Gamete Formation: Spermatogenesis and Oogenesis

Gamete Formation: Sperm and Eggs

Meiosis

  • Meiosis is crucial for sexual reproduction due to:
    • Recombination: Shuffles genes, enabling species improvement.
    • Independent Assortment: With 23 chromosomes, about 8,000,000 possible chromosome assortments exist per cell.
    • Random Fertilization: Ova and sperm each have 8,000,000 possible chromosome combinations.
    • You are the product of approximately a 1 in 64 quadrillion event.

Oogenesis and Spermatogenesis: Commonalities

  • Products undergo meiosis.
  • Extensive morphological differentiation occurs.
  • Limited survival time without fertilization.

Oogenesis vs. Spermatogenesis: Differences

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

Meiosis Explained

  • Meiosis I (Reductive Division):
    • Chromosomes are separated.
    • Genetic material is doubled in the preparatory stage.
    • Sister chromatids do not separate; chromosomes do.
  • Meiosis II:
    • Sister chromatids are separated.
    • Results in haploid cells.

Potential Meiotic Errors: Nondisjunction

  • Nondisjunction: Improper chromosome separation, leading to aneuploidy (changes in chromosome number).
  • Sex Chromosome Aneuploidy:
    • Turner Syndrome: Single X chromosome (XO). Phenotypically female, may have developmental impacts (e.g., shorter stature, webbed neck).
    • Klinefelter Syndrome: Two X chromosomes and one Y chromosome (XXY). Phenotypically male, often taller, may go undiagnosed until later in life.
    • X chromosome silencing in females (and in XXY males) minimizes the impact of extra X chromosomes.
  • Autosomal Chromosome Aneuploidy:
    • More severe developmental impacts.
    • Often results in spontaneous miscarriage before ten weeks of gestation.
    • Exceptions (compatible with development, but with significant impacts):
      • Down Syndrome (Trisomy 21): Smallest chromosome, compatible with life.
      • Trisomy 18 and Trisomy 13: Survivable through gestation, but infants usually survive only about a year due to developmental impacts.

Spermatogenesis

  • Location: Testes (outside the body for temperature regulation).
  • Sperm mature in the epididymis, gaining the ability to swim.
  • Process within Seminiferous Tubules:
    • Spermatogonial stem cells divide at the basement membrane, maintaining the stem cell population, and producing cells that undergo spermatogenesis.
    • Spermatogenesis proceeds through the tubule width.
    • Mature sperm are released into the lumen.
  • Cellular Stages:
    • Spermatogonium: Stem cell.
    • Spermatocyte: DNA duplicated.
    • Secondary Spermatocytes: Post-meiosis I cells.
    • Spermatids: Compacted cells.
    • Spermatozoa: Mature sperm with compacted head, midpiece (mitochondria), and tail.
  • Sertoli Cells:
    • Supporting somatic cells, also known as nurse cells.
    • Named after Enrico Sertoli (discovered in 1865).
    • Express SRY gene, driving gonadal development towards testes.
    • Produce anti-Müllerian hormone (AMH) to regress Müllerian ducts.
    • Regulate hormonal and stem cell functions.
    • Form the blood-testis barrier, protecting spermatogenic cells from immune surveillance.
  • Summary of Spermatogenesis Stages:
    • Spermatogonium → Primary Spermatocyte → Secondary Spermatocytes → Spermatids → Spermatozoa.
  • Syncytium Formation:
    • Germ cells maintain connections during spermatogenesis, forming a syncytium.
    • Allows sharing of RNAs and proteins produced by X and Y chromosomes until the end of spermatogenesis.
  • Morphological Changes:
    • Cells undergo significant changes to become sperm, including flagella development, mitochondrial arrangement, and cytoplasm removal.
    • End product: Acrosome (from Golgi), compacted nucleus, centriole, and mitochondria.
  • Hormonal Control:
    • Testosterone (produced by Leydig cells) impacts reproductive structures and other body cells.
    • Sertoli cells produce hormones like AMH.

Oogenesis

  • Ovarian Reserve:
    • At five weeks gestational age: 700-1,300 germ cells.
    • Peak: About 6-7 million germ cells per ovary.
    • Decline: Oocyte reserve decreases before puberty.
    • At 50 years: Very few germ cells remain.
  • Follicle Development:
    • Oogonium is surrounded by granulosa cells as it enters the ovary.
    • Thecal cells form around the outside.
    • Granulosa cells support the oocyte; theca cells produce hormones (estrogen).
  • Monthly Process:
    • Unlike continual spermatogenesis, oogenesis is monthly in humans.
    • About 50 oocytes start maturation each month, but only one dominates; the rest die.
    • The dominant follicle is defined around seven days before ovulation.
  • Hormonal Control:
    • Complex process, but specific hormonal details are not required for this course.
  • Gamete Size:
    • Oocyte is about 100,000 times the size of the sperm.
    • Oocyte provides everything needed for the start of development (RNAs, proteins, nutrients).
    • Sperm contributes a compact nucleus with little else.

Germ Cells

  • Primordial germ cells start the same.
  • In the gonad, germ cells change depending on the environment (testis vs. ovary).
  • They are surrounded by support cells (Sertoli or granulosa cells) and hormone-producing cells (Leydig or theca cells).
  • Germ cells undergo meiosis to produce haploid cells that form a new individual.