Oocyte (ovum)

موضوع: Oocyte (ovum) الأحد أغسطس 28, 2011 1:53 pm

OOCYTE

Oocyte (ovum)
– female germ cell was described by Karl fon Ber in 1827. It is the largest cell (130-150 mkm in
diameter) in human body which can not move.

Oocytes
are developing in ovary in special vesicles – follicles, which undergo
prominent changes due to maturation of ovum. Mature follicle (Graaphian) wall
has special follicular (granulose) cells and antrum is filled with estrogens.
Mature oocyte (II) is settled over cumulus oophorus and is covered with a few
tunics. From outside they are: corona radiata (tunica granulosa),
zona pellucida and oolemma. The last tunica oolemma is the proper cytolemma. Zona pellucida
is a special chemical membrane, which contains much glycoproteins and
glycosoaminglicans. The outer layer consits of numerous small follicular cells,
which give their processes to the ovum through the previous tunic. (Corona radiata makes special crown-like
feature arround oocyte.) Their functions are protection and nutrition of the
oocyte and regulation of its maturation.

Ovulation
is the process by which an oocyte is released from the ovary. Beginning from
puberty, usually one follicle matures each month and ovulation occurs. After
ovulation, oogenesis is completed outside the ovary, in the uterine tube. In
the case of fertilization the secondary oocyte completes second meio.

Nucleus of
human ovum lies eccentrically and contains 23-d chromosomes. There are all the
organells of a general meaning in the ooplasm: mitochondria, endoplasmic
reticulum, few lysosomes. But cell center is absent, so oocyte needs this
organell for successful maturation (in fertilization spermatozoon will bring
it). Smooth endoplasmic reticulum and Golgi apparatus is especially well
developed and produce yolk inclusions. These inclusions are arranged in small
granules, which are dispersed in cytoplasm.

In the
peripheral layer of cytoplasm just under the plasmalemma numerous cortical
granules may be seen. They take an active part in the process of fertilization.