OOGENESIS
The final generation of oogonia, which are created by the mitotic proliferation of primordial germ cells, continue to grow until they reach the prophase of the first meiotic division.
As primary oocytes at this stage, they go into a protracted resting phase of development. This resting phase, known as the “dictyate stage,” is specific to female gametes. These dictyate oocytes require the preovulatory LH surge to resume meiosis. Oogonia in domestic animals mature into primary oocytes either prior to or soon after birth. Oocytes begin to grow along with the primordial follicles, and by the time the follicles reach the preantral stage, the oocytes have finished growing and formed the zona pellucida and vitelline membrane. The glycoprotein structure known as zona is secreted by the inner granulosa cells. The material secreted by follicular cells and the oocyte are incorporated into the zona, which thickens as the follicle grows. The main components of a primary oocyte are the germinal vesicle, a large spherical nucleus, and the cytoplasm, or vitellus. The oocyte can resume meiosis when the preovulatory surge of LH occurs because it eliminates the inhibition caused by the OMI. It finishes the first meiotic division, extrudes the first polar body, moves on to the second meiotic division’s metaphase, and then stops once more. It is referred to as the secondary oocyte at this point.Similar to spermatogenesis, the first meiotic division - known as reduction division - does not establish the haploid state because chromosome replication occurs before this division. Only after being activated by the sperm during fertilization does the secondary oocyte resume and finish the second meiotic division and extrude the second polar body. Momentarily, the oocyte is an Ootid. The haploidy is established by the second meiotic division, and the oocyte’s cytoplasmic maturation happens concurrently with nuclear maturation. Since the granulosa and other cells surrounding the oocyte begin to loosen at this point, it is likely that the follicular cells’ release of an inhibitory influence is what causes this cytoplasmic maturation. The first meiotic division in the majority of domestic animals is finished a few hours before ovulation. However, in bitch, the first meiotic division is finished two to three days following ovulation. This could be the cause of the oocytes in bitch oviducts having a longer fertile life span - up to seven days. The first meiotic division in a mare can be completed either before or after ovulation.
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