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Fetal Membranes

OBJECTIVESBy the end of the lecture the student should be able to:List the components of the fetal membranes.Describe the stages of development of the components.Describe the structure and function of the components.Describe their fate and the possible congenital anomalies.

ComponentsUmbilical cord (Connecting Stalk)AmnionAmniotic FluidYolk SacAllantois

ProtectionNutritionRespirationExcretionSynthesis of HormonesFunctions

Umbilical CordIt is a pathway which connects the ventral aspect of the embryo with the placenta (chorion)It is a soft tortuous cord measuring (30- 90) cm in length (average 55) ,(1-2) cm in diameter.It has a smooth surface because it is covered by the amnion

Structure of Umbilical Cord1-Connecting stalk: Alantois + two Umbilical arteries + two Umbilical veinsThe extra embryonic mesoderm forms Whartons jelly2-Yolk stalk (Vitello-intestinal duct):A narrow, elongated duct which connects gut to yolk sac It contains Vitelline VesselsLater on , it is obliterated and the vitelline vessels disappear.

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Attachment of Umbilical CordNormally, it is attached to a point near the centre of the fetal surface of the placenta

Anomalies of Umbilical Cord (1) Abnormal Attachment:a-Battledore placenta : The UC is attached to the margin of the placenta (it is not dangerous). b-Velamentous insertion of the cord : UC is attached to the amnion away from placenta, (It is dangerous to the fetus due to rupture of blood vessels during labor)

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(2) Abnormalities in Length:a-Very Long Cord: It is dangerous , it may surround the neck of the fetus and causes its death. b-Very Short Cord: It is dangerous because it may cause premature separation of placenta, or the cord itself may rupture

(3) False and True knots of umbilical cord: a-False knots: UC looks tortuous due to twisting of umbilical vessels (umbilical vessels are longer than the cord), these knots are normal and do not cause any harm to the fetus b-True knots:Are rare (1%) of pregnancy, but very dangerous because they may cause obstruction to blood flow in umbilical vessels, leading to fetal anoxia & fetal deathTrue Knots in 20-weeks fetus

Allantois3rd week: Appears as a diverticulum from caudal wall of Y.S. that extends into connecting stalk. 2nd month: Its extra- embryonic part degenerates. 3rd month: Its intra-embryonic part extends from UB to UC as thick tube , (urachus) After birth: the urachus is obliterated and fibrosed to form median umbilical ligament, that extends from apex of UB to umbilicus.

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Functions of AllantoisBlood formation in its wall during 3rd to 5th week.Its blood vessels persist as the umbilical vein & arteries.

Yolk SacIt is essential in the transfer of nutrients to the embryo during 2nd & 3rd weeks, when the uteroplacental circulation is not established.It does not contain any yolk.Its development passes through three stages:Primary yolk sac.Secondary yolk sac.Definitive yolk sac.

Primary Yolk Sac Appears in the Blastocyst stage at 10-days, it lies ventral to the embryonic plate. Its roof is formed by hypoblast (primary endoderm), Its wall is formed by exocoelomic membrane, it lines the inner surface of the cytotrophoblast, and separated from it by the extraembryonic mesoderm

Appears in the chorionic vesicle stageIts roof is formed by hypoblast (embryonic endoderm), its wall is formed by exocoelomic membrane + inner layer (splanchnic layer) of the extraembryonic mesoderm.At day 16: a diverticulum appears from its dorsocaudal end (Allantois) into the substance of the connecting stalk

Secondary Yolk Sac

Definitive Yolk SacAfter folding, part of Yolk Sac is enclosed within the embryo to form the Gut (Foregut, Midgut & Hindgut).The remainder of Yolk Sac that remains outside the embryo becomes the Definitive Yolk SacThe midgut is temporarily connected to Definitive Yolk Sac by a narrow duct Vitello-intestinal duct (Yolk stalk), which is incorporated inside the umbilical cord. This is fibrosed and degenerated by the end of (6th week)

Functions of Yolk Sac3rd week: (a) Blood formationtFirst formed in the extra-embryonic mesoderm covering the wall of the yolk sac, until hemopoietic activity begins in the liver during 6th week4th week: endoderm of yolk sac is incorporated into the embryo to form primordial gutEpithelium of Respiratory system &G.I.T.

(b)Primordial germ cells in the endodermal lining of the wall of caudal end of the yolk sac migrate into the developing sex glands to differentiate into germ cells (spermatogonia or oogonia)

Fate of Yolk Sac

Yolk stalk detached from midgut by the end of 6th week. In (2%) of adults, its proximal intra-abdominal part persists as ileal diverticulum (Meckel diverticulum).At 10 week, small definitive yolk sac lies in the chorionic cavity between amniotic & chorionic sacs At 20 weeks, as pregnancy advances, definitive yolk sac atrophies and becomes a very small cyst.In unusual cases, it persists under the amnion near the attachment of Umbilical cord, on the fetal surface of the placenta. Its persistence is of no significance

Amnion It is a thin, transparent & tough fluid-filled, membranous sac surrounding the embryo.At First : It is seen as a small cavity lying dorsal to the embryonic plate.At Stage of Chorionic Vesicle: The amnion becomes separated from the chorion by chorionic cavity or extra embryonic coelom. After Folding: the amnion expands greatly and is becomes on the ventral surface of the embryo. As a result of expansion of the amnion, the extra embryonic coelom is gradually obliterated and amnion forms the epithelial covering of umbilical cord.

Amniotic Fluid It is a watery fluid inside the amniotic cavity (sac). It has a major role in fetal growth & developmentIt increases slowly, to become (700-1000) ml by full term (37) weeks.Composition:99% of amniotic fluid is waterIt contains un-dissolved material of desquamated fetal epithelial cells + organic + inorganic saltsAs pregnancy advances, composition of amniotic fluid changes as fetal excreta (meconium = fetal feces & urine) are added

Sources of amniotic fluidFetal & Maternal Sources:Initially, some amniotic fluid is secreted by amniotic cells. Most of fluid is derived from Maternal tissue by: 1-Diffusion across amnio-chorionic membrane from placenta. 2-Diffusion across chorionic plate (chorionic wall related to placenta) from the maternal blood in the intervillous spaces. Later, it is derived from Fetus through: Skin, Fetal Respiratory Tract & mostly by Excreting Urine (at beginning of 11th week)

Functions of amniotic fluidProvides symmetrical external growth of the embryoActs as a barrier to infection (it is an aseptic medium)Permits normal fetal lung developmentPrevents adherence of embryo to amnionIt protects embryo against external injuriesKeeps the fetal body temperature constantAllows the embryo to move freely, aiding muscular development in the limbsIt is involved in maintaining homeostasis of fluids & electrolytesIt permits studies on fetal enzymes, hormones and diagnosis of fetal sex and chromosomal abnormalities

Circulation & Fate of amniotic fluidAmniotic fluid remains constant & in balance--Most of fluid is swallowed and few passes into lungs by fetus, and absorbed into fetal blood, where it is metabolised -- Part of fluid passes through placental membrane into maternal blood in intervillus space, Other part of fluid is excreted by fetal kidneys into amniotic sac

Anomalies of Volume of Amniotic Fluid(1) Oligohydramnios:The volume is less than litersCauses :Placental insufficiency with low placental blood flow Preterm rupture of amnio-chorionic membrane occurs in 10% of pregnanciesRenal Agenesis (failure of kidney development)Obstructive Uropathy (urinary tract obstruction) lead to absence of fetal urine (the main source)Complications : Fetal abnormalities (pulmonary, facial & limb defects)

(2) Polyhydramnios (Hydramnios):The volume is more than 2 liters, it is diagnosed by Ultrasonography. Causes Fetal ( 1-20% ) : Esophageal atresia. Maternal (2-20%) : defects in maternal circulation. Idiopathic (3-60%)

25PLACENTAThis is a fetomaternal organ.It has two components:Fetal part develops from the chorionic sac ( chorion frondosum )Maternal part derived from the endometrium ( functional layer decidua basalis )The placenta and the umbilical cord are a transport system for substances between the mother and the fetus.( vessels in umbilical cord )

Function Of The Placenta:ProtectionNutritionRespirationExcretionHormone production

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26DECIDUA

DECIDUA is the endometrium of the gravid (pregnant) uterus.It has four parts:Decidua basalis: it forms the maternal part of the placentaDecidua capsularis: it covers the conceptusDecidua parietalis: the rest of the endometriumDecidua reflexa:Junction between capsularis & parietalis.

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27DEVELOPMENT OF PLACENTA

Until the beginning of the 8th week, the entire chorionic sac is covered with villi.After that, as the sac grows, only the part that is associated with Decidua basalis retain its villi.Villi of Decidua capsularis compressed by the developing sac.Thus, two types of chorion are formed:Chorion frondosum (villous chorion) Chorion laeve bare (smooth) chorionAbout 18 weeks old, it covers 15-30% of the decidua and weights about 1\ 6 of fetus

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28DEVELOPMENT OF PLACENTA

The villous chorion ( increase in number, enlarge and branch ) will form the fetal part of the placenta.The decidua basalis will form the maternal part of the placenta.The placenta will grow rapidly.By the end of the 4th month, the decidua basalis is almost entirely replaced by the fetal part of the placenta.

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29FULL-TERM PLACENTA

Cotyledons about 15 to 20 slightly bulging villous areas. Their surface is covered by shreds of decidua basalis from the uterine wall.After birth, the placenta is always inspected for missing cotyledons. Cotyledons remaining attached to the uterine wall after birth may cause severe bleeding.Grooves formerly occupied by placental septa

Maternal side

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30FULL-TERM PLACENTA( Discoid shape -500- 600 gm- Diameter 15-20 cm Thickness of 2-3 cm)

Fetal surface:This side is smooth and shiny. It is covered by amnion.The umbilical cord is attached close to the center of the placenta.The umbilical vessels radiate from the umbilical cord.They branch on the fetal surface to form chorionic vessels.They enter the chorionic villi to form arteriocapillary-venous system.

Fetal side

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31PLACENTAL MEMBRANEThis is a composite structure that consists of the extra-fetal tissues separating the fetal blood from the maternal blood.It has four layers:SyncytiotrophoblastCytotrophoblastConnective tissue of villusEndothelium of fetal capillariesAfter the 20th week, the cytotrophoblastic cells disappear and the placental membrane consists only of three layers.

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32TRANSFER ACROSS THE PLACENTAL MEMBRANE Viruses: measles;poliomyelitis Microorganism: treponema pallidum of syphilis ; T.g which produce destructive change in the eye; brain . IgG( gamma globulin) , IgS;IgM ( immunoglobulin S;M )

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Placental endocrine synthesisThe syncytiotrophoblast synthesizes protein & steroid hormones The protein hormones1- human chorionic gonadotropin2- human chorionic somatomammotropin3- human chorionic thyrotropin4- human chorionic corticotropinThe steroid hormonesProgesterone & Estrogens

34 Third trimester bleeding is the common sign of these anomaliesPlacental anomalies

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