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Division of cells in the early embryo
In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a compact mass called the morula. Cleavage ends with the formation of the blastula, or of the blastocyst in mammals. Depending mostly on the concentration of yolk in the egg, the cleavage can be holoblastic (total or entire cleavage) or meroblastic (partial cleavage). The pole of the egg with the highest concentration of yolk is referred to as the vegetal pole while the opposite is referred to as the animal pole. Cleavage differs from other forms of cell division in that it increases the number of cells and nuclear mass without increasing the cytoplasmic mass. This means that with each successive subdivision, there is roughly half the cytoplasm in each daughter cell than before that division, and thus the ratio of nuclear to cytoplasmic material increases.

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).

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.

A cell can only be indeterminate (also called regulative) if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes—when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.

In holoblastic cleavage, the zygote and blastomeres are completely divided during the cleavage, so the number of blastomeres doubles with each cleavage. In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells (cells with a small, even distribution of yolk) or in mesolecithal cells or microlecithal cells (moderate concentration of yolk in a gradient)—bilateral holoblastic, radial holoblastic, rotational holoblastic, and spiral holoblastic, cleavage. 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.

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.

Radial cleavage is characteristic of the deuterostomes, which include some vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte.

Rotational cleavage involves a normal first division along the meridional axis, giving rise to two daughter cells. The way in which this cleavage differs is that one of the daughter cells divides meridionally, whilst the other divides equatorially. Mammals display rotational cleavage, and an isolecithal distribution of yolk (sparsely and evenly distributed). Because the cells have only a small concentration of yolk, they require immediate implantation into the uterine wall in order to receive nutrients. The nematode C. elegans, a popular developmental model organism, undergoes holoblastic rotational cell cleavage.

Mammals have a slow rate of division that is between 12 and 24 hours. These cellular divisions are asynchronous. Zygotic transcription starts at the two , four , or eight cell stage. Cleavage is holoblastic and rotational. In human embryonic development at the eight cell stage, having undergone three cleavages the embryo goes through some changes as it develops into a blastocyst. At the eight cell stage the blastomeres are round, and only loosely adhered. With further division in the process of compaction the cells become flattened, and develop an inside out polarity that optimises the cell to cell contact between them. They begin to tightly adhere as gap junctions are formed, and tight junctions are developed with the other blastomeres. At the 16–32 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). 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. This compaction serves to make the structure watertight, to contain the fluid that the cells will later secrete. In the human 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 from the maternal environment to become blastocoelic fluid. Hydrostatic pressure of the fluid creates a large cavity in the morula called a blastocoel. Embryoblast cells also known as the inner cell mass form a compact mass of cells at the embryonic pole on one side of the cavity that will go on to produce the embryo proper. The embryo is now termed a blastocyst. 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.