SPERMATOGENESIS AND OOGENESIS
Spermatogenesis:
Sperm Production: Each day, about 300 million sperm complete the process of spermatogenesis. A sperm cell is about 60 μm long and structurally adapted for reaching and fertilizing a secondary oocyte.
Structure of Sperm:
- The head is about 4–5 μm long and contains the nucleus with 23 highly condensed chromosomes.
- The head is covered by the acrosome, a cap-like vesicle filled with enzymes such as hyaluronidase and proteases that help the sperm penetrate the oocyte.
- The tail is divided into four parts — neck, middle piece, principal piece, and end piece.
- The neck contains centrioles that form microtubules of the tail.
- The middle piece contains mitochondria arranged in a spiral to produce ATP for sperm motility.
- The principal piece forms the longest portion of the tail, while the end piece is the terminal tapering part.
Once ejaculated, most sperm do not survive for more than 48 hours in the female reproductive tract.
Process of Spermatogenesis:
- Spermatogenesis takes about 65–75 days in humans and begins with spermatogonia (diploid cells).
- Spermatogonia act as stem cells; some remain undifferentiated, while others develop into primary spermatocytes.
- Primary spermatocytes (2n) undergo meiosis I to form secondary spermatocytes (haploid, n).
- Secondary spermatocytes undergo meiosis II to produce spermatids, four haploid cells from each primary spermatocyte.
- Spermatogenic cells remain connected by cytoplasmic bridges for synchronized development.
- The final stage, spermiogenesis, involves the transformation of spermatids into mature sperm cells without further cell division.
- During spermiogenesis, the nucleus elongates, an acrosome forms, a flagellum develops, and mitochondria multiply.
- Sperm are released into the lumen of seminiferous tubules during spermiation and moved forward by fluid secreted from sustentacular cells.
OOGENESIS:
Introduction:
Oogenesis is the process of forming female gametes (ova) and begins before birth inside the fetus. The initial steps, up to the formation of primary oocytes, occur before birth. These oocytes remain dormant until puberty.
Stages of Oogenesis:
- Pre-natal Stage: Primary oocytes grow and are arrested at meiosis I. Granulosa cells form a stratified layer around them and secrete glycoproteins to form the zona pellucida.
- Antral Stage: Granulosa cells secrete fluid, forming the antrum. These become secondary follicles that develop under the influence of FSH and LH during each menstrual cycle.
- Pre-ovulatory Stage: Triggered by the LH surge, meiosis I completes, producing a large secondary oocyte and a small polar body. The secondary oocyte enters meiosis II but is arrested in metaphase until fertilization.
Ovulation:
Oocyte formation occurs in the ovaries. Each oocyte is surrounded by follicular cells forming follicles. As the cycle begins, primary oocytes enlarge, and follicles grow. Usually, one follicle matures fully and releases a secondary oocyte during ovulation. In rare cases, more than one ovum matures, leading to fraternal twins.
Fertilization:
Upon fertilization, meiosis II completes, forming a mature ovum and another polar body. Without fertilization, the oocyte degenerates within 24 hours after ovulation.
Development of Oogonia and Oocytes:
- All oogonia (diploid germ cells) form before birth in the female fetus.
- Most oogonia degenerate before birth; the remaining ones enter meiosis I and become primary oocytes.
- Primary oocytes remain arrested in prophase I until puberty.
- After puberty, during each menstrual cycle, a few primary oocytes resume meiosis I, producing a secondary oocyte and a polar body.
- The secondary oocyte proceeds to meiosis II but pauses at metaphase II until fertilization.
- After fertilization, meiosis II completes, yielding a haploid ovum and a second polar body.
Detailed Notes:
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