After fertilization, growth of multicellular organism proceeds by the procedure known aa cleavage, a series of mitotic divisions whereby huge volume of egg cytoplasm is divided in many smaller, nucleated cells. These cleavage-stage cells are known as blastomeres. In most species rate of cell division and placement of blastomeres with respect to one another is entirely under control of proteins and mRNAs stored in oocyte by mother. Zygotic genome, transmitted by mitosis to all new cells, doesn't function in early-cleavage embryos. Few, if any, mRNAs are made until comparatively late in cleavage, and embryo can separate properly even when chemicals are utilized experimentally to inhibit transcription. During cleavage, though, cytoplasmic volume doesn't increase. Rather, huge volume of zygote cytoplasm is divided in increasingly smaller cells. First egg is separated in half, then quarters, then eighths, and so on. This division of egg cytoplasm without increasing volume is achieved by eliminating growth period between cell divisions (i.e., G1 and G2 phases of cell cycle).
One outcome of this rapid cell division is that ratio of cytoplasmic to nuclear volume gets increasingly smaller as cleavage progresses. In several kinds of embryos, this decrease in cytoplasmic to nuclear volume ratio is vital in timing activation of certain genes. For instance, in frog Xenopus laevis, transcription of new messages is not activated until after 12 divisions. At that time, rate of cleavage decreases, blastomeres turn out to be motile, and nuclear genes start to be transcribed. This phase is known as mid-blastula transition. Cleavage starts soon after fertilization and ends shortly after phase when embryo gets the new balance between nucleus and cytoplasm.
From fertilization to cleavage:
Transition from fertilization to cleavage is caused by activation of mitosis promoting factor (MPF). MPF was first found as major factor liable for resumption of meiotic cell divisions in ovulated frog egg. It continues to play the role after fertilization, regulating biphasic cell cycle of early blastomeres. Blastomeres usually progress through the cell cycle comprising of immediately two steps: M (mitosis) and S (DNA synthesis). MPF goes through cyclical changes in level of activity in mitotic cells. MPF activity of early blastomeres is highest during M and untraceable during demonstrated that DNA replication (S) and mitosis (M) are driven exclusively by gain and loss of MPF activity. Cleaving cells can be experimentally trapped in S phase by incubating them in the inhibitor of protein synthesis. When MPF is microinjected in these cells, they enter M. Their nuclear envelope breaks down and their chromatin condenses in chromosomes. Mitosis-promoting factor has two subunits. Large subunit is known as cyclin B. It is this component which illustrates periodic behavior, accumulating during S and then being degraded after cells have reached. Cyclin B is frequently encoded by mRNAs stored in oocyte cytoplasm, and if translation of this message is specially inhibited, cell will not enter mitosis. Presence of cyclin B depends on synthesis and its degradation. Cyclin B regulates small subunit of MPF, cyclin-dependent kinase. This kinase activates mitosis by phosphorylating numerous target proteins, comprising histones, nuclear envelope lamin proteins, and regulatory subunit of cytoplasmic myosin. This brings about chromatin condensation, nuclear envelope depolymerization, and organization of mitotic spindle. Without cyclin, the cyclin-dependent kinase won't function. Presence of cyclin is handled by numerous proteins which make sure periodic synthesis and degradation. Embryo now enters mid-blastula transition, in which numerous new phenomena are added to biphasic cell divisions of embryo. First, growth stages (G1 and G2) are added to cell cycle, allowing cells to grow. Before this time, egg cytoplasm was being divided in smaller and smaller cells, but total volume of the organism remained unaffected. Xenopus embryos add those phases to cell cycle shortly after twelfth cleavage.
Pattern of embryonic cleavage:
Cell, manifestly, entertains the very different opinion. Indeed, different organisms go through cleavage in distinctly different ways. Pattern of embryonic cleavage specific to a species is determined by two main parameters: amount and distribution of yolk protein inside cytoplasm, and factors in egg cytoplasm which influence angle of mitotic spindle and timing of the formation. Amount and distribution of yolk determines where cleavage can take place and relative size of blastomeres. When one pole of the egg is comparatively yolk-free, cellular divisions take place there at faster rate than at opposite pole. Yolk-rich pole is referred to as vegetal pole; yolk concentration in animal pole is relatively low. Zygote nucleus is frequently displaced toward animal pole. Generally, yolk inhibits cleavage.
At one extreme are eggs of sea urchins, mammals, and snails. These eggs have sparse, equally spaced yolk and are therefore isolecithal (Greek, "equal yolk"). In these species, cleavage is holoblastic (Greek holos, "complete"). The eggs of insects have their yolk in center (that is, they are centrolecithal), and divisions of cytoplasm take place only in rim of cytoplasm around periphery of cell (that is superficial cleavage). Eggs of birds and fishes have only one small area of the egg that is free of yolk (telolecithal eggs), and thus, cell divisions take place only in this small disc of cytoplasm, giving rise to discoidal pattern of cleavage. These are general rules, though, and closely associated species can evolve different patterns of cleavage in the different environment.
Though, yolk is just one factor influencing the species' pattern of cleavage. There are also inherited patterns of cell division which are superimposed on constraints of yolk. This can willingly be seen in isolecithal eggs, in which very little yolk is present. In absence of the large concentration of yolk, four main cleavage types can be seen: radial holoblastic, spiral holoblastic, bilateral holoblastic, and rotational holoblastic cleavage.
The blastula stage of sea urchin development starts at 128-cell stage. Here cells form the hollow sphere nearby the central cavity, or blastocoels. By this time, all cells are similar size, micromeres having slowed down cell division. Every cell is in contact with proteinaceous fluid of blastocoel on inside and with hyaline layer on outside. At this time, tight junctions join the once loosely connected blastomeres in a seamless epithelial sheet which wholly encircles blastocoels. As cells continue to separate, blastula remains one cell layer thick, thinning out as it expands. This is achieved by adhesion of blastomeres to hyaline layer and by the influx of water which expands blastocoels. These rapid and invariant cell cleavages last through ninth or tenth cell division, depending on species. After that time, there is a mid-blastula transition, when synchrony of cell division ends, new genes become expressed, and several of non-dividing cells develop cilia on their outer surfaces. Ciliated blastula starts to rotate within fertilization envelope. Soon later, differences are seen in cells. Cells at vegetal pole of blastula start to thicken, forming the vegetal plate. Cells of the animal half synthesize and secrete the hatching enzyme which digests fertilization envelop. Embryo is now free swimming hatched blastula.
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