Describe the process of 'oogenesis' in humans.

Oogenesis is the process of formation and development of female gametes (ova) in humans. It begins during fetal life and continues until menopause. The process involves several key stages:

1. Formation of Oogonia: Primordial germ cells migrate to the gonadal ridge during fetal development and differentiate into oogonia. These are diploid cells (2n) that undergo rapid mitotic divisions to increase their numbers.

2. Formation of Primary Oocytes: By the third month of fetal life, oogonia enter meiosis I and become primary oocytes. These primary oocytes are arrested in the prophase I stage (dictyate stage) until puberty. At birth, a human female has approximately 1–2 million primary oocytes, which decrease to about 40,000 by puberty.

3. Development of Primary Follicles: Each primary oocyte is surrounded by a single layer of follicular cells, forming a primary follicle. These follicles remain dormant until puberty, when hormonal changes stimulate their development.

4. Meiosis I Completion and Formation of Secondary Oocytes: During each menstrual cycle, a few primary follicles mature into Graafian follicles under the influence of follicle-stimulating hormone (FSH). One primary oocyte completes meiosis I, producing a secondary oocyte (haploid, n) and a small first polar body (which degenerates). The secondary oocyte is arrested in metaphase II and is ovulated from the ovary.

5. Meiosis II Completion and Formation of Ovum: If fertilization occurs, the secondary oocyte completes meiosis II, producing a mature ovum (haploid) and a second polar body (which also degenerates). The ovum is now fully mature and capable of fusion with a sperm cell.

6. Degeneration of Polar Bodies: The first and second polar bodies are non-functional and eventually degenerate, ensuring that only one functional ovum is produced per cycle.

Key differences between oogenesis and spermatogenesis include:

• Oogenesis produces one functional ovum per cycle, while spermatogenesis produces four functional sperm cells.
• Oogenesis involves unequal cytoplasmic division, resulting in a large ovum and small polar bodies.
• Oogenesis begins before birth, while spermatogenesis begins at puberty.