Spermatogenesis
Spermatogenesis is the process of sperm formation in the seminiferous tubules of the testes. It is continuous from puberty and maintained throughout the male’s reproductive life. It is divided into three phases:
- Proliferative phase: spermatogonia divide by mitosis to maintain the stem cell population and generate primary spermatocytes.
- Meiotic phase: primary spermatocytes undergo two meiotic divisions. The first reduces the chromosome number by half, forming secondary spermatocytes; the second generates haploid spermatids.
- Spermiogenesis: spermatids differentiate into mature spermatozoa. The acrosome (enzymes to penetrate the oocyte), the flagellum (motility), and condensed genetic material in the head are formed.
Sertoli cells provide physical and nutritional support, as well as forming the blood-testis barrier. Leydig cells produce testosterone, essential for maintaining spermatogenesis and sexual behavior.
Oogenesis
Oogenesis is the process of oocyte formation in the ovaries. Unlike spermatogenesis, it is not continuous but limited by the follicular reserve established during fetal life. It is characterized by meiotic arrests that synchronize the reproductive cycle.
- Proliferative phase: oogonia multiply by mitosis during fetal life and differentiate into primary oocytes.
- Meiosis I: primary oocytes initiate meiosis but remain arrested in prophase I until puberty.
- Meiotic resumption: under LH stimulus, oocytes complete meiosis I and form secondary oocytes and the first polar body.
- Meiosis II: begins but is arrested in metaphase II. It is only completed if fertilization occurs.
The process is accompanied by follicular development, where granulosa and theca cells produce estrogens and progesterone. These hormones regulate oocyte maturation and prepare the female reproductive tract for fertilization and gestation.
Hormonal Regulation
Gametogenesis in both sexes is regulated by the hypothalamic-pituitary-gonadal axis, which coordinates hormone production and synchronizes reproductive processes.
- GnRH: secreted by the hypothalamus, stimulates the anterior pituitary to release FSH and LH.
- FSH:
- In males: stimulates Sertoli cells, promoting spermatogenesis and the production of proteins that nourish spermatozoa.
- In females: promotes the growth and maturation of ovarian follicles.
- LH:
- In males: stimulates Leydig cells to produce testosterone.
- In females: triggers ovulation and the formation of the corpus luteum, which produces progesterone.
- Testosterone: regulates spermatogenesis, the development of secondary sexual characteristics, and libido.
- Estrogens: promote follicular development, endometrial proliferation, and female sexual behavior.
- Progesterone: maintains the endometrium in optimal conditions for implantation and regulates the luteal phase.
- Inhibin: produced by Sertoli and granulosa cells, inhibits FSH secretion to prevent overstimulation.
The interaction of these hormones ensures that gametes are produced in adequate quantity and quality, and that the reproductive tract is prepared for fertilization and gestation.
Differences between Species
| Species | Spermatozoon shape | Estrous cycle | Particularities |
|---|---|---|---|
| Dog | Oval head | 6–7 months | Penile bone (os penis), fractionated ejaculation |
| Cat | Prominent acrosome | Seasonal, polyestrous | Penis with spicules that induce ovulation |
| Bovine | Elongated head | 21 days | Fibroelastic penis with spiral deviation |
| Pig | Rhomboid head | 21 days | Spiral-shaped penis, large semen volume |
| Horse | Rounded head | 21 days | Musculocavernous penis, explosive ejaculation |
| Hen/Rooster | Filiform spermatozoon | Daily ovulation | Internal testes, cloacal fertilization |
Conclusion
Gametogenesis integrates cellular processes, anatomical bases, and hormonal regulation that guarantee the formation of viable gametes. Spermatogenesis is continuous and abundant, while oogenesis is cyclical and limited. Differences between species reflect specific reproductive adaptations, essential for understanding fertility and applying assisted reproduction techniques in veterinary medicine.
References
Hafez, E.S.E. Reproduction in Farm Animals. Wiley-Blackwell.
Senger, P.L. Pathways to Pregnancy and Parturition. Current Conceptions