Lecture 6A - Third to Eighth Weeks Development – II - Dr. Ayesha Fatima - 24 April 2024 PDF

rd th 3 -8 weeks of development Part 2 Embryonic period (organogenesis- II) Dr. Ayesha Fatima, Department of Anatomy, CMMS - AGU Intrauterine development in humans takes 280 days or 40 weeks or 9 months a. Pre-embryonic period 1st & 2nd Weeks b. Embryonic period 3rd to 8th Weeks EMBRYO c. Fetal per...

rd th 3 -8 weeks of development Part 2 Embryonic period (organogenesis- II) Dr. Ayesha Fatima, Department of Anatomy, CMMS - AGU Intrauterine development in humans takes 280 days or 40 weeks or 9 months a. Pre-embryonic period 1st & 2nd Weeks b. Embryonic period 3rd to 8th Weeks EMBRYO c. Fetal period 3rd to 9th Months Major events that take place during the Embryonic period: 1) Changes in the embryonic disc (Gastrulation, Formation of notochord, derivatives of embryonic disc) 2) Changes in the intraembryonic coelom 3) Changes in the trophoblast (Chorion) 4) Folding of the embryo 5) External appearance during the embryonic period. Dr.Ayesha F 3.Changes in the trophoblast (Chorion) Dr.Ayesha F Syncytiotrophoblast The Chorion The chorion is a double-layered membrane which forms the fetal part of placenta. Chorion is formed from 3 structures: 1. Syncytiotrophoblast. 2. Cytotrophoblast. 3. Somatopleuric layer of extraembryonic mesoderm Extra Embryonic Mesoderm (chorionic plate). The syncytiotrophoblast proliferate and erode the maternal blood vessels. They become surrounded by intervillous lacunae containing maternal blood Dr.Ayesha F Primary villi: Three successive stages of chorionic villi formation: Syncytiotrophoblast Cytotrophoblast 1. Primary chorionic villi: at the end of the second week (2 layers: syncytio+cyto). 2. Secondary chorionic villi: beginning of the third week (3 layers: syncytio+cyto+mesoderm). 3. Tertiary chorionic villi: end of the third week (4 layers :syncytio+cyto+meso+endothelium of fetal capillary).-placental barrier Secondary villi: Extraembryonic mesoderm Endothelium of Fetal capillaries Dr.Ayesha F This. can be asked ex: now many layers in Secondam chronic villus Dr.Ayesha F PLACENTAL BARRIER/ MEMBRANE It is the protective barrier that prevents mixing of maternal blood (in the intervillous spaces) and fetal blood corpus luteam of carrying Pregnancy out function of Placenta Placental Barrier/membrane consists of four layers (from outside inward) : Layers of placenta in early pregnancy Syncytiotrophoblast Cytotrophoblast 1.Syncytiotrophoblast 2.Cytotrophoblast. 3.Extraembryonic mesoderm. 4.Endothelial lining of fetal capillaries Mesoderm Endothelial lining Dr.Ayesha F Fetal Capillaries Layers of the placenta in late pregnancy By the 4th month, 2nd & 3rd layers disappear and the Syncytiotrophoblast Cytotrophoblast and mesoderm will disappear endothelial layer of capillaries come in contact with the syncytial layer and endothelial3 syncytial layers come in contact with each other barrier becomes thinner the barrier becomes relatively thinner. This facilitates the exchange of certain substances across it e.g. gas exchange, nutrients hormones, drugs, and viruses. Transport of substances occurs only were the endothelial layer of capillaries come in contact with the syncitial layer (only 2 layers) Dr.Ayesha F Endothelial lining of fetal capillaries. Cytotrophoblastic Shell The cytotrophoblast at the tip of the tertiary villus penetrates the syncytiotrophoblast & extends in lateral directions to fuse with similar cytotrophoblast of near by villi thus forming a continuous CYTOTROPHOBLASTIC SHELL surrounding the embryonic vesicle to prevent further penetration of the decidua by syncytiotrophoblast. Cytotrophoplastic cells limit Vaginal/uterus eroding. Decidua Cytotrophoblastic shell Syncytiotrophoblast Cytotrophoblast Cytotrophoblast Intervillous space filled with maternal blood forms Cytotrophoblastic shell to prevent Chorion frondosum further penetration Fetal blood vessels after fertilization decidua : endometrium Decidua basalis Decidua capsularis Fate of chorionic villi: 1. The villi opposite the decidua basalis enlarge forming the Chorion Frondosum. 2. The villi opposite the decidua capsularis atrophy giving the Chorion Leave. Chorion leave Dr.Ayesha F capillaries in villi communicate with capillaries Fetal–placental circulation in mesoderm and blood vessels with During the 4th week of development, newly formed capillaries in the villi communicate with those formed in the mesoderm of chorionic plate and connecting stalk(Primordium of umbilical cord). The blood vessels in the connecting stalk communicate with those formed in the embryo. This connects the vasculature of the embryo with that of the placenta, and thus fetal-placental circulation is established. Dr.Ayesha F thus those , in connecting connecting formed connects and placenta In stalk stalk communicate embryo Vasculature of embryo 4.Folding of The Embryo Flat trilaminar disc Time: 3rd week = same time of somites appearance. Amnion Process: The flat trilaminar embryonic disc becomes folded on itself forming cylindrical embryo. Cylindrical embryo Causes: 1. Extensive, rapid growth of the cranial end of the neural tube (brain). 2. Faster growth of the axial part of the embryonic disc than its periphery. 3. Enlargement of the amnion. 4. Diminishing of the yolk sac. Dr.Ayesha F Embryonic disc Yolk sac Ectoderm Mesoderm Endoderm Flat trilaminar embryonic disc Cephalocaudal folding Tail fold Head fold It occurs in two planes: 1. Median plane: the head and tail Lateral folding folds(CEPHALOCAUDAL FOLDING). 2. Horizontal plane: the 2 lateral folds(LATERAL FOLDING). Dr.Ayesha F Lateral fold Head fold Cephalo-caudal folding Head Fold Septum transversum Pericardial coelom Tail fold Head Fold : (due to growth of the brain.) The primitive heart, pericardial cavity and the oropharyngeal membrane moves on the ventral surface of the embryo Developing brain Buccopharyngeal membrane Developing heart The septum transversum (diaphragm) becomes caudal to the heart Endoderm forms the primitive foregut (from pharynx to duodenum) Foregut Heart Dr.Ayesha F Buccopharyngeal membrane Pericardial cavity Septum transversum Cephalo-caudal folding Connecting stalk Tail Fold: (growth of the caudal part of the neural tube): Cloacal membrane Allantois Endoderm forms the primitive hindgut including the cloacal membrane (anus) Connecting stalk (primordium of the umbilical cord) lies in the ventral surface of the embryo Hindgut Cloacal membrane Dr.Ayesha F Umbilical cord Dr.Ayesha F Lateral folding Lateral Folds: (formed due to growth of the spinal cord and somites). Amniotic cavity Amniotic cavity Midgut It leads to: Cylindrical embryo IEC Vitellointestinal duct Yolk sac Endoderm forms the midgut (duodenum to large intestine). A. Folding is initiated Yolk sac will narrow and is connected to midgut by vitello-intestinal duct then disappears in further development. Amniotic cavity Obliteration of the extraembryonic coelom due to the enlargement of the amniotic cavity, Surface ectoderm IEC Gut Formation of the epithelial lining of the umbilical cord from extraembryonic coelom. IEC Body wall Yolk sac Splanchnopleuric mesoderm B. The midgut is connected to yolk sac. C. The amniotic sac expands to surround the embryo. CROSS SECTIONS THROUGH EMBRYOS SHOWING THE STEPS OF LATERAL FOLDING. Dr.Ayesha F 5.External Appearance of Embryo during Embryonic period Dr.Ayesha F Pharyngeal arches External appearance At the end of 1st month The embryo is characterized by the presence of: 1. Somites. 2. Pharyngeal arches. 3. Pericardial bulge. 4. Limb buds. 5. Tail. Pericardial bulge ‫اﻟﻣﺿﻐﺔ‬ Arm bud Somites Tail 28 days- embryo Leg bud 29 days- embryo External appearance at the end of 2nd month The major features of the body form and established as: Rounded contour. Absence of the tail. Head formation with the appearance of the primordium of the eye, ear, nose. Fingers start to appears in the limbs. Head Ears Nose Eyes Fingers& Toes Dr.Ayesha F 8 weeks- embryo THANK YOU & GOOD LUCK!