Embryo Development: Cleavage Cleavage is an early embryonic phase consisting of rapid mitotic cell division without intervening growth periods It resulting cell called blastomeres Cleavage process also called Blastulation It begin within an our after fertelization. The fertilization envelope is visible as a large "halo" around the embryo. C. E. 8-cell. D. 16-cell. a cell zygote. Chuck Ettensohn . The arrow points to the site of sperm penetration. B. 32-cell. F.A. . 2-cell. Hatched blastula (F is courtesy Dr. Cleavage Pattern Cleavage pattern of different animal are correlated with Amount and distribution of yolk in their egg Polarity of egg . holoblastic cleavege produces similar sized blastomes Lot of yolk in vegetal pole. Holoblastic cleavage Produces different sized blastomeres Amount of distribution of yolk in an egg have major impact on it cleavage pattern.Amount and distribution of yolk in their egg Little yolk. . 4 term of yolk distribution mesolecithal telolecithal Isolecithal centrolecithal . ascidians & mammals. Eg: type of egg in the most echinoderms.Isolecithal Egg that have a small amount of evenly distributed yolk in the cytoplasm. molluscs. . Cleavage slower.Mesolecithal Egg with moderate amount of yolk present mostly in the vegetal hemisphere Vegetal pole-mostly yolk. Animal pole-mostly cytoplasm Eg. Amphibian egg . reptiles and bird egg .Telolecithal Egg with large amount of yolk filling the entire egg except for small area near the animal pole Eg. fish. the egg of all insect and many other arthropods.Centrolecithal Egg in which the yolk is concentrated in the central cytoplasm are Eg. . slower cleavage rate Animal pole: Portion of embryo housing primary tissue.Polarity of egg Animal pole: Yolk free. Vegetal pole: Portion of embryo containing yolk . faster cleavage free Vegetal pole: Yolk rich. . Types of cleavage Depending on the amount and Pattern of yolk and correlated with polarity of egg Term of cleavage pattern Holoblastic: Eggs cell are cleavage totally Meroblastic: Eggs cell are cleavage partially . Holoblastic cleavage pattern this term indicates that the entire egg is cleaved during cytokinesis Holoblastic Cleavage pattern Radial cleavage Eg: sea urchins. amphibians Spiral cleavage Eg: in moluscs & Other group animal Bilateral Cleavage Eg: in ascidians Rotation cleavage Eg: in mammal . reptiles.bird . a large amount of yolk-rich cytoplasm at the vegetal pole remain uncleaved ex>fish.• There are 2 major types of meroblastic cleavage: i) Discoidal cleavage -when it is limited to a disc of cytoplasm at animal pole /restricted to a small disc of yolk-free cytoplasm at the animal pole. ii) superficial cleavage -cytokinesis is limited to a surface layer of clear cytoplasm while the yolk-rich inner cytoplasm remains uncleaved . isolethical and mesolethical eggs undergo a type of cleavage known as holoblastic cleavage (whole germ) teloethical characterized by discoidal cleavage Centrolethical eggs characterized by superficial cleavage . CLEAVAGE PATTERNS OF REPRESENTATIVE ANIMALS Cleavage pattern of the: -sea urchins -molluscs -amphibians -chick/birds -fish/zebrafish -mammals . Leads to the production of large vegetal macromeres and small animal micromeres. the cell divisions proceed more rapidly in the animal pole.CLEAVAGE OF THE AMPHIBIANS Meroblastic. Blastocoele cavity becomes apparent at about the 32-cell stage. although it actually begins to form earlier . but the whole embryo does divide. Because of the yolk concentrated in the vegetal pole. CLEAVAGE DIVISION 8 cell frog embryo The first cleavage vertical through the animal and vegetal poles. . The 2nd cleavage vertical 3rd cleavage horizontal but unequal Embryo consists of four smaller blastomeres at animal poles and four larger blastomeres at the vegetal poles. . Early Frog Blastula (surface view) . Early Frog Blastula (sagittal section) . DIFFERENCES BETWEEN CLEAVAGE OF AMPHIBIANS AND THE OTHER ANIMALS Animals Characteristics Types of cleavage Amphibians Mammals Birds Chick Meroblastic Holoblastic Meroblastic (discoidal) Meroblastic (discoidal) Yolk distribution Mesolechital Isolecithal Telolecithal Telolechital Polarity of eggs .