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Chapter 5:

Cleavage

Gerald M. Salas, MSDepartment of Sciences, College of Arts and SciencesPampanga State Agricultural University

Results of Fertilization

Restoration of the diploid number of chromosomes

Determination of the sex of the new individual

Initiation of cleavage

Cleavage

Cleavage

Cleavage is a series of mitotic divisions that results in an increasein cells, blastomeres, which become smaller with each division.After three divisions, blastomeres undergo compaction tobecome a tightly grouped ball of cells with inner and outer layers.

Compacted blastomeres divide to form a 16-cell morula. As themorula, enters the uterus on the third or fourth day afterfertilization, a cavity begins to appear, and the blastocystforms.The inner cell mass, which is formed at the time of compactionand will develop into the embryo proper, is at one pole of theblastocyst. The outer cell mass, which surrounds the inner cellsand the blastocyst cavity, will form the trophoblast.

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Cleavage

Once the zygote has reached the two-cell stage, it undergoes a series ofmitotic divisions, increasing the numbers of cells. These cells, which become

smaller with each cleavage division, are known as blastomeres. Until theeight-cell stage, they form a loosely arranged clump.

After the third cleavage, however, blastomeres maximize their contact witheach other, forming a compact ball of cells held together by tight junctions.This process, compaction, segregates inner cells, which communicateextensively by gap junctions, from outer cells.

Approximately 3 days after fertilization, cells of the compacted embryo divideagain to form a 16-cell morula(mulberry). Inner cells of the morula constitutethe inner cell mass, and surrounding cells compose the outer cell mass. Theinner cell mass gives rise to tissues of the embryo proper, and the outer cellmass forms the trophoblast, which later contributes to the placenta.

Blastocyst Formation

About the time the morula enters the uterine cavity, fluid begins topenetrate through the zona pellucida into the intercellular spaces of the

inner cell mass. Gradually, the intercellular spaces become confluent,and finally, a single cavity, the blastocele, forms.

At this time, the embryo is a blastocyst. Cells of the inner cell mass,now called the embryoblast, are at one pole, and those of the outer cell

mass, or trophoblast, flatten and form the epithelial wall of the

blastocyst.

The zona pellucida has disappeared, allowing implantation to begin.

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Cleavage Patterns

Cleavage Patterns

The resulting tightly packed mass of about 32 cells iscalled a morula,and each individual cell in the morulais referred to as a blastomere.

As the blastomeres continue to divide,they secrete afluid into the center of the morula. Eventually, ahollow ball of 500 to 2000 cells, the blastula,isformed.

The fluid-filled cavity within the blastula is known asthe blastocoel.

Eggs

Isolecithal/Microlecithal - The yolk is evenly distributed, so thecleavage of the egg cell cuts through and divides the egg intocells of fairly similar sizes.

Mesolecithal - The yolk is concentrated in one part of the egg (thevegetal pole), with the cell nucleus and most of the cytoplasm inthe other (the animal pole).

Macrolecithal - Eggs with a large yolk

Telolecithal - The yolk is concentrated at one pole of the egg separate fromthe developing embryo.

Centrolecithal - the placement of the yolk in the centre of the cytoplasm of

ovums

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Holoblastic Cleavage

Holoblastic cleavage occurs when the cleavage furrows passthrough the entire egg

Cleavage can either be equal, where the resulting cellscontain the same amount of yolk, or unequal, in which somecells contain more yolk than others:

equal cleavage occurs in microlecithal eggs

unequal cleavage occurs in mesolecithal eggs

cleavage results in the formation of a ball of cells(blastomeres) surrounding an internal cavity (blastocoel)

Holoblastic with Isolecithal eggs

Observed in eggs with low amounts of yolk.

Yolk homogeneously distributed in egg andblastomeres are often the same size.

Holoblastic with mesolecithal eggs

Yolk is distributed heterogeneously (concentrated inthe vegetal pole).

Meroblastic Cleavage

Meroblastic cleavage occurs more in macrolecithaleggs

cleavage takes place only in a disk at the animal pole

the cleavage furrows do not extend into the yolk

results in the formation of the blastodisc that lies onthe top of the yolk

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Meroblastic Cleavage

Incomplete cleavage with telolecithal and centrolecithal yolk distribution

The eggs produced by reptiles, birds, and some fish are composed almost

entirely of yolk, with a small amount of cytoplasm concentrated at onepole.