07 GASTRULATION ACumbul (Med).pdf

GASTRULATION 46 THE DEVELOPING HUMAN Ectoderm of amnion Extraembryonic mesoderm Epiblast Embryonic ectoderm Embryonic mesoderm Primitive streak (see Chapter 4) Bilaminar embryonic disc Notochordal process Embryonic endoderm Endoderm of umbilical vesicle Extraembryonic mesoderm Hypoblast...

GASTRULATION 46 THE DEVELOPING HUMAN Ectoderm of amnion Extraembryonic mesoderm Epiblast Embryonic ectoderm Embryonic mesoderm Primitive streak (see Chapter 4) Bilaminar embryonic disc Notochordal process Embryonic endoderm Endoderm of umbilical vesicle Extraembryonic mesoderm Hypoblast FIGURE 3–6 Origin of embryonic tissues. The colors in the boxes are used in drawings of sections of conceptuses. Learning Objectives • Define the embryological differences of gastrulation • Learn the derivate of gastrulation • Learn the embryological features of primitive streak • Define the formation of notochord THIRD WEEK • Gastrulation • Somitogenesis • Angiogenesis and Haematogenesis CARNEGIE STAGE FOR III. WEEK Stage Days Size (approx) (mm) Images Somite Numbe Events r 7 8 9 13 -16 16 -18 18 – 20 Gastrulation, Notochordal Process 0,4 Primitive Pit, Notochordal canal 11,5 1,5 2,5 1-3 Neural Folds, Cardiac primordium, Head Fold Gastrulation • • • Gastrulation (or more precisely, germ layer formation) is the process by which the bilaminar embryonic disc (composed of the epiblast and hypoblast) is turned into the trilaminar embryonic disc. Formation of the primitive streak and the notochord are the two important processes occurring during gastrulation. The three tissue layers produced by gastrulation are called embryonic germ layers. – The ectoderm forms the outer layer of the gastrula. • Outer surfaces, neural tissue – The endoderm lines the embryonic digestive tract. – The mesoderm partly fills the space between the endoderm and ectoderm. • Muscles, reproductive system https://youtu.be/3AOoikTEfeo Gastrulation STAGE DAY SIZE (mm) MESODERMAL EVENTS VIb (Week II) 12-13 0,2 Primitive Streak Development Begin VII (Week III) (Early) 13-16 0,4 Gastrulation Begin (three germ layer, primitive node and groove developed) Notochordal Process Development Begin VIII (Week III) (Mid) 16-18 1-1,5 IX (Week III) (Late) 18-20 1,5-2,5 Primitive Pit Forms Notochordal Canal Developed Notochord formed Primitive Streak Chapter 5 Third Week of Development: Trila At the beginning of the 3rd week, thickened linear band of epiblast on the dorsal caudal aspect of embryonic disc. -Primitive node (and primitive pit) is on its cranial end. -Primitive groove on the primitive streak. -Mesenchyme (from epiblast cells through primitive groove) à A -intraembryonic mesoderm (mesoblast) -intraembryonic endoderm -The process of forming mesoderm through primitive streak slows down after the end of the 2nd week and degenerates in the sacrococcygeal region of embryo. Oropharyngeal membrane Cut edge of amnion Prenotochordal cells Primitive node (the organizer) Primitive streak Cloacal membrane Primitive node Primitive streak Epiblast B A Hypoblast Invaginating mesoderm cells Primitive node Primitive streak Epiblast Detaching cells Hypoblast C Primitive Streak Primitive Streak During third week of development two faint depression appear in the ectoderm; 1.One at the cranial end of the embryo adjacent to the prechordal plate is called buccopharyngeal membrane. Prechordal plate à oropharyngeal membraneà mouth 2.Other at the caudal end behind the primitive streak is called cloacal membrane. Cloacal membraneàanus Notochord Formation Amnion Bilaminar embryonic disc Connecting stalk Embryonic ectoderm Umbilical vesicle Notochordal process under ectoderm • • • • • The notochordal process elongates by invagination of cells from the primitive pit. The primitive pit extends into the notochordal process, forming a notochordal canal The notochordal process is now a cellular tube The floor of the notochordal process fuses with the underlying embryonic endoderm The notochordal canal into communication with the umbilical vesicle Notochordal Connecting stalk process Primitive pit Plane of sections B, C & E Cranial end A Cloacal membrane Prechordal plate Notochordal process Neural plate B Cardiogenic (heart-forming) area Primitive pit in primitive node Primitive pit Primitive streak Embryonic endoderm Intraembryonic mesoderm Allantois Neural plate Intraembryonic mesoderm Allantois C D Prechordal plate Level of section D Primitive pit Notochordal canal Primitive streak Notochordal process Cloacal membrane Primitive groove Notochordal canal Amnion Allantois E F Notochordal Level of section F Embryonic Intraembryonic mesoderm Umbilical vesicle Notochord Formation • • • • The remains of the notochordal process form a flattened, grooved notochordal plate Beginning at the cranial end of the embryo, the notochordal cells proliferate and the notochordal plate infolds to form the notochord The proximal part of the notochordal canal persists temporarily as the neurenteric canal (between the amniotic and umbilical vesicle cavities). The notochord becomes detached from the endoderm of the umbilical vesicle, which again becomes a continuous layer 60 THE DEVELOPING HUMAN Cut edge of amnion Amniotic cavity Connecting stalk Notochordal process Plane of sections B, C, & E Allantois Primitive pit Connecting stalk Primitive streak Umbilical vesicle Prechordal plate Primitive pit A B Neurenteric canal (arrow) Neural plate Primitive streak C Embryonic ectoderm Neural groove Intraembryonic mesoderm Umbilical vesicle D Oropharyngeal membrane Level of section D Notochord Embryonic endoderm Trilaminar embryonic disk Neural groove Notochordal plate intercalated in the embryonic endoderm Notochordal plate infolding Neurenteric canal Cloacal membrane F Intermediate mesoderm E Level of section G Level of section F Lateral mesoderm G Neural groove Neural fold Paraxial mesoderm Embryonic endoderm Notochord https://youtu.be/73k0k8qXAow Functions and Fates of Notochord •1.Structure - acts as a rigid axis around which the embryo develops •2.Skeletal - foundation upon which the vertebral column (vertebral bodies) will form •3.Induction - will bring about formation of the neural tube (future nervous system) • • • Fate of the Notochord It is the structure around vertebral column in the future, and degenerates as nucleus pulposus in the intervertebral disc. Notochord induces the development of neural plate (future central nervous system). A small passage, neuroenteric canal connects amniotic cavity with yolk sac but will be sealed after notochord completes PRIMITIVE STREAK OVERVIEW •Primitive streak: Gastrulation begins when cells of the epiblast migrate to the median plane of the dorsum of the embryonic disc. • Primitive node: The primitive streak grows by adding new cells to its caudal end. About half-way towards the prechordal plate, the cells of the primitive streak proliferate. • Primitive groove: Running longitudinally within the primitive streak • Primitive pit: Primitive groove which ends in a central indentation of the primitive node . •PRIMITIVE STREAK FUNCTION –Primitive streak defines body axis •Rostrocaudal (head tail) •Left Right NOTOCHORDAL DEVELOPMENT STAGE DAY SIZE (mm) NOTOCHORDAL EVENTS VII (Week III) 15-17 0,4 VIII (Week III) 17-19 1-1,5 IX (Week III) 20-21 1,5-2,5 Notochord formed IX (Week III) 20-21 1,5-2,5 Notochord formed X (Week IV) 22-23 2-3,5 Notochordal Process Development Begin Notochordal Canal Developed Notochord detached endoderm Cell Movements & Rearrangements during Embryonic Development • • • • • • • Ingression: cells break away from the tissue and migrate as individuals Delamination: layers of cells separate from each others more or less as sheets of cells. Intercalation: two cell layers connect with each other. Epiboly: a form of cell spreading in which cells flatten out; this allows them to cover a much larger surface area. Invagination (Evagination): a tissue layer folds in (out). Involution: cells move over a lip of tissue and into the interior. Convergent Extension: cells reorganize to form less layers allowing the cells to extend out from a point. ROLE OF GASTRULATION –Gastrulation will covert the bilaminate epiblast into the three primary embryonic germ layers –Ectoderm: outside; this embryonic layer more or less surrounds the other germ layers. Outer epithelia and nervous system. –Endoderm: inside; this germ layer lies at the most interior of the embryo. Epithelial linings of the respiratory and digestive tracts; including the glandular cells of the liver and pancreas. –Mesoderm: middle; this germ layer lies between the ectoderm and mesoderm. Smooth muscular coats, connective tissue, skeleton, striated muscle, reproductive and excretory organs, the vessels supplying these organs, and bloods cells and the bone marrow. –Subsequently neurulation will form epithelial and neural ectoderm from the ectoderm OVERHEAD LIST OF STRUCTURES FORMED SUMMARIZE : Ectoderm Mesoderm Endoderm central nervous system peripheral nervous system sensory epithelium of ear, nose and eye epidermis glands subcutaneous mammary pituitary Tooth enamel supporting tissues cartilage bone muscle connective tissue blood and lymph cells walls of heart, blood and lymph vessels kidneys, gonads and associated ducts suprarenal gland cortex Spleen Tooth dentin epithelial lining of gut respiratory tract urinary bladder and urethra tympanic cavity and auditory tube parenchyma of thyroid parathyroid glands liver stroma of tonsils and thymus ALWAYS BE HAPPY…