Placenta And Fetal Membranes - Tulane University

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Placenta and Fetal MembranesAmnion - Epiblast / Extraembryonic MesodermYolk Sac - Hypoblast / Extraembryonic MesodermAllantois - Embryonic HindgutChorion - Trophoblasts / Extraembryonic MesodermPlacenta - Chorion / Maternal Decidua

AmnionAmnionic membrane is two cell layers1) epiblast derived extraembryonic ectodermal layer2) thin non-vascular extraembryonic mesodermAs the amnion enlarges it encompasses the embryo on theventral side, merging around the umbilical cord.Amnion forms the epithelial layer of the umbilical cordWith embryo growth the amnion obliterates the chorioniccavityAmnionic sac is fluid filled called amnionic fluid: theembryo is bathed in the fluid

Extraembryonic Tissues8 days14 days9 days

Amnion

Amnion

Amnionic FluidUp to week 20 - fluid is similar to fetal serum (keratinization)After 20 weeks – Contribution from urine, maternal serum filteredthru endothelium of nearby vessels, filtration from fetalvessels in cordNear birth - can contain fetal feces called meconiumNear birth – amnionic fluid (500-1000 ml) exchanges every 3 hrs1) across the amnion – exchange with maternal fluids.2) fetal swallowing (20 ml/hour) – to gut – adsorption byfetus – out the umbilical cord to placenta.Hydraminos – Excess fluid ( 2000 ml), esophageal atresiaOligohydramnios – Insufficient fluid ( 500 ml), renal agenesis

Amnion FunctionMechanical protection: hydrostatic pressureAllows free movement - which aids in neuromusculardevelopmentAntibacterialAllow for fetal growthProtection from adhesions

Amnion Band Syndrome (ABS)

Placenta and Fetal MembranesAmnion - Epiblast / Extraembryonic MesodermYolk Sac - Hypoblast / Extraembryonic MesodermAllantois - Embryonic HindgutChorion - Trophoblasts / Extraembryonic MesodermPlacenta - Chorion / Maternal Decidua

Yolk SacHypoblast - the primary yolk sac or Heuser's membrane.Day 12 - Second wave of cell migration - forms definitive yolk sacComposed of extrembryonic endodermEarly nutrition (2-3 weeks) for the embryo - later shrinking nonfunctional – Meckels diverticulum (outpocketing of smallintestine)Connects to midgut via the yolk sac stalkDerivatives:Early blood cells forms from blood islandsPrimordial germ cellsThe early gut, epithelium of the respiratory and digestivetracts

Placenta and Fetal MembranesAmnion - Epiblast / Extraembryonic MesodermYolk Sac - Hypoblast / Extraembryonic MesodermAllantois - Embryonic HindgutChorion - Trophoblasts / Extraembryonic MesodermPlacenta - Chorion / Maternal Decidua

AllantoisEndodermal origin – caudal outpocketing of the yolk sacInvades the connecting stalk (extraembryonic mesoderm)that suspends the embryo in the chorionic cavityInvolved in early hematopoiesis (up to 2 months)The allantois blood vessels - artery and vein - becomesthe umbilical vesselsRemnants of Allantois becomes the urachus ligament thatconnects the belly button to the bladder

Placenta and Fetal MembranesAmnion - Epiblast / Extraembryonic MesodermYolk Sac - Hypoblast / Extraembryonic MesodermAllantois - Embryonic HindgutChorion - Trophoblasts / Extraembryonic MesodermPlacenta - Chorion / Maternal Decidua

Chorion

ChorionChorionic cavity (extraembryonic coelom)- lined withextraembryonic mesodermChorionic cavity expands separating amnion fromcytotrophoblastChorionic sac consist of:cytotrophoblastic layersyncytiotrophoblastic layerextraembryonic somatic mesodermThe Chorion / maternal endometrium forms the placentaChorion forms stem villi

Stem Villi

Stem VilliChorionic Plate – Stem villi extends from this tissuePrimary stem villi (day 11-13) - finger-like protrusionsinto endometrium - contains syncytiotrophoblast,cytotrophoblast.Secondary stem villi (day 16) - extraembryonicmesoderm invasion into villi core.Tertiary stem villus (21 day) - extraembryonic vessels chorionic arteries and veins derived fromextraembryonic mesoderm.Hemichorial type placenta – maternal blood baths villi

Stem VilliCytotrophoblastic cell column – terminal villi, solid massof trophoblastCytotrophoblastic shell – surrounds embryo; direct contactwith maternal decidual cellsAnchoring Villi – give off cytotrophoblastic extensions anchoring because they represent the real maternalembryo linkFloating Villi – branches off anchoring villi – danglesfreely in maternal blood

Chorion

Placenta and Fetal MembranesAmnion - Epiblast / Extraembryonic MesodermYolk Sac - Hypoblast / Extraembryonic MesodermAllantois - Embryonic HindgutChorion - Trophoblasts / Extraembryonic MesodermPlacenta - Chorion / Maternal Decidua

DeciduaDecidual Reaction – stromal cells – accumulate glycogenand lipid, called Decidual CellsDecidua basalis - forms maternal component of theplacenta; associates with the chorion frondosomDecidua capsularis - superfical layer overlying the entireembryoblast - this layer eventually degenerates;associates with the chorion laeveDecidua parietalis - all remaining parts of the endometrium- not associated with the embryo

Deciduas

Making the PlacentaBy 8 weeks - chorionic stem villi over the entire surface ofthe chorionic sacThose villi associated with the decidua basalis increase insize and more villi form.Enlargement includes further branching of the anchoringvillus - chorion frondosum.The villi continue to enlarge during most of gestation.The villi project into a blood filled intervillous spaceresulting from the erosion of the decidua basalis.Endometrial vessels - spiral arteries and endometrial veinsVilli associated with the decidua capsularis degenerate this region is called the chorion laeve

Deciduas

PlacentaThe erosion of the decidua basalis is incomplete - unerodedregions called decidual septa.The decidual septa define regions of the placenta calledcotyledon.

Placental Blood Flow

Placental Anatomy

Umbilical CordOne umbilical vein, twoumbilical arteriesWharton’s jelly – mucoidconnective tissuesurrounding vesselsAllantoisYolk Stalk (vitelline duct) andvitelline vessels (early)Intestinal loop – umbilical hernia(late)

Placental CirculationFetal – Contained within vesselsUmbilical Arteries – chorionic plate – branches tostem villi – capillaries in terminal villi – return viaumbilical veinMaternal – Free-flowing lakeSpiral arteries open into intervillous space and baththe villi150 ml of maternal bloodExchanged - 3-4 times/minuteReduced blood pressure in intervillous spaceOxygenated blood to the chorionic plate, returnbaths the villi

Placental Anatomy

Placental barrier decreases with gestationPlacental Barrier – syncytiotrophoblast basal lamina, basal lamin fetal capillary endotheliumSyncytiotrophoblasts – many microvilli, no majorhistocompatibility antigens

O2H2 OCO2FeH2Osaltssaltscarbohydrates, amino acids, lipidsurea, uric acidvitamins, hormones, antibodiescreatininedrugs, alcoholbilirubin, hormones,viruses (rubella, varicella-zoster, HIV)RBC antigens

Placenta as an Endocrine OrganHuman Chorionic Gonadotropin – Corpus Luteum (declines after 8weeks)Progesterone – High levels by the end of first trimesterEstrogen – Synthesis involves enzymatic activity of fetal adrenalgland and liverChorionic Somatomammotropin – Human Placental Lactogen –similar to GH (growth, lactation, lipid and carbohydratemetabolism)Placental Growth Hormone – similar to GH – Replaces materrnal GHby 15 wks – enhances blood glucose levelsChorionic Thyrotropin, Chorionic Corticotropin

Multiple iamnionic

Hydatiform Mole

Erythroblastosis fetalisFetus / newborn - hemolytic disease (anemia)Rh factor is a RBC surface antigenRh- mother with Rh 1st baby – Maternal antibodiesare induced after birthAt risk is second Rh babyMaternal Rh antibodies cross placentaHemolysis of fetal Rh RBC

Placental Circulation Fetal – Contained within vessels Umbilical Arteries – chorionic plate – branches to stem villi – capillaries in terminal villi – return via umbilical vein Maternal – Free-flowing lake Spiral arteries open into intervillous space and bath the villi 150 ml of maternal blood Exchanged - 3-4 times/minute

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