types of cleavage, blastula formation, gastrula , morphogenetic movements etc
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1. Meroblastic cleavage is incomplete cell division that occurs in eggs with large amounts of yolk, like those of reptiles and birds. It results in two main types - discoidal cleavage, restricted to the cytoplasmic disc, and superficial cleavage, limited to the thin surface area. 2. During gastrulation, massive cell proliferation, movement and rearrangement occurs in the blastula, forming the three germ layers - ectoderm, endoderm and mesoderm. This involves morphogenetic cell movements that shape the embryo and form the archenteron cavity. 3. Types of morphogenetic movements include epiboly such as extension and intercalation that thin and spread
types of cleavage, blastula formation, gastrula , morphogenetic movements etc
- 1. S.Y.B.Sc ZOOLOGY SEMESTER IV COURSE X UNIT 1 COMPARATIVE EMBRYOLOGY LECTURE :03
- 2. Meroblastic cleavage • Incomplete divisions due to large amount of yolk present • Telolecithal, centrolecithal (Macroleithal eggs) • 2 Main types of Meroblastic Cleavage: Discoidal & Superficial Discoidal cleavage • Seen in animals having discoidal egg (cytoplasm less & yolk more) • Cleavage is restricted to only cytoplasmic disc • Eg: Reptiles and birds Superficial cleavage • Limited to thin surface area of cytoplasm • Cleavage doesn’t takes place in cytoplasm but only nucleus divides (Kayokinesis) • Divided nuclei are called Energids. • These energids migrtae towards the periphery of egg , syncytial blastoderm • Cellularization occur at about 14th nuclear division and create cellular blastoderm • Eg : Insects (centrolecithal eggs)
- 5. TYPES OF BLASTULAE • COELOBLASTULA • DISCOBLASTULA • BLASTOCYST • Blastula : Cleavage leads to the formation of multicellular embryonic stage called Blastula • The cells of blastula are called Blastomeres • Central cavity in blastula: Blastocoel/ segmentation cavity (surrounded by blastoderm)
- 7. Coeloblastula • Common in eggs having low yolk content • Eccentric/centric blastocoel • Blasstoderm : unilayered/multilayered • Eg; Frogs, Amphioxus • In some cases coleoblastula itself will play role of larva : Sponges (Stereoblastula, Parenchymula, Amphiblastula directly develop into adult
- 10. DISCOBLASTULA • Heavy yolk content : clevage occurs at Animal Pole hence leads to the formation of polar blastomeres • Blastodisc : a flat, topical multilayered disc of blastomeres • Shows presence of subgerminal cavity or primary cavity
- 11. Blastocyst • Marsupial & Placental Mammals (No/negligible amount of yolk) • Cells at VP divide faster than cells at AP • Cells at VP divide rapidly and constitute a thin outer wall called : Trophoblast • Shows presence of inner cell mass (confined to one pole) • ICM is covered by layer of Rauber’s cell. • ICM : Hypo & Epiblast
- 13. GASTRULATION • Making & shaping of embryo by massive proliferation, movement & rearrangements of cells occurred in simple multicellular blastula. • The cell movements lead to formation of germ layers (embryonic layers: ectoderm, endoderm & mesoderm) which play vital role in organogenesis • Cell Movements (motility, shaping & adhesion)- Morphogenetic movements • Archenteron : new cavity developed during morphogenetic movements • Archenteron opens to exterior by blastopore. • 2 germ layers : Diplobalstic • 3 germ layers: Triploblastic
- 17. MORPHOGENETIC MOVEMENTS EPIBOLY • Process of thinning and extension of epithellium which occurs through different movements • Extension: Cells get stretched & increase the germ layer length • Intercalation: Merging of 2 epithelia in which cells from 2 adjacent epithelia move between one another and merge to form single layer • Convergence/divergence: Intercalation of cells from multilayered epithellium in a highly directional & Organized manner • Cells Congrete at one point : Convergence • Cells move away from each other: Divergence EMBOLY • Movement of cell into interior of embryo • Invagination • Ingression • Infiltration • Delamination • Proliferation • Involution
- 18. Types of morphogenetic movements