Cleavage (embryo)

Cleavage mechanisms and types are governed by four general laws, derived from early studies of embryonic development patterns: 1) '''Pfluger's Law''': the spindle, once formed, will elongate in the direction where the resistance is least (minimal). 2) '''Balfour's Law''': In holoblastic cleavage, the rate at which cleavage progresses tends to reflect the amount of yolk present. Yolk slows down division of both cytoplasm and nucleus. 3) '''Sack's Law''': Daughter cells are of equal size; and successive planes of division are at right angles to one another. 4) '''Hertwig's Law': The nucleus (and spindle) tend to locate at the center of the active protoplasm; and the spindle tends to align with the longest dimension (axis) of the cytoplasmic mass. The subsequent division cleaves the mass accordingly. (Thus, the mother cell cleaves across its longest axis, which is then the smallest dimension of the daughter cells. When they subsequently cleave, their spindles will align across'' that axis: hence Sack's Law). == Mechanism ==

The rapid cell cycles are facilitated by maintaining high levels of proteins that control cell cycle progression such as the cyclins and their associated cyclin-dependent kinases (CDKs). The complex cyclin B/CDK1 also known as MPF (maturation promoting factor) promotes entry into mitosis. The processes of karyokinesis (mitosis) and cytokinesis work together to result in cleavage. The mitotic apparatus is made up of a central spindle and polar asters made up of polymers of tubulin protein called microtubules. The asters are nucleated by centrosomes and the centrosomes are organized by centrioles brought into the egg by the sperm as basal bodies. Cytokinesis is mediated by the contractile ring made up of polymers of actin protein called microfilaments. Karyokinesis and cytokinesis are independent but spatially and temporally coordinated processes. While mitosis can occur in the absence of cytokinesis, cytokinesis requires the mitotic apparatus. The end of cleavage coincides with the beginning of zygotic transcription. This point in non-mammals is referred to as the midblastula transition and appears to be controlled by the nuclear-cytoplasmic ratio (about 1:6). == Types of cleavage ==

Determinate Determinate cleavage (also called mosaic cleavage) is in most protostomes. It results in the developmental fate of the cells being set early in the embryo development. Each blastomere produced by early embryonic cleavage does not have the capacity to develop into a complete embryo. These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavages. In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here, the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms. Bilateral The first cleavage results in bisection of the zygote into left and right halves. The following cleavage planes are centered on this axis and result in the two halves being mirror images of one another. In bilateral holoblastic cleavage, the divisions of the blastomeres are complete and separate; compared with bilateral meroblastic cleavage, in which the blastomeres stay partially connected. Spiral Spiral cleavage is conserved between many members of the lophotrochozoan taxa, referred to as Spiralia. Most spiralians undergo equal spiral cleavage, although some undergo unequal cleavage (see below). This group includes annelids, molluscs, and sipuncula. Spiral cleavage can vary between species, but generally the first two cell divisions result in four macromeres, also called blastomeres, (A, B, C, D) each representing one quadrant of the embryo. These first two cleavages are not oriented in planes that occur at right angles parallel to the animal-vegetal axis of the zygote. With each successive cleavage cycle, the macromeres give rise to quartets of smaller micromeres at the animal pole. The divisions that produce these quartets occur at an oblique angle, an angle that is not a multiple of 90 degrees, to the animal-vegetal axis. == Mammals ==

. p.gl. Polar bodies a. Two-cell stage b. Four-cell stage c. Eight-cell stage d, e. Morula stage Compared to other fast developing animals, mammals have a slower rate of division that is between 12 and 24 hours. Initially synchronous, these cellular divisions progressively become more and more asynchronous. Zygotic transcription starts at the two-, four-, or eight-cell stage depending on the species (for example, mouse zygotic transcription begins towards the end of the zygote stage and becomes significant at the two-cell stage, whereas human embryos begin zygotic transcription at the eight-cell stage). Cleavage is holoblastic and rotational. In human embryonic development at the eight-cell stage, having undergone three cleavages the embryo starts to change shape as it develops into a morula and then a blastocyst. At the eight-cell stage the blastomeres are initially round, and only loosely adhered. With further division in the process of compaction the cells flatten onto one another. At the 16–cell stage the compacted embryo is called a morula. Once the embryo has divided into 16 cells, it begins to resemble a mulberry, hence the name morula (Latin, morus: mulberry). Concomitantly, they develop an inside-out polarity that provides distinct characteristics and functions to their cell-cell and cell-medium interfaces. As surface cells become epithelial, they begin to tightly adhere as gap junctions are formed, and tight junctions are developed with the other blastomeres. With further compaction the individual outer blastomeres, the trophoblasts, become indistinguishable as they become organised into a thin sheet of tightly adhered epithelial cells. They are still enclosed within the zona pellucida. The morula is now watertight, to contain the fluid that the cells will later pump into the embryo to transform it into the blastocyst. In humans, the morula enters the uterus after three or four days, and begins to take in fluid, as sodium-potassium pumps on the trophoblasts pump sodium into the morula, drawing in water by osmosis from the maternal environment to become blastocoelic fluid. As a consequence to increased osmotic pressure, the accumulation of fluid raises the hydrostatic pressure inside the embryo. Hydrostatic pressure breaks open cell-cell contacts within the embryo by hydraulic fracturing. Initially dispersed in hundreds of water pockets throughout the embryo, the fluid collects into a single large cavity, called blastocoel, following a process akin to Ostwald ripening. The trophoblasts will eventually give rise to the embryonic contribution to the placenta called the chorion. A single cell can be removed from a pre-compaction eight-cell embryo and used for genetic screening, and the embryo will recover. Differences exist between cleavage in placental mammals and other mammals. == References ==