Formation of Placenta and Foetal Membranes PDF

Fertilization Zygote development Implantation Formation of placenta Formation of membranes Early development of fetus Functions of placenta  Fertilization:  Occurs in the ampulla of fallopian tube  Within 12 hours after ovulation  Takes 24 hours  Sperms pass through corona radi...

Fertilization Zygote development Implantation Formation of placenta Formation of membranes Early development of fetus Functions of placenta  Fertilization:  Occurs in the ampulla of fallopian tube  Within 12 hours after ovulation  Takes 24 hours  Sperms pass through corona radiata then through  Zona pellucida , attach to ovum membrane (zona inhibits entry of further sperms+ ovum completes second maturity division- female pronucleus). Sperm enters the ovum, tail degenerates, head is the male pronucleus. Male pronucleus fuses with female pronucleus  Zygote 46 chromosomes.  Zygote development:  Zygote divides 2 cells:30hrs after fertilization 16 cell (morula): 3rd day after fertilization Blastocyst: fluid filled space  inner cell mass: embryo+amnion  trophoblast: placenta+chorion.  Implantation:  Morula enters the uterine cavity on 3rd day  Endometrium develops, progesterone.  Rich in glands and blood vessels  Implants at the body of uterus- anterior or posterior  The blastocyst attaches to the endometrium on day 7-8.  Then the trophoblast differentiates into Cytotrophoblast Syncytiotrophoblast; derived from cytotrophoblast  The blastocyst penetrates inside the endometrium by its embryonic pole  Implantation takes 3 days  Blastocyst inside the endometrium and site of entery closed  Process of implantation is completed by the 10 th or 11 th postovulatory day.  Decidua:  When implantation is complete, the endometrium changes into the decidua- progesterone  Endometrial stromal cells changes to decidual cells- large cells.  Decidua contains blood vessels, glands regress.  Decidua  Decidua basalis under embryonic plate  Decidua capsularis : covers blastocyst  Decidua parietalis: covers the rest of uterus  Functions:  Prevents the invasion of maternal tissues by fetal tissues  Fetal nutrition  Produces hormones  Prostaglandins+ Prolactin. Formation of placenta  Cytotrophoblast + syncytiotrophoblast  Proliferates in all directions-villi chorionic  Syncytiorophoblasts degenerate neighbouring tissues- the decidua.  Laceration of maternal blood vessels in decidua Chorionic Villus:  Primary villus: Formed of cytotrophoblasts+ Syncytiotrophoblasts  Secondary villus: Primary villus invaded by mesenchymal cells from the extraembryonic mesoderm.  Tertiary villus: Secondary villus. Chorionic blood vessels formed from the mesoderm  Intervillous Spaces: Between chorionic villi  Formation of lacunae full of maternal blood Lacunae intervillous spaces.  Chorionic villi degenerate in all directions except the area near the myometrium Chorion frondosum, near myometrium. Placenta Chorion leave. Form the chorion.  Maternal blood enters the intervillous spaces through arterioles.  Fetal circulation through the chorionic vessels capillaries.  Exchange between maternal and fetal blood across the villous (placental) membrane. Normally never mixes.  The chorionic blood vessels form the umbilical vein and arteries. (2 arteries +one vein).  Vein carries oxygenated blood. Artery carries deoxygenated blood.  Villous trees float in a lake of maternal blood.   Anchoring villi: Reaches the Basal plate: the distal part of the villus stem is formed by cytotrophoblast which spread to form the trophoblastic shell.  Growth and maintenance of syncytiotrophoblasts depend on cytotrophoblast.  Trophoblasts continous turn over:  Cytotrophoblasts Syncytiotrophoblasts The Placenta:  Originates from chorion forndosum  Formed of a large number of chorionic villi which originate from the chorionic plate and float in a lake of maternal blood  Lobule: Derived from a secondary stem villus, breaking into tertiary stem villus and finally terminal villi. Each lobule supplied by a single utero- placental artery.  Placental bed: Maternal side (fetal trophoblast+ decidua- Based plate)  Placenta achieves its definitive form at 4th month and no further anatomical modification  Growth continues until term due to arborization of the villus tree and formation of fresh villi.  Disc like- umbilical cord inserted in fetal side.  Diameter 22 cm: weight:500g.  Fetal side covered by amnion. Chorionic vessels continue with the umbilical cord in a star like.  Chorionic plate:fetal side  Basal plate: maternal side. Trophblast shell  At maragins: Chorionic+Basal fuse to form the chorionic membrane.  Functions of the Placenta  Respiratory:  Oxygen, carbon dioxide  Simple diffusion Nutrient:  Glucose:Facilitated diffusion  Aminoacids: active  Fatty acids: simple diffusion Liporoteins Steroids  Water+ electrolytes: simple diffusion  Proteins: Pelatively impermeable to plasma proteins except IgG. Endocrine: Oestrogen, progesterone, prolactin, human placental lactogen, Human chorionic gonadotrophin. Inner cell mass:  Rapid proliferation from 10 th day after fertilization  The cavity of the blastocyst changes into the chorionic cavity. Lined by the chorion.  Two cavities appear around the embryonic plate  Amnion amniotic sac. Adjacent to the embryonic ectoderm.Basal from ectoderm ,walls and roof from cytotrophoblast  Yolk sac. Adjacent to embryonic endoderm-Arise from endoderm.  Two cavities separated by the embryonic plate which differentiate into:  Ectoderm- direction of amnion  Endoderm-direction of yolk sac  Mesoderm- middle layer-from ectoderm. Fetal membranes: 1. Chorion: Originate from the trophoblast-continous with the placenta. 2. Amnion: Develops from ectoderm and trophoblast, enlarges rapidly to from the amniotic sac which covers the embryo. The amnion covers the umbilical cord and the fetal surface of the placenta it secretes the amniotic fluid. Amnio chorionic membrane from fetal to maternal  Amniotic membrane  Extra-embryonic mesoderm  Loose reticular layer (coelom)  Extra embryonic mesoderm  Chorionic membrane.