Spermatogenesis and oogenesis are crucial processes in creating male and female gametes. These complex journeys involve stem cell division, meiosis, and cellular differentiation, resulting in sperm and eggs ready for fertilization.

The timing and outcomes of these processes differ significantly between sexes. While males continuously produce sperm throughout adulthood, females form all their eggs during fetal development, with final maturation occurring only after fertilization.

Stages of spermatogenesis

Stem cell division and meiosis

• Spermatogenesis produces male gametes in seminiferous tubules of testes through mitosis, meiosis, and cellular differentiation
• Spermatogonial stem cells divide mitotically maintaining stem cell population and generating primary spermatocytes
• Primary spermatocytes undergo meiosis I forming secondary spermatocytes
• Secondary spermatocytes complete meiosis II yielding haploid spermatids
  • Meiosis reduces chromosome number from diploid (46) to haploid (23)
  • Genetic recombination occurs during prophase I, increasing genetic diversity

Spermiogenesis and maturation

• Spermatids transform into mature spermatozoa through spermiogenesis involving:
  • Nuclear condensation compacts genetic material
  • Acrosome formation creates enzyme-filled vesicle for egg penetration
  • Flagellum development provides motility
• Mature spermatozoa release into seminiferous tubule lumen via spermiation
• Entire spermatogenesis process spans approximately 64 days in humans
  • Continuous production throughout adult male life
• Sertoli cells support developing sperm cells by:
  • Providing structural and nutritional support
  • Forming blood-testis barrier protecting germ cells from immune system
  • Secreting factors essential for sperm development (androgen-binding protein)

Process of oogenesis

Fetal development and meiotic arrest

• Oogenesis initiates female gamete formation during fetal development
• Oogonial stem cells divide mitotically producing finite primary oocyte population
• Primary oocytes enter meiosis I prophase, arresting at diplotene stage
  • Arrested oocytes surrounded by follicular cells form primordial follicles
  • Meiotic arrest can last decades until puberty
• Oocytes protected by zona pellucida, crucial glycoprotein layer for:
  • Sperm-egg interaction during fertilization
  • Early embryo development and implantation

Follicular development and ovulation

• At puberty, hormonal signals trigger resumption of meiosis in select oocytes
• Primary oocyte completes meiosis I, forming:
  • Secondary oocyte (larger cell retaining most cytoplasm)
  • First polar body (small cell with minimal cytoplasm)
• Secondary oocyte begins meiosis II, arresting at metaphase II
• Ovulation releases secondary oocyte from ovary
• Fertilization triggers completion of meiosis II, producing:
  • Mature ovum (fertilized egg)
  • Second polar body

Spermatogenesis vs oogenesis

Gamete production and timing

• Spermatogenesis yields four functional sperm per meiotic event
• Oogenesis produces one functional ovum and three polar bodies per meiotic event
  • Polar bodies eventually degenerate, conserving cytoplasmic resources
• Spermatogenesis occurs continuously throughout adult male life
• Oogenesis initiates in fetal development, pauses, resumes at puberty
  • Completes only after fertilization in mature females

Meiotic progression and gamete characteristics

• Spermatogenesis progresses through meiosis without interruption
• Oogenesis involves two distinct meiotic arrests:
  • Prophase I arrest (fetal development to puberty)
  • Metaphase II arrest (until fertilization)
• Sperm cells small, motile, minimal cytoplasm
• Ova large, non-motile, abundant cytoplasm (energy reserves for early development)
• Sperm fully mature before leaving male reproductive tract
• Ova complete final maturation only after fertilization

Hormonal regulation of gametogenesis

Hypothalamic-pituitary-gonadal axis

• Hypothalamic-pituitary-gonadal axis regulates gametogenesis
• Gonadotropin-releasing hormone (GnRH) from hypothalamus stimulates anterior pituitary
• Anterior pituitary releases follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
• FSH and LH act on gonads (testes in males, ovaries in females)

Sex-specific hormone actions

• Male spermatogenesis:
  • FSH stimulates Sertoli cells supporting developing sperm
  • LH stimulates Leydig cells to produce testosterone
  • Testosterone and FSH maintain spermatogenesis, promote sperm development
• Female oogenesis:
  • FSH stimulates follicular development and estrogen production
  • LH triggers ovulation and corpus luteum formation
  • Estrogen and progesterone provide feedback to hypothalamus and pituitary
• Inhibin produced by Sertoli cells (males) and granulosa cells (females)
  • Acts as negative feedback regulator of FSH secretion in both sexes