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Download Lecture 11 Outline - Napa Valley College
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Biology 240 – General Zoology Lecture 11 Outline - Development I I. Embryology A. Processes of Development 1. Cell division (mitosis) 2. Differentiation 3. Morphogenesis (pattern formation, organogenesis) Process: fertilization Stage: zygote → cleavage gastrulation neurulation organogenesis 2,4,8-cell… → blastula → gastrula → neurula → → organism Patterns of Cleavage - depends on type of egg : isolecithal - sparse yolk, evenly distributed → radial, spiral or rotational cleavage mesolecithal - moderate amount of yolk at one end → displaced radial cleavage (frogs) telolecithal - large amount of yolk → discoidal cleavage (fishes, reptiles, birds) blastula – hollow ball of cells (50-200), undifferentiated (blastodisc in reptiles & birds; blastocyst in mammals) blastopore – site of invagination to form a gastrula B. Gastrulation - invagination of cell layers of blastula from surface to interior - forms archenteron = embryonic gut cavity; blastocoel disappears - establishes 3 germ layers: ectoderm → epidermis of skin, CNS (brain and spinal cord) , PNS (nerves, ganglia) mesoderm → dermis of skin, notochord, bones, muscles, heart, blood, blood vessels, kidneys, gonads, , connective tissues, coelomic membranes endoderm → lining of GI tract, lungs, gills, liver, pancreas, lining of bladder echinoderms (sea urchin) - simple gastrulation of hollow blastula at the blastopore frog embryo - gastrulation proceeds from the dorsal lip of the blastopore reptiles, birds, mammals - gastrulation of discoidal embryo occurs along the primitive streak C. Neurulation (chordates) - infolding of dorsal ectoderm to form a neural tube neural plate → neural groove and neural folds → neural tube → spinal cord and brain - neural crest cells break off, migrate to form nerves, ganglia, adrenal medulla, melanocytes Cross-section of vertebrate embryo after neurulation Biology 240, Lecture 11 D. Mesoderm Development (late gastrula and neurula) notochord - embryonic “backbone”, forms early in development - characteristic structure of chordates somites - segmented blocks of mesoderm, lateral to notochord - form vertebrae, most muscles, dermis of skin intermediate mesoderm - forms kidneys and gonads lateral plate mesoderm - splits to form coelomic membranes (parietal and visceral layers) - migrating visceral mesoderm forms heart, blood vessels, muscle of GI tract mesenchyme - undifferentiated embryonic connective tissue, gives rise to most mature CTs E. Amniotic Egg - characteristic of amniotes (reptiles, birds, mammals) - “land egg” supports embryonic development away from water (“enclosing the pond”) - embryo is surrounded by extraembryonic membranes: 1. amnion - encloses fluid-filled cavity around the embryo, functions for support & hydration 2. chorion - outermost membrane, functions for gas exchange; embryonic placenta in mammals 3. allantois - surrounds cavity off embryonic gut, functions for waste storage and gas exchange (chorioallantoic membrane); forms embryonic bladder and umbilical cord in mammals 4. yolk sac - surrounds yolk for nutrition in birds & reptiles, continuous with gut; empty in mammals (but gives rise to primordial germ cells) amnion, chorion and allantois are unique structures of the amniotic egg; yolk sac is also present in fish and amphibian embryos. Note embryological homology of extraembryonic membranes of amniotes Study Questions 1. Summarize the process of gastrulation in a vertebrate embryo and compare gastrulation in embryos from eggs with little yolk (sea star), moderate yolk (frog), and large yolk (bird). 2. What major organs of the body (or parts of major organs) are formed from endoderm? …from mesoderm? …from ectoderm? 3. Draw a vertebrate embryo post-neurulation, label the major structures, and indicate whether they are formed of ectoderm, mesoderm, or endoderm. 4. Identify organs or specific structures in a vertebrate that develop from the following embryonic structures: neural tube, notochord, somites, archenteron, intermediate mesoderm, lateral plate mesoderm 5. Name the four extraembryonic membranes in an amniote and describe their location and function. What embryonic structure in a mammal is formed by the chorion? What structure is formed by the allantois?
