Overview
This lecture covers oogenesis (egg formation) and folliculogenesis (follicle development) in the female reproductive system, with comparative insights from spermatogenesis.
Spermatogenesis Overview
- Spermatogenesis is the process of forming sperm from spermatogonia stem cells.
- Each spermatogonium contains 23 pairs of chromosomes (diploid).
- DNA replication occurs, followed by recombination (crossing over) between homologous chromosomes.
- Meiosis I separates homologous chromosomes; meiosis II separates chromatids, resulting in four unique haploid spermatids.
- Spermatogenesis starts at puberty and continues throughout life.
Oogenesis Overview
- Oogenesis is the formation of ova (egg cells) from oogonia stem cells.
- Oogonia replicate and begin meiosis during fetal life, pausing at prophase I (primary oocyte).
- Each primary oocyte is arrested at prophase I from fetal development until puberty.
- At puberty, meiosis I completes, producing a large secondary oocyte and a small polar body.
- Secondary oocyte starts meiosis II and arrests at metaphase II, only completing upon fertilization.
- Oogenesis begins prenatally, but ovulation/egg maturation is cyclical from puberty to menopause.
Folliculogenesis Overview
- Folliculogenesis is the development of ovarian follicles that support and protect oocytes.
- Primordial follicles: primary oocyte surrounded by flat pre-granulosa cells, present from fetal life.
- Primary follicles: oocyte surrounded by cuboidal granulosa cells and the zona pellucida.
- Secondary follicles: more granulosa layers, addition of theca cells, and larger oocyte.
- Antral (Graafian) follicles: formation of a fluid-filled cavity (antrum) and further maturation.
- Most follicles undergo atresia (degeneration); few reach full maturation.
Hormonal Regulation
- Puberty triggers hypothalamic release of GnRH, stimulating anterior pituitary to secrete FSH and LH.
- FSH acts mainly on granulosa cells; LH acts mainly on theca cells.
- Granulosa and theca cells together produce estrogen, which thickens the endometrium and regulates feedback.
- Rising estrogen inhibits further FSH/LH release (negative feedback), except for a surge that triggers ovulation (positive feedback).
- Ovulation is caused by a spike in LH.
Key Terms & Definitions
- Oogenesis — process of egg (ovum) formation in females.
- Folliculogenesis — development of ovarian follicles that house and support oocytes.
- Spermatogenesis — process of sperm formation in males.
- Meiosis — specialized cell division producing haploid gametes.
- Primary oocyte — immature egg cell arrested in prophase I of meiosis.
- Secondary oocyte — egg cell after meiosis I, arrested in metaphase II.
- Primordial follicle — earliest stage follicle with primary oocyte and pre-granulosa cells.
- Granulosa cell — follicular cell type nourishing the oocyte and producing estrogen.
- Theca cell — outer follicular cell producing androgens for estrogen synthesis.
- Zona pellucida — glycoprotein layer surrounding the oocyte.
- Atresia — degeneration and loss of immature ovarian follicles.
- GnRH (Gonadotropin-releasing hormone) — hormone prompting pituitary to release FSH/LH.
- FSH (Follicle-stimulating hormone) — hormone stimulating follicle development.
- LH (Luteinizing hormone) — hormone triggering ovulation.
Action Items / Next Steps
- Review stages of meiosis and follicle development.
- Study hormonal regulation during the menstrual cycle.