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The subdivision of eggs into cells called blastomeres. It occurs in eggs activated by fertilization or parthenogenetic agents. Cleavages follow one another so rapidly that there is little opportunity for daughter cells to grow before they divide again. Consequently the size of blastomeres diminishes progressively, although many times unequally, during cleavage. By contrast, the nucleus of each daughter cell enlarges following each cleavage with the result that the ratio of the volume of the nucleus to the volume of cytoplasm (the nucleoplasmic ratio) progressively increases. The cleavage period is said by some authorities to terminate when the nucleoplasmic ratios of various blastomeres attain values characteristic of adult tissues. Cells continue to divide thereafter, but each daughter cell then undergoes a period of growth prior to its division with the result that the nucleoplasmic ratio tends to remain approximately constant for each cell type following termination of cleavage. According to others, cleavage terminates with formation of the definitive blastula. Cleavage appears to be an essential step in development. Although some differentiation occurs in eggs of certain animals when cleavage is blocked experimentally, it is limited and infrequent. See Blastulation
Cleavage does more than merely subdivide the substance of the egg quantitatively into smaller units, the blastomeres, which are then of such a size that they can readily undergo the subsequent events of blastulation, gastrulation, and interaction that are involved in formation of tissues and organs. Sooner or later, cleavage segregates different cytoplasmic areas into different blastomeres, thus subdividing the substance of the egg qualitatively. These qualitative cytoplasmic differences among blastomeres are then sufficient to account for the initial establishment of different lines of differentiation in the progeny of different blastomeres, even though the genetic content of all blastomeres is identical. See Cell lineage
cleavagesee POLITICAL CLEAVAGE, CLASS CLEAVAGE.
a series of successive divisions of an egg into increasingly smaller cells (blastomeres).
Cleavage is an essential stage in the development of all multicellular animals. It usually begins after male and female pronuclei unite (fertilization) and combine their chromosomes on the spindle of the first cleavage. Cleavage of unfertilized eggs (parthenogenesis) occurs in some animals. Sometimes fertilized eggs remain dormant for a while (diapause), stimulated later to development by a change in external conditions (for example, a change in ambient temperature). At first, during the period of synchronous division, the nuclei in all the blastomeres divide at an identical and constant rate, and the nuclear cycle is short. This period varies in duration from one group of animals to another; it is absent in mammals altogether. Then, in the period of asynchronous division, or blastulation, the nuclear cycle lengthens, synchronism in the division of the various nuclei is disrupted, the synthesis in them of ribonucleic acid (RNA) begins (in the interphase stage), and their morphogenetic function is revealed. Fission of the cytoplasm (cytotomy) follows division of the nuclei (karyotomy) but generally fails to keep pace with it. Cleavage does not result in growth, and the embryo retains the original size of the egg. Upon the completion of cleavage the embryo reaches the blastula stage.
The amount and distribution of yolk in the cytoplasm of the egg affects the nature of cleavage. Homolecithal eggs, containing comparatively little evenly distributed yolk, undergo complete, uniform cleavage. More often the yolk is distributed unevenly through the cytoplasm (as in telolecithal and centrolecithal eggs). The region containing more yolk divides more slowly than the area poor in yolk, leading to complete but uneven cleavage, or it does not divide at all, leading to only partial cleavage.
The eggs that undergo complete cleavage are called holoblastic; those that undergo partial cleavage are called meroblastic. Holoblastic eggs include homolecithal eggs (for example, those of many invertebrates, lancelets, and mammals) and some telolecithal eggs (for example, those of certain arthropods and most amphibians), which undergo complete but uneven cleavage (small blastomeres are called micromeres; medium-sized, mesomeres; and large, macromeres). Meroblastic eggs include some telolecithal and centrolecithal eggs with a large amount of yolk. Only the animal part of the egg, poor in yolk, divides in these telolecithal eggs, splitting successively into two, four, or more blastomeres, which form a disk of cells on the surface of the noncleaving yolk (discoidal cleavage); this is characteristic of the eggs of scorpions, cephalopods, sharks and teleost fish, birds, reptiles, and lower mammals. Discoidal cleavage results in the formation of the discoblastula, whose cavity is limited by the dimensions of the blastoderm. Partial cleavage is also characteristic of the centrolecithal eggs of most arthropods. The nucleus begins to divide after fertilization. Following several synchronous divisions, the nuclei and the surrounding cytoplasm move along cytoplasmatic bridges to the surface layer of the cytoplasm (initially symplasm); later, a separate cell forms around each nucleus. This gives rise to an embryo whose wall consists of a single layer of cells (blastoderm), while the center is occupied by undivided yolk with its cells (vitellophags) within. Such an embryo is called a periblast, and the cleavage, superficial or syncytial.
The nature of cleavage is also affected by the properties of the egg cytoplasm, which determine the position of the spindles and, therefore, the position of the blastomeres relative to one another, since the plane of cleavage is always perpendicular to the axis of the spindle. According to the relative position of the blastomeres in complete cleavage, a distinction is made between radial, spiral, bilateral, and bisymmetrical cleavage. In radial cleavage, which is characteristic of many coelenterates, echinoderms, and amphibians, the blastomeres are arranged such that any plane that could be drawn through the animal-vegetal axis of the egg is a plane of symmetry. The first two fissures usually proceed meridionally and the third equatorially, whereupon the meridional and equatorial divisions alternate. Multicellular vesicles with a cavity, or coeloblastula, are formed as a result of radial cleavage.
In spiral cleavage, which is characteristic of most turbellarians, annelids, nemerteans, and mollusks, the micromeres separating from the first four blastomeres (macromeres) are arranged in the spaces between them. The blastomeres of the upper story shift either to the right of those of the lower story (dexiotropic cleavage) or to their left (levotropic cleavage). In spiral cleavage the embryo in the blastula stage may have a cavity (uneven coeloblastula), or it may not (stereoblastula).
In bilateral cleavage (in roundworms and ascidians) and in the late stages of spiral cleavage, divisions occur in such a way that the embryo has only a single plane of symmetry.
Bisymmetrical cleavage is very rare (ctenophores); it is characterized by two planes of symmetry. A particular type of cleavage is usually characteristic of a majority of the representatives of a given class of animals, although sometimes there are different types of cleavage within a class. For example, among amphibians, most of which have complete, uneven cleavage, the apodals are characterized by discoidal cleavage; mammals have both discoidal (monotremes) and complete (all higher mammals) cleavage. In several respects (isolation of the embryonic disk and extraembryonic part) the latter is similar to discoidal cleavage (from which it originated). The blastocyst results from complete cleavage; part of its wall, which consists of a dense cluster of cells, forms the embryonic disk; the remainder is called the trophoblast.
In the process of cleavage the nuclei divide evenly (the nuclei of all blastomeres carry a full set of genetic information, equal to one another as well as to the nucleus of the zygote) and the cytoplasm divides unevenly. The differences in the properties of the cytoplasm of the first blastomeres are not equally pronounced in different animals; they depend on the level of differentiation of the cytoplasm in oogenesis. In some animals, when the first two blastomeres are divided artificially, a whole embryo forms from each; in other animals only part of the embryo forms, since the cytoplasm in the eggs of different animals attains different degrees of differentiation by the start of cleavage (the earliest differentiation is characteristic of eggs with spiral, bilateral, or surface cleavage). Regulation and mosaic eggs are sometimes distinguished accordingly.
During cleavage, nuclei equivalent in genotype interact with cytoplasm that is qualitatively different in the various blastomeres, a condition of the differential realization in them of genetic information.
REFERENCESIvanov, P. P. Rukovodstvo po obshchei i sravnitel’noi embriologii. Leningrad, 1945.
Tokin, B. P. Obshchaia embriologiia [2nd ed.]. Moscow, 1970.
T. A. DETLAF
the splitting of rock into thin sheets, occurring in places where there are linear folds in the layers of the earth’s crust that have been caused by tectonic movements.
Cleavage is especially manifest in clay shales, where the sheets can be fractions of a millimeter in thickness (roofing slate). Thicker and coarser sheets are formed in sandstones and limestones. Although the conditions for cleavage formation are not completely clear, it is known that mineral recrystallization through directed compression is involved. The crystals are flattened into planes perpendicular to the compression axis, and the rock acquires a plane-parallel oriented inner structure. Since linear folds are formed in the earth’s crust under the influence of compression perpendicular to the axial planes of the folds, the planar orientation of the crystals takes the same direction, thereby causing the rock to be split into plates along the same lines. Also important in the formation of cleavage is the water extruded by minerals during metamorphism. Extensive recrystallization results in schistosity, the formation of flat oriented crystals over 1 mm in size. Sometimes rock jointing is incorrectly termed cleavage.
V. V. BELOUSOV