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Cleavage

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Definition of Cleavage

  • It is the repeated mitotic division of the zygote resulting in an increasing number of cells.
  • The daughter cells generated by these divisions are termed blastomeres
  • The process begins soon after fertilization and continues as the zygote moves down the uterine tube (diagram)
  • During early cleavage, the cell number doubles with each division and since the zygote is still contained within the zona pellucida, successive generations of blastomeres become progressively smaller or compacted.

Early Zygotes

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A newly fertilized ovum with polar bodies in the zona pellucida on the left. Once cleavage begins, the cells double in number with each division. A zygote at the four cell stage on the right. Note that in early cleavage, the zygote is still contained within the zona pellucida.

Compaction occurs when the zygote reaches the nine cell stage and the blastomeres change shape and tightly align themselves with one another.
This allows for greater cell to cell interaction and subsequent re-aggregation of the cells into inner and outer cell masses.
It is probably mediated by
adhesion glycoproteins.

Morula

  • The zygote reaches the morula stage when it is comprised of between 16 and 32 cells.
  • The term morula means mulberry, which is what the cell mass resembles. Click on the image to reveal the labels.
  • The morula stage is reached about the third day of development around the same time the zygote reaches the lumen of the uterus
  • The cells of the morula are segregated into an inner cell mass and an outer cell mass and will eventually become hollow to form the blastocyst.
  • The blastocyst “hatches” from the zona pellucida before it begins to invade into the endometrium.

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Blastocyst

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  • Around day 4 of development, fluid begins to accumulate in the morula, causing the formation of a cavity within its center.
  • The cavity is called the blastocoele and the morula is now known as a blastocyst.
  • Cells of the inner cell mass become aggregated at one pole of the blastocyst
  • Since these cells will give rise to the germ layers that will form the tissues and organs of the embryo, they become known as the embryoblast
  • Cells of the outer cell mass become flattened and surround the blastocoele and the embryoblast
  • These cells become known as the trophoblast and will contribute to the formation of the extraembryonic supportive membranes by forming the embryonic contribution to the placenta
  • Around this time, the zona pellucida degenerates resulting in “hatching” of the blastocyst from its protective, but restrictive coat.
  • Once the blastocyst is rid of the zona pellucida it is free to begin implantation into the wall of the uterus.

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