CLEAVAGE histology

موضوع: CLEAVAGE histology الأحد أغسطس 28, 2011 2:00 pm

CLEAVAGE

Cleavage (fissio) is the next process after
fertilization – special mitotic divisions (short G1 phase) without growth of
daughter cells.
It results in rapid increase in the number of cells with diminution in their
size. These embryonic cells are called blastomeres. Cleavage
directly depend
on the amount and distribytion of yolk inclusions in the oocyte (type of ovum)
and varies according to whether the eggs are alecital, oligolecital and
polilecital.

Process of
cleavage is characterized by the next features: all zygote is dividing or part
of it, size of appearing cells-blastomeres and time of their appearance.

Cleavage
may be full (complete or holoblastic) and partial (meroblastic).

Complete
cleavage means full division of zygote into blastomeres. Such cleavage is
typical for oligolecital primary isolecital cells, which have moderate amount
of yolk inclusions dispersed in cytoplasm. Partial cleavage includes division
of animal pole of blastula,
vegetative pole of blastula with nutrients is not dividing. It is typical for birds polilecital and telolecital eggs –
a lot of yolk inclusions are condensed together, blastula is known as
diskoblastula.

Complete
cleavage may result in origin of similar or different blastomeres – so called
equal and unequal cleavage. The last one takes place in amphibian blastula.
Some time cells are similar but not absolutely the same–subequal cleavage
(female ovum).

In the case of synchronic cleavage all the
blastomeres are dividing at the same time and in the case of asynchronic one –
at a different time

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