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In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. (2025). 9780878932436, Sinauer Associates. ISBN 9780878932436 The of many species undergo rapid 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 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 complete cleavage) or meroblastic (partial or incomplete 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 Cell nucleus mass without increasing the 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 (2025). 9780521783378, Cambridge University Press. ISBN 9780521783378
1) Pfluger's Law states how the spindle formed will elongate in the direction where the resistance is least or minimal.
2) Balfour's Law discovered that cleavage tends to perform at a rate based on the amount of yolk present and the yolk's inverse ratio in holoblastic cleavage. The law also covers how the yolk restricts or interferes with division in the cytoplasm and nucleus.
3) Sack's Law articulates the equal part division of cells during cleavage and describes the two-part division through right angles of the previous planes to make new planes.
4) Hertwig's Law governs the discovery of the nucleus's general location and its spindles in the active protoplasm's center. The law covers every axis of any division's spindle and is generally located at the longest axis of the protoplasmic masses. Like in Sack's law, the discovery of divisions by right angles makes an appearance again, but with the cut of the protoplasmic masses by right angles to their axes.
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 . The asters are nucleated by 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 . 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 and appears to be controlled by the NC ratio (about 1:6).
The nematode C. elegans, a popular developmental model organism, undergoes holoblastic rotational cell cleavage. (2025). 9781605354705, Sinauer. ISBN 9781605354705
Specification of the D macromere and is an important aspect of spiralian development. Although the primary axis, animal-vegetal, is determined during oogenesis, the secondary axis, dorsal-ventral, is determined by the specification of the D quadrant. The D macromere facilitates cell divisions that differ from those produced by the other three macromeres. Cells of the D quadrant give rise to dorsal and posterior structures of the spiralian. Two known mechanisms exist to specify the D quadrant. These mechanisms include equal cleavage and unequal cleavage.
In equal cleavage, the first two cell divisions produce four macromeres that are indistinguishable from one another. Each macromere has the potential of becoming the D macromere. After the formation of the third quartet, one of the macromeres initiates maximum contact with the overlying micromeres in the animal pole of the embryo. This contact is required to distinguish one macromere as the official D quadrant blastomere. In equally cleaving spiral embryos, the D quadrant is not specified until after the formation of the third quartet, when contact with the micromeres dictates one cell to become the future D blastomere. Once specified, the D blastomere signals to surrounding micromeres to lay out their cell fates.
In unequal cleavage, the first two cell divisions are unequal producing four cells in which one cell is bigger than the other three. This larger cell is specified as the D macromere. Unlike equally cleaving spiralians, the D macromere is specified at the four-cell stage during unequal cleavage. Unequal cleavage can occur in two ways. One method involves asymmetric positioning of the cleavage spindle. This occurs when the aster at one pole attaches to the cell membrane, causing it to be much smaller than the aster at the other pole. This results in an unequal cytokinesis, in which both macromeres inherit part of the animal region of the egg, but only the bigger macromere inherits the vegetal region. The second mechanism of unequal cleavage involves the production of an enucleate, membrane bound, cytoplasmic protrusion, called a polar lobe. This polar lobe forms at the vegetal pole during cleavage, and then gets shunted to the D blastomere. The polar lobe contains vegetal cytoplasm, which becomes inherited by the future D macromere.
B. Mesolecithal (moderate vegetal yolk disposition)
B. Centrolecithal (yolk in center of egg)
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 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 cell polarity that provides distinct characteristics and functions to their cell-cell and cell-medium interfaces. As surface cells become epithelium, they begin to tightly Cell adhesion as are formed, and are developed with the other blastomeres.
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 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. 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.
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 placentalia and other mammals.
Meroblastic
In the presence of a large concentration of yolk in the fertilized egg cell, the cell can undergo partial, or meroblastic, cleavage. Two major types of meroblastic cleavage are discoidal and superficial.
(2025). 9781605354705, Sinauer Associates, Inc. ISBN 9781605354705+ Summary of the main patterns of cleavage and vitellogenesis (after (2025). 9780878932580, Sinauer. ISBN 9780878932580 and (2025). 9780070607507, McGraw-Hill. ISBN 9780070607507).A. Isolecithal (sparse, evenly distributed yolk)
A. Telolecithal (dense yolk throughout most of cell)
Mammals
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.
(2025). 9780702052309, Elsevier Limited. ISBN 9780702052309 (2025). 9781455706846, Churchill Livingstone. ISBN 9781455706846 With further compaction the individual outer blastomeres, the , become indistinguishable as they become organised into a thin sheet of tight junction . 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.
(2025). 9780781790697, Lippincott William & Wilkins. ISBN 9780781790697 The trophoblasts will eventually give rise to the embryonic contribution to the placenta called the chorion.
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