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B.Sc. Hons Part II Zoology Development Biology CLEAVAGE

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B.Sc. Hons Part II Zoology Development Biology CLEAVAGE B.Sc. Hons Part II Zoology Development Biology CLEAVAGE OR SEGMENTATION The fertilized egg starts dividing by mitosis to form a large number of cells which organize themselves into a ball of cells. This process is termed cleavage or segmentation or blastulation. Cleavage differs from mitosis in the sense that there is no growth phase. Therefore, although the number of cells increase, there is a progressive decrease in the cell size. It is to be borne in mind that the relative size of fertilized egg remains the same until organogenesis commences. The fertilized egg cell or zygote is a single diploid cell. It undergoes mitosis. The nucleus divides first into two and it is followed by cytokinesis. The first division is on the vertical plane passing from the animal pole to the vegetal pole. This division cuts the cell into two complete blastomeres. As the division is complete it is called holoblastic cleavage or total segmentation. The second division is also on the vertical plane, but, the plane of division is at right angles to the plane of the first division. Now four cells have been formed. The third division is on the horizontal plane. This divides the four cells into eight cells (octant). This division is usually in the equatorial plate region but in most of the animals it is more towards the animal pole. As a result, the resulting eight cells are not of the same size. The four cells towards the animal pole are smaller and are called micromeres. The four cells towards the vegetal pole are larger and are termed the mega or macromeres. Further divisions follow making the embryo 16 celled, 32 celled, 64 celled and so on. These divisions may start as radial cleavage (all cells of the tiers splitting into two as in frog and other vertebrates) or as spiral cleavage (cells one after another splitting in a spiral way as in Annelida and Mollusca. At the end of cleavage a ball of cells called morula is formed. Later a cavity develops inside the morula making it look like a hollow ball called the blastula. The cavity inside the blastula is called the blastocoel or segmentation cavity. Cleavage in bird's egg differs from that of micro and mesolecithal eggs. While the micro and mesolecithal eggs show holoblastic cleavage the bird's egg which is extremely telolecithal shows meroblastic cleavage. Here the first division appears at the animal pole but does not divide the egg into two cells completely. It appears as a furrow at the animal pole and cannot proceed further because of the yolk. So it is called incomplete segmentation or meroblastic cleavage. It is at this stage the embryo reaches the uterine cavity. The blastocyst, as the embryo is now called begins to attach itself to the wall of the uterus. This process is called implantation. The trophoblast as it comes in contact with the uterine wall secretes some proteolytic enzymes that destroy the uterine epithelium. Meanwhile the trophoblast multiplies and gets transformed into two layers, the inner cell layer referred to as cyto-trophoblast and an outer layer, syncytial trophoblast. The blastocyst gets embedded into the uterine wall reaching the endometrial layer of the uterus drawing its nourishment from the uterine blood vessels. DEVELOPMENT OF BODY CAVITY OR COELOM Coelom refers to the body cavity in which the various organs remain suspended. In lower animals like the platyhelminthes there is no coelom. The space between the alimentary canal and the body wall is occupied by a parenchyma-like tissue, Such animals are called Acoelomate (no coelom) (e.g.). Tapeworm. The round worms such as Ascaris has a body cavity but it does not arise from the mesoderm. It is a part of the blastocoel and is not lined by mesoderm. It is also partly occupied by the processes of giant cells. It is therefore referred to as false coelom or pseudocoelom and the animals are pseudocoelomate animals. In the annelids, coelom arises by the splitting of mesoderm. A cavity appears in the mesoderm and coelom, lined on both sides by the mesoderm is called a truecoelom. Eg. Earthworm. These animals are eucoelomate. In vertebrates too the coelom arises from the mesoderm. In Amphioxus, the alimentary canal gives rise to lateral pouches, which later on get separated. Inside these pouches is found a cavity which was formerly connected with the enteron. The pouches give rise to the mesoderm and the cavity becomes the coelom. Such an origin of coelom is said to be Enterocoelic. In higher vertebrates the mesoderm is formed from the chordamesoderm and coelom appears due to splitting of the mesoderm. This split causes mesoderm to become two layered enclosing a cavity in between (i.e.) the coelom. This type of origin of coelom is termed schizocoelic.(Schizo=splitting). (A) Acoelomates lack coelom. Spaces between the various organs are filled with special mesodermal tissue, the mesenchyme or parenchyma. (B) Pseudocoelomates like round worms are characterized by the presence of a cavity Pseudocoel, between the gut and the body wall. This cavity is so called because the coelom is not lined with mesoderm. (C) Coelomates (all other higher phyla) have a true coelom with the mesoderm surrounding the body cavity entirely; (D) In coelomate animals the internal organs (e.g. heart, lungs, kidney) are suspended in the body cavity and they are lined by the mesoderm (called mesenteries). second cleavage is right angles to the plane of first little division further division form a sheet of cells lying above the is also yolk, creating a cavity called blastocoel. Further cells are added to form a layer of cells above the yolk called blastodisc or blastoderm a sheet of cells in contact with yolk is called area opaca and the sheet of cells in the center forms the lifted surface cells on the yolk is called area pellucida. In mammals, the zygote divides first into 2 cells by holoblastic cleavage. Then follows the second division which is at right angles to the previous one. It is not uncommon to come across a 3-celled stage in mammals as one of the two cells may complete its division before the other one due te lack. of synchronization. Finally, a solid mass of cells called Morula is produced. Morula consists of micromeres and macromeres. The macromeres become flattened and form a covering. This is now referred to as trophoectoderm. The interior of morula is occupied by the micromeres. They are called inner cell mass . Soon a cavity appears in the cell mass. The cavity increases in size and gets filled up with a fluid. The cells are pushed towards the animal pole. This is the blastula stage. The inner cell mass is attached at the embryonic knob at the animal pole. The embryo develops from this knob. The general outer layer of cells which encloses blastocoel and knob is the trophoblast. The trophoblast cells overlying the embryonic knob are called cells of Rauber. .
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