12.2 Gametogenesis and reproductive cycles

Gametogenesis is the process of forming sex cells. In males, sperm develop through spermatogenesis in the testes. In females, eggs develop through oogenesis in the ovaries. Both processes involve complex cell divisions and transformations.

Reproductive cycles regulate fertility in mammals. The estrous cycle occurs in many animals, with females becoming receptive during estrus. Humans and some primates have menstrual cycles, involving hormonal changes and uterine lining shedding if fertilization doesn't occur.

Gametogenesis

Sperm Development

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• Spermatogenesis produces male gametes called spermatozoa through a series of cell divisions and differentiation events in the seminiferous tubules of the testes
  • Involves mitotic divisions of spermatogonia to produce primary spermatocytes
  • Primary spermatocytes undergo meiosis I to form secondary spermatocytes
  • Secondary spermatocytes complete meiosis II, resulting in haploid spermatids
• Spermiogenesis is the final stage of spermatogenesis during which spermatids differentiate into mature spermatozoa
  • Involves condensation of the nucleus, formation of the acrosome, and development of the flagellum (tail)
  • Mature spermatozoa are released into the lumen of the seminiferous tubules (spermiation)

Egg Development

• Oogenesis produces female gametes called ova (egg cells) in the ovaries
  • Begins during fetal development with the formation of primary oocytes through mitotic divisions of oogonia
  • Primary oocytes enter meiosis I but are arrested in prophase I until puberty
  • Each menstrual cycle, a cohort of primary oocytes resumes meiosis I, forming secondary oocytes and the first polar body
• Meiosis in oogenesis is asymmetric, with the oocyte retaining most of the cytoplasm and organelles
  • Secondary oocyte is arrested in metaphase II until fertilization occurs
  • Upon fertilization, the secondary oocyte completes meiosis II, producing a mature ovum and a second polar body

Reproductive Cycles

Estrous Cycle

• The estrous cycle is a recurring period of sexual receptivity and fertility in many female mammals (dogs, cats, rodents)
  • Consists of four stages: proestrus, estrus, metestrus, and diestrus
  • Estrus is the stage of sexual receptivity and ovulation, typically accompanied by behavioral changes (increased activity, vocalizations)
• Length and frequency of the estrous cycle vary among species (rats: 4-5 days, dogs: 2-3 weeks)

Menstrual Cycle

• The menstrual cycle is a recurring series of physiological changes in the female reproductive system of humans and some primates
  • Averages 28 days in length, but can range from 21 to 35 days
  • Regulated by the interplay of hormones from the hypothalamus, pituitary gland, and ovaries (hypothalamic-pituitary-gonadal axis)
• Follicular phase begins on the first day of menstruation and lasts until ovulation (typically day 14)
  • Characterized by the growth and development of ovarian follicles under the influence of follicle-stimulating hormone (FSH)
  • Rising estrogen levels from the growing follicles lead to the proliferation and thickening of the endometrium (uterine lining)
• Ovulation occurs around day 14 of the menstrual cycle, triggered by a surge in luteinizing hormone (LH)
  • The mature follicle ruptures, releasing the secondary oocyte into the fallopian tube
• Luteal phase follows ovulation and lasts until the beginning of the next menstrual period
  • The ruptured follicle transforms into the corpus luteum, which secretes progesterone to maintain the endometrium
  • If fertilization does not occur, the corpus luteum degenerates, leading to a decline in progesterone and the shedding of the endometrium (menstruation)