Embryo Development: The Placenta PDF

Chapter The Placenta |6 which is a syncytium {cytoplasm with nuclei, but no cell boundaries). A developing e m b r y o gets attached to the...

Chapter The Placenta |6 which is a syncytium {cytoplasm with nuclei, but no cell boundaries). A developing e m b r y o gets attached to the The first-formed villi are called primary villi. u t e r i n e e n d o m e t r i u m. T h i s is called They consist of a central core of c y t o t r o - implantation. phoblast covered by syncytiotrophoblast. In human beings the embryo gets buried in the substance of the endometrium. This type of Secondary villi have three layers. From inside implantation is called interstitial implantation. o u t these are e x t r a - e m b r y o n i c m e s o d e r m , cytotrophoblast and syncytiotrophoblast. After implantation the endometrium is called the decidua. In tertiary villi, blood capillaries are formed in the extra-embryonic mesoderm. The placenta is formed partly from embryonic structures and partly from the decidua. It is Villi are surrounded by an intervillous space responsible for t r a n s p o r t of n u t r i e n t s and which contains maternal blood. As the placenta enlarges, septa grow into the intervillous space oxygen to the fetus, and for removal of waste dividing the placenta into lobes. The fully products. formed placenta is about six inches in diameter T h e essential elements of the placenta are and about 500 g in weight. chorionic villi. The villi are surrounded by maternal blood. Fetal blood circulates through The placenta is normally attached to the upper capillaries in villi. part of the body of the uterus. A placenta attached lower down is called placenta praevia. The maternal and fetal blood are separated by It can cause problems during child birth. a very thin placental membrane (or barrier). All substances passing from mother to fetus The embryo is surrounded by three large cavities. (and vice versa) traverse this membrane. These are the a m n i o t i c cavity, t h e e x t r a - e m b r y o n i c coelom, and the uterine cavity. The fetal tissue that takes part in forming the Enlargement of the amniotic cavity obliterates placenta is chorion. It consists of trophoblast the extra-embryonic coelom, leading to fusion (one layer of cells) resting on extra-embryonic of amnion and chorion. Further enlargement mesoderm. of a m n i o t i c cavity o b l i t e r a t e s the uterine Proliferation of cells of the trophoblast leads cavity. Fused a m n i o n a n d c h o r i o n (called to formation of two layers: cytotrophoblast, membranes) bulge into the cervical canal which is cellular and syncytiotrophoblast, (during child birth) and help to dilate it.  Human Embryology IMPLANTATION - Biasiocyst After the ovum is shed from the ovary, it travels through the uterine tube t o w a r d s the uterus. If - Uterine gland fertilization occurs, segmentation of the ovum - Stroma begins. By the time the fertilized ' o v u m ' reaches the uterus, it has already become a morula. The morula is still surrounded by the zona pellucida, which prevents it from 'sticking 1 to the wall of the Fig. 6.1 Relationship of blastocyst to uterine uterine tube. The cells lining the surface of the endometrium. morula, constitute the trophoblast. The trophoblast has t h e p r o p e r t y of a t t a c h i n g itself t o , a n d f e r t i l i z a t i o n. T h e t r o p h o b l a s t of the h u m a n invading, any tissue it comes in contact with. Once blastocyst invades the endometrium of the uterus. the zona pellucida disappears, the cells of the The blastocyst burrows deeper and deeper into trophoblast stick to the uterine endometrium. This the uterine mucosa till the whole of it comes to lie is called implantation (Fig. 6.1). In h u m a n s , within the thickness of the endometrium (Fig. 6.2). i m p l a n t a t i o n begins on the 6 t h day after This is called interstitial implantation (Fig. 6.3). Trophoblast OffifflffiffifflHI^ $XU1T Stroma (Decidua) Fig. 6.2 Stages in implantation of blastocyst.  The Placenta DECIDUA Decidua basalis Amnion After the implantation of the embryo, Choroin the uterine endometrium is called the decidua. When the morula reaches the uterus, the endometrium is in the secretory phase. After implantation, the features of the endometrium, which are seen during the secretory phase of the menstrual cycle, are maintained Extra-embryonic and intensified. The stromal cells coelom enlarge, become vacuolated, and store Uterine lumen glycogen and lipids. This change in the stromal cells is called the decidual Myometrium reaction. Fig. 6.3 Manner of implantation in the human uterus. This The portion of the decidua where the type of implantation is interstitial. Various other placenta is to be formed (i.e. deep to the types occur in other mammals. developing blastocyst) is called the decidua basalis (Fig. 6.4). The part of The process of implantation is aided by proteo- the decidua that separates the embryo from the lytic enzymes produced by the trophoblast. The uterine lumen is called the decidua capsularis, uterine mucosa also aids the process. The while the part lining the rest of the uterine trophoblastic cells which are situated over the cavity is called the decidua parietalis. The inner cell mass, start penetrating the epithelium decidua basalis consists predominantly of large of the endometrium. The implantation results decidual cells which contain large amounts of due to the mutual interaction between tropho- lipids and glycogen (that presumably provide blastic cells and endometrium. This interaction a source of nutrition for the embryo). The is mediated by the receptors present on uterine decidua basalis is also referred to as the epithelium and the secretion of 'L-selectin1 and decidual plate, and is firmly united to the 'integrins' by the trophoblast cells. chorion. At the end of pregnancy, the decidua Myometrium is shed off, along with the placenta and membranes. It is this shedding off which Decidua basalis gives the decidua its name (c.f. Chorion deciduous trees). Decidua capsularis FORMATION OF CHORIONIC VILLI Decidua parietalis The essential functional elements of the Uterine cavity placenta are very small finger-like processes or villi. These villi are surrounded by maternal blood. In the substance of the villi, there are Fig. 6.4 Subdivisions or' decidua. capillaries through which the fetal blood  Human Embryology 1m circulates. Exchanges between the maternal and fetal circulations take place through the tissues forming the walls of the villi (Fig. 6.5). The villi are formed as offshoots from the surface of the trophoblast. As the trophoblast, along with.Capillary carrying the underlying extra-embryonic mesoderm, fetal blood constitutes the chorion, the villi, arising from it, s Intervillous space are called chorionic villi. containing maternat blood The chorionic villi are first formed all over the trophoblast and grow into the surrounding decidua Blood vessels in extra-embryonic (Fig. 6.6A). Those villi related to the decidua 1 L^> mesoderm capsularis are transitory. After some time these degenerate. This part of the chorion becomes Fig. 6.5 Scheme to show that fetal b l o o d s m o o t h a n d is called t h e chorion laevae. In circulating through capillaries of villi is i contrast, the villi that grow into the decidua basalis close relation to maternal b l o o d in the intervillous space. undergo considerable development. Along with the tissues of the decidua basalis these villi form a disc-shaped mass which is called the placenta (Fig. 6.6B).The part ofthe chorion that helps form Decidua basalis the placenta is called the chorion frondosum. T h e essential features of the f o r m a t i o n of - Chorionic villi chorionic villi are as follows. The trophoblast is at first m a d e u p of a single l a y e r of cells (Fig. 6.7A). As these cells multiply, two distinct layers are formed (Fig. 6.7B). The cells that are nearest to the decidua (i.e. the most superficial cells) lose t h e i r cell b o u n d a r i e s. T h u s , o n e continuous sheet of cytoplasm containing many- n u c l e i is f o r m e d. Such a t i s s u e is c a l l e d a syncytium. Hence, this layer o f t h e trophoblast is called the syncytiotrophoblast or plasmodio- trophoblast. Deep to the syncytium, the cells of the trophoblast retain their cell walls and form the second layer called the cytotrophoblast (also called Langhan's layer). The cytotrophoblast rests on extra-embryonic mesoderm. All these elements (syncytium, cytotrophoblast and mesoderm) take Chorion laevae part in forming chorionic villi. T h e following three stages in formation of chorionic villi are seen: Fig. 6.6 Two stages in the formation of chorionic (a) Primary villi consist of a central core of v i l l i. Note their relationship to the c y t o t r o p h o b l a s t c o v e r e d by a layer of decidua. In {B) note that the villi over the s y n c y t i o t r o p h o b l a s t. Adjoining villi are decidua capsularis have disappeared. separated by an intervillous space.  The Placenta Details of the process of villus f o r m a t i o n a r e as -Trophoblast follows: _ Extraembryonic 1. The syncytiotrophoblast mesoderm grows rapidly and becomes thick. Small - Syncytiotrophoblast cavities (called lacunae) a p p e a r in t h i s l a y e r (Fig. 6.7C). Gradually, B the lacunae increase in - Cytotrophoblast size. At first they are irregularly arranged (Fig. 6.7D), but gradu- ally they c o m e t o lie r a d i a l l y (Figs 6. 8 A , 6.9) a r o u n d the blasto- Lacunae in thickened cyst. T h e lacunae are " syncytiotrophoblast s e p a r a t e d from one another by partitions of syncytium, which are called trabecular The lacunae gradually com- municate with each other, so that eventually o n e l a r g e s p a c e is formed. Each trabe- D c u l e is now surrounded - Lacunae enlarge all around by this lacu- nar space (Fig. 6.8B). The syncytiotrophoblast (in which these changes are o c c u r r i n g ) g r o w s Fig. 6.7 Early stages in formation of chorionic villi: into the endometrium. (A) Cytotrophoblast in contact with decidua; (B) Syncytiotrophoblast formed; As the endometrium is (C) Lacunae appear in syncytiotrophoblast; e r o d e d , s o m e of its (D) The lacunae enlarge. blood vessels are open- ed up, and blood from them fills the lacunar (b) Secondary villi show three layers: outer space (Fig. 6.10). syncytiotrophoblast, an intermediate layer Each t r a b e c u l u s is, initially, m a d e up of cytotrophoblast, and an inner layer of entirely of syncytiotrophoblast (Fig. 6.10). extra-embryonic mesoderm N o w the cells of the cytotrophoblast begin (c) Tertiary villi arc like secondary villi except to multiply and grow into each trabeculus t h a t t h e r e are b l o o d c a p i l l a r i e s in t h e (Fig. 6.11 A). T h e trabeculus thus comes mesoderm. to have a central core of cytotrophoblast  Human Embryology 4. Extra-embryonic meso- A derm invades the centre of each primary villus (Fig. 6.12A). The villus mf¥TlOE ^>Lacunar space now has a core of meso- derm (Fig. 6.12B) covered._ by cytotrophoblast and by syncytium. This structure is called a secondary villus. Trabeculae 5. Soon thereafter, blood vessels can be seen in the B mesoderm forming the 0 Lacunar space completely core of each villus. With their appearance, the * surrounding i each trabeculus villus is fully formed and is called a tertiary villus (Fig. 6.13). The blood Fig. 6.8 Formation of chorionic v i l l i. The trabeculae are n o w vessels of the villus estab- regularly arranged. (B) is a transverse section across (A) in the lish connections with the plane XY. circulatory system of the embryo. Fetal blood now circulates through the villi, while maternal blood -Decidua basalis circulates through rhc intervillous space..— Cytotrophoblast 6. From Figs 6.11A, 6.12A and 6.13A it will be seen Connecting stalk that the cytotrophoblast, that grows into the tra- Syncytiotrophoblast beculus (or villus) does not containing radially arranged trabeculae penetrate the entire thickness of syncytium Extra-embryonic coelom and, therefore, does not come in contact with the Decidua capsulars decidua. At a later stage, however, the cytotropho- Fig. 6.9 Figure showing radial arrangement of trabeculae and blast emerges through the lacunae around the blastocyst, syncytium of each villus. The cells of the cytotro- covered by an outer layer of syncytium. It phoblast now spread out to form a layer is surrounded by maternal blood, filling that completely cuts off the syncytium from the lacunar space. The trabeculus is now the decidua. This layer of cells is called called a primary villus (Fig. 6.11) and the the cytotrophoblasticshell (Fig. 6.14). The lacunar space is now called the intervillous cells of this shell multiply rapidly and the space. placenta increases in size.  The Placenta The villi that are first formed (as. Uterine described above) are attached on the blood vessel feta! side (Fig. 6.15) to the embryonic mesoderm and on the maternal side to the cytotrophoblastic shell. They are, Maternal blood in lacunar space therefore, called anchoring villi. Each a n c h o r i n g villus consists of a stem {truncus chorii); this divides into a number of branches {rami chorii) which in t u r n divide i n t o finer b r a n c h e s iramuli chorii). The ramuli are attached to the cytotrophoblastic shell. Lacunar spaces The anchoring villi give off numerous - seen in A' form one continuous space branches which grow into the fifled with maternal blood. i n t e r v i l l o u s s p a c e as free villi (Fig. 6.16). N e w villi also sprout from the chorionic side of the intervillous space. Ultimately, almost the whole Fig. 6.10 Uterine b l o o d vessels in the decidua open into the intervillous space becomes filled with lacunar space and fill it w i t h maternal b l o o d. (B) is a villi. As a result, the surface a r e a transverse section through trabeculae. a v a i l a b l e for e x c h a n g e s b e t w e e n maternal and fetal circulations becomes enormous. These, newly formed, villi at first consist only of syncytiotrophoblast. They are subsequently invaded by Syncytiotrophoblast cytotrophoblast, mesoderm, and Maternal blood blood vessels, and pass through the in lacunar space stages of primary, secondary and Cytotrophoblast tertiary villi, as described above. Extra-embryonic mesoderm FURTHER DEVELOPMENT OF THE PLACENTA Primary villus The placenta now becomes subdivided - Syncytiotrophoblast into a number of lobes, by septa that „ Core of cytotrophoblast grow into the intervillous space from the maternal side (Fig. 6.17). Each such Maternal blood lobe of the placenta is often called a in lacunar space maternal cotyledon. If the placenta is viewed from the maternal side, the Fig. 6.11 Cells of cytotrophoblast grow into the bases of the septa are seen as grooves syncytiotrophoblast of each trabeculus. The (Fig. 6.18) while the cotyledons appear trabeculae are now called primary v i l l i. as c o n v e x a r e a s b o u n d e d by t h e  Human Embryology g r o o v e s. T h e n u m b e r of l o b e s - Decidua generally varies between 15 and 20. Each lobe c o n t a i n s a n u m b e r of -Secondary villus anchoring villi and their branches. O n e such villus a n d its b r a n c h e s - Syncytiotrophoblast constitute a fetal cotyledon. The fully - Cytotrop he-blast formed placenta has 6 0 - 1 0 0 such _ Extra-embryonic fetal cotyledons. The placenta n o w mesoderm forms a compact mass and is disc- Maternal blood in shaped (Figs 6.17, 6.18). intervillous space At full term (9 months after onset of p r e g n a n c y ) the placenta has a diameter of 6 to 8 inches and weighs - Secondary villus about 500 g. After the birth of the - Syncytiotrophoblast child, the placenta is shed off along with the decidua. The maternal surface (formed by the decidual plate) - Cytotrophoblast is r o u g h a n d is s u b d i v i d e d i n t o Extra-embryonic mesoderm c o t y l e d o n s. T h e fetal s u r f a c e (chorionic plate) is lined by amnion. Fig. 6.12 Extra-embryonic mesoderm grows into the It is smooth and is not divided into cytotrophoblastic core of each primary villus, cotyledons. The umbilical cord is converting it into a secondary villus. attached to this surface. Decidua Placental Membrane Tertiary villus In the placenta, maternal blood circu- lates through the intervillous space Intervillous space and fetal blood circulates through Syncytiotrophoblast blood vessels in the villi. The mater- Cytotrophoblast nal and fetal blood do not mix with Capillary in extra- each other. They are separated by a mem- embryonic mesoderm brane, made up of the layers of the wall of the villus (Fig. 6.19A). These Maternal blood in (from the fetal side) are as follows: intervillous space (a) the endothelium of fetal blood v e s s e l s , a n d its b a s e m e n t Tertiary villus membrane. Syncytiotrophoblast (b) surrounding mesoderm (con- nective tissue). (c) cytotrophoblast, a n d its base- Cytotrophoblast Capillaries in extra-embryonic ment membrane. mesoderm (d) syncytiotrophoblast. Fig. 6.13 Blood capillaries invade the extra-embryonic mesoderm of These structures c o n s t i t u t e the each secondary villus thus converting it into a tertiary villus. placental membrane or barrier. All  The Placenta Maternal blood vessels of the membrane is increased, in decidua by d i s a p p e a r a n c e of t h e Cytotrophoblastic shell cytotrophoblastic layer from I II II UE m o s t villi, a n d by c o n s i - Syncvtiotrophoblast d e r a b l e t h i n n i n g of t h e Maternal blood in connective tissue (Fig 6.19B). intervillous space This membrane, which is at Cytotrophoblast first 0. 0 2 5 m m t h i c k , is Capillary in villus r e d u c e d ro 0. 0 0 2 m m. Fetal blood vessels in However, towards the end of extra-embryonic mesoderm pregnancy, a fibrinoid d e p o s i t a p p e a r s on the Fig. 6.14 Formation of cytotrophoblastic shell. Note that after membrane, and this reduces formation of this shell the syncytiotrophoblast is no longer in contact w i t h maternal tissues. its efficiency. Functions of Placenta Maternal surface 1 1. The placenta enables the a - ^ ^ ^ ^ ^ ^ ^ ^ n _ t r a n s p o r t of o x y g e n , water, electrolytes and nutrition (in the form of carbohydrates, lipids, f W Ramus ^ - ^ ^ JFRamuli y\g J^. polypeptides, amino acids ^ ^ M chorii ^k and vitamins) from ^M Truncus Mi Intervillus mp ^^L maternal to fetal blood. A chorii "" "' space ~~~f~^. ^ ^ full term fetus takes up about 25 ml of oxygen per m i n u t e from m a t e r n a l b l o o d. Even a s h o r t Fetal surface i n t e r r u p t i o n of oxygen Umbilical cord s u p p l y is fatal for t h e fetus. Fig. 6.15 Arrangement of anchoring villi and intervillous spaces within the placenta. Note the subdivisions of each anchoring illus. 2. It a l s o p r o v i d e s for e x c r e t i o n of c a r b o n i n t e r c h a n g e s of o x y g e n , n u t r i t i o n a n d w a s t e dioxide, urea and other products take place through this membrane. waste products produced by the fetus into the maternal blood. The total area of this membrane varies from 4 3. Maternal antibodies (IgG, gamma globulins t o 14 square metres. It is interesting to note or I m m u o g l o b u l i n s ) reaching the fetus that this is equal to the total absorptive area of through the placenta give the fetus immunity the adult intestinal tract. As in the gut, the against some infections (e.g. diphtheria and effective absorptive area is greatly increased measles). by the presencejpf numerous microvilli on the 4. The placenta acts as a barrier and prevents surface of the syncytiotrophoblast. many bacteria and other harmful substances In the later part of pregnancy, the efficiency from reaching the fetus. However, most  Human Embryology diethylstilbestrol) easily cross the placenta and can have adverse effects on the fetus (including carcinoma in later life). 5, While permitting the exchange of several substances between the maternal and fetal blood, it keeps these blood streams separate, thereby, preventing antigenic reactions between them. 6. The placenta synthesizes several hormones. These are probably produced in the syncytiotrophoblast. Progesterone secreted by the placenta is essential for maintenance of pregnancy after the fourth month (when the corpus luteum degenerates). Oestrogens (mainly estriol) produced by Fig. 6.16 Free villi arising from an anchoring villus. the placenta reach maternal blood and viruses (including poliomyelitis, measles and promote uterine growth and development rubella) and some bacteria can pass across of the mammary gland. it. Drugs taken by the mother may also enter the fetal circulation and can produce CLINICAL CORRELATION congenital malformations. Human chorionic gonadotropin (hCG) produced As a rule, maternal hormones do not by the placenta is similar in its actions to reach the fetus. However, synthetic luteinizing hormone of the hypophysis cerebri. progestins and synthetic oestrogens (e.g. Gonadotropins are excreted through maternal Maternal vessel opening Intervillous space into intervillous space 0ne cotyledon-, Septum \ [....1 i Anchoring villus Umbilical cord Fig. 6.17 Structure of a fully formed placenta. Each lobe (labelled cotyledon) contains a number of anchoring villi but only one is shown here for the sake of simplicity.  The Placenta urine where their presence is used as a test to detect a pregnancy in its early stages. Somatomammotropin (bCS) h a s an a n t i - insulin effect on the mother leading to increased plasma levels of glucose and amino acids in t h e m a t e r n a l c i r c u l a t i o n. In t h i s w a y it increases availability of these materials for the fetus. It also enhances glucose utilization by the fetus. Circulation of Blood through the Placenta Blood flow t h r o u g h l a c u n a r spaces in the syncytiotrophoblast begins as early as the 9th day of pregnancy. Thereafter, the maternal blood in the intervillous spaces is constantly in circulation. Blood enters the intervillous space Fig. 6.18 Placenta after shedding, through maternal arteries that open into the the maternal aspect. space. The pressure of blood drives it right up to the c h o r i o n i c p l a t e. Blood from the intervillous spaces is drained by veins that also Layers of barrier open into the same spaces. Syncytiotrophoblast In the fully funned placenta, the intervillous Cytotrophoblast spaces contain about 150 ml of blood which is Mesoderm replaced in 15 to 20 seconds {i.e. three to four Endothelium of times per minute). fetal blood vessel Fetal blood in capillary ' Maternal blood around villus NORMAL SITE OF IMPLANTATION OF THE OVUM The uterus can be divided into an upper part, Layers of barrier consisting of the fundus and the greater part of the Syncytiotrophoblast b o d y , a n d a l o w e r p a r t , c o n s i s t i n g of t h e Endothelium of lower part of the body and the cervix. These are fetal capillary c a l l e d t h e upper uterine segment, and the lower uterine segment, respectively. It is the upper uterine segment that enlarges during pregnancy. Fig. 6.19 Diagrams of placental barrier: (A) in early The placenta is normally attached only to the part of pregnancy, and (B) in later part. upper uterine segment (Fig. 6.21).  Human Embryology — Upper uterine segment Vagina Fig 6.21 Upper and lower uterine segments and their normal relationship to the placenta. Intervillus space Umbilical cord Fig. 6.20 Scheme to show h o w maternal blood circulates through the placenta. Villi are ABNORMAL SITES OF not drawn for the sake of simplicity. IMPLANTATION OF THE OVUM Abnormal Implantation within the Uterus The attachment of the placenta may extend partially, or completely, into the lower uterine segment. T h i s condition is called placenta praevia. It causes difficulty during childbirth and may cause severe bleeding. Various degrees of placenta praevia may be recognized, as given below: 1. First degree: The attachment of the placenta extends into the lower uterine segment, bur does not reach the internal os (Fig. 6.22A). 2. Second degree: The margin of the placenta reaches the internal os, but does not cover it (Fig. 6.22B). 3. Third degree: T h e edge of the placenta covers the internal os, but when the os Third dilates during childbirth, the placenta no degree longer occludes it (Fig. 6.22C). 4 Fourth degree: T h e placenta completely covers the internal os, and occludes the os Fig. 6.22 Types of placenta praevia. even after it has dilated (Fig. 6.22D).  The Placenta 2. lobed, w h e n it is d i v i d e d i n t o l o b e s (Fig.6.24B); 3. diffuse, when chorionic villi persist all round the blastocyst: the placenta is thin and does not assume the shape of a disc (Fig. 6.24C); 4. placenta succenturiata, when a small part of the placenta is separated from the rest of it (Fig. 6.24D); 5. fenestrated, when there is a hole in the disc (Fig. 6.24E); and 6. ctrcumvallate, when the peripheral edge of Fig. 6.23 Abnormal sites of implantation: the placenta is covered by a circular fold of (1) Normal site, (2) Placenta praevia, decidua (Fig. 6.24F). (3) Interstitial tubal implantation, (4) Tubal implantation, (5) A b d o m i n a l The umbilical cord is normally attached to the implantation, (6) O v a r i a n implantation. placenta near the centre (Fig. 6.25A). However, this attachment may be: Implantation outside the Uterus When the ovum gets implanted at any site outside the uterus, this is called an ectopic pregnancy. This may be as follows: 1. Tubal pregnancy: T h e b l a s t o c y s t gets i m p l a n t e d in t h e u t e r i n e t u b e. Such a pregnancy cannot go on to full term, and may result in rupture of the tube. After r u p t u r e , the b l a s t o c y s t may a c q u i r e a secondary implantation in the abdominal c a v i t y (Fig. 6. 2 3 ) , g i v i n g rise t o an abdominal pregnancy. 2. Interstitial tubal implantation: The blastocyst may get implanted in the part of the uterine tube passing through the uterine wall. 3. Implantation in the ovary: Fertilization and implantation may occur while the ovum is still in the ovary. Other Anomalies of Placenta Instead of being shaped like a disc, the placenta may be: Fig. 6.24 Anomalies of placenta: (A) Bidiscoidal; / / Note that amnion I T~"""^ / and chorion have fused to obliterate the 1 J A\ extra-embryonic coelom. Fig. 6.27 Relationship of amniotic cavity and uterine cavity after obliteration o f the extra-embryonic c o e l o m.  Human Embryology aspects of the process of childbirth. The changing membrane. From Fig. 6.27 it will be seen that the relationships will be best u n d e r s t o o d by first wall of the amniotic cavity is now formed by (i) reviewing Figs 4.6, 4.7 and 4.13 and then by amnion, (ii) chorion, and (Hi) decidua capsularis, studying Figs 6. 2 6 - 6. 2 8. all three being fused t o one another. In Fig. 6.26 we see three cavities, namely, the Further expansion of the amniotic cavity occurs uterine cavity, the extra-embryonic coelom, and at the expense of the uterine cavity. Gradually, the amniotic cavity. The outer wall of the extra- the decidua capsularis fuses with the decidua embryonic coelom is formed by chorion and the parietalis, and the uterine cavity is also obliterated inner wall by a m n i o n. As the amniotic cavity (Fig. 6.28). Still further expansion of the amniotic enlarges, the extra-embryonic coelom becomes cavity is achieved by enlargement of the uterus. smaller and smaller. It is eventually obliterated, H n l a r g e m e n t of t h e a m n i o t i c c a v i t y is by fusion of a m n i o n and c h o r i o n. T h e fused accompanied by an increase in the a m o u n t of chorion and amnion form the amniochorionic amniotic fluid. Decidua capsularis Decidua parietalis (dark green; (light green) Uterine cavity has been obliterated by 'Membranes' fusion of decidua (fused amnion, chorion, capsularis and and decidua capsularis) decidua parietalis lie over the internal os. Fig. 6.28 A m n i o t i c cavity after obliteration of the extra-embryonic c o e l o m and uterine cavity.  The Placenta At the time of parturition (childbirth), the fused transferred through the placenta to maternal amnion and chorion (amniochorionic membrane) blood. When the fetal kidneys start working (along with the greatly thinned out decidua the fetus passes urine into the amniotic fluid. capsularis), constitute what are called the This does not cause harm because fetal urine is membranes. As the uterine muscle contracts, made up mostly of water (metabolic wastes increased pressure in the amniotic fluid causes these being removed from blood by the placenta and membranes to bulge into the cervical canal. This not through the kidneys). bulging helps to dilate this canal. The bulging In some cases hydramnios is associated with membranes can be felt through the vagina and atresia of the oesophagus, which prevents are referred to as the bag of waters. Ultimately swallowing of amniotic fluid by the fetus. the membranes rupture. Amniotic fluid flows out Oligamnios is sometimes associated with renal into the vagina. After the child is delivered, the agenesis as no urine is added to the amniotic placenta and the membranes, along with all parts fluid. of the decidua, separate from the wall of the uterus and are expelled from it. TIMETABLE OF SOME EVENTS DESCRIBED IN THIS CHAPTER AMNIOTIC FLUID Age (in Days) Developmental Events Amniotic fluid provides support for the delicate 8th day Trophoblast differentiates tissues of the growing embryo or fetus. It allows into cytotrophoblast and free movement and protects the fetus from external syncytiotrophoblast. injury. It also avoids adhesion of the fetus to amnion. As pregnancy advances the quantity of 9th day Lacunae appear in rhe this fluid increases, till at full term it is about one syncytium. litre. 11 th day Embryo gets completely The condition in which there is too much implanted in the amniotic fluid (over 1500 ml) is called endometrium. hydramnios; and when the fluid is too little it is 13th day Primary villi are formed. called oligamnios. Both conditions can cause 16th day Secondary and tertiary villi abnormalities in the fetus. They can also cause are seen. difficulties during childbirth. 2nd month Villi are seen all around the trophoblast. There is constant exchange of water between 4th month A definitive placenta is the amniotic fluid and maternal blood, the water formed. being completely replaced every three hours. Some time in the fifth month the fetus begins to Full term Placenta is shed about half swallow amniotic fluid. This fluid is absorbed an hour after birth of the (through the gut) into fetal blood and is baby.