Second Week Embryonic Development PDF

Events of the Second Week of Embryonic Development Trophoblast and chorion  Key Events: - Completion of implantation Overview of - Formation of bilaminar germ disc Second Week - Development of extraembryonic cavitie...

Events of the Second Week of Embryonic Development Trophoblast and chorion  Key Events: - Completion of implantation Overview of - Formation of bilaminar germ disc Second Week - Development of extraembryonic cavities and membranes - Establishment of the chorionic sac  Definition of implantation: The blastocyst penetrates the superficial (Compact) layer of the endometrium of the Implantation uterus. Site: the posterior wall of the uterine body near the fundus. Time: ; begins about the 6th day after fertilization. ; completed by the 11th or 12th day. embryoblast endometrium wall after implantation is called decidual  abembryonic pole embryonic pole where implantation happen where prolifiration of cells happen there is no cell membrane around it loss of cell membrane collection of nucli empeded in a shell of cytoblasm  the part of endometrium that is near the implantation site where it will form placenta in the future fusion point between decidua parietalis and capsularis the part that is away from the implantation point Endometrium at the time of implantation: 5mm in thickness by secretory phase 2 step of implantation progesterone apposition and then invasion 1) contact between 2) infiltration between blastocyst & - At embryonic pole endometrial cells endometrium Completion of Implantation Description: - By day 9, the blastocyst is fully blastocystic cavity blastocele embedded in the endometrial lining. epiblast culumnal cells hypoblast flat cells - Trophoblast differentiates into cytotrophoblast and syncytiotrophoblast. - Syncytiotrophoblast invades maternal shedding occur separation of the inner cell mass and go down tissues and erodes uterine blood vessels, forming lacunae. ammiongenic cells/layer exocoelomic membrane from hyoblast cells primitive yolk sac extraembryonic coelom  Function of Trophoblast: Immune barrier Trophoblast secretes human chorionic gonadotropin (hCG) that helps the corpus luteum maintain the uterine lining Endometrium → secretory phase Decidual reaction Lacunar network → maternal sinusoid → uteroplacental circulation  Amniotic Amniotic Cavity: - Appears as a small cavity within the epiblast. Cavity and - Cells lining the cavity become the amniotic membrane. Yolk Sac Yolk Sac: - Hypoblast cells migrate and form the exocoelomic membrane, which lines the primary yolk sac.  Formation of the Bilaminar Germ Disc Description: Epiblast and hypoblast form the bilaminar germ disc. Epiblast cells give rise to the amniotic cavity. mass of the mesoderm connecting embryo with triangle that will give rise to placenta Hypoblast cells contribute membranes to the formation of the primary yolk sac. trophoblast cells are highly proleferating cells start to do secretions of another mass outside the embryo only near the yolk sac called (Extraembryonic mesoderm) spaces and fusion of the spaces form a cavity embryoblast  cranial end and caudal end ? cranial end: epiblast and hypoblast stick to eachother fused epiblast and hypoblast at the cranial end of embryo hypoblast cells elongate and form prochodral plate Development of Extraembryonic Mesoderm Description: Extraembryonic mesoderm forms between the cytotrophoblast and the yolk sac/amniotic cavity. Cavities appear in the extraembryonic mesoderm, coalescing to form the extraembryonic coelom (chorionic cavity). Formation of the Chorionic Sac Description: - The chorionic cavity expands, surrounding the embryo and yolk sac. - The somatic extraembryonic mesoderm lines the trophoblast, while the visceral mesoderm surrounds the yolk sac. Lacunae Formation and Early Uteroplacental Circulation Description: Syncytiotrophoblasts form lacunae that fill with maternal blood. Establishment of early uteroplacental circulation to nourish the embryo.  folding every time part of yolk sac enter inside the uretra this yolk sac will give the gut of the embryo (GIT) benching to a part of yolk sac (‫)ﯾﺸﺪ ﺟﺰء ﻣﻨﮭﺎ‬ GUT lateral folding and antroposterior folding of embryo amniotic cavity enlarge and yolk sac shrinks incorporation of yolk sac inside the developing embryo surround by amniotic cavity cytotrophoblast start to proliferate and rise upward and do invasion inside the syncytiotrophoblast mesoderm ‫ﻟﻮ دﺧﻞ ﻓﯿﮭﺎ‬ mesoderm‫ﻟﻮ ال‬ is invaded by blood vessels 4th stage umbirical vesicle Summary of Second Week Development Key Points: - Implantation is completed. - Bilaminar germ disc forms (epiblast and hypoblast). - Amniotic cavity and yolk sac develop. - Extraembryonic mesoderm and chorionic cavity form. - Early uteroplacental circulation begins. References 1. Larsen's Human Embryology, 5th Edition 2. Moore & Persaud's The Developing Human: Clinically Oriented Embryology, 11th Edition. 3. https://slideplayer.com/slide/14568484/