Lecture 5 - Third Eighth Weeks Development – I - Dr. Ayesha Fatima - 22 April 2024.pdf

rd th 3 -8 weeks of development Part 1 Embryonic period (organogenesis -I) Dr. Ayesha Fatima, Department of Anatomy, F CMMSDr. Ayesha - AGU Dr. Ayesha F 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 We...

rd th 3 -8 weeks of development Part 1 Embryonic period (organogenesis -I) Dr. Ayesha Fatima, Department of Anatomy, F CMMSDr. Ayesha - AGU Dr. Ayesha F 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 & neural tube, 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. & the bilaminar disk Dr. Ayesha F 1. Changes in the embryonic disc- Gastrulation: It is most characteristic event seen in the 3rd week of IUL. It results in the formation of all three germ layers It is the ~ formation of 3 germ layers ectoderm , endoderm , mesoderm (trilaminar germ disc)- Ectoderm, Endoderm, and Intraembryonic Mesoderm. It begins with the formation of primitive streak on the surface of epiblasts. Primitive Streak: It is a linear band of thickened epiblast that first appears at the caudal end of the embryo and grows cranially. Its cranial end forms the primitive node with a pit in its center. Primitive node Epiblast Hypoblast Dr. Ayesha F Gastrulation(Continued..) The three germ layers i.e., Ectoderm, Endoderm and Mesoderm originate from the Epiblast Cells from the epiblast migrate towards the primitive streak. At the primitive streak they get detached from the epiblast and invaginate beneath it and form intraembryonic MESODERM Few cell displace the hypoblast and form the ENDODERM. The rest of the epiblast cells form the ECTODERM. & - cells from epiblast start moving towards the primitive streak. - - then they will go underneath the epiblast layer and form the mesoderm some and - of the cells will replace form the endoderm the rest forms the the hypoblastic layer ectoderm - origin Is of mesoderm the epiblast , endoderm , and ectoderm cell Dr. Ayesha F The intraembryonic mesoderm spreads through out the disc except in the region of prechordal plate (near Cranial end) and cloacal membrane(near caudal end). At the prechordal plate and the cloacal membrane the ectoderm and endoderm are in contact with each other. The prechordal plate later forms the oropharyngeal or buccopharyngeal membrane. The cloacal membrane forms the urogenital and anal membranes mesoderm will except prechordal Plate in spread Prechordal and prechordal throught dis cloacal membrane will come plate will later cloacal plate the and cloacal membrane will form later form : : in contact Oropharyngeal urogenital/anal with each other buccopharyngeal membranes membranes Buccopharyngeal membrane Intraembryonic mesoderm Primitive node with pit Primitive streak Cloacal membrane Dr. Ayesha F NOTOCHORD formation of notochord Notochord is the basis of axial skeleton and its remnants form part of intervertebral disc. Notochord also induces the overlying ectoderm to thicken and form neural plate (neuroectoderm). Formation of Notochord The notochordal process is a solid cord of cells which grows from the primitive pit and proceeds cranially between the ectoderm and endoderm in the midline, till it reaches the prechordal plate. The primitive pit sends extension (canal) into the notochordal process, changing it to notochordal canal. Dr. Ayesha F Formation of notochord (Continue..) Amniotic cavity The floor of the notochordal canal degenerates forming the neurenteric canal which temporarily connects the amniotic and the yolk sac cavities. Yolk sac cavity The remaining roof forms the notochordal plate which proliferates and detaches from the endoderm. Then they form a solid cord of cells, the definitive notochord. A floor degenerates forming , neurocentric can al * neurocentric canal : temporally and connects amniotic yolk sac cavities * roof proliferate and detaches from the endoderm then they form the definitive notochord; cord of cells a solid Dr. Ayesha F FORMATION OF NEURAL TUBE- NEURULATION The ectodermal cells overlying the notochord thicken to form form the neural plate. neural plate: ectodermal cells thicken neural groove inwards The neural plate invaginates inwards forming the neural groove. Its edges are called neural folds which contain cells called the neural crests. : u neural folds: the MELa! Crest neural Plate Invaginates snape) Neural (forming : edges the Cells tubes : folds come close and fuse The folds come close & fuse thus form the neural tube. Neural groove The neural tube and crests become buried and separated from the surface ectoderm. The cranial and caudal ends of the neural tube remain patent(open) forming the anterior and posterior neuropores which close later (the anterior on 25th day and the posterior on 27th day). The neural tube will give rise to the central nervous system (Brain and Spinal cord). Dr. Ayesha F Neural folds AC YC Dr. Ayesha F Sacrococcygeal teratoma Dr. Ayesha F Changes in embryonic disc The ectoderm and endoderm remain in contact at two regions, i.e. still bilaminar: 1) Anteriorly- At Buccopharyngeal membrane (Prechordal plate) 2) Posteriorly – At Cloacal membrane (Urogenital and anal membranes) Allantois is a diverticulum that extends into the connecting stalk from the yolk sac(it has no role in humans) Certains organs start their development in the third week: 1) The nervous system (Neurulation), 2) The primitive cardiovascular system. These two systems start to develop early as they are important for the reflex activities and the nutrition of the developing embryo. Dr. Ayesha F DERIVATIVES OF EMBRYONIC DISC In 3rd-8th week ORGANOGENESIS: The cells from the three germ layers(ectoderm, endoderm, and mesoderm) formed from gastrulation form different organs of the various system. mesodermWle I. The intraembryonic mesoderm on either side of the midline proliferates gets differentiated into 3 parts(from medial to lateral) 1. Paraxial mesoderm 2. Intermediate mesoderm 3. Lateral plate mesoderm (continuous with the extraembryonic mesoderm covering the amniotic cavity and yolk sac cavity). parts Dr. Ayesha F I Ist mesoderm Derivatives of paraxial mesoderm Paraxial mesoderm gets divided in craniocaudal direction, into 42 segments, called SOMITES ( ) The number of somites indicates the age of the embryo (eg. 4 somites=21 days). It gives rise to: bone(vertebral column),cartilage,muscle(trunk),connective tissue and dermis of the skin Dr. Ayesha F 2nd mesoderm Derivatives of intermediate mesoderm(nephrogenic cord) It gives rise to: excretory elements of the urinary system, and the gonads(ovary or testis) 3rd mesoderm Derivatives of lateral plate mesoderm A cavity known as Intraembryonic coelom develops in the lateral plate mesoderm. Lateral plate mesoderm gives rise to: smooth muscles of the gut, serous membranes of pleura, peritoneum and, pericardium, primitive heart, blood cells, spleen and cortex of adrenal gland. Intraembryonic cavity Dr. Ayesha F Intraembryonic Coelom(IEC) It divides lateral plate mesoderm into 2 layers: Parietal/somatic layer(continuous with the extraembryonic mesoderm covering amniotic cavity) Visceral /Splanchnic layer(continuous with the splanchnic extraembryonic mesoderm covering the yolk sac) 3 body cavities are derived from IEC: 1. 2. 3. Pericardial cavity Pleural cavity Peritoneal cavity IEC is connected to the extraembryonic coelom except at the pericardial area, where the two cavities are separated by a layer of mesoderm called septum transversum, which gives rise to the central tendon of diaphragm and liver stroma Dr. Ayesha F Dr. Ayesha F Derivatives of embryonic disc: II. Ectoderm: It is divided into : a- Surface ectoderm: gives rise to epidermis of skin and other structures (eg: nail,hair..) b- Neurectoderm: it gives rise to nervous system and other structures (eg: retina,medulla of adrenal gland) Dr. Ayesha F III. Endoderm: This layer gives rise to epithelial lining of organs such as gut,respiratory tract,urinary bladder. It also gives rise to parenchyma of organs like liver,pancreas,tonsils.. Dr. Ayesha F Dr. Ayesha F