Fetal Membranes

1. Fetal Membranes Dr Rania Gabr

2. Objectives • By the end of this lecture , the student should be able to: • Define the fetal membranes • Define and enumerate types of decidua • Describe the development , types ,functions and anomalies of chorion • Describe the development , types ,functions and anomalies of amnion • Describe the development , types ,functions and anomalies of yolk sac • Describe the development , types ,functions and anomalies of allantois

3. Fetal Membranes • The membranous structures closely associated with or surrounding the embryo during its developmental period . • Include the amnion, chorion, allantois, yolk sacand umbilical cord. • Develop from the zygote • Since such membranes are external to the embryo proper, they are called extraembryonic membranes.

4. Fetal Membranes • They function in the embryo's protection, nutrition, respiration, and excretion • The chorion & amnion do not take part in the formation of the embryo or fetus • Part of the yolk sac is incorporated into the embryo as the primordium of the gut • The allantois forms a fibrous cord called urachus

5. Decidua • Decidua means: The endometrium after implantation. The decidua is differentiated into 3 parts: • Decidua basalisis the part of the decidua between the blastocyst and the musculature of the uterus • Decidua capsularisis the part of the decidua between the blastocyst and the uterine cavity • Decidua parietalisis the uninvolved uterine mucosa i.e the rest of the decidua lining the cavity of the uterus

8. Chorion • The outermostof the two fetal membranes (amnion is the inner one) • Develops in the early second week, as a three layered membrane : 1- Extraembryonic mesoderm 2- Cytotrophoblast 3- Syncytiotrophoblast • Forms the wall of the chorionic cavity (the original extraembryoniccelome)

9. Chorionic Villi • On day 13-14 the primary villi appear as cellular extensions from the cytotrophoblast that grow into the syncytio-trophoblast. • Shortly after their apperance, the primary villi begin to branch • A mesodermal core appears in the cytotrophoblast, this is the secondary villi

11. Chorionic Villi • Blood vessels appear in the mesodermal core of the villi that are now called the tertiary villi. • These blood vessels connect up with vessels that develop in the chorion and connecting stalk and begin to circulate embryonic blood about the third week of development. secondary villus tertiary villus primary villus

12. Functions of the villi • Nutrition of the embryo (free villi). • Fixation of the embryo (anchoring villi). • Respiration of the embryo. • Excretion of the embryo.

13. As the embryo grows and the amniotic fluid increases in amount, the decidua capsularisbecomes extremely stretched. The chorionic villi in this region become atrophied and disappear leaving a smooth chorion (chorionlaeve) The villi in the region of decidua basalisgrow rapidly, branch, and become highly vascular. This region of chorion is called chorionfrondosum (villous chorion)

14. Chorionic cavity embryo Chorionic villi

15. Amnion extraemryonic mesoderm amniotic ectoderm amniotic cavity • A thin protective membrane that surrounds the embryo/ fetus • Starts developing, in the early 2nd week (8th day) after fertilization, as a closed cavity in the embryoblast • This cavity is roofed in by a single layer of flattened cells, the amnioblasts(amniotic ectoderm), and its floor consists of the epiblast of the embryonic disc • Outside the amniotic ectoderm is a thin layer of extraembryonic mesoderm epiblast

16. Amnion cont’d • It is attached to the margins of the embryonic disc • As the embryonic disc grows and folds along its margins , the amnion and the amniotic cavity enlarge and entirely surround the embryo • From the ventral surface of the embryo it is reflected onto the connecting stalk and thus forms the outer covering of the future umbilical cord

17. The amniotic fluid increases in quantity and causes the amnion to expand The amnion ultimately adheres to the inner surface of the chorion, so that the chorionic cavity is obliterated The fused amnion and chorion form the amnio-chorionic membrane Amniochorionic membrane

18. Further enlargement of amniotic cavity results in obliteration of uterine cavity and fusion of amniochorionic membrane (covered by decidua capsularis), with the decidua parietalis Amniochorionic membrane usually ruptures just before birth Amniochorionic membrane

19. Amniotic Fluid: Origin • Initially some fluid is secreted by the amniotic cells • Later most of it is derived from the maternal tissue fluid by diffusion: • Across the amniochorionic membrane from the decidua parietalis • Through the chorionic plate from blood in the intervillous space of the placenta • By 11th week, fetus contributes to amniotic fluid by urinating into the amniotic cavity; in late pregnancy about half a liter of urine is added daily. • After about 20 weeks, fetal urine makes up most of the fluid.

20. Amniotic Fluid: Composition • Amniotic fluid is a clear, slightly yellowish liquid • 99% of fluid in the amniotic cavity is water • Suspended in this fluid are undissolved substances e.g. desquamated fetal epithelial cells, proteins, carbohydrates, fats, enzymes, hormones and pigments • As pregnancy advances the composition of amniotic fluid changes as fetal waste products (meconium & urine) are added

21. Amniotic Fluid: Circulation • The water content of the amniotic fluid changes every three hours • Large volume moves in both directions between the fetal & maternal circulations mainly through the placental membrane • It is swallowed by the fetus, is absorbed by respiratory & GIT and enters fetal circulation. It then passes to maternal circulation through placental membrane. During final stages of pregnancy fetus swallows about 400ml of amniotic fluid per day • Excess water in the fetal blood is excreted by the fetal kidneys and returned to the amniotic sac through the fetal urinary tract

22. Amniotic Fluid: Volume • By the beginning of the second trimester the amniotic sac contains 50 ml of the amniotic fluid • The volume of amniotic fluid increases gradually, reaching about 1000ml by 37th week. • High volume of amniotic fluid i.e. more than 2000 ml is called Polyhydramnios. It results when the fetus does not swallow the usual amount of amniotic fluid e.g. in esophageal atresia • Low volume of amniotic fluid i.e. less than 400 ml is called Oligohydramnios. Renal agenesis (failure of kidney formation) is the main cause of oligohydramnios

23. Amniotic Fluid: Functions • The fetus floats in the amniotic fluid. It allows fetus to move freely, aiding development of muscles and bones. • Prevents adherence of the amnion to the embryo • Acts as a cushion to protect embryo from injuries • Acts as a barrier to infection • Permits normal lung development • Permits symmetrical external growth of the embryo • Regulates fetal water/electrolyte balance • Assists in regulation of fetal body temperature

24. Yolk Sac • At 32 days: a large structure • 10 weeks: small, shrunk pear-shaped, lies in the chorionic cavity, connected to midgut by a narrow yolk stalk • Atrophies as pregnancy advances • By 20 weeks: very small, and thereafter usually not visible • Very rarely it persists as a small structure on the fetal surface of placenta, under the amnion, near the attachment of umbilical cord. Its persistence is of no significance

25. Yolk Sac: Significance • Source of nutrition for the embryo during 2-3 weeks • Blood development first occurs in the mesodermal layer of the yolk sac (early 3rd week) and continues until hemopoietic activity begins in the liver (6th week) • Primordial germ cells appear in the endodermal lining of the wall of the yolk sac (3rd week) and then migrate to the developing gonads • Part of yolk sac is incorporated into the embryo as the primitive gut (4th week)

26. Yolk Stalk (Vitelline Duct) • A tubular connection between the midgut and the yolk sac • Initially wide, becomes narrow with the folding of the embryo • Becomes one of the contents of the developing umbilical cord • Attached to the tip of the midgut loop • Usually detaches from midgut loop by the end of the 6th week

27. Abnormalities Related to Yolk Stalk • In about 2% of cases, the proximal intra-abdominal part persists as a small diverticulum attached to the ileum of the small intestine as ileal diverticulum (Meckel`s diverticulum) • Clinical significance: It is usually mistaken for appendicitis

28. Meckeldiveticulum may: • Remain connected to umbilicus by cordlike thevitelline ligament • Persist as a smallvitelline cyst • Open on the umbilicus asvitelline fistula

29. Allantois • Appears in 3rd week as a diverticulum from the caudal wall of the yolk sac, that extends into the connecting stalk • During folding of the embryo, a part of allantois is incorporated into the hindgut • During the 2nd month, the extra-embryonic part of allantois degenerates

30. Allantois cont’d • The intraembryonic part runs from the umbilicus to the urinary bladder. • As bladder enlarges, this part involutes and changes to a thick tube called URACHUS • After birth,theurachus becomes a fibrous cord, the median umbilical ligament, that extends from the apex of the bladder to the umbilicus

31. Allantois: Significance • Blood formation occurs in its walls during the 3rd week • Its blood vessels persist as umbilical vessels • Allantois: Anomalies Allantois may not involute properly and give rise to: • Urachal fistula • Urachal cyst • Urachal sinus