REPRODUCTIVE 3

Overview

  • Discussion of the oversimplification of biological sex
    • Recognition of a dynamic range between male and female
    • Explanation of the need for simplification in educational settings
    • Emphasis on feedback from students shaping the course content

Objectives of the Lecture

  • Topics covered in today's lecture:
    • Sperm production
    • Hormonal control of spermatogenesis
    • Male factor infertility

Anatomy of the Testes

  • Introduction to key structures in the testes using an electron micrograph of a seminiferous tubule:
    • Basement membrane: Gray area of the tubule
    • Interstitial cells (Leydig cells): Important for hormonal control of spermatogenesis
    • Seminiferous tubules: Site of sperm precursor cells
    • Mature sperm: Indicated by yellow coloring in micrograph, ready for release into the lumen to fertilize

Spermatogenesis

  • Definition: The process of sperm production occurring in the seminiferous tubules and only after puberty.
  • Comparison with oogenesis:
    • Oogenesis starts in females before birth and reaches a maximal component of oocytes (6-7 million) by six months gestation.
    • Spermatogenesis starts at puberty in males with continuous sperm production.
  • Statistics:
    • Production rates:
    • Approximately 300 to 600 sperm per gram of testis tissue per second
    • Example calculation:
      • Assuming 500 sperm produced per gram, and a class of 1,000 students (~500 with testes):
      • 250,000 sperm produced every second in the class.
      • Average testicle weight: 20 grams.
      • Calculated total:
        • 10,000,000 sperm per second for the class
        • 600 million sperm per minute and approximated 30 billion sperm in a 50-minute lecture.

Phases of Spermatogenesis

  • Three main phases of spermatogenesis:

    1. Mitotic cell division
    • Process involves spermatogonia stem cells on the basement membrane before puberty.
    • Asymmetrical division to maintain a stem cell reserve.
    • Spermatogonia undergo mitosis in the basal compartment of the seminiferous tubule.
    1. Meiotic cell division
    • Primary spermatocytes undergo meiosis I (46 chromosomes) to produce secondary spermatocytes (23 chromosomes).
    • Secondary spermatocytes undergo meiosis II to produce spermatids (4 spermatids with 23 chromosomes each).
    1. Cytodifferentiation/spermiogenesis
    • Round spermatids reshape into spermatozoa, characterized by the development of tail, midpiece (packed with mitochondria), and acrosome (enzyme-containing pouch on the head).

  • Spermiogenesis: Major morphological changes in spermatids:

    • Formation of tail for swimming.
    • Mitochondria in midpiece provide energy.
    • Acrosome over the head contains enzymes for fertilization.

Hormonal Control of Spermatogenesis

  • Hypothalamus role: Produces gonadotropin-releasing hormone (GnRH) which stimulates anterior pituitary.

  • Hormones produced by gonadotropes:

    • Follicle-stimulating hormone (FSH)
    • Luteinizing hormone (LH)

  • LH action:

    • Travels to Leydig cells, promoting the production of testosterone, which facilitates spermatogenesis and male secondary sexual characteristics.
    • Dihydrotestosterone (DHT): A more potent androgen influencing characteristics.

  • FSH action:

    • Binds to Sertoli cells in seminiferous tubules to support sperm development.
    • Induces production of androgen binding protein for hormone transport and retention within seminiferous tubules.

Feedback Mechanism

  • Negative feedback:
    • Testosterone inhibits GnRH and reduces FSH/LH production.
    • Inhibin from Sertoli cells also specifically reduces FSH production.
  • Role of kisspeptin: Peptide hormone that stimulates GnRH, although control above this level is still unclear.

Male Factor Infertility

  • Definition: Infertility is a disease with many causes, impacting 1 in 6 couples.

  • Causes of male factor infertility:

    • Low sperm count (oligospermia): Less than 20 million sperm/mL considered abnormal.
    • Absence of sperm (azoospermia): A complete lack of sperm.
    • Immotile sperm: Impaired ability to swim toward the egg.

  • Assisted reproductive technologies:

    • In Vitro Fertilization (IVF): Requires motile sperm to fertilize the egg in an external environment.
    • Intracytoplasmic Sperm Injection (ICSI): Allows direct injection of single sperm into an egg, useful for cases of immotility or absence of sperm in ejaculate.
    • Testicular Sperm Extraction (TESE): Biopsy of testes may yield sperm even in cases of azoospermia from absent vas deferens.

Conclusion

  • Summary of spermatogenesis processes and hormonal controls leading to sperm production and implications for male fertility treatments.