Gamete Development

SEQUENCE:

Primordial germ cell < spermatogonia (Mitosis) < primary spermatocyte (Growth phase) < secondary spermatocyte (Meiosis 1) < spermatid (Meiosis 2) < Mature Sperm (differentiation)

Enters testis as primordial germ cell during embryonic development
From birth to puberty remain as spermatogonia
Spermatogonia proliferate by mitosis
Number of spermatids is maximum number of sperm
1 germ cell results in the production of 4 mature sperm

Germinal Epithelium – epithelial layer filled with primordial germ cells, surrounds Sertoli Cells

Seminiferous Tubules – Spermatogonia move toward lumen of the tubule, travel through tubule until they become primary sperm cells (spermatozoa)

Efferent Ducts – spermatazoa transported to epididymis (lacking functioning tails to give them mobility)

Epidydimis – final maturation/differentiation of sperm

Sperm:

To the right is a cross section of a seminiferous tubule. The Sertoli cells (part of the 2 cell 2 gonadotropin theory) provide growth factors needed for sperm cell growth and maturation. As the spermatogonia mature, they move toward the center of the tubules.

Oocytes:

Sperm make up about 10% of the ejaculate and can survive for 20 - 60 days in the epidydimis, depending on the species. Temperature of the testes should be 2 - 10 degrees Celsius lower than normal body temperature, otherwise sperm will not develop properly.

Germ cell < Oogonia < Primary oocyte < Secondary oocyte + polar body < 1 ripe egg + 3 polar bodies

Primordial germ cells that differentiate to oogonia begin outside of the gonads and migrate into the developing gonads from the embryonic yolk sac
The number of oocytes a woman is born with is her maximum egg-producing capacity
1 germ cell results in 1 ripe egg + 3 polar bodies
Oogenesis occurs in outermost layer of ovary, the cortex

One germ cell in the female has the potential to produce just one mature egg in contrast to four sperm in the male. The female is born with hundreds of thousands of oocytes, which develop in clusters or waves. Animals such as pigs will ovulate around 16 eggs at once, while humans will release just one or two eggs per ovulation.

Theca and granulosa cells are comparable to Leydig and sertoli cells in the male (see 2 cell 2 gonadotropin theory). They nurture egg maturation and make up the majority of ovarian tissue.

Histology of an ovarian follicle (viewed with a light microscope at 100 to 1000x magnification) shows the developing oocyte supported by a bed of Granulosa cells within the follicle. Once released from the ovary, the egg will survive only 12 - 48 hours in the female reproductive tract.

The hypothalamus is the center of control for hormonal regulation of reproductive development. It controls the timing of development of secondary sex characteristics, hormonal control of the gonads, and responds to signals from the gonads to elucidate a response for gamete production. For more details on hormones, click here.