ZOO: DSC-353 Reproductive and Developmental Biology
Unit 2: Hormonal Regulation of Female Reproductive Cycles
Table of Contents
1. Gametogenesis: Spermatogenesis and Oogenesis
Gametogenesis is the biological process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes.
Spermatogenesis
The process of forming mature male gametes (sperm) in the testes.
- Phase 1: Spermatocytogenesis - Spermatogonia (2n) undergo mitosis to form primary spermatocytes (2n).
- Phase 2: Meiosis - Primary spermatocytes undergo Meiosis I to form secondary spermatocytes (n), and Meiosis II to form spermatids (n).
- Phase 3: Spermiogenesis - Transformation of spherical spermatids into elongated, motile spermatozoa.
Oogenesis
The process of forming mature female gametes (ova) in the ovaries.
- Unlike spermatogenesis, oogenesis begins before birth.
- Primary oocytes are arrested in Prophase I of meiosis until puberty.
- Each cycle, one oocyte completes Meiosis I, producing a secondary oocyte and a first polar body.
- Meiosis II is only completed if fertilization occurs.
2. Epididymal Function and Sperm Maturation
Spermatozoa leaving the testes are immotile and incapable of fertilization. Maturation occurs during their passage through the epididymis.
- Storage: The epididymis stores sperm until ejaculation.
- Motility: Sperm acquire the ability for forward progressive motility.
- Membrane Changes: Alterations in surface proteins and lipids prepare the sperm for interaction with the egg.
- Decapacitation factors: Proteins are added to prevent premature acrosome reactions.
3. Reproductive Cycles and Their Regulation (Rat and Human)
The female reproductive system undergoes cyclic changes regulated by the Hypothalamo-Hypophyseal-Gonadal (HHG) axis.
Human Menstrual Cycle
| Phase | Primary Hormone | Major Event |
|---|---|---|
| Menstrual | Low Estrogen/Progesterone | Shedding of the uterine lining (endometrium). |
| Follicular/Proliferative | FSH and Estrogen | Growth of follicles and thickening of the endometrium. |
| Ovulatory | LH Surge | Release of the secondary oocyte from the Graafian follicle. |
| Luteal/Secretory | Progesterone | Preparation of the endometrium for implantation. |
Rat Estrous Cycle
Rats exhibit an estrous cycle rather than a menstrual cycle, typically lasting 4-5 days.
- Proestrus: Follicular growth and high estrogen.
- Estrus: Period of sexual receptivity ("heat") and ovulation.
- Metestrus: Early corpus luteum formation.
- Diestrus: Longest phase, period of luteal activity.
4. Ovum and Sperm Transport
Ovum Transport
- After ovulation, the fimbriae of the fallopian tube capture the oocyte.
- Transport through the fallopian tube is facilitated by ciliary beating and muscular contractions.
Sperm Transport in the Female Tract
- Sperm must navigate the cervix, uterus, and fallopian tubes.
- Capacitation: Physiological changes in the female tract that enable sperm to fertilize the egg.
- Only a small fraction of deposited sperm reaches the site of fertilization (ampulla).
Exam-Oriented Focus
Exam Tip: Be prepared to compare and contrast Spermatogenesis and Oogenesis. Focus on the timing (continuous vs. interrupted) and the number of functional gametes produced (four vs. one).
Frequently Asked Questions (FAQs)
- Q: What is the "LH Surge"?
A: It is a dramatic increase in Luteinizing Hormone that triggers ovulation in humans. - Q: Where does fertilization typically occur?
A: In the ampulla of the fallopian tube. - Q: What is capacitation?
A: The final step of sperm maturation that occurs within the female reproductive tract.
Common Mistake: Do not use the terms "Menstrual Cycle" and "Estrous Cycle" interchangeably. Humans/Primates have menstrual cycles; most other mammals (like rats) have estrous cycles.