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. CLEAVAGE (SEGMENTATION), term cleavage refers to a series of rapid mitotic divisions of the zygote following, , forming a many-celled, Versus Typical Mitosis, , fertilization,, , .Cleavage, , The cleavage divisions, sis in, , a, , are no, , few significant points., , blastula., , doubt mitotic, , as, , they produce diploid cells, they differ, , from, , typical, , Divisions occur in quick succession., a nterphase in cleavage divisions is short and does not involve growth so that the resulting blastomeres, hecome smaller in size as their number increases. The total cytoplasmic mass remains the same as in, cells (less, zygote. The zygote, as stated earlier, is a much larger cell (0<$E >1 mm) than the, than of 0-01 mm) of the parents, and cleavage progressively reduces the size of the blastomeres to, that of the parental cells., , de, , body, , LDNA synthesis occurs faster during cleavage to facilitate duplication of chromosomes for the rapidly, , forming new blastomeres. This occurs at the expense of cytoplasmic materials. As a result, the nuclear/, , cytoplasmic ratio, which is very low for the zygote, increases and finally reaches that for the ordinary, cells of the adult., , ) Shape of the embryo does, , not, , change during cleavage because there is no movement of blastomeres.

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Differences between Cleavage and Typical Mitosis, , ABLE 3.13., , Typlcal Mitosls, , Cleavage (Segmentation), 1., , Cleavage, , occurs, , in the, , zygote,, , or, , 1. Mitosis occurs in most body cells., , parthenogenetic, , egg., 2., , Interphase, , is short and blastomeres do not grow, , during it., , 3. Size of blastomeres decreases, , 2., , increases., 4. The total mass of blastomeres remains the same, as, , that of the, , 5. DNA, , occurs, , cells, , grow. to, , nearly, , constam, nt, , 4. The total, , mass of daughter cells, than that of parent cell., , zygote., , synthesis, , daughter, , 3. Size of daughter cells remains, due to growth., , their number, , as, , is long and, normal size during it., , Interphase, , becomes mo, , 5. DNA synthesis occurs at the normal rate., , much faster to form, , chromosomes for new blastomeres., 6., , Nuclear/cytoplasmic, , ratio increases, , as, , 6., , cleavage, , Nuclear/cytoplasmic, , ratio does not, , progresses., , change, , (Oxygen consumption greatly increases during cleavage to provide adequate energy for activte, taking place in rapid cell division., In, but, , short, during cleavage,, , the cells, , undergo M (mitosis) and S (synthesis of DNA) phases of cell cyelk, , practically skip the G, and G, phases., 3.11.2. Cleavage in Human Zygote, (Fig. 3.22), , FIGURE 3, , Cleavage in the human zygote, occurs, , during, , its passage, , through, , the, , fallopian tube to the uterus. It is, holoblastic, i.e., it divides the zygote and, the blastomeres completely into, daughter, cells. However, the blastomeres differ in, size from the start. Their number does not, increase by a regular, doubling sequence, as, happens in frog. The first cleavage, takes place about, 30 hours after, fertilization. It is, coinciding, with the, animal-vegetal pole axis. It, , Female, Zona nhromosomes, , Pellucida, , remain adhered, , cleavage, , to, , occurs, , fertilization., , each other. The second, within 60 hours after, , It is at, , plane, , right angles, , dand9Chromo, , that there is a, transitory "3-cell", before the characteristic "4-cell" stage, stage of, the embryo is reached., Third, so, , takes, , place, , cleavage, , about 72 hours after, , fertilization. Subsequent cleavage, divisions follow, , one, , after another in, , an, , Somes each with, 2 chromatids, , Ooplasm, , Spindle, Polar Bodles, , Polar Bodies, , Early Zygote, Large, Polar Body, , Late Zygote, , Blastomere, , Polar Body, , Zona Pellucida, Products of, large, blastomere, , ma, , Blastomere, , Small, , 2-Cell Stage Blastomere, , Zona Pellucida, 3-Cell Stage, , Blastomeres, , to the, , of the first, and divides, each, blastomere into two by, a, forming mitotic, spindle in each. The larger blastomere, divides a little sooner than the smaller one, , PellucidaOoplasm, , Pronucleus, , meridional,, , produces two blastomeres, one slightly, larger than the other. The two blastomeres, , Zona, , Male, , Blastomeres, Zona Pellucida, , 4-Cell Stage, Zona Pellucida, , Zona, , 8-Cell Stage, , Blastomeres, Early Morula, , Cleavage stages, , Pellucida

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manner, but, , derlv, , order, , embryo now, , surface, , looks, , in a less, , IKe a, , precise orientation. Cleavage, produces, and, , mulberTy,, of the embryo at this, , is O W n as, A, , known, , stage., , morula (Latin,, (, , /117, , solid ball of small blastomeres. The, little mulberry). It refers to the lobed, , a, , is, , Cleavage radialand indeterminate in the human, zygote. In radial cleavage,, uDper 4 cells are direculy above the lower 4 cells., In, , 8-cell stage, the, cleavage, each blastomere formed by, complete embryo. It is the indeterminate, at, , indeterminate, early cleavage divisions retains the capacity to develop, leavage of the human zygote that makes identical twins intó a, possible., Compaction. It is an important event during early mammalian, , hlastomeres,, , hitherto loosely held, tightly adhere, , cohesions, on their surface., , to one, , cleavage., , another due, , Significance of Cleavage. (i) Cleavage restores the cell size and, , to, , It occurs at, , production, , of, , 8-cell stage. The, , proteins,, , called, , nucleo-cytoplasmic, , ratio characteristic, result in growth,, it increases cell number., though, cleavage, cellular, activity is still mainly controlled by the organelles and molecules received During, the, from, secondary oocyte's, cytoplasm, but some of the developing organism's genes become active., , species., , of the, , It does, , not, , (ii) Cleavage, besides producing a large number of, çells by, substances present in the cytoplasm into different cells. rapid divisions, also segregates different, These substances determine how, the various, , cells develop later., , (ii) Cleavage produces a large number of cells, , that the, , building of the offspring's body may begin., Q. What is the source of centrioles in an egg?, Ans. Mammalian, egg lacks centrosome. Basal body of the sperm's flagellum divides, and forms two centrosomes, , NOTABLE, , QUESTION, , mitotic, , spindle, , for, , so, , (with centrioles) in, , the, , cleavage., Morula. The morula consists of 16 to 32 cells, but, it is not larger than the, zygote. Its cells are of two types., , There is, , zygote. These generate the, , FIGURE 3.23, , an outer, , layer of smaller, clearer cells around an, of larger cells (Fig. 3.23A). The morula reaches, the uterus about 4 to 6, days after fertilization. It is stil, Surroundedby the zona pellucida, that prevents its sticking, to the uterine, wall., inner mass, , Outer layer of cells, Inner cell mass, , Zona, , Blastocyst (Blastodermic Vesicle, Blastula). The, , Pellucida, , outer layer of cells of the morula now absorbs the nutritive, , Tluid secreted by the uterine mucous membrane, and is, , Cells of, , Teinafter called, , Rauber, , the, , trophoblast, or, absorbed by the trophoblast collects in a cavity, the, , trophoectoderm., ne cells of the trophoblast become flattened., The fluid, , Dastocoel, or blastocvst cavity, that separates the, pnoblast from the inner cell mass except on one side., the quantity of fluid increases, the morula enlarges, , pldly and assumes the shape of a cyst. It is then called, c blastocyst, Inner cell mass now looks like a small, knob, at one ypole. The knob gives rise to the embryo and, at, , Embryonic, Pole, , EmbryonalKnob, , Blastocoel, , (With fluid), , Trophoblast, , ro, , Abembryonic, Pole, , blast, is called the embryonal knob (Fig. 3.23B)., hed, Theis side, known, of, , blastod, , to, , which, , Formation of blastocyst, , knob is attached is known, embryon, as, the, embryonic, take nayonic or animal pole, and the opposite side as the abembryonic pole. The trophoblast does not, , take, P t in, TH:, , the formation of the embryo proper. It remains external to the embryo and gives rise to the

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and nourishment, , extraembryonic membranes, namely, chorion and amnion, for the protection, known as cells of Rauber., The trophoblast cells in contact with the embryonal knob are, As, , a, , blastocyst, , is formed,, , zona, , of the emh, , dryo', , pellucida becomes thinner and finally disappears., , 3.12. IMPLANTATION, , FIGURE 32, , Implantation is the attachment of the blastocyst, to the uterine wall. It takes place, days after fertilization., , Fallopian, Tube, , about seven, Cleavage, , 100 cells. The portion of, the blastocyst where the embryonal knob is located, lies against the endometrium. This contact stimulates, rapid divisions of the trophoblastic cells which destroy, the endometrial cells by releasing lytic enzymes and, engulf the eroded cells. The blastocyst sinks into a pit, formed in the endometrium (Fig. 3.24) and then gets, completely buried in the endometrium that grows, around it (Fig. 3.25). Thus, implantation involves, activities of both the blastocyst and the uterus. The, , The blastocyst has, , over, , embedded blastocyst forms villi all round it to obtain, , (Day 1), , Morula, , (Day 3), Implanting, , blastocyst, , Ovary, , in endometrium, , (Day 7), , Fallopian, , Uterus, /Endometrium, , Funnel, , Stages of human development from fertilization, , o to dd, , to, , implantation, , nourishment. This establishes a parasitic relationship, , and the uterus, in other words the mother. Later, the embryo projects into the uterne, cavity and loses villi from the free surface., of te, zona pellucida is to prevent the implantation, the, of, function, The, Pellucida., Zona, of, Role, cells till the blastocys, blastocyst at an abnormal site. It does not expose the sticky and phagocytic trophoblast, between the, , embryo, , reaches the proper implantation site., t h e rudiment