Mitosis, Meiosis, and Spermatogenesis

Mitosis

• Definition: Part of somatic cell division.

• Outcome: Produces two diploid (2n) daughter cells.

  • Both daughter cells have identical chromosome numbers and pairs.

  • Chromosome pairs consist of homologous chromosomes.

Meiosis

• Definition: A special form of cell division characterized by the production of gametes.

• Types of Gametes:

  • Sperm in males.

  • Oocytes in females.

Key Features of Meiosis

• Crossing Over: An exchange of maternal and paternal genetic material that increases genetic variation among offspring.

• Outcome: Produces 4 haploid (n) daughter cells (gametes) with only 23 chromosomes.

• Fertilization: Fusion of male and female gametes produces a zygote with 46 chromosomes.

• Phases: Involves two division cycles:

  • Meiosis I.

  • Meiosis II.

Spermatogenesis

• Definition: The process of sperm production.

• Timeline: Begins at puberty and continues past the age of 70; the complete process takes about 64 days.

• Location: Begins in the periphery of seminiferous tubules and progresses towards the lumen.

• Process Overview: Involves three main steps:

  1. Mitosis.

  2. Meiosis.

  3. Spermiogenesis.

Cells Involved in Spermatogenesis

1. Spermatogonia (Stem Cells):

   • Divide by mitosis to produce two daughter cells.

   • One remains a spermatogonium while the other differentiates into a primary spermatocyte.

   • Takes about 16 days.

2. Primary Spermatocytes:

   • Undergo meiosis I to form secondary spermatocytes.

   • Each has 23 chromosomes (haploid).

   • Takes about 24 days.

3. Secondary Spermatocytes:

   • Differentiate into spermatids, which are immature gametes.

   • The transformation from spermatids to mature sperm takes a few hours.

4. Spermiogenesis:

   • Spermatids undergo maturation into sperm, losing contact with the wall of the seminiferous tubule, and enter the fluid in the lumen (process called spermiation).

   • Takes about 24 days.

Components of Seminiferous Tubules

• Types of Cells:

  • Spermatogonia.

  • Spermatocytes at various stages of meiosis.

  • Spermatids.

  • Mature sperm.

  • Large nurse cells (Sertoli cells).

Nurse Cells (Sertoli Cells)

• Role: Play a critical role in the process of spermatogenesis.

• Functions:

  • Maintain the blood-testis barrier.

  • Support mitosis and meiosis.

  • Support spermiogenesis.

  • Secretion of inhibin and androgen-binding protein (ABP).

Blood Testis Barrier

• Function: Isolates seminiferous tubules from the general circulation.

• Structure: Maintained by nurse cells through tight junctions creating compartments.

  • Outer Compartment: Contains spermatogonia.

  • Inner Compartment: Site for spermiogenesis and meiosis; produces unique fluid and provides immune protection.

Stimuli for Nurse Cells

• Hormonal Influence:

  • Stimulated by follicle-stimulating hormone (FSH) and testosterone.

  • Support division of spermatogonia and meiotic division of spermatocytes.

  • Provide nutrients and chemical stimuli for development; phagocytize cytoplasm shed by developing spermatids.