCs) finally got there.

“The analysis revealed that cells resembling all three types of the early embryo were generated by our unique reprogramming system at the same time," he said. "Our analysis showed, in great detail, that our reprogrammed cells had engaged nearly all early embryo cells, while turning off the genes of the cell type they came from. The most important analysis was comparing our reprogrammed cells to real embryonic cells and finding that, across incredibly rich data, our cells were nearly identical."

The breakthrough has given Kime, Tomoda and their team a portal into what was once unthinkable. Because epiblast stem cells are easily reproduced, they can carry out studies on a much larger scale. They will also be able to explore things that would have not been considered ethical otherwise, such as getting a more in-depth look at how reprogramming happens and screening for gene mutations and other things that could cause a pregnancy to terminate itself. So while their work may be venturing into a brave new world, the intent is the total opposite of the sinister motives in Brave New World.

Whether strawberry ice cream soma will ever be a thing still remains to be seen.

Watch the video: Fetal membranes (July 2022).


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