SPERMATOGENESIS

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Figure 10-5, see below)

It takes place in the seminiferous tubule and consists of the sum of all cellular transformation in developing germ cells

A. Spermatogenesis consists of three distinct phases

Spermatocyctogenesis, meiosis, and spermiogenesis

1) Spermatocyctogenesis (Proliferation)

- It takes place in the basal compartment of the somniferous tubule

- Mitotic cell division and proliferation and maintenance of spermatogonia

- Spermatogonia undergo several mitotic divisions with the last division resulting in primary spermatocytes

- Three types spermatogonia found in the basal compartment are spermatogonia A, spermatogonia intermediate, spermatogonia B

- Duration of spermatocytogenesis varies in different species:

bull ~21 days, ram ~18 days, stallion ~21 days

2) Meiosis

- It takes place in the adluminal compartment of the seminiferous tubule

- Reduction of the number of chromosomes in the gamete in half (from diploid to the haploid state)

- Primary spermatocytes undergo meiosis I and become secondary spermatocytes and subsequently undergo meiosis II resulting in round spermatid

- The lifespan of spermatocytes is the longest of all sperm cell types

- Secondary spermatocytes is short-lived (1-2 days)

3) Spermiogenesis (Differentiation; Figures 10- 6 and to 10- 7)

- It takes place in the adluminal compartment of the seminiferous tubule

- Round spermatids mature and become elongated spermatids

- DNA becomes highly condensed, the acrosome is formed, flagellum (tail) is formed, and cells become potentially motile

- Elongated spermatids move closer to the lumen of the seminiferous tubule

4) Four phases of spermiogenesis (Figure 10-7)

I) Golgi phase: acrosomic vesicle formation

II) Cap phase: acrosomic vesicle spreads over the nucleus of the round spermatid and the flagellum starts to form

III) Acrosomal phase:  the spermatid nucleus and cytoplasm elongates, acrosome covers the majority of the anterior nucleus

IV) Maturation phase: Mitochondria are assembled around the flagellum and the flagellum is completely formed