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.

Oogenesis: Production of Eggs

1. Location: Ovaries.
2. Timing: Begins before birth; completes only if fertilization occurs.
3. Number of gametes: One functional egg produced per primary oocyte.
4. Cytoplasm: Abundant, egg is large and nutrient-rich.
5. Timing: Begins before birth. One egg released per menstrual cycle from puberty to menopause.
6. Purpose: Produce a large, well-resourced gamete capable of supporting early embryonic development.

Stage 1: Mitosis and Cell Growth

1. Oogonia (diploid stem cells) divide by mitosis during fetal development (before birth).
2. Oogonia grow into primary oocytes (diploid cells).
3. Primary oocytes begin meiosis I but are arrested in prophase I until puberty.

Stage 2: Meiosis

1. Meiosis I:
   1. At puberty, during each menstrual cycle, hormones stimulate one primary oocyte to complete meiosis I.
   2. Unequal cytokinesis produces:
      1. Secondary oocyte (haploid), receives most of the cytoplasm.
      2. First polar body (haploid), small, with minimal cytoplasm. Degenerates.
2. Meiosis II:
   1. The secondary oocyte begins meiosis II but is arrested in metaphase II.
   2. Meiosis II is completed only if fertilization occurs.
   3. Unequal cytokinesis produces:
      1. Mature ovum (egg) (haploid), large, with abundant cytoplasm and organelles.
      2. Second polar body (haploid), small. Degenerates.
3. Total: One primary oocyte produces one functional egg and two or three polar bodies (which degenerate).

Stage 3: Differentiation

1. Unlike sperm, eggs do not undergo significant structural differentiation after meiosis.
2. The mature ovum is large, with abundant cytoplasm containing:
   1. Nutrients (proteins, lipids, carbohydrates) for the early embryo.
   2. Organelles (mitochondria, ribosomes, ER) to support cellular processes before implantation.
   3. mRNA and proteins needed for early development.
3. Result: The egg is a large, resource-rich cell ready to support embryonic development.

Comparing Spermatogenesis and Oogenesis