Oogenesis: It is a cycle of female gametes formation. Before conception, this cycle starts inside the Embryo. Before birth, the steps in Oogenesis up to primary oocyte production occur. The primary oocytes are not further divided. They either become or degenerate into secondary oocytes. It occurs in the outermost layers of the ovaries. Oogenesis starts with a germ cell called oogonium and undergoes mitosis to increase in number.
Stages of Oogenesis
- Pre-natal Stage: The primary oocyte grows while being arrested in meiosis-I. The follicular cells proliferate and form a stratified cuboidal epithelium. Such cells are known as granulosa cells. These cells secrete glycoproteins to form zona pellucida around the primary oocyte.
- Antral Stage: The fluid-filled spaces between granulosa Cells merge together to form a central fluid-filled space called the antrum. These are referred to as secondary follicles. Those secondary follicles develop during each monthly cycle under the influence of follicle-stimulating hormone and luteinizing hormone.
- Pre-ovulatory Stage: LH surge induces this stage and meiosis-I complete here. Two haploid cells of unequal sizes are formed within the follicle. One of the daughter cells that receive less cytoplasm forms a polar body. This cell does not participate in ovum formation. The other daughter cell is known as the secondary oocyte. The two daughter cells undergo meiosis-II. The polar body replicates to form two polar bodies, while the secondary oocyte arrests in the metaphase stage of meiosis-II.
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