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Allantois Definition

The allantois is a hollow sac-like structure that is filled with transparent fluid, which is part of the concept of developing an amniotic membrane. The allantois is also known as allantoic. It is present in all embryos and extra-embryonic tissues. It helps the embryo exchange gas and helps in disposing of the liquid waste. The allantois, along with the amniotic membrane and chorionic membrane help in the development of the embryo. They are the other outer embryonic membranes, that are present in humans and other mammals and reptiles including birds. Among vertebrates, only amphibians and non-tetrapods do not have this structure. We will further learn about the allantois function, allantois development, and allantoic diverticulum. The term allantoic can also be used to pronounce allantois. 


Allantois Function 

This is a sac-like structure and its name is derived from the Greek word allantoin that means sausages. It is mainly involved in nutrition and excretion and is surrounded by blood vessels. The allantois function is to collect liquid waste from the embryo and exchange the gas used by the embryo. In reptiles, birds, and monotremes this structure first evolved to become a deposit of nitrogenous waste in reptiles and birds and is also a means of oxygenation of embryos. 

The allantois in human embryo is very important to structure for the development of the zygote. Oxygen is absorbed by the allantois through the shell of the egg. In marsupials, the allantoic is avascular and has no blood vessels, but is still used to store nitrogenous waste that is ammonia. Furthermore, most marsupial allantoises do not fuse with the chorion. One exception is the bandicoot, which has a vascular system and fuses with the chorion. In placental mammals, the allantoic diverticulum is part of the development of the umbilical cord and forms the axis of the development of the umbilical cord. The allantoic in mice is composed of mesoderm tissue, which undergoes angiogenesis to form mature umbilical arteries and veins. 

The human allantoic is the tail exit sac of the yolk sac, surrounded by the connecting mesoderm stem called the body. The vascular system of the trunk becomes the umbilical artery, which carries deoxygenated blood to the placenta. It is continuous with the rectum on the outside and the cloaca on the inside. The allantoic diverticulum becomes the fetal urach, which connects the fetal bladder that is developing from the cloaca with the yolk sac. The urach removes nitrogenous wastes from the fetal bladder.

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Amniotic Membrane 

The amniotic membrane is the membrane that tightly covers the embryo when it first forms. It is filled with amniotic fluid, which expands the amniotic membrane and becomes the amniotic sac, providing a protective environment for the developing embryo or fetus. The amniotic membrane together with the chorion, the yolk sac, and the allantoic sac form a protective sac around the embryo. In birds, reptiles, and monotremes, the protective sac is enclosed in a shell. In marsupials and placental mammals, it is enclosed in the uterus

About Amnion

Amniotic membrane is a feature of vertebrate clade amniotic membrane, including reptiles, birds, and mammals. Amphibians and fish are not amniotic animals and therefore lack an amniotic membrane. The amniotic membrane comes from the outer mesoderm of extraembryonic somatic cells and the inner extraembryonic ectoderm or trophoblast. 

In human allantois and embryos, the early stages of amniotic membrane formation have not been observed. In the youngest embryo studied, the amniotic membrane already exists as a closed sac and appears as a cavity in the inner cell mass. This cavity is covered by a layer of flat ectoderm cells, the ectoderm of the amniotic membrane, and its lower part is composed of the prismatic ectoderm of the embryonic disc. 

Continuity is established between the upper and lower part at the edge of the embryonic disc. Outside of the amniotic membrane, the ectoderm is a thin mesoderm, which is continuous with the mesoderm of the body pleura and is connected to the mesoderm lining of the chorionic stem through the body stem. When initially formed, the amniotic membrane is in contact with the body of the embryo, but around the fourth or fifth week, amniotic fluid begins to accumulate in it. The amount of this fluid increases and causes the amniotic membrane to swell and eventually adhere to the inner surface of the chorionic membrane, causing the extra-embryonic part of the body cavity to disappear. 

The amount of amniotic fluid increases in the sixth or seventh month of pregnancy and then decreases. Amniotic fluid allows the fetus to move freely in the later stages of pregnancy and protects it by reducing the risk of external harm. It contains less than 2% solids and is composed of urea and other extracts, inorganic salts, small amounts of protein, and trace sugars. The fact that the fetus has ingested some amniotic fluid can be proved by the fact that epidermal fragments and hair are found in the contents of the fetus’s digestive tract. 

Clinical Significance of Amnion

Extraamniotic pregnancy is a rare condition in which the rupture of the amniotic membrane causes the fetus to develop in the extraembryonic body cavity. In reptiles, birds, and many mammals, the development of an amniotic membrane is as follows. At the contraction point where the embryo’s primitive digestive tract connects with the yolk sac, a reflection or fold occurs above the body pleura. This amniotic fold first appears at the head, then at the tail and sides of the embryo, and gradually increases. Its different parts meet and fuse on the back of the embryo. They surround a cavity, the amniotic cavity. This amniotic membrane is called the pleural amniotic membrane that is formed by folding, and not the schizont amniotic membrane that is formed by layers. After the edges of the amniotic folds fuse, the two layers of folds separate completely, the inner layer forms an amniotic membrane, and the outer layer forms a pseudo-amniotic or serous membrane.

Amnion in Animals

Cats and dogs were born in the amniotic membrane, it was cut and eaten by its mother. 

In elephants, the amniotic membrane continues from the bottom of the umbilical cord to the allantoic membrane. The allantoic membrane is quite large, between the chorionic membrane and the amniotic membrane to prevent any part of the amniotic membrane from reaching the surface internal layer of the allantoic placenta. The amniotic membrane consists of two layers of composition in which one layer is the granular layer, which continues to the inner surface of the allantoic or fetal surface, and then the umbilical cord and the other is a smooth outer layer, which continues the outer surface of the allantoic or chorionic surface. Allantoic, and then on the inner surface of the chorionic membrane. 

Application of Amnion

Amniotic membrane as a biological dressing to heal incurable wounds. To do this, the placenta is harvested from the cesarean section and, under aseptic conditions, the amniotic membrane is separated and packaged, and sold commercially. In effective commercial products, to prevent the spread of viral infections, such as HIV, hepatitis, and similar diseases, donor (mother) blood is tested. According to the regulations of the Food and Drug Administration of the producing country, products generally pass sterility and endotoxin testing. 


Chorion is the outermost layer of the fetal membrane around the embryo of mammals, birds, and reptiles that are amniotic animals. It is developed from the outer folds on the surface of the yolk sac, which is located outside the zona pellucida in mammals and is called the yolk membrane in other animals. In insects, it develops from follicular cells, while the egg is in the ovary. 

Structure of Chorion 

In humans and other mammals excluding monotremes, the chorion is one of the fetal membranes that exist between the developing fetus and the mother during pregnancy. The chorionic membrane and amniotic membrane together form the amniotic sac. In humans, it consists of the outer mesoderm of the embryo and two trophoblasts surrounding the embryo and other membranes. Chorionic villi emerge from the chorion, invade the endometrium, and transfer nutrients from the maternal blood to the fetal blood. The layer of the chorion consists of two layers that are the outer layer which is formed by the trophoblast, and the inner layer which is formed by the somatic mesoderm. The trophoblast is composed of an inner layer of cubic or prismatic cells that are cytotrophoblast or Langhans layer cells and an outer protoplast that are syncytiotrophoblast cells that are rich in nuclei. 

Growth of Chorion

The chorion undergoes rapid proliferation and forms many processes, namely chorionic villi, which invade and destroy the decidua, and at the same time, they absorb nutrients for embryo growth. The chorion is initially very small, without blood vessels, and consists only of trophoblasts, but their size and branches increase and the mesoderm with branches of umbilical blood vessels grows in them and becomes blood vessels. Blood is transported to the chorionic villi by paired umbilical arteries, which branch into chorionic arteries and enter the chorionic villi as cotyledon arteries. After circulating through the capillaries of the villi, blood returns to the embryo through the umbilical vein. Until about the second month of pregnancy, the villi cover the entire chorionic membrane and are almost the same size. However, after this, their development is uneven. 

Parts of Chorion 

The part of the chorion that is in contact with the capsular and decidua atrophy, so there is almost no trace of villi by the fourth month. This part of the chorion becomes soft and is called chorion laeve. Because it does not participate in the formation of the placenta, it is also called the non-placental part of the chorion. As the chorion grows, the chorion comes into contact with the decidua parietalis, and these layers fuse. The size and complexity of the embryonic villi in contact with the decidua basalis increase considerably, which is why this part is called the chorion. Thus, the placenta develops from the chorionic chorion and the decidua basalis. 

Monochorionic Twins 

Monochorionic twins are twins who share the same placenta. This occurs in 0.3% of pregnancies and 75% of single-eggs that are identical twins. When separation occurs on or after the third day after fertilization. The remaining 25% of identical twins become double chorionic double amniotic membranes. This condition can affect any type of multiple births, leading to multiple births.

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FAQs on Allantois

Q1. What is Allantois Meaning?

Answer: The allantois is an extension of the posterior wall of the yolk sac. Although their blood vessels have in fact become umbilical cord blood vessels, their exact role in development is unclear. It produces a structure called urach, which helps form the upper wall of the bladder or the superior wall of the urinary bladder. The distal end of the urach becomes a fibrous cord after birth and exists in the body as the median ligament of the umbilical cord. The umbilical ligament is located on the anterior abdominal wall and runs between the bladder and the umbilicus.

Q2. What is the Role of the Yolk Sac in the Human Body?

Answer: The role of nutrient transfer in the embryo during the second to third week is helped by the yolk sac. From the third week, the blood develops in its wall and continues to form there until the hematopoietic activity in the liver starts around the fifth week. During the week, the dorsal part of the embryo is incorporated into the embryo as the endoderm tube, the primordial intestine, and produces the epithelium of the trachea, bronchus, lung, and digestive tract. Primordial germ cells appear in the yolk of the cyst wall at the third week and migrate to the gonads in the developmental zone. Gonads become germ cells that are oogonia and spermatogonia. 

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