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[Academic Script] Extra Embryonic Membranes, Types & Physiology of Placenta Subject: Zoology Course: B.Sc. 3rd Year Paper No. & Title: Z-305B Developmental Biology Topic No. & Title: Topic – 5 Extra Embryonic Membranes, Types & Physiology of Placenta Lecture Title: Extra Embryonic Membranes, Types & Physiology of Placenta Academic Script Extra embryonic Membranes In vertebrate embryonic development, only a part of the egg or the cleavage mass of cells forms the actual embryo, while other parts lying outside the embryonic territory develop into extra-embryonic regions, called embryonic or foetal membranes. Embryonic membranes are auxiliary organs, which have arisen partly for protection of the embryo, and more specially to provide for its nutrition, respiration and excretion until the independent existence is attained. Extra embryonic membranes in chick In chick, the presence of an enormous amount of yolk and embryonic life to be spent within a shell is correlated with the development of extra-embryonic membranes. Original blastoderm is a small disc, which spreads by peripheral expansion and eventually covers the entire surface of the egg. But only the most central region is directly connected with the formation of the embryo proper. All the remainder of the blastoderm is extra-embryonic. These are mainly four types. They are: 1. Yolk sac 2. Amnion 3. Chorion 4. Allantois Development of Yolk SAC In reptiles and splanchnopleure birds, develop the from somatopleure the periphery of and the blastodisc. These usually spread peripherally over the yolk mass. Soon afterwards, the embryo undergoes series of folds, which appear all around the body of the embryo. These folds are termed as the body folds. The extra embryonic splanchnopleure (splanchnic mesoderm + endoderm) constantly spreads over the yolk mass and eventually yolk sac encloses the mass of yolk in a large measure. The yolk sac, however, not surrounds the yolk fully. A small passage is left on the ventral side for the embryo to absorb the remains of albumen at a later stage. Immediately with the formation of the yolk sac, the intra embryonic splanchnopleure is subjected to fold resembling with the more superficial body folds i.e., the intra embryonic folds. The intra embryonic folds give rise to walled digestive tract, or gut, in the body of the embryo. The middle of the embryonic gut remains open to the yolk beneath. At this level, yolk sac is connected to the digestive tract by a constricted yolk stalk. Development Of Amnion & Chorion The amnion and chorion are developed jointly as upward projecting folds, the amniotic folds of the extra embryonic somatopleure. The amniotic folds are named according to their location. They are amniotic head fold and amniotic tail fold. The amniotic fold first appears as a transverse fold in front of the head. It is called amniotic head fold. It grows upwards and then bends backwards, over the anterior end of the head and covers it as with a hood. Another fold develops behind the embryo. It is termed as amniotic tail fold. All these folds finally cover an embryo in two sheets of somatopleure. The inner somatopleuric sheet becomes the amnion and the outer, the chorion. The amnion consists of a layer of extra embryonic ectoderm on the inside and a layer of extra embryonic somatic mesoderm on the outside whereas, the chorion is made up of a layer of extra embryonic ectoderm on the outside and a layer of extra embryonic somatic mesoderm on the inside. The cavity between the amnion and the embryo is called the amniotic cavity. In between, the amnion and the chorion is the chorionic cavity or extra embryonic coelom. DEVELOPMENT OF ALLANTOIS The allantois arises as a ventral outgrowth of the splanchnopleure from the hindgut on the third day of incubation. It slowly enlarges as holosac and expands inside exocoel. Its walls are formed of an outer splanchnic mesoderm and inner endoderm. The proximal part of the allantois forms a slender neck or the allantoic stalk with which it remains connected with the hindgut of the embryo. The distal part of the allantois expands and penetrates between the amnion and the yolk sac on one side and the chorion on the other side. By the middle of the incubation period, the allantois spreads all around the egg underneath the chorion. The mesoderm on the external surface of the allantois fuses with that of the chorion forming a conjoined chorio-allantoic membrane. Extra embryonic membranes in Mammals The developing embryo of rabbit and other eutherian mammals are provided with four foetal or extra-embryonic membranes namely, amnion, chorion, allantois and yolk sac as in chick. But in marsupial and placental mammals, embryo depends on the mother for food and oxygen and elimination of its own wastes. The foetal membranes begin to develop while the contact with the uterine wall is being established. Different extra-embryonic membranes of mammals develop in the following manner: DEVELOPMENT OF AMNION & CHORION Amnion and chorion develops simultaneously as upward projecting folds of somatopleure called amniotic folds. Two types of amniotic folds develop. They are named according to their location. They are the amniotic head fold, and the amniotic tail fold. First of all, the amniotic tail fold appears. It grows over the embryo dorsally and then grows forward. Afterwards, a transverse fold appears in front of the head. It is known as amniotic head fold. It grows upwards and then backwards over the embryo. Lastly the head fold and the tail fold meet and fuse together. The place of their final fold develops. First fusion is marked by a provisional connection called seroamniotic connection or seroamniotic raphe. After the fusion of the folds, the inner and outer layers separate and the inner layer becomes the chorion. The further development is same as in chick. DEVELOPMENT OF YOLK SAC There is very less or no yolk in marsupial and eutherian eggs, yet a yolk sac develops in their embryos which points to their reptilian ancestor. In the mammals, yolk sac begins to form during early gastrulation. In the blastocyst stage, the hypoblast endoderm cells found in the inner cell mass starts to extend along the inside surface of the trophoblast and encircles the inner cavity of the blastocyst. The cavity of the blastocyst within the enveloping endodermal layer is known as the yolk sac. DEVELOPMENT OF ALLANTOIS The development of allantois in mammals is same as in chick. FUNCTIONS OF EXTRA EMBRYONIC MEMBRANES FUNCTION OF YOLK SAC IN BIRDS & MAMMALS: The function of yolk sac in birds is to digest the yolk and transfer the product of digestion to the embryo. In mammals, the yolk sac is very little or with no yolk. FUNCTION OF AMNION IN BIRDS & MAMMALS: The primary function of amnion is to protect the embryo from dessication and provides “private salty” to an embryo to float. Amniotic fluid serves as an efficient shock absorber. FUNCTION OF CHORION IN BIRDS AND MAMMALS: A part of chorion forms finger like out growths known as chorionic villi that penetrate into the wall of uterus for exchange of substances between embryo and uterus in the body of the mother. FUNCTION OF ALLANTOIS IN BIRDS & MAMMALS: In birds, cavity of allantois serves as an urinary bladder. In mammals, the original function of allantois as urinary bladder becomes all together lost. The carbon dioxide produced by the embryo diffuses into the maternal blood and is excreted by the kidney of the mother. Types & physiology of Placenta The placenta is a composite structure produced by the development and apposition of the extra embryonic membranes with the uterine endometrium for the purpose of physiological exchange. In between these two parallel plates, a huge blood sinus, the intervillous space, contains an enormous number of chorionic villi. The placenta consists of two parts: 1. Foetal placenta 2. Maternal placenta 1. Foetal placenta: It is formed by extra embryonic membrane chorion which is the principal component of foetal placenta. It establishes a vascular link between the embryo and the maternal tissues. 2. Maternal placenta: Uterine endometrium is a solitary component of maternal placenta. Placenta is also found in diverse groups of the animal kingdom such as in Peripatus, Salpa, Elasmobranchia and certain lizards. In each case, mode of origin and the structure of placenta is different. CLASSIFICATION OF MAMMALIAN PLACENTA The mammalian placenta is classified into four different types. A. TYPES OF PLACENTA ACCORDING TO THE NATURE OF THE FOETAL MEMBRANES TAKING PART IN THE FORMATION OF PLACENTA 1. Chorio-vitelline or “Yolk-sac” placenta-It is a primitive type of placenta found in some of the marsupials. E.g., Opossum and Kangaroo. In this type of placenta, the allantois remains comparatively tiny and never makes fusion with the chorion, while the yolk sac becomes very huge and combines broadly with the chorion. 2. Chorio-allantoic placenta- In chorio-allantoic placenta, the yolk sac remains undeveloped. The fusion found between the uterine wall and the embryo is lined by chorion and allantois. So, allantois furnishes the chorionic circulation. Since the placenta is formed of chorion and allantois, it is termed as chorio-allantoic placenta. E.g., Parameles, Dasyurus. 3. Chorionic placenta- The chorionic placenta is formed of thickened layer of chorion containing sinuses filled with maternal blood. Chorionic “villi” containing foetal connective tissue and capillaries expand and cross the chorionic lacunae. Chorionic type of placenta is found in human beings. The portion of the trophoblast which is nearer to the embryo is known as cytotrophoblast. The more external lying part of the trophoblast is called syncytiotrophoblast as it is a syncytium of irregular strands with interstices in between. The part of the uterine wall to which the placenta is joined is termed as decidua basalis. The portion of uterine mucosa and epithelium which is over the blastocyst forming a capsule, is known as decidua capsularis, the left over part of the uterine wall with which the chorion finally comes in touch, is called as decidua vera. B. TYPES OF PLACENTA ACCORDING TO THE DEGREE OF INTIMACY BETWEEN THE FOETAL AND MATERNAL TISSUE Five sub-types of placenta can be distingushed. 1. Epithelio-chorial placenta- It’s the simplest type of placenta where the villi of the chorion dip into the crypts of the uterine wall. Eg., All marsupials, some ungulates and lemurs. Therefore, the molecules of oxygen and other nutrients diffuse from the blood of the mother to that of the embryo through I. Maternal endothelium II. Maternal connective tissue III. Maternal uterine epithelium IV. The Epithelium of chorion V. Foetal connective tissues and VI. Foetal endothelium 2. Syndesmochorial Placenta- In this type of placenta, the uterine epithelium is eroded. The chorion comes in contact with maternal connective tissue. Thus the nutrients pass through maternal endothelium, maternal connective tissue, chorion, foetal connective tissue and foetal endothelium. This type of placenta is found in ruminant ungulates. 3. Endotheliochorial placenta-In this type, uterine epithelium and maternal connective tissues are eroded. Thus the nutritive materials pass through the maternal endothelium, chorion, the foetal connective tissues and foetal endothelium. This condition is found in carnivores. 4. Haemochorical placenta- Here, in addition to the uterine epithelium and maternal connective tissues, the maternal endothelium is also eroded. Thus the chorionic villi directly dip in maternal blood. This type of placenta is found in lower rodents, insectivores, bats and man. 5. Haemoendothelial placenta- In this type of placenta all the three maternal tissues and two foetal tissues i.e., chorionic epithelium and chorionic tissue are completely eroded. The foetal blood vessels dip into blood lacunae of the uterus. The number of barriers between the maternal and foetal blood streams, therefore is reduced to just one. E.g., Higher rodents and rabbit. C. TYPES OF PLACENTA ACCORDING TO THE DEGREE OF CONTACT BETWEEN CHORIONIC VILLI AND THE ENDOMETRIUM Two sub-types of placenta may be recognized. 1. Non-deciduate placenta- The chorionic villi are simple projections. They have loose connections with crypts in the uterine epithelium. At the time of birth, the chorion peels off from the uterine wall by pulling the villi out of the crypts. So no bleeding occurs at parturition. It is found in pig, cattle, horse and other ruminants. 2. Deciduate placenta- In higher Eutherian mammals including dog, rabbit and man, the union between the chorion and the uterine epithelium is much more close. The villi are so closely united with the uterine wall that at parturition, a large part of the uterine tissue is lost along with the foetal membranes. A large amount of bleeding also occurs. As the uterine wall participates in the formation of placenta, such a placenta is called decidua. The decidua has three types as follows: (i) Decidua basalis: The upper part of uterine wall to which the embryo becomes attached is called decidua basalis. (ii) Decidua capsularis: The part which surrounds the blastocyst and separates it from the cavity of the uterus is called decidua capsularis. (iii) Decidua parietalis: The parts which form the inner lining of the uterine wall is called decidua parietalis. D. TYPES OF PLACENTA ACCORDING TO THE DISTRUBUTION PATTERN OF CHORIONIC VILLI The placenta is classified into four sub- types according to the distribution and arrangement of villi on the surface of the chorion. 1. Diffuse Placenta- The villi are uniformly distributed all over the surface of the blastocyst. It is found in pig, horse and lemurs. 2. Cotyledonary placenta- The villi are arranged in patches. Each patch of villi is known as cotyledon. The uterine wall is provided with thickened sockets into which the cotyledons fit. E.g., Sheep, cattle and deer. 3. Zonary Placenta- The villi are arranged in the form of a belt around the middle of the chorionic sac. E.g., Cat and dog. 4. Discoidal Placenta- The villi are restricted to a small disc shaped area of the blastocyst and hence known as discoidal placenta. E.g., Insectivores, rodents, anthropoid apes and bats. Physiology of placenta 1. Placenta forms a physiological barrier and a semipermeable membrane between the mother and the foetus. It prevents the straight mixing of the maternal and the foetal blood. It prevents the entry of harmful materials. 2. It allows smaller molecules to diffuse. It provides nourishment and oxygen to the embryo. Oxygen, water and small molecules such as monosaccharides, salts of sodium, potassium and magnesium diffuse from the maternal blood into the foetal blood through the placenta. Macromolecules of polysaccharides, lipids and proteins may be engulfed by trophoblast cells by pinocytosis. 3. The placenta provides immunity to the foetus against certain diseases such as diphtheria, scarlet fever, small pox and measles. The antibodies which have developed in the blood of mother against these diseases are passed to the foetal placenta. Similarly Rh antibody also pass through placenta. Blood proteins cannot pass through the placenta because they are large molecules, so they are broken into aminoacids and transmitted. The foetus rebuilds complex proteins. 4. The most important function of placenta is the transfer of food stuffs from the mother to the foetus. 5. Many drugs consumed by the mother penetrate the placental barrier and cause most adverse effects in the embryo. Eg- Thalidomide used as a sedative by women in early pregnancy causes extensive deficiencies. 6. Certain pathogenic organisms and viruses can penetrate through the placenta and infect the foetus if the mother is infected. This is known to happen in infections with syphilis, smallpox, chickenpox and measles. SUMMARY The extra embryonic membranes are protective membranous structure that appear in parallel with the embryo and play an important role in embryonic development. It consists of chorion, amnion, yolk sac and allantois. The chorion participates in exchange of gases between the embryo and its surroundings whereas the amnion protects the embryo. The yolk sac is sole source of food containing yolk and allantois stores the metabolic waste of an embryo. Thus, the developing embryo is able to carry on essential metabolism while sealed within the egg and womb. The placenta is a materno-foetal temporary organ that develops at the implantation and is required for development of embryo and foetus. Its principal activities are metabolism, respiratory gas exchange, transfer of nutrients, elimination of waste products and endocrine secretion for maintenance of foetus during pregnancy.