FUHSO PLACENTATION, TWINNING AND 4TH-8TH WEEK OF DEVELOPMENT PDF

GENERAL EMBRYOLOGY LECTURER’S NAME: Prof. AKINLOLU, A. A. Department of Anatomy, FUHSO Date: 7TH February, 2024. 8am - 10am and 2 - 4pm LECTURE TOPIC 1: PLACENTATION AND MULTIPLE PREGNANCIES...

GENERAL EMBRYOLOGY LECTURER’S NAME: Prof. AKINLOLU, A. A. Department of Anatomy, FUHSO Date: 7TH February, 2024. 8am - 10am and 2 - 4pm LECTURE TOPIC 1: PLACENTATION AND MULTIPLE PREGNANCIES LECTURE TOPIC 2: 4TH - 8TH WEEKS OF DEVELOPMENT OUTLINE 1. Review of 1st - 3rd weeks of development: Villous System 2. Placentation 3. Fate of Fetal Membranes 4. Twinning and Multiple Pregnancies 5. Embryonic Folding 6. Events of 4th – 8th weeks of development 7. Applied anatomy 8. Class Assignment 1 Attachment of the blastocyst to the endometrial epithelium during the early stages of its implantation. A, At 6 days: the trophoblast is attached to the endometrial epithelium at the embryonic pole of the blastocyst. B, At 7 days: the syncytiotrophoblast has penetrated the epithelium and has started to invade the endometrial connective tissue. 2 Implantation of a blastocyst in the endometrium. The actual size of the conceptus is 0.1 mm, approximately the size of the period at the end of this sentence. A, Drawing of a section through a blastocyst partially implanted in the endometrium (approximately 8 days). Note the slitlike amniotic cavity. B, An enlarged three-dimensional sketch of a slightly older blastocyst after removal from the endometrium. Note the extensive syncytiotrophoblast at the embryonic pole (side of the blastocyst containing the embryonic disc). C, Drawing of a section through a blastocyst of approximately 9 days implanted in the endometrium. Note the lacunae appearing in the syncytiotrophoblast. The term yolk sac is a misnomer because it contains no yolk. 3 Implanted blastocysts. A, At 10 days; B, at 12 days. This stage of development is characterized by communication of the blood-filled lacunar networks. Note in B that coelomic spaces have appeared in the extraembryonic mesoderm, forming the beginning of the extraembryonic coelom. 4  Observe that (1) the defect in the endometrial epithelium has disappeared; (2) a small secondary umbilical vesicle has formed; (3) a large cavity, the extraembryonic coelom, now surrounds the umbilical vesicle and amnion, except where the amnion is attached to the chorion by the connecting stalk; and (4) the extraembryonic coelom splits the extraembryonic mesoderm into two layers: extraembryonic somatic mesoderm lining the trophoblast and covering the amnion and the extraembryonic splanchnic mesoderm around the umbilical vesicle. A, At 13 days, illustrating the decrease in relative size of the primary umbilical vesicle and the early appearance of primary chorionic villi. B, At 14 days, showing the newly formed secondary umbilical vesicle and the location of the prechordal plate in its roof. C, Detail of the prechordal plate outlined in B. Chorion = Extraembryonic mesoderm/chorionic plate + cytotrophoblast + syncytiotrophoblast. DEVELOPMENT OF CHORIONIC VILLI Primary chorionic villi (trophoblast + syncytiotrophoblast) begin to branch after appearing at the end of 2nd week. Secondary villi form when mesenchyme grows into primary villi to form a core of mesenchymal tissue early in 3rd week (trophoblast + mesenchyme core + syncytiotrophoblast). It covers the entire surface of the chorionic sac. Tertiary villi form when capillaries & blood cells appear in mesenchymal core. The capillaries in the chorionic villi fuse to form arteriocapillary networks, which connect with the embryonic heart through vessels that differentiate in the mesenchyme of the chorion and connecting stalk. By the end of 3rd week, embryonic blood begins to flow through capillaries of chorionic villi. Oxygen and nutrients in the maternal blood in the intervillous space diffuse through walls of the villi and enter the embryo's blood. Carbon dioxide and waste products diffuse from blood in the fetal capillaries through the wall of the chorionic villi into the maternal blood. Concurrently, cytotrophoblastic cells of the chorionic villi proliferate and extend through syncytiotrophoblast to form a cytotrophoblastic shell, which surrounds the chorionic sac and attaches it to the endometrium. Villi that attach to the maternal tissues through cytotrophoblastic shell are stem chorionic villi (anchoring villi). The villi that grow from the sides of the stem villi are branch chorionic villi (terminal villi). It is through the walls of the branch villi that the main exchange of material between the blood of the mother and the embryo takes place. The branch villi are bathed in continually changing maternal blood in the intervillous space. 5 Presomite embryo and the trophoblast at the end of the third week. Tertiary and secondary stem villi give the trophoblast a characteristic radial appearance. Intervillous spaces, which are found throughout the trophoblast, are lined with syncytium. Cytotrophoblastic cells surround the trophoblast entirely and are in direct contact with the endometrium. The embryo is suspended in the chorionic cavity by means of the connecting stalk. Development of a villus. A. Transverse section of a primary villus showing a core of cytotrophoblastic cells covered by a layer of syncytium. B. Transverse section of a secondary villus with a core of mesoderm covered by a single layer of cytotrophoblastic cells, which in turn is covered by syncytium. C. Mesoderm of the villus showing a number of capillaries and venules. 6 Longitudinal section through a villus at the end of the third week of development. Maternal vessels penetrate the cytotrophoblastic shell to enter intervillous spaces, which surround the villi. Capillaries in the villi are in contact with vessels in the chorionic plate and in the connecting stalk, which in turn are connected to intraembryonic vessels. Human embryo at the beginning of the second month of development. At the embryonic pole, villi are numerous and well formed; at the abembryonic pole, they are few in number and poorly developed. 7 Diagrams illustrating development of secondary chorionic villi into tertiary chorionic villi. Early formation of the placenta is also shown. A, Sagittal section of an embryo (approximately 16 days). B, Section of a secondary chorionic villus. C, Section of an implanted embryo (approximately 21 days). D, Section of a tertiary chorionic villus. The fetal blood in the capillaries is separated from the maternal blood surrounding the villus by the endothelium of the capillary, embryonic connective tissue, cytotrophoblast, and syncytiotrophoblast. 8  THE PLACENTA The placenta is a fetomaternal organ whose early development is characterized by rapid proliferation of the trophoblast and development of the chorionic sac and chorionic villi. A fully developed placenta covers 15% to 30% of the decidua and weighs approximately one sixth that of the fetus. Fetal Membranes: The chorion (Extraembryonic mesoderm/chorionic plate + cytotrophoblast + syncytiotrophoblast), amnion, umbilical vesicle, and allantois. Placenta and fetal membranes are expelled from the uterus as the afterbirth. Decidua (gravid endometrium): Functional layer of the endometrium in a pregnant woman that separates from the remainder of the uterus after parturition (childbirth). Has 3 regions in relation to implantation site: Decidua basalis: part of the decidua deep to the conceptus that forms the maternal part of the placenta. Decidua capsularis: superficial part of the decidua overlying the conceptus. Decidua parietalis: all the remaining parts of the decidua. Relation of fetal membranes to wall of the uterus. A. End of the second month. Note the yolk sac in the chorionic cavity between the amnion and chorion. At the abembryonic pole, villi have disappeared (chorion laeve). B. End of the third month. The amnion and chorion have fused, and the uterine cavity is obliterated by fusion of the chorion laeve and the decidua parietalis. DECIDUA REACTION - Implantation triggers decidual reaction i.e. cellular and vascular changes occurring in the endometrium as the blastocyst implants; accumulation of lipids & glycogen in decidual cells in response to increased progesterone levels; decidual cells enlarge & become pale-staining. - Many decidual cells degenerate near the chorionic sac in the region of the syncytiotrophoblast and, together with maternal blood and uterine secretions, provide a rich source of nutrition for the embryo. - Decidual cells possibly protect the maternal tissue against uncontrolled invasion by the syncytiotrophoblast - Decidual regions are clearly recognizable with ultrasonography, & are important in diagnosing early pregnancy. 9 - The fetal part of the placenta is formed by the villous chorion. The chorionic villi that arise from it project into the intervillous space containing maternal blood. - The maternal part of the placenta is formed by the decidua basalis. By the end of 4th month, the decidua basalis is almost entirely replaced by the fetal part of the placenta. - Fetal part is attached to maternal part by the cytotrophoblastic shell. - Endometrial arteries and veins pass freely through gaps in the cytotrophoblastic shell and open into the intervillous space. Estimation of Age using Chorionic sac: Chorionic sac with median sac diameter of 2 to 3 mm indicate that the gestational age is 4 weeks and 3 to 4 days, that is, approximately 18 days after fertilization. Drawing of a sagittal section of a gravid uterus at 4 weeks shows the relation of the fetal membranes to each other and to the decidua and embryo. The amnion and smooth chorion have been cut and reflected to show their relationship to each other and the decidua parietalis. 10 Structure of villi at various stages of development. A. During the 4th week, the extraembryonic mesoderm penetrates the stem villi in the direction of the decidual plate. B. During the 4th month, in many small villi, the wall of the capillaries is in direct contact with the syncytium. C,D. Definitive Placenta barrier: final villus development. DEFINITIVE PLACENTA - It is discoid in shape & is divided into irregular convex cotyledons by placenta septa (formed via erosion of decidua basalis by chorionic villi). Each cotyledon consists of 2 or more stem villi & their branching villi. - Glia cells missing-1 (Gcm-1) in trophoblast stem cells regulate branching processes to form vascular networks. - Chorion dvelop where related to decidua basalis (bushy chorion) but not developed where related to decidua capsularis (smooth chorion). - Fetal enlargement causes decidua capsularis (DC) to contact & fuse with decidua parietalis obliterating the uterine cavity. By 22 to 24 weeks, DC degenerates due to reduced blood supply allowing smooth chorion to fuse with decidua parietalis. Blood accumulation may separate smooth chorion from decidua parietalis. - Intervilous space contains maternal blood in sinusoids. Cotyledons of intervillous space communicate freely because placenta septa do not touch chorionic pate. - Spiral endometrial arteries pass through cytotrohpoblastic shells to release blood into intervillous space. - Endometrial veins enter through cytotrophoblastic shells to drain intervillous spaces. - Amniotic cavity enlarges faster than chorionic cavity causing amnion & chorion to fuse to form amniochorionic membrane which first with decidua capsularis & later with decidua parietalis (after degeneration of decidua capsularis). This membrane ruptures during labor allowing escape of amniotic fluid. Preterm rupture is one of the causes of premature labor. 11 Spontaneously aborted human chorionic sacs. A, At 21 days. The entire sac is covered with chorionic villi. B, At 8 weeks. Actual size. As the decidua capsularis becomes stretched and thin, the chorionic villi on the corresponding part of the chorionic sac gradually degenerate and disappear, leaving a smooth chorion. The remaining villous chorion forms the fetal part of the placenta. A full-term placenta. A. Fetal side. The chorionic plate and umbilical cord are covered by amnion. B. Maternal side showing the cotyledons. In one area, the decidua has been removed. The maternal side of the placenta is always carefully inspected at birth, and frequently one or more cotyledons with a whitish appearance are present because of excessive fibrinoid formation and infarction of a group of intervillous lakes. 12 PLACENTA MEMBRANE - Until approximately 20 weeks, the placental membrane consists of four layers: syncytiotrophoblast, cytotrophoblast, connective tissue of villus, and endothelium of fetal capillaries. - After the 20th week, the placental membrane consists of two layers: syncytiotrophoblast and endothelium of fetal capillaries forming a vasculosyncytial placental membrane. - Only a few substances, endogenous or exogenous, are unable to pass through the placental membrane in detectable amounts. - The placental membrane acts as a barrier only when the molecule is of a certain size, configuration, and charge such as heparin and bacteria. - Some metabolites, toxins, and hormones, although present in the maternal circulation, do not pass through the placental membrane in sufficient concentrations to affect the embryo/fetus. - Most drugs and other substances in the maternal plasma pass through the placental membrane and enter the fetal plasma. - Electron micrographs of the syncytiotrophoblast show that its free surface has many microvilli, more than 1 billion/cm2 at term, that increase the surface area for exchange between the maternal and fetal circulations. - During the third trimester, numerous nuclei in the syncytiotrophoblast aggregate to form multinucleated protrusions called nuclear aggregations or syncytial knots which continually break off and are carried from the intervillous space into the maternal circulation. Some knots lodge in capillaries of the maternal lung where they are rapidly destroyed by local enzyme action. - Toward the end of pregnancy, fibrinoid material forms on the surfaces of villi. This material consists of fibrin and other unidentified substances that stain intensely with eosin. Fibrinoid material results mainly from aging and appears to reduce placental transfer. Diagrammatic illustration of transfer across the placental membrane (barrier). The extrafetal tissues, across which transport of substances between the mother and fetus occurs, collectively constitute the placental membrane. Inset, Light micrograph of chorionic villus showing a fetal capillary (arrow) and the placental membrane. 13 Placental Circulation: The branch chorionic villi of placenta provide a large surface area for exchange of materials across the very thin placental membrane (interposed between the fetal and maternal circulations). Fetal Placental Circulation: Poorly oxygenated blood leaves the fetus and passes through umbilical arteries to placenta. At the site of attachment of the umbilical cord to the placenta, umbilical arteries divide into chorionic arteries that branch freely in the chorionic plate before entering the chorionic villi. The blood vessels form an extensive arteriocapillary-venous system within the chorionic villi, which brings the fetal blood extremely close to the maternal blood. There is normally no intermingling of fetal and maternal blood; however, small amounts of fetal blood may enter maternal circulation when minute defects develop in the placental membrane. Well-oxygenated fetal blood in the fetal capillaries passes into thin-walled veins that follow chorionic arteries which unite to form umbilical vein at the site of attachment of the umbilical cord. [[[[[[ Maternal Placental Circulation: - The maternal blood in the intervillous space is temporarily outside the maternal circulatory system. - It enters the intervillous space through 80 to 100 spiral endometrial arteries in the decidua basalis. - Spiral arteries discharge into the intervillous space through gaps in the cytotrophoblastic shell. - Blood flow from the spiral arteries is pulsatile and is propelled by the maternal blood pressure. - The entering blood is at a higher pressure than that in the intervillous space and spurts toward the chorionic plate forming the "roof" of the intervillous space. - As blood pressure reduces, the blood flows slowly over the branch villi, allowing an exchange of metabolic and gaseous products with the fetal blood. - Maternal blood eventually returns through the endometrial veins to the maternal circulation. - The welfare of the embryo/fetus depends more on the adequate bathing of the branch villi with maternal blood than on any other factor. Reductions of uteroplacental circulation result in fetal hypoxia and intrauterine growth restriction (IUGR) or death (in severe cases). - The intervillous space of the mature placenta contains approximately 150 mL of blood that is replenished three or four times per minute. - Oxygen transfer to the fetus is decreased during uterine contractions, but does not stop its transfer because they do not force significant amounts of blood out of the intervillous space. - Drawing of a stem chorionic villus showing its arteriocapillary-venous system. The arteries carry poorly oxygenated fetal blood and waste products from the fetus, whereas the vein carries oxygenated blood and nutrients to the fetus. B and C, Drawings of sections through a branch villus at 10 weeks and full term, respectively. The placental membrane separates the maternal blood in the intervillous space from the fetal blood in the capillaries in the villi. Note that the placental membrane becomes very thin at full term. Hofbauer cells are thought to be phagocytic cells. 14 Schematic drawing of a transverse section through a full-term placenta, showing (1) the relation of the villous chorion (fetal part of placenta) to the decidua basalis (maternal part of placenta), (2) the fetal placental circulation, and (3) the maternal placental circulation. Maternal blood flows into the intervillous space in funnel- shaped spurts from the spiral endometrial arteries, and exchanges occur with the fetal blood as the maternal blood flows around the branch villi. It is through these villi that the main exchange of material between the mother and embryo/fetus occurs. The inflowing arterial blood pushes venous blood out of the intervillous space into the endometrial veins, which are scattered over the surface of the decidua basalis. Note that the umbilical arteries carry poorly oxygenated fetal blood (shown in blue) to the placenta and that the umbilical vein carries oxygenated blood (shown in red) to the fetus. Note that the cotyledons are separated from each other by placental septa, projections of the decidua basalis. Each cotyledon consists of two or more main stem villi and many branch villi. In this drawing, only one stem villus is shown in each cotyledon, but the stumps of those that have been removed are indicated. Placental Changes at the end of Pregnancy: (a) Increase in fibrous tissue in the core of the villus, (b) Thickening of basement membranes in fetal capillaries, (c) Obliterative changes in small capillaries of the villi, and (d) Deposition of fibrinoid on the surface of the villi in the junctional zone and in the chorionic plate. Excessive fibrinoid formation frequently causes infarction of an intervillous lake or sometimes of an entire cotyledon. The cotyledon then assumes a whitish appearance. 15  FUNCTIONS OF THE PLACENTA Metabolism (e.g., synthesis of glycogen); Transport of gases and nutrients & Endocrine secretion (e.g. hCG) which are essential for maintaining pregnancy and promoting normal fetal development. Placental Metabolism: In early pregnancy, it synthesizes glycogen, cholesterol, and fatty acids, which serve as sources of nutrients and energy for the embryo/fetus. Placental Transfer: It permits transport of substances in both directions between the fetal and maternal blood via simple diffusion, facilitated diffusion, active transport, and pinocytosis. Other transfer mechanisms include (i) passing of fetal RBCs into the maternal circulation during parturition, through microscopic breaks in the placental membrane. Maternal RBCs have also been found in the fetal circulation. (ii) Some cells cross the placental membrane under their own power, e.g., maternal leukocytes and Treponema pallidum (the organism that causes syphilis). (iii) Some bacteria and protozoa such as Toxoplasma gondii infect the placenta by creating lesions and then cross the placental membrane through the defects that are created. Transfer of Gases: O2, CO2 & CO cross the placental membrane by simple diffusion. Quantity of oxygen reaching the fetus is primarily flow limited rather than diffusion limited; hence, fetal hypoxia results primarily from factors that reduce either uterine blood flow or fetal blood flow. Nutritional Substances: Water (simple diffusion), Glucose (facilitated diffusion), Free fatty acids (in very small amount), Amino acids (active transport) & Vitamins (Water-soluble vitamins cross more quickly than fat-soluble ones). There is little or no transfer of maternal cholesterol, triglycerides, or phospholipids.. Hormones: Protein hormones do not reach the embryo/fetus in significant amounts. Thyroxine & triiodothyronine (slow transfer), Unconjugated steroid hormones (free passage); Testosterone & certain synthetic progestins (cross placental membrane and may cause masculinization of female fetuses in some cases). Electrolytes: Freely exchanged across the placental membrane in significant quantities, each at its own rate. Electrolytes in intravenous fluids pass to the fetus and affect its water and electrolyte status. Maternal Antibodies: The fetus produces only small amounts of antibodies because of its immature immune system. Some passive immunity is conferred via maternal antibodies against some diseases such as diphtheria, smallpox, and measles. No immunity is acquired to pertussis (whooping cough) or varicella (chickenpox). IgG gamma globulins are readily transported to the fetus by transcytosis. Transferrin (a maternal protein) crosses the placental membrane and carries iron to the embryo or fetus. Hemolytic Disease of the Newborn: Hemolysis of fetal Rh-positive blood cells, jaundice, and anemia in the fetus may occur if small amounts of fetal Rh positive blood pass to the maternal Rh negative blood through microscopic breaks in the placental membrane. The fetal blood cells may stimulate the formation of anti-Rh antibodies by the immune system of the mother. Rh (D) immunoglobulin given to the mother usually prevents development of this disease in the fetus. Waste Products: Urea and uric acid pass through the placental membrane by simple diffusion. Conjugated bilirubin (which is fat soluble) is easily transported by the placenta for rapid clearance. 16  Drugs and Drug Metabolites: Most drugs and drug metabolites cross the placenta by simple diffusion, except those with a structural similarity to amino acids, such as methyldopa and antimetabolites. Some drugs such as Thalidomide cause major congenital anomalies. Fetal drug addiction may occur after maternal use of drugs such as heroin and 50% to 75% of these newborns experience withdrawal symptoms, but no psychic dependence occurs. Infectious Agents: Cytomegalovirus, rubella, and coxsackie viruses, and viruses associated with variola, varicella, measles, and poliomyelitis may pass through the placental membrane and cause fetal infection. Rubella virus may cause congenital anomalies such as cataracts. Treponema pallidum, which causes syphilis, and Toxoplasma gondii, which produces destructive changes in the brain and eyes, also cross the placental membrane, often causing congenital anomalies and/or death of the embryo or fetus. Placental Endocrine Synthesis and Secretion: Syncytiotrophoblast of the placenta synthesizes protein and steroid hormones. Protein hormones: hCG, Human chorionic somatomammotropin or human placental lactogen, Human chorionic thyrotropin & Human chorionic corticotropin. Steroid hormones: Progesterone formed from maternal cholesterol or pregnenolone and Estrogens. The ovaries of a pregnant woman can be removed after the first trimester without causing an abortion because the placenta takes over the production of progesterone from the corpus luteum. Uterine Growth during Pregnancy: The uterus of a nonpregnant woman lies in the pelvis. It increases in size & in weight to accommodate the growing fetus making its walls to become thinner. During the first trimester, the uterus moves out of the pelvis and by 20 weeks reaches the level of the umbilicus. By 28 to 30 weeks, the uterus reaches the epigastric region. Uterine increase in size results from hypertrophy of preexisting smooth muscular fibers and partly from the development of new fibers. Drawings of median sections of a woman's body. Note that as the conceptus enlarges, the uterus increases in size to accommodate the rapidly growing fetus. A- Not pregnant. B- 20 wks pregnant: uterus and fetus reach the level of the umbilicus. C- 30 wks pregnant: uterus and fetus reach the epigastric region. The mother's abdominal viscera are displaced and compressed, and the skin and muscles of her anterior abdominal wall are stretched. 17  FATE OF FETAL MEMBRANES Fate of Umbilical Vesicle (Yolk Sac): - It can be observed sonographically early in 5th week & is recognizable in ultrasound examinations until the end of first trimester. - At 32 days, the umbilical vesicle is large, but by 10 weeks, it has shrunk to a pear-shaped remnant approxly 5 mm in diameter which is connected to midgut by a narrow omphaloenteric duct (yolk stalk). - The omphaloenteric duct usually detaches from the midgut loop by the end of 6th week. In approxly 2% of adults, the proximal intra-abdominal part of the omphaloenteric duct persists as an ileal diverticulum (Meckel diverticulum). - By 20 weeks, it becomes very small & usually not visible. Significance of the Umbilical Vesicle: - It has a role in the transfer of nutrients to the embryo during the second and third weeks when the uteroplacental circulation is being established. - Blood development first occurs in the well-vascularized extraembryonic mesoderm covering the wall of the umbilical vesicle beginning in 3rd week until hemopoietic activity begins in the liver during 6th week. - During the fourth week, the endoderm of the umbilical vesicle (derived from epiblast) is incorporated into the embryo as the primordial gut & it forms epithelium of the trachea, bronchi, lungs, and digestive tract. - Primordial germ cells appear in the endodermal lining of the wall of the umbilical vesicle in the third week and subsequently migrate to the developing gonads to form spermatogonia & oogonia. Illustrations showing how the amnion enlarges, obliterates chorionic cavity, and envelops umbilical cord. Part of the umbilical vesicle is incorporated into the embryo as primordial gut. Formation of the fetal part of the placenta and degeneration of chorionic villi are also shown. A, At 3 weeks; B, at 4 weeks; C, at 10 weeks; D, at 20 weeks. 18 Fate of Allantois: - It appears as a sausage-like diverticulum from the caudal wall of the umbilical vesicle that extends into the connecting stalk in the 3rd week. - By 2nd month, the extraembryonic part of the allantois degenerates. Significance of Allantois: It is not functional in human embryos, but it is important for three reasons: - Blood formation occurs in its wall during the third to fifth weeks. - Its blood vessels persist as umbilical vein & arteries. - The intraembryonic part of the allantois runs from the umbilicus to the urinary bladder, with which it is continuous. As the bladder enlarges, the allantois involutes to form a thick tube, the urachus. After birth, the urachus becomes a fibrous cord, the median umbilical ligament that extends from the apex of the urinary bladder to the umbilicus. Allantoic Cysts: A cystic mass in the umbilical cord may represent the remains of the extraembryonic part of the allantois. These cysts usually resolve but may be associated with omphalocele-congenital herniation of viscera into the proximal part of the umbilical cord. Illustrations of the development and usual fate of the allantois. A, A 3-week embryo. B, A 9-week fetus. C, A 3- month male fetus. D, Adult female. The nonfunctional allantois forms the urachus in the fetus and the median umbilical ligament in the adult. 19 Amnion and Umbilical Cord - The yolk sac proper occupies the space between the amnion and chorionic plate. - Enlargement of amniotic cavity at the expense of the chorionic cavity cause the amnion to envelop the connecting and yolk sac stalks, crowding them together to form primitive umbilical cord. - Distally, the cord contains the yolk sac stalk and umbilical vessels (2 arteries & 1 vein). - More proximally, it contains some intestinal loops and the remnant of the allantois. - The abdominal cavity is temporarily too small for the rapidly developing intestinal loops, and some of them are pushed into extraembryonic space in umbilical cord forming a physiological umbilical hernia. - Around the end of 3rd month, the loops are withdrawn into the body of the embryo, and the cavity in the cord is obliterated. - When the allantois and the vitelline duct and its vessels are also obliterated, all that remains in the cord are the umbilical vessels surrounded by the jelly of Wharton (rich in proteoglycans & functions as a protective layer for the blood vessels). Later stage in the development of umbilical cord. A. 5-wk embryo showing structures passing through primitive umbilical ring. B. Primitive umbilical cord of a 10- wk embryo. Amniotic Fluid: - Partly produced by amniotic cells but derived primarily from maternal blood. Volume increases from approxly 30 mL at 10 wks of gestation to 450 mL at 20 wks to 800 to 1,000 mL at 37 wks. - In early pregnancy, the embryo is suspended by its umbilical cord in this fluid, which serves as a protective cushion. - The fluid (a) absorbs jolts, (b) prevents adherence of the embryo to the amnion, and (c) allows for fetal movements. - The volume of amniotic fluid is replaced every 3 hours. From the beginning of 5th month, the fetus swallows its own amniotic fluid (about 400 mL a day, about half of the total amount). - Fetal urine is added daily to the amniotic fluid in the fifth month, but this urine is mostly water, since the placenta is functioning as an exchange for metabolic wastes. - During childbirth, amniochorionic membrane forms a hydrostatic wedge that helps dilate cervical canal. 20  MULTIPLE PREGNANCES - Risks of chromosomal anomalies and fetal morbidity and mortality are higher in multiple gestations than in single gestations. Triplets, quadruplets etc are not common. - Multiple births are more common now because of greater access to fertility therapies, including induction of ovulation. Twins and Fetal Membranes: Dizygotic (DZ) twins or fraternal twins versus Monozygotic (MZ) twins or identical twins. The fetal membranes and placentas vary according to the origin of the twins. Frequency of Types of Placentas and Fetal Membranes in Monozygotic (MZ) and Dizygotic (DZ) Twins Dizygotic (DZ) Twins: - Result from fertilization of two oocytes, DZ twins develop from two zygotes and may be of the same sex or different sexes, who are ‘womb mates’ but genetically like brothers & sisters. - Zygotes implant separately in the uterus with each having its own placenta, amnion & chorionic sac. - DZ twins always have 2 amnions & 2 chorions, but walls of chorions & placentas may fuse if apposed. - DZ twinning shows a hereditary tendency. - Each DZ twin may possess RBCs of two different types (erythrocyte mosaicism), indicating that fusion of the two placentas was so intimate that red cells were exchanged. - The incidence of DZ twinning shows ethnic variation: approxly 1 in 500 in Asians, 1 in 125 in whites, and as high as 1 in 20 in some African populations. Development of dizygotic twins. Normally, each embryo has its own amnion, chorion, and placenta (A), but sometimes the placentas are fused (B). Each embryo usually receives the appropriate amount of blood, but on occasion, large anastomoses shunt more blood to one of the partners than to the other. 21 Monozygotic (MZ) Twins: - MZ twins result from fertilization of one oocyte & develop from one zygote. MZ twins are of the same sex, genetically identical, and very similar in physical appearance. - Physical differences between MZ twins are environmentally induced, e.g., because of anastomosis of placental vessels. - MZ twinning occur at different stages: (1) Two-cell stage - Two separate zygotes develop & blastocysts implant separately with two amnions, two chorions, and two placentas that may or may not be fused. Arrangement of membranes resembles that of DZ twins, but differ with strong resemblance in blood groups, fingerprints, sex, and external appearance, such as eye and hair color in the MZ twins. (2) Early blastocyst stage- Inner cell mass splits into 2 separate groups of cells (2 embryonic primordia) within the same blastocyst cavity. The two embryos have a common placenta and a common chorionic cavity, but separate amniotic cavities - a monochorionic-diamniotic twin placenta. (3) Bilaminar germ disc stage – Separation occurs just before the appearance of the primitive streak resulting in formation of two twins with a single placenta and a common chorionic and amniotic sac. Although the twins have a common placenta, blood supply is usually well balanced. Possible relations of fetal membranes in monozygotic twins. A. Splitting occurs at the two-cell stage, and each embryo has its own placenta, amniotic cavity, and chorionic cavity. B. Splitting of the inner cell mass into two completely separated groups. The two embryos have a common placenta and a common chorionic sac but separate amniotic cavities. C. Splitting of the inner cell mass at a late stage of development. The embryos have a common placenta, a common amniotic cavity, and a common chorionic cavity. 22 Discordance in MZ Twinning: May be caused by: - Environmental differences and chance variation. - Mechanisms of embryologic development, such as vascular abnormalities. - Postzygotic changes, such as somatic mutation leading to discordance for cancer, or somatic rearrangement of immunoglobulin or T cell-receptor genes - Chromosome aberrations originating in one blastocyst after the twinning event - Uneven X chromosome inactivation between female MZ twins, with the result that one twin preferentially expresses the paternal X and the other the maternal X. Parturition (Birth) - The uterine myometrium does not respond to signals for parturition until in the last 2 to 4 weeks of pregnancy, when the myometrium prepares for onset of labor resulting in (i) a thickening of myometrium in the upper region of uterus & (ii) softening & thinning of lower region & cervix. - Labor itself is divided into three stages: (1) effacement (thinning and shortening) and dilatation of the cervix (this stage ends when the cervix is fully dilated), (2) delivery of the fetus, and (3) delivery of the placenta and fetal membranes. - Stage 1 is produced by uterine contractions that force the amniotic sac against the cervical canal like a wedge, or if the membranes have ruptured, then pressure will be exerted by the presenting part of the fetus, usually the head. - Stage 2 is also assisted by uterine contractions, but the most important force is provided by increased intra-abdominal pressure from contraction of abdominal muscles. - Stage 3 requires uterine contractions and is aided by increasing intra-abdominal pressure. - As the uterus contracts, the upper part retracts, creating a smaller and smaller lumen, while the lower part expands, thereby producing direction to the force. - Contractions usually begin about 10 minutes apart; then, during 2nd stage of labor, they may occur less than 1 minute apart and last from 30 to 90 seconds. Their occurrence in pulses is essential to fetal survival, since they are of sufficient force to compromise uteroplacental blood flow to the fetus. Applied Anatomy Hydramnios or polyhydramnios/Oligohydramnios: Hydramnios - Excess of amniotic fluid (1,500 to 2,000 mL), while Oligohydramnios refers to a decreased amount (less than 400 mL). Both conditions are associated with an increase in the incidence of birth defects. Primary causes of hydramnios include idiopathic causes (35%), maternal diabetes (25%), and congenital malformations, including CNS disorders (e.g., anencephaly) and gastrointestinal defects (atresias, e.g., esophageal) that prevent the infant from swallowing the fluid. Oligohydramnios is a rare occurrence that may result from renal agenesis, & it may cause clubfoot and lung hypoplasia following amnion rupture. Causes of rupture are largely unknown, but in some cases, trauma plays a role. Twin Defects: Twin pregnancies have a high incidence of perinatal mortality and morbidity and a tendency for preterm delivery. Approxly 12% of premature infants are twins, and twins are usually small at birth. Low birth weight and prematurity place infants of twin pregnancies at great risk, and approxly 10% to 20% of them die, compared with only 2% of infants from single pregnancies. Many twins die before birth, and only 29% of women pregnant with twins actually give birth to two infants. Twin transfusion syndrome: Occurs in 5% to 15% of monochorionic monozygotic pregnancies characterized by placental vascular anastomoses, which occur in a balanced arrangement in most monochorionic placentas, resulting in one twin receiving most of the blood flow while flow to the other is compromised. Hence, one twin is larger than the other. Death of both twins occur in 60% to 100% of cases. 23 Monozygotic twins with twin transfusion syndrome. Placental vascular anastomoses produced unbalanced blood flow to the two fetuses. Conjoined twins: Twinning occurring at bilaminar stage of development involving partial splitting of the primitive node and streak may result in formation of conjoined (Siamese) twins. These twins are classified according to the nature and degree of union as thoracopagus (pagos, fastened), pygopagus, and craniopagus. Occasionally, monozygotic twins are connected only by a common skin bridge or by a common liver bridge. Misexpression of genes, such as Goosecoid, may also result in conjoined twins. 24 Preterm Birth (delivery before 34 weeks): May be caused by factors in which pregnancy-supporting factors (e.g., hormones) are withdrawn, or active induction caused by stimulatory factors targeting the uterus or both. The mechanisms not yet clearly understood. It is due to premature rupture of the membranes, premature onset of labor, or pregnancy complications requiring premature delivery. Maternal hypertension and diabetes as well as abruptio placenta are risk factors. Maternal infections, including bacterial vaginosis, are also associated with an increased risk. Umbilical Cord Abnormalities: At birth, the umbilical cord is approxly 2 cm in diameter and 50 to 60 cm long. It is tortuous, causing false knots. An extremely long cord may encircle the neck of the fetus, usually without increased risk, whereas a short one may cause difficulties during delivery by pulling the placenta from its attachment in the uterus. One umbilical artery may be present in some cases (1 in 200) with affected babies having approxly a 20% chance of having cardiac and other vascular defects. The missing artery either fails to form (agenesis) or degenerates early in development. Amniotic Bands: Tears in the amnion result in amniotic bands that may encircle part of the fetus i.e. the limbs and digits resulting in amputations, ring constrictions, and other abnormalities, including craniofacial deformations. Amniotic bands may result from infection or toxic insults that involve either the fetus, fetal membranes, or both. Bands then form from the amnion, like scar tissue, constricting fetal structures. Limb abnormalities caused by amniotic bands. A. Limb constriction ring. B. Limb amputation. 25  ORGANOGENETIC PERIOD: EVENTS OF 4TH TO 8TH WEEKS OF DEVELOPMENT Embryonic Folding and Flexion of the Embryo I. Embryonic folding: the flat trilaminar embryonic disk becomes a more cylindric embryo due to the longitudinal and transverse folding that occurs as a result of embryonic growth, especially of the neural tube. The foldings occur simultaneously and are not separate sequential events. Flexion, a process of curving, transforms the embryo into a sort of "tube" and isolates it from the embryonic membranes, to which it is eventually attached only by a thin stalk, the umbilical cord. The embryo increases rapidly in its long axis due to central growth being greater than peripheral growth, and the dorsal region of the embryo grows more rapidly than its ventral region, resulting in the embryo curving itself around the umbilical region. The dorsal region also thickens, especially in the midline, and the edges of the disk swing ventrally carrying the amnion with them. Thus, the embryo is surrounded by its amniotic cavity A. LONGITUDINAL FOLDING produces both head- and tailfolds, or flexion, and creates a cranial and caudal region to the embryo 1. Headfold: neural folds (end of week 3) begin to develop into the brain and project dorsally into the amniotic cavity a. The forebrain grows cranially beyond the oropharyngeal membrane and overhangs the primitive heart. At the same time, the septum transversum (a mass of mesoderm cranial to the pericardial coelom), the heart, the pericardial coelom, and the oropharyngeal membrane turn under onto the ventral surface b. During folding, part of the yolk sac is incorporated as the foregut (between brain and heart, ending blindly at the oropharyngeal membrane). The membrane separates the foregut from the stomodeum or primitive mouth cavity c. After folding, the septum transversum lies caudal to the heart and develops into a major portion of the diaphragm d. Before folding, the intraembryonic coelom is a flattened horseshoe-shaped cavity. After folding, the pericardial coelom lies ventrally and the pericardioperitoneal canals run dorsally over the septum transversum to join the peritoneal coelom which, on each side, communicates with the extraembryonic coelom 2. The tailfold (caudal end) takes place later than the headfold and results from the dorsal and caudal growth of the neural tube a. As the embryo grows, the tail region projects over the cloacal membrane which eventually comes to lie ventrally b. During folding, part of the yolk sac is incorporated into the embryo as the hindgut, the terminal portion of which soon dilates and forms the cloaca, separated from the amniotic cavity by the cloacal membrane c. Before folding, the primitive streak lies cranial to the cloacal membrane, but, after folding, lies caudal to it d. The connecting stalk now attaches to the ventral embryonic surface, and the allantois is partly incorporated into the embryo B. TRANSVERSE FOLDING (FLEXION) produces right and left lateral folds 1. Each lateral body wall (somatopleure) folds toward the midline, rolling the edges of the embryonic disk ventrally to form a cylindric embryo 2. As lateral and ventral body walls form, part of the yolk sac is incorporated into the embryo as the midgut; simultaneously, the connection of the midgut with the yolk sac is reduced to a yolk stalk or vitelline duct 3. After folding, the area of the amnion attachment to the embryo is reduced to a narrow umbilicus on its ventral surface 4. As the midgut is separated from the yolk sac, it attaches to the dorsal abdominal wall via a thin dorsal mesentery 5. As the umbilical cord forms, the ventral fusion of the lateral folds reduces the area of communication between the intra- and extraembryonic coelom 6. As the amniotic cavity enlarges and obliterates the extraembryonic coelom, the amnion forms an outer covering for the umbilical cord. 26 27 Fourth Week: Major changes in body form occur during the fourth week. - Early 4th wk, the embryo is almost straight & has 4 - 12 somites that produce visible surface elevations. - Neural tube is formed opposite the somites, but it is widely open at the rostral and caudal neuropores. - By 24 days, the first two pharyngeal arches are visible: first (mandibular arch) and second (hyoid arch). - The embryo is now slightly curved because of the head and tail folds. - The heart produces a large ventral prominence and pumps blood. - Three pairs of pharyngeal arches are visible by 26 days, and the rostral neuropore is closed. - The forebrain produces a prominent elevation of the head, and folding of the embryo has given the embryo a C-shaped curvature. - Upper limb buds are recognizable by day 26 or 27 as small swellings on the ventrolateral body walls. The otic pits, the primordia of the internal ears, are also visible. - Ectodermal thickenings (lens placodes) indicating future lenses of eyes are visible on sides of the head. - 4th pair of pharyngeal arches and the lower limb buds are visible by the end of 4th week. - Toward the end of the fourth week, a long tail-like caudal eminence is a characteristic feature. - Rudiments of many of the organ systems, especially the cardiovascular system, are established. - By the end of the fourth week, the caudal neuropore is usually closed. A and B, Drawings of dorsal views of embryos early in 4TH week showing 8 and 12 pairs of somites, respectively. C, D, and E, Lateral views of older embryos showing 16, 27, and 33 pairs of somites, respectively. The rostral neuropore is normally closed by 25 to 26 days, and the caudal neuropore is usually closed by the end of the fourth week. 28 Fifth week: Changes in body form are minor during 5th week compared with 4th week. - Growth of the head exceeds that of other regions. Enlargement of the head is caused mainly by the rapid development of the brain and facial prominences. - The face soon contacts the heart prominence. The rapidly growing second pharyngeal arch overgrows the third and fourth arches, forming a lateral ectodermal depression on each side-the cervical sinus. - Mesonephric ridges indicate the site of mesonephric kidneys (interim excretory organs in humans). A, Lateral view of an embryo at Carnegie stage 14, approximately 32 days. The second pharyngeal arch has overgrown the third arch, forming a depression known as the cervical sinus. The mesonephric ridge indicates the site of the mesonephric kidney, an interim kidney. B, Illustration of the structures shown in A. The upper limb buds are paddle shaped and the lower limb buds are flipper-like. 29 Sixth Week: - The embryos show reflex response to touch. - Development of the lower limbs occurs 4 to 5 days later than that of the upper limbs. - The upper limbs begin to show regional differentiation as the elbows and large handplates develop. - The primordia of the digits (fingers), called digital rays, begin to develop in the handplates, which indicate the formation of digits. - Embryos in the sixth week show spontaneous movements, such as twitching of the trunk and limbs. - Several small swellings -auricular hillocks- (contribute to auricle formation) develop around pharyngeal groove between the first two pharyngeal arches. This groove forms external acoustic meatus. Largely because retinal pigment has formed, the eye is now obvious. - The head is now much larger relative to the trunk and is bent over the heart prominence. This head position results from bending in the cervical (neck) region. - The trunk and neck have begun to straighten. - The intestines enter the extraembryonic coelom in the proximal part of the umbilical cord. This umbilical herniation is a normal event in the embryo. The herniation occurs because the abdominal cavity is too small at this age to accommodate the rapidly growing intestine. A, Lateral view of a 27-somite embryo at Carnegie stage 12, approximately 26 days. The embryo is curved, especially its tail-like caudal eminence. Observe the lens placode (primordium of lens of eye) and the otic pit indicating early development of internal ear. B, Illustration of the structures shown in A. The rostral neuropore is closed, and three pairs of pharyngeal arches are present. Seventh week - The limbs undergo considerable change in the seventh week. - Notches appear between the digital rays in the handplates, clearly indicating the future digits. - Communication between the primordial gut and umbilical vesicle is now reduced to a relatively slender duct, the omphaloenteric duct. - By the end of the seventh week, ossification of the bones of the upper limbs has begun. 30 Eight Week: Final week of the embryonic period. - Digits of the hand are separated but noticeably webbed during early 8th week. Notches are now clearly visible between the digital rays of the feet. - By the end of the eighth week, all regions of the limbs are apparent, the digits have lengthened and are completely separated. - Both hands and feet approach each other ventrally. - Purposeful limb movements first occur during this week. Ossification begins in the femur. - The intestines are still in the proximal portion of the umbilical cord. - The caudal eminence is still present but stubby. - All evidence of the caudal eminence has disappeared by the end of the eighth week. - Early 8th week, the scalp vascular plexus has appeared and forms a characteristic band around the head. - At the end of the eighth week: (i) the embryo has distinct human characteristics; however, the head is still disproportionately large, constituting almost half of the embryo. (ii) The neck region is established, and the eyelids are more obvious. (iii) The eyelids are closing, and by the end of the eighth week, they begin to unite by epithelial fusion. (iv) The auricles of the external ears begin to assume their final shape. (v) Although there are sex differences in the appearance of the external genitalia, they are not distinctive enough to permit accurate sexual identification. A, Lateral view of an embryo at Carnegie stage 13, approximately 28 days. The primordial heart is large, and its division into a primordial atrium and ventricle is visible. The rostral and caudal neuropores are closed. B, Drawing indicating the structures shown in A. The embryo has a characteristic C-shaped curvature, four pharyngeal arches, and upper and lower limb buds. 31 Drawing of an embryo at Carnegie stage 13, approximately 28 days. B, Photomicrograph of a section of the embryo at the level shown in A. Observe the hindbrain and otic vesicle (primordium of internal ear). C, Drawing of same embryo showing the level of the section in D. Observe the primordial pharynx and pharyngeal arches. 32 A, Drawing of an embryo at Carnegie stage 13, approximately 28 days. B, Photomicrograph of a section of the embryo at the level shown in A. Observe the parts of the primordial heart. C, Drawing of the same embryo showing the level of section in D. Observe the primordial heart and stomach. 33 Estimation of Gestational and Embryonic Age - Gestational age: Pregnancy Date from the first day of the LNMP. - Embryonic age begins at fertilization, approxly 2 weeks after the LNMP. - Fertilization age is used in patients who have undergone in vitro fertilization or artificial insemination. - Probability of error in dating the LNMP is highest in women who become pregnant after cessation of oral contraception because the interval between discontinuance of hormones and the onset of ovulation is highly variable. - In some cases, slight uterine bleeding ("spotting"), which sometimes occurs during implantation of the blastocyst, may be incorrectly regarded by a woman as light menstruation. - Oligomenorrhea (scanty menstruation), pregnancy in the postpartum period (i.e., several weeks after childbirth), and use of intrauterine devices may affect dating of LNMP. - The day on which fertilization occurs is the most accurate reference point for estimating age; this is commonly calculated from the estimated time of ovulation because the oocyte is usually fertilized within 12 hours after ovulation. - Records on embryonic age should indicate the reference point used i.e.days after the LNMP or after the estimated time of fertilization. - Appearance of the developing limbs is a helpful criterion for estimating embryonic age. - Because embryos of 3rd & early 4th weeks are straight, measurements indicate the greatest length. - The crown-rump length is most frequently used for older embryos. Because no anatomic marker clearly indicates the crown or rump, one assumes that the longest crown-rump length is the most accurate. - Standing height, or crown-heel length, is sometimes measured for 8-week embryos. - The Carnegie Embryonic Staging System is used internationally; because it allows comparisons to be made between the findings of one person and those of another. A, Lateral view of an embryo at Carnegie stage 17, approximately 42 days. Digital rays are visible in the handplate, indicating the future site of the digits. B, Drawing illustrating the structures shown in A. The eye, auricular hillocks, and external acoustic meatus are now obvious. 34 AGE CARNEGIE NO. OF LENGTH (DAYS) STAGE SOMITES (MM)* MAIN EXTERNAL CHARACTERISTICS†20-21 9 1-3 1.5-3.0 Flat embryonic disc. Deep neural groove and prominent neural folds. One to three pairs of somites present. Head fold evident. 22-23 10 4-12 1.0-3.5 Embryo straight or slightly curved. Neural tube forming or formed opposite somites, but widely open at rostral and caudal neuropores. First and second pairs of pharyngeal arches visible. 24-25 11 13-20 2.5-4.5 Embryo curved owing to head and tail folds. Rostral neuropore closing. Otic placodes present. Optic vesicles formed. 26-27 12 21-29 3.0-5.0 Upper limb buds appear. Rostral neuropore closed. Caudal neuropore closing. Three pairs of pharyngeal arches visible. Heart prominence distinct. Otic pits present. 28-30 13 30-35 4.0-6.0 Embryo has C-shaped curve. Caudal neuropore closed. Upper limb buds are flipper-like. Four pairs of pharyngeal arches visible. Lower limb buds appear. Otic vesicles present. Lens placodes distinct. Tail-like caudal eminence present. ‡ 31-32 14 5.0-7.0 Upper limbs are paddle shaped. Lens pits and nasal pits visible. Optic cups present. 33-36 15 7.0-9.0 Handplates formed; digital rays visible. Lens vesicles present. Nasal pits prominent. Lower limbs are paddle shaped. Cervical sinuses visible. 37-40 16 8.0-11.0 Footplates formed. Pigment visible in retina. Auricular hillocks developing. 41-43 17 11.0-14.0 Digital rays clearly visible in handplates. Auricular hillocks outline future auricle of external ear. Trunk beginning to straighten. Cerebral vesicles prominent. 44-46 18 13.0-17.0 Digital rays clearly visible in footplates. Elbow region visible. Eyelids forming. Notches between the digital rays in the hands. Nipples visible. 47-48 19 16.0-18.0 Limbs extend ventrally. Trunk elongating and straightening. Midgut herniation prominent. 49-51 20 18.0-22.0 Upper limbs longer and bent at elbows. Fingers distinct but webbed. Notches between the digital rays in the feet. Scalp vascular plexus appears. 52-53 21 22.0-24.0 Hands and feet approach each other. Fingers are free and longer. Toes distinct but webbed. 54-55 22 23.0-28.0 Toes free and longer. Eyelids and auricles of external ears more developed. 56 23 27.0-31.0 Head more rounded and shows human characteristics. External genitalia still have sexless appearance. Distinct bulge still present in umbilical cord, caused by herniation of intestines. Caudal eminence ("tail") has disappeared. *The embryonic lengths indicate the usual range. In stages 9 and 10, the measurement is greatest length; in subsequent stages, crown-rump measurements are given. †Based on Nishimura et al (1974), O'Rahilly and Maller (1987), Shiota (1991), and Gasser (2004). ‡At this and subsequent stages, the number of somites is difficult to determine and so is not a useful criterion. 35 A, Lateral view of an embryo at Carnegie stage 19, about 48 days. The auricle and external acoustic meatus are now clearly visible. Note the relatively low position of the ear at this stage. Digital rays are now visible in the footplate. The prominence of the abdomen is caused mainly by the large size of the liver. B, Drawing indicating the structures shown in A. Observe the large hand and the notches between the digital rays, which clearly indicate the developing digits or fingers. 36 A, Lateral view of an embryo at Carnegie stage 21, approximately 52 days. Note that the feet are fan shaped. The scalp vascular plexus now forms a characteristic band across the head. The nose is stubby and the eye is heavily pigmented. B, Illustration of the structures shown in A. The fingers are separated and the toes are beginning to separate. C, A Carnegie stage 20 human embryo, approximately 50 days after ovulation, imaged with optical microscopy (left) and magnetic resonance microscopy (right). The three-dimensional data set from magnetic resonance microscopy has been edited to reveal anatomic detail from a mid-sagittal plane. 37 A, Lateral view of an embryo at Carnegie stage 23, approximately 56 days. The embryo has a distinct human appearance. B, Illustration of the structures shown in A. C, A Carnegie stage 23 embryo, approximately 56 days after ovulation, imaged with optical microscopy (left) and magnetic resonance microscopy (right). References (for contents and diagrams): 1. 1. Keith Moore, T. V. N. Persaud & Mark Torchia. The Developing Human: Clinically Oriented 2. Embryology, 11th Edition. Published by ELSEVIER, 2018. 3. 2. Sadler, T. W. Langman's Medical Embryology. 12th edition. Published by Wolters Kluwers/ Lippincott 4. Williams & Wilkins, 2011. 3. Heil JR, Bordoni B. Embryology, Umbilical Cord. [Updated 2023 Apr 17]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557490/ Class Assignment 1. Give the embryological basis of twinning. Add a note on applied anatomy. 2. An ultrasound at 7 months’ gestation shows too much space (fluid accumulation) in the amniotic cavity. What is this condition called, and what are its causes? 3. A pregnant lady who is addicted to illegal drugs refused to stop taking illegal drugs because she held on to a traditional belief that the placenta barrier protects the baby from illegal drugs taking by the mother. Is she correct? Give reasons for your answer. 38