Human Reproduction Anatomy and Spermatogenesis

Introduction

• Human reproduction involves both male and female participation
• Males produce gametes and assist in transporting them through the female reproductive tract

Anatomical Components

• Gonads: Testes
• Ducts: Epididymis, vas deferens, ejaculatory ducts, urethra
• Glands: Seminal vesicles, prostate, bulbourethral glands
• Penis

Testicle Anatomy and Histology

• Oval-shaped; adult size ~4.5 cm x 2.5 cm, volume ~18 mL
• Located within the scrotum
• Surrounded by a fibrous capsule (tunica albuginea)
• Divided into lobules (250-300) containing seminiferous tubules

Seminiferous Tubules

• Each lobule contains 2-4 seminiferous tubules (~165µm diameter, 30-80 cm long)
• Seminiferous tubules converge into the rete testis
• Connect to efferent ductules, then to the epididymis

Spermatogenesis

• Process: Production of sperm from spermatogonia
• Stages:
  • Multiplication phase: spermatogonia multiply
  • Meiotic division phase: spermatocytes divide, forming spermatids
  • Spermiogenesis phase: spermatids mature into sperm
• Takes ~74 days in males
• Production: ~50-150 million sperm/day at age 20, declining to about 3.8 million at 90

Hormonal Control of Spermatogenesis

• Hormones Involved: FSH (Follicle-Stimulating Hormone) and testosterone. Both have both direct and indirect effects on Sertoli cells.
• Hormonal Action:
  • FSH stimulates Sertoli cells
  • Testosterone potentiates this action by increasing gene expression levels for proteins and enzymes.
• Local Control: Feedback mechanisms involving factors exchanged between Sertoli cells and germ cells.

Testicle Endocrine Function

• Hormone Production: Primarily testosterone by Leydig cells (95%), with some androgens produced by other tissues.
• Testosterone Metabolism: primarily in the liver and other tissues. Some testosterone is converted into dihydrotestosterone (DHT), while some is converted to estrogen.

Testosterone Effects

• Internal & External Gender Development: Involves significant internal and external differences in development of the male reproductive system both at fetal/placental stages and during puberty.
• Secondary Sexual Characteristics: Development of male secondary sexual characteristics, such as deepening of the voice, body hair, and muscular development.
• Anabolic Effects: Increased protein synthesis and muscle mass.

Function of the Penis

• Erection: A physiological mechanism involved in sexual intercourse and involves vascular changes to the penis.
  • Requires activation of parasympathetic nerves and subsequent release of nitric oxide (NO)
  • Blood flow into the corpora cavernosa causes an erection.
  • Sympathetic impulses reduce blood flow to the corpora cavernosa and cause the return to the flaccid state.
  • The degree of relaxation and contraction of the muscles of the corpora cavernosa is involved.
• Ejaculation: expulsion of semen involves muscular contractions in the epididymis, seminal vesicles, prostate and urethra.

Hormonal Regulation of Male Reproductive Function

• Hypothalamus & Pituitary Gland: Crucial roles in initiating and controlling the release of reproductive hormones.
• Feedback Mechanisms: Regulation involves negative feedback loops to maintain hormonal homeostasis.

Life Span Changes in Testicular Function

• Fetal Period: Significant testosterone production and development of male reproductive tract.
• Adulthood: Relatively stable hormone levels, but decreased sperm production is gradual after 20-30 years of age.
• Aging: Gradual decline in testicular function and hormone production.

Epididymis, Seminal Vesicles and Prostate Functions

• Epididymis: Maturation and storage of sperm, and facilitating the movement and transport of spermatozoa to the vas deferens.
• Seminal Vesicles: Contribute fluid to the ejaculate, rich in fructose for sperm energy.
• Prostate: Secretion contributes to the seminal fluid, and it contains various components vital to sperm viability.