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Fetal Membranes. Dr. Zeenat Zaidi. Fetal Membranes. The membranous structures closely associated with or surrounding the embryo during its developmental period . Include the amnion , chorion , allantois , yolk sac and umbilical cord . Develop from the zygote - PowerPoint PPT Presentation


  • Fetal MembranesThe membranous structures closely associated with or surrounding the embryo during its developmental period .Include the amnion, chorion, allantois, yolk sac and umbilical cord. Develop from the zygoteSince such membranes are external to the embryo proper, they are called extraembryonic membranes.

  • Fetal MembranesThey function in the embryo's protection, nutrition, respiration, and excretionThe chorion & amnion do not take part in the formation of the embryo or fetusPart of the yolk sac is incorporated into the embryo as the primordium of the gutThe allantois forms a fibrous cord called urachus

  • The Amnion & the Amniotic Fluid

  • AmnionA thin protective membrane that surrounds the embryo/ fetusStarts developing, in the early 2nd week (8th day) after fertilization, as a closed cavity in the embryoblastThis 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 discOutside the amniotic ectoderm is a thin layer of extraembryonic mesoderm

    amniotic cavity amniotic ectodermextraemryonic mesodermepiblast

  • Amnion contdIt is attached to the margins of the embryonic discAs the embryonic disc grows and folds along its margins , the amnion and the amniotic cavity enlarge and entirely surround the embryoFrom the ventral surface of the embryo it is reflected onto the connecting stalk and thus forms the outer covering of the future umbilical cord

  • The amniotic fluid increases in quantity and causes the amnion to expandThe amnion ultimately adheres to the inner surface of the chorion, so that the chorionic cavity is obliteratedThe fused amnion and chorion form the amnio-chorionic membrane

    Amniochorionic membrane

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

    Amniochorionic membrane

  • Amniotic Fluid: OriginInitially some fluid is secreted by the amniotic cellsLater most of it is derived from the maternal tissue fluid by diffusion:Across the amniochorionic membrane from the decidua parietalisThrough the chorionic plate from blood in the intervillous space of the placentaBy 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.

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

  • Amniotic Fluid: CirculationThe water content of the amniotic fluid changes every three hoursLarge volume moves in both directions between the fetal & maternal circulations mainly through the placental membraneIt 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 dayExcess water in the fetal blood is excreted by the fetal kidneys and returned to the amniotic sac through the fetal urinary tract

  • Amniotic Fluid: VolumeBy 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 atresiaLow 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

  • Amniotic Fluid: FunctionsThe 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 embryoActs as a cushion to protect embryo from injuriesActs as a barrier to infectionPermits normal lung developmentPermits symmetrical external growth of the embryoRegulates fetal water/electrolyte balanceAssists in regulation of fetal body temperature

  • AmniocentesisAmniocentesis is the removal of a small amount of amniotic fluid from the sac around the baby. This is usually performed at 16 weeks in pregnancy. A fine needle is inserted under ultrasound guidance through the mothers' abdomen into a pool of amniotic fluid.

  • Studies of cells in the amniotic fluid permit:Diagnosis of sex of the fetusDetection of chromosomal abnormalities e.g. trisomy 21 (Downs syndrome)DNA studiesDevelopmental problems e.g. Spina BifidaInherited disorders e.g. Cystic FibrosisHigh levels of alpha-fetoproteins in the amniotic fluid indicate the presence of a severe neural tube defect. Low levels of alpha-fetoproteins may indicate chromosomal abnormalities

  • Abnormalities Related to AmnionAmniotic bands syndrome: Fibrous bands of the amniotic sac become entangled around a developing fetus. The bands may wrap around any part of the fetus, but more commonly occur around a limb, fingers or toes, creating severe constrictionsPremature rupture of membranes (leaking membranes)Amniotic bands

  • ChorionThe outermost of the two fetal membranes (amnion is the inner one)Develops in the early second week, as a three layered membrane (extraembryonic mesoderm & two layers of trophoblast)Forms the wall of the chorionic cavity (the original extraembryonic celome)

  • Chorionic VilliOn day 13-14 the primary villi appear as cellular extensions from the cytotrophoblat that grow into the syncytio-trophoblast. Shortly after their apperance, the primary villi begin to branch In early 3rd week, the extraembryonic mesodermal cells grow into the primary villi forming a core of loose mesenchymal tissue. At this stage the villi are called the secondary villi and they cover the entire surface of the chorionic sac

  • Chorionic VilliBlood 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.primary villussecondary villustertiary villus

  • As the embryo grows and the amniotic fluid increases in amount, the decidua capsularis becomes extremely stretched. The chorionic villi in this region become atrophied and disappear leaving a smooth chorion (chorion laeve)The villi in the region of decidua basalis grow rapidly, branch, and become highly vascular. This region of chorion is called chorion frondosum (villous chorion)

  • Chorionic villiembryoChorionic cavity

  • Yolk SacAt 32 days: a large structure10 weeks: small, shrunk pear-shaped, lies in the chorionic cavity, connected to midgut by a narrow yolk stalkAtrophies as pregnancy advancesBy 20 weeks: very small, and thereafter usually not visibleVery 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 significant

  • Yolk Sac: SignificanceSource of nutrition for the embryo during 2-3 weeksBlood 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 gonadsPart of yolk sac is incorporated into the embryo as the primitive gut (4th week)

  • Yolk Stalk (Vitelline Duct)A tubular connection between the midgut and the yolk sacInitially wide, becomes narrow with the folding of the embryoBecomes one of the contents of the developing umbilical cordAttached to the tip of the midgut loopUsually detaches from midgut loop by the end of the 6th week

  • Abnormalities Related to Yolk StalkIn 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 diverticulum)Meckel diveticulum may:Remain connected to umbilicus by cordlike the vitelline ligamentPersist as a small vitelline cystOpen on the umbilicus as vitelline fistula

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

  • Allantois contdThe intraembryonic part runs from the umbilicus to the urinary bladder. As bladder enlarges, this part involutes and changes to a thick tube called urachusAfter birth, urachus becomes a fibrous cord, the median umbilical ligament, that extends from the apex of the bladder to the umbilicus

  • Allantois: SignificanceBlood formation occurs in its walls during the 3rd weekIts blood vessels persist as umbilical vesselsAllantois: Anomalies Allantois may not involute properly and give rise to:Urachal fistulaUrachal cystUrachal sinus

  • Umbilical Cord

  • Umbilical CordCord like structureConnects fetus to the placentaAttached to the ventral surface of the fetal body and to the smooth chorionic plate of the placenta

  • Umbilical Cord: FormationDevelo


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