Gamete Formation: Meiosis, Spermatogenesis, and Oogenesis

Meiosis

  • Meiosis shuffles genes through recombination, providing opportunities for species improvement.
  • Independent assortment during meiosis:
    • With 23 chromosomes, there are approximately 8,000,000 possible chromosome assortments for each cell.
    • Random fertilization leads to about one in 64,000,000,000,000 possible combinations.

Similarities Between Oogenesis and Spermatogenesis

  • Both involve meiosis.
  • Both undergo morphological differentiation.
  • Neither product survives long without fertilization.

Differences Between Oogenesis and Spermatogenesis

  • Spermatogenesis:
    • One cell results in four haploid spermatozoa.
  • Oogenesis:
    • One cell results in one functional oocyte and three polar bodies.
  • Timing of maturation:
    • Oogenesis starts and stops throughout life.
    • Spermatogenesis starts at puberty and continues for life.

Meiosis Overview

  • Meiosis I:
    • Reductive division where chromosomes are separated (not sister chromatids).
  • Meiosis II:
    • Sister chromatids are separated, resulting in haploid cells.

Issues with Meiosis

  • Nondisjunction events: can lead to aneuploidy (changes in chromosome number).
  • Turner Syndrome:
    • One X chromosome (XO).
    • Phenotypically female.
    • May have impacts on phenotype depending on mosaicism (some cells with XX, some with XO).
    • Possible impacts: shorter stature, webbed neck, intellectual development.
  • Klinefelter Syndrome:
    • Two X chromosomes and one Y chromosome (XXY).
    • Phenotypically male.
    • Often undiagnosed until later in life.
    • Limited developmental impact due to X chromosome silencing.

Aneuploidy of Autosomal Chromosomes

  • Often results in spontaneous miscarriage before ten weeks of gestation.
  • Surviveable trisomies:
    • Down Syndrome (Trisomy 21):
      • Smallest chromosome with the fewest genes.
      • Compatible with life.
    • Trisomy 18 and Trisomy 13:
      • May survive through gestation, but with significant developmental impacts.
      • Most children do not survive past one year.

Spermatogenesis

  • Takes place in the testes, which are located outside the body to maintain the correct temperature for sperm production.
  • Sperm mature in the epididymis, acquiring the ability to swim.
  • Seminiferous tubules:
    • Spermatogonial stem cells divide at the basement membrane.
    • One daughter cell maintains the stem cell population; the other enters spermatogenesis.
      • Spermatocytes:
        • DNA and cell contents duplicated.
      • Secondary spermatocytes:
        • Undergo meiotic process.
      • Spermatids:
        • Compact down to form sperm.
    • Mature sperm are released into the lumen.
  • Sertoli cells (nurse cells):
    • Support spermatogenesis.
    • Express SRY, driving gonads to the testis pathway.
    • Produce anti-Müllerian hormone (AMH) to regress Müllerian ducts.
    • Regulate hormones and stem cells.
    • Form the blood-testis barrier to protect spermatogenic cells from immune surveillance.
  • Spermatogenesis stages:
    • Spermatogonium → primary spermatocyte → secondary spermatocytes → spermatids → spermatozoa.
  • Germ cells:
    • Maintain connection (syncytium) during spermatogenesis to share RNAs and proteins produced by the X and Y chromosomes.
    • Ensures equal distribution of X and Y chromosome products until the end of the process.
  • Morphological changes:
    • Flagella develop.
    • Mitochondria arrange along the flagella.
    • Cytoplasm is removed.
    • Final components:
      • Acrosome (from Golgi).
      • Compacted nucleus.
      • Centriole.
      • Mitochondria.
  • Hormonal Control:
    • Testosterone produced by Leydig cells impacts reproductive structures.
    • Sertoli cells produce hormones like AMH.

Oogenesis

  • At five weeks of gestation: 700 to 1,300 germ cells.
  • Up to 6,000,000 germ cells per ovary.
  • Oocyte reserve decreases before birth through puberty.
    • Likely a selection process.
  • Oocyte population is fixed at birth.
  • Continual loss of oocytes during the menstrual cycle.
  • By age 50: few remaining germ cells, unlikely to produce functional gametes.
  • Oogenesis stages:
    • Oogonium enters the ovary.
    • Oocyte surrounded by granulosa cells and thecal cells.
      • Granulosa cells: support.
      • Theca cells: produce estrogen.
  • Monthly process in humans (unlike the continual process of spermatogenesis).
    • Approximately 50 oocytes start to mature each month.
      • One dominates, while the rest die.
    • Dominant follicle is defined about seven days before ovulation.
    • Oocyte is ovulated into the fallopian tube for potential fertilization.
  • Hormonal control of female germ cell maturation exists, but specific details are not needed for this lecture.

Size Difference Between Gametes

  • Oocyte is about 100,000 times the size of the sperm.
  • Oocyte provides everything needed for the start of development.
    • RNAs and proteins for early cleavage events.
    • Nutrients until the blastocyst stage.
  • Sperm carries few products into the egg.

Primordial Germ Cells

  • Start out the same in both sexes.
  • Change once in the gonad.
  • Surrounded by support cells (Sertoli or granulosa cells).
  • Hormone-producing cells (Leydig or theca cells).
  • Undergo significant maturation.
  • Are the only cell types in the body to undergo meiosis except for yeast cells during a yeast infection.
  • Produce haploid germ cells that combine to form a new individual.