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Amniotic cavity Endometrium Maternal blood vessels Proliferating syncytiotrophoblast Cytotrophoblast Amniotic cavity Bilayered embryonic disc: • Epiblast • Hypoblast Endometrial epithelium Lacuna (intervillus space) containing maternal blood Chorionic villus Primary germ layers: • Ectoderm • Mesoderm Chorion • Endoderm Amnion Forming body stalk Allantois Yolk sac Extraembryonic mesoderm Chorion being formed Extraembryonic coelom Lumen of uterus (a) 71/2-day implanting blastocyst (b) 12-day implanted blastocyst (c) 16-day embryo Changes in trophoblast( formation of chorion) 1) During implantation, trophoblast forms 2 layers: - Outer syncytiotrophoblast. - Inner cytotrophoblast. The syncytio trophoblast sends finger like projections(villi) that invade the endometrium(decidua). These villi erode the decidual blood vessels forming lacunae in intervillous space fil led with maternal blood(lacunar stage). At day 12 cells of syncytiotrophoblast penetrate more deepl y, the lacunea becomes continous with maternal blood sinsoids so, maternal blood enters lacunar system→ uteroplacental circulation begins. by the appearance of extra embryonic mesoderm the wall of choronic vesicle is called chorion . The chorion = sycyt. + cytotroph blast.+outer layer of 3 stages of chorionic villi appear: 1)primary villi: a core of cytotrophblast surrounded by syncytiotrophobl ast. 2) secondary : as the 1ry but invaded with a core of mesoderm. 3rd eek 3) tertiary : as the 2ry + fetal blood capillaries. 3rd week Types of 3ry villi: a) stem(anchoring) villi :fix the ch. Vesicle to the decidua. b) Free (absorbing) villi : in the intervillous space to increase surface area for exchange between fetal and maternal blood. Fate of chorionic villi: 1) Villi opposite the decidua basalis enlarge forming chorion frondosu m→ fetal part of placenta. 1) Villi opposite decidua capsularis atrophy→ chrion laeve Cytotrophoblastic shell : the cytotrophblast at the end of anchoring villous penetrate the sycytiotrophoblast and extend to fuse with similar cytotrophoblast From other villous Forming a continuous shell surrounding the embryonic vesicle to prevent further penetration of the decidua by syncytiotrophoblast .also it Changes in the Trophoblast By the beginning of the 2nd month, trophoblast is characterized by a great number of 2° & 3° villi that give it a radial appearance. Chorionic plate: the extreembryonic mesoderm lining the inside of cytotrophoblast. That border the placenta from fetal side. Decidual plate: decidua basalis border the placenta from maternal side. Outer cytotrophoblast shell attaches the villi to maternal decidua. Lacunar or intervillous space in between the villi Syncytial knots – the thin syncytium breaks off and drop into intervillous blood lakes. Usually degenerates and causes no harm Chorion Frondosum & Decidua Basalis Chorion frondosum (bushy or villous chorion): well developed villi on embryonic pole. This Chorion laeve (smooth chorion): villi on the aembryonic pole Decidua basalis: decidua over ch. frondosum Decidua capsularis: over aembryonic pole Decidua parietalis (decidua vera): Amniochorionic membrane:fusion of amnion and chorion that obliterates the chorionic cavity . It is the thin membrane that ruptures during labour. Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic cavity Umbilical cord Decidua capsularis Extraembryonic coelom (f) 13-week fetus Uterus Lumen of uterus Structure of the Placenta By the beginning of the 4th month, placenta has 2 components: Fetal portion –: chorion frondosum Maternal portion – decidua basalis Chorionic & decidual plates: borders placenta on fetal and maternal side Junctional zone: in between 2 plates , contain intervillous space Decidual septa that projects into intervillous space but not reach chorionic plate. cotyledons: the placenta is divided by decidual septum into compartements Full-term Placenta At full term, placenta is discoid with a diameter of 15~25 cm, is approximately 3 cm thick, & weighs about 500~600 g. Sources: fetal part + maternal part. Has 2 surfaces: 1)fetal surface – smooth, coverd by amnion, umb. Cord attached to its centre. 2)Maternal surface – irregular, divided into 15 – 20 lobes or cotyledons separted by placental septa. Circulation of the Placenta The intervillous spaces of a mature placenta contain approximately 150 ml of blood, which is replenished about 3 or 4 times per minute. Fetal blood arrives to the placenta by 2 umb. Arteries that branch to form capillaries in the villi where gaseous exchange occur with maternal blood in intervillous space. finally oxygenated blood returns to fetus via lt. umbilical vein Placental membrane / barrier (x): maternal in the intervillous spac and fetal blood in the capillaries don’t mix. They are separated by: – syncytiotrophoblast - cytotrophoblast. – extraembryonic mesoderm - endothelium of fetal capillaries. By 4th month: layer 2,3 disappears Function of the Placenta Exchange of metabolic & gaseous products between maternal & fetal bloodostreams Exchange of gases . Exchange of nutrients & electrolytes . Transmission of maternal antibodies Production of hormones: progestrone, estrogen, relaxin, placental lactogen and HCG. • Exchange ofwaste products • Placental membrane protects the fetus as maternal and fetal blood don’t mix. • Substances cross that membrane: - organisms: AIDS, geman measles - Drugs: thialodomide→ short limbs Chemicals: insecticides. Maternal antibodies. Rhesus factor: if the mother is Rh negative and the baby i s Rh posithive, the mother produces anti Rh antibodies which causes hemolysis of fetal red blood cells. - Usually 1st baby is not harmed but subsequent babies suffer . Anomalies: 1) Placenta previa: centralis , marginalis, lateralis. 2) Accessory placenta. 3) Bipartite , tripartite placenta. 4) Batteldore placenta: cord is inserted at periphery o • Velamentous insertion of the cord: the cord is inserted outsi de the placenta, i.e in the membranes. Chorion epithelioma: malignant tumour of the placenta. Placenta accreta, increta and placenta percreta: villi deeply invading the endometrium, myometrium and whole thickness of the uterus respectively. Dizygotic -2/3 of the twins. - results from two oocytes fertilized by different sperms. -Each have its own placenta, amnion and chorionic cavity. Twins Monozygotic Twins - From a single fertilized ovum. - they result from splitting of t he zygote at various stages of development . a) Spliiting at early blastocyst stage. a) Splitting of inner cell mass within same blastocyst. c) Splitting of bilaminar germ di sc Frequency of Types of Placentas & Fetal Membranes in Monozygotic (MZ) & Dizygotic (DZ) Twins Single Chorion Two Chorions Zygosity Single Amnion MZ Very rare 65% 25% 10% DZ - - 40% 60% Two Fused Two Amnions Placentas Placentas Conjoind (Siamese) twins