Cleavage and Blastulation PDF

Associate Professor Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University  The series of mitotic divisions of the early embryo within the confines of the zona pellucida giving rise to daughter...

Associate Professor Department of Morphophysiology and Pharmacology College of Veterinary Science and Medicine Central Luzon State University  The series of mitotic divisions of the early embryo within the confines of the zona pellucida giving rise to daughter cells, called blastomeres.  Compared with the usual form of mitosis, the daughter cells in cleavage become progressively smaller with each division. After a few cell divisions, the embryo takes the shape of a small ball of cells referred to as morula.  HOLOBLASTIC CLEAVAGE Complete cleavage The type of cleavage during which the whole of the egg becomes subdivided into blastomeres  MEROBLASTIC CLEAVAGE Incomplete/partial cleavage The type of cleavage in which only a part of the egg is subdivided into blastomeres An ovum that undergoes partial cleavage has large amount of yolk material Based on the amount Based on the of yolk distribution of yolk Oligolecithal Isolecithal Mesolecithal Telolecithal Polylecithal Centrolecithal  MIOLECITHAL (OLIGOLECITHAL) Ovum with very small amount of yolk  MEDIALECITHAL (MESOLECITHAL) Ovum with a moderate amount of yolk  MEGALECITHAL (POLYLECITHAL) Ovum with huge amount of yolk  ISOLECITHAL Ovum with evenly distributed yolk  CENTROLECITHAL Ovum in which the yolk is concentrated in the interior of the egg and the cytoplasm is distributed as a thin coat on the external surface  TELOLECITHAL Ovum in which the amount of yolk present displaces the embryo- forming cytoplasm into a small area at the animal pole  Type of Egg Type of Animals Cleavage Isolecithal/ Holoblastic Mammals,molluscs Oligolecithal Mesolecithal Holoblastic Amphibians, some fishes Telolecithal/ Meroblastic Avians, reptiles, Polylecithal most fishes Centrolecithal Meroblastic Insects, arthropods  Commonly called “ball of cells” After the Latin name for mulberry. Cellular mass of the embryo occupies most of the perivitelline space  Outer blastomeres Outer blastomeres Form tight junctions Destined to become trophoblast or trophoectoderm Inner blastomeres Inner Form gap junctions blastomeres Destined to become inner cell mass  Blastomeres coalesced forming a compact mass Embryo mass occupies 60 –70% of the perivitelline space which means a larger perivitelline space than in morula.  Consist of: Inner cell mass (embryoblast)- a collection of cells localized inside one pole (end) of the blastula. Embryoblast/ICM – where embryonic development begins Trophoblasts- the surface cells of the blastocyst Blastocoele- the fluid cavity  The portion of the trophoectoderm covering the inner cell mass is referred to as the polar trophoectoderm whereas the rest is known as the mural trophoectoderm.  Diameter of the embryo dramatically increased with concurrent thinning of the zona pellucida ET is best done on blastocyst stage  Zona pellucida is digested enzymatically The blastocyst leaves the zona pellucida and attaches (adheres) to the uterine mucosa, followed by the implantation and the placentation of the embryo.  Horses, dogs, cats and rabbits- marked round to oval expansion of the blastocyst Cattle, sheep, pig- marked thread-like expansion Primates, rodents, guinea pigs- little expansion occurs because blastocyst invades the endometrium  ICM differentiates into two cell populations: Those facing the blastocyst cavity become flattened and delaminate, forming an inner cell sheet referred to as the hypoblast. The remaining cells form the multilayered epiblast. Th an k you !