D3.1.14 Spermatogenesis and oogenesis in humans (HL) Notes

Spermatogenesis and Oogenesis Are Both Forms of Gametogenesis

1. Both spermatogenesis and oogenesis involve four main stages:
   1. Mitosis: Germ cells divide to produce more cells.
   2. Cell growth: Cells increase in size and prepare for meiosis.
   3. Meiosis I and II: Two divisions reduce chromosome number from diploid to haploid.
   4. Differentiation: Cells develop specialized features for their function.
2. Despite these shared stages, spermatogenesis and oogenesis differ significantly in timing, number of gametes produced, and cytoplasm distribution.

Spermatogenesis: Production of Sperm

1. Location: Seminiferous tubules in the testes.
2. Timing: Continuous from puberty throughout adult life.
3. Number of gametes: Four sperm produced per primary spermatocyte.
4. Cytoplasm: Minimal, sperm are streamlined for movement.
5. Timing: Continuous from puberty, with millions of sperm produced daily.
6. Purpose: Maximize the number of gametes to increase chances of fertilization.

Stage 1: Mitosis and Cell Growth

1. Spermatogonia (diploid stem cells) divide by mitosis in the germinal epithelium (outer layer of seminiferous tubules).
2. Some spermatogonia remain as stem cells; others grow and differentiate into primary spermatocytes (larger diploid cells).

Stage 2: Meiosis

1. Meiosis I:
   1. Each primary spermatocyte (diploid) undergoes meiosis I.
   2. Produces two secondary spermatocytes (haploid).
2. Meiosis II:
   1. Each secondary spermatocyte undergoes meiosis II.
   2. Produces two spermatids (haploid).
3. Total: One primary spermatocyte produces four haploid spermatids.

Stage 3: Differentiation (Spermiogenesis)

1. Spermatids undergo differentiation to become mature sperm.
2. Changes during differentiation:
   1. Develop a flagellum (tail) for motility.
   2. Form an acrosome (cap containing enzymes to penetrate the egg).
   3. Condense nucleus to streamline the cell.
   4. Shed excess cytoplasm to reduce size and increase efficiency.
3. Result: Mature sperm are small, mobile cells with minimal cytoplasm.