Lesson 3 of 9 · 7 min
Gametogenesis: spermatogenesis
NCERT §2.3
One of Dev's primary spermatocytes will, after two rounds of division, become four separate sperm cells — all four viable, none discarded. Oogenesis, as the next section shows, works very differently.
The lesson in notes
In short
Gametogenesis is the making of sperms in the testis and ova in the ovary. In males it begins at puberty.
Spermatogonia on the inner wall of the seminiferous tubules multiply by mitosis. Each is diploid with 46 chromosomes. Some of them become primary spermatocytes, which enter meiosis.
Meiosis I of one primary spermatocyte gives two equal, haploid secondary spermatocytes with 23 chromosomes each; meiosis II of these gives four equal, haploid spermatids.
Spermiogenesis is the change of a spermatid into a sperm (spermatozoon); no division occurs. Spermiation is the release of sperms from the seminiferous tubules after spermiogenesis.
Hormonal control: at puberty the hypothalamus markedly increases its release of GnRH, which acts on the anterior pituitary to release LH and FSH.
LH acts on Leydig cells and makes them produce androgens, which in turn stimulate spermatogenesis. FSH acts on Sertoli cells and makes them release factors that help spermiogenesis.
A sperm has a head, neck, middle piece and tail, all wrapped in a plasma membrane. The head has an elongated haploid nucleus capped in front by the acrosome, which carries enzymes needed for fertilisation.
The middle piece is packed with mitochondria that supply energy for the tail's movement, which gives the sperm the motility it needs to reach the egg.
About 200–300 million sperms are released in one ejaculation. Normal fertility requires that at least 60 per cent of them be of normal shape and size, and that at least 40 per cent move vigorously.
Sperms from the seminiferous tubules travel through the accessory ducts, and the secretions of the epididymis, vas deferens, seminal vesicles and prostate are needed for their maturation and motility. Sperms plus seminal plasma make up semen. The ducts and glands depend on androgens to work.