Gametogenesis – production of mature gametes (eggs and sperm)

• Mitosis of diploid germ cell
• Migration to developing gonad
• Meiosis (Þ haploid)
• Maturation Þ viable eggs and sperm

Primordial Germ Cell – diploid progenitor of gametes

• Migrate from epiblast through yolk sac endoderm and along the dorsal mesentery of the hindgut to the genital ridges (genital ridges » primary sex chords)
• The germ cells proliferate as they migrate
• Once at the genital ridges, proliferate for next few weeks to ~7 million germ cells
• Next, initiate oogenesis or spermatogenesis
• 1 germ cell Þ 1 egg or 4 sperm

• (1) Start meiosis and develop primordial follicle
• 6 weeks of gestation
• migrate to gonad and proliferate (~7million)
• 8 weeks of gestation
• 1^st germ cells enter meiosis and become oogonia
• arrest in prophase of M1 until just prior to ovulation
• follicle cells (somatic cells) proliferate and envelop oocytes forming primordial follicles (primary follicles)
• many oocytes begin to degenerate (atresia) and by the time of birth go from 7 million Þ ~1 million
• primordial follicles enter a period of quiescence until at least puberty
• (2) Folliculogenesis and oocyte growth
• Oocyte becomes egg in roughly 85 days via folliculogenesis
• Stores nutrients at this time for the voyage
• After puberty, pituitary hormone causes a small group of oocytes to resume growth each month
• Follicle growth (Fig 2)
• Follicle cells promote oocyte growth via a nutritive role; acting as a source of metabolites via gap junctions
• Storage of materials in the oocyte
• Cortical granules containing hydrolytic enzymes are formed in the cytoplasm
• Ribosomes accumulate
• rRNA, tRNA, mRNA and proteins are synthesized and stored
• Process takes ~85 days
• Each month one follicle matures and is ovulated while others degenerate (atresia)
• 4 million Þ 400 eggs
• (3) Completion of Meiosis
• Surge of leutinizing hormone (LH) stimulates the dominate follicle to ovulate
• Just prior to ovulation, the oocyte resumes meiosis
• Completes M1 and arrest at metaphase of M2
• primary oocyte Þ secondary oocyte Þ ovulates Þ egg
• completes meiosis at fertilization

• Diploid germ cells Þ haploid gametes
• 1 round DNA synthesis Þ 2 divisions
• prophase M1 Þ recombination between homologous chromosomes
• M1 division – one remains in oocyte and other forms 1st polar body (Fig 3)

• Most aneuploids spontaneously abort during 1^st trimester
• trisomy 13, 18, 21 can survive to term
• Error frequency influenced by maternal age
• <25: ~2% trisomic
• 30s: ~20% trisomic
• 40s: >30% trisomic