Formation of Haploid ovum
Formation of haploid ovum in ovary by meiosis is called as oogenesis. During prenatal period, mitotic proliferation of primordial germ cells occurs. This ends at the time of birth.
Oogonia undergo mitotic division until the final generation of oogonia enters the prophase of meiosis-I. At this point it is called primary oocyte. This is under the influence of meiosis initiating factor
In domestic animals (except Female dog and Queen), oogonia develops into primary oocyte shortly before or after birth. At this stage primary oocyte are surrounded by a layer of follicular epithelium to form primary follicle.
The nucleus of primary oocyte enters the dictyate / resting stage of meiotic prophase I and does not compete until the animal reaches maturity. Resumption of meiosis, beyond dictyate stage depends on preovulatory surge of LH.
After puberty, the rising level of FSH causes follicular cells to develop. The primary oocyte grows by RNA synthesis.
Contact between oocyte and granulose cells prevent maturation of oocyte beyond dictyate stage through oocyte maturation inhibitor (OMI) produced by granulosa cell. OMI concentration declines as follicle matures.
Ovulatory LH surge block the transfer of OMI from cumulus cells to the oocyte allowing meiosis to resume. Half of chromatin and small amount of cytoplasm are extruded as first polar body
First meiotic division completes few hours before ovulation and the cell is now secondary oocyte. In bitch and few mares, completion of first meiotic division and extrusion of first polar body occur after ovulation.
The 2nd day oocyte enters II meiosis. It completes when spermatozoa penetrate Zona pellicida to activate the oocyte. When oocyte is activated 2nd maturation division is completed with the formation of 2nd polar body.
Now the germ cell is called as ootid. Occasionally first polar body undergoes division in to form two PBs.
