IB Biology Topic D3.1.14: Spermatogenesis and Oogenesis in Humans Notes

Spermatogenesis and Oogenesis Are Both Forms of Gametogenesis

1. Both processes pass through four shared stages.
   1. Mitosis: germ-line 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 the chromosome number from diploid to haploid.
   4. Differentiation: cells develop the specialised features they need.
2. The two processes differ in timing, number of gametes, and cytoplasm distribution.

Spermatogenesis Produces Four Small Motile Sperm

1. Location: the seminiferous tubules of the testes.
2. Timing: continuous from puberty, producing millions of sperm daily.
3. Number of gametes: four functional sperm per primary spermatocyte.
4. Cytoplasm: minimal, so sperm stay streamlined for movement.
5. Purpose: maximise gamete number to raise the chance of fertilisation.

Stage 1: Mitosis and Cell Growth

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

Stage 2: Meiosis

1. Meiosis I
   1. Each primary spermatocyte (diploid) undergoes meiosis I.
   2. This produces two secondary spermatocytes (haploid).
2. Meiosis II
   1. Each secondary spermatocyte undergoes meiosis II.
   2. This produces two spermatids (haploid).
3. In total, one primary spermatocyte produces four haploid spermatids.

Stage 3: Differentiation (Spermiogenesis)

1. Spermatids differentiate into mature sperm.
2. Four changes occur during differentiation.
   1. A flagellum (tail) develops for motility.
   2. An acrosome forms, a cap of enzymes that helps the sperm penetrate the egg.
   3. The nucleus condenses to streamline the cell.
   4. Excess cytoplasm is shed to reduce size.
3. The result is a small, mobile cell with minimal cytoplasm.

Oogenesis Produces One Large Egg Plus Polar Bodies

1. Location: the ovaries.
2. Timing: begins before birth, and one egg is released per menstrual cycle from puberty to menopause.
3. Number of gametes: one functional egg per primary oocyte.
4. Cytoplasm: abundant, so the egg is large and nutrient-rich.
5. Purpose: produce one well-resourced gamete that can support 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 two unequal products.
      1. A secondary oocyte (haploid) that keeps most of the cytoplasm.
      2. A first polar body (haploid) with little cytoplasm, which degenerates.
2. Meiosis II
   1. The secondary oocyte begins meiosis II but is arrested in metaphase II.
   2. Meiosis II completes only if fertilisation occurs.
   3. Unequal cytokinesis again produces two unequal products.
      1. A mature ovum (egg) (haploid) with abundant cytoplasm and organelles.
      2. A second polar body (haploid), which degenerates.
3. In total, one primary oocyte produces one functional egg and two or three polar bodies that degenerate.

Stage 3: Differentiation

1. Unlike sperm, the egg does not undergo major structural change after meiosis.
2. The mature ovum is large and its cytoplasm stores three things for the embryo.
   1. Nutrients such as proteins, lipids, and carbohydrates.
   2. Organelles such as mitochondria, ribosomes, and endoplasmic reticulum.
   3. mRNA and proteins needed for early development.
3. The result is a large, resource-rich cell ready to support the embryo.

Spermatogenesis and Oogenesis Compared Side by Side