Finfish Breeding and Hatchery Management (2+1)

Histology of fish testis

The testis of fish is sac-like and folded.
Like the oocyte, male gametes (sperm cells) develop and mature through a series of stages.
Various types of germ cells can be seen within a testis during its maturity cycle and they are: male germ cells (resting or primary spermatogonium), secondary spermatogonium, spermatocytes, spermatids and spermatozoa.

Resting or primary spermatogonia or male sperm cells : these are the largest of germinal cells, lodged within the lobules, closed to the lobular wall. These cells have clear cytoplasm and centrally placed nucleus.

Secondary spermatogonia : the primary spermatogonia divide mitotically to produce secondary spermatogonia. These resemble primary spermatogonia, but possess chromatin threads (in the nucleus) which radiate from nucleolus and are smaller in size.

                          

Figure 5: Photomicrograph (10x) of immature testis showing         Figure 6 :  Photomicrograph (20x) of maturing testis showing
different stages of spermatogenesis. Sl: Seminiferous lobule,       different stages of spermatogenesis. Sc: Spermatocytes,
Sg: Spermatogonia                                                                              Sm: Spermatids, Sg: Spermatogonia, Sp: Spermatozoa

Figure 7 : Photomicrograph (10x) of  spent testis showing
Sl: Seminiferous lobule (empty), Rsp: Residual spermatozoa,
Sg: Spermatogonia

Spermatocytes : these are of two types viz., primary and secondary spermatocytes and are centripetally (tend to move towards centre) arranged in the lobules (usually in cysts) secondary spermatocytes are smaller but their cysts are comparatively larger. They possess cytoplasm and central chromatin materials. Distinct nuclear membrane is often lacking.

Spermatids : these are round, darkely stained cells usually filling the entire lobule. Each spermatid comprises of a prominent nucleus with in a small layer of cytoplasm.

Spermatozoa : these are derived from spermatids and each spermatozoans has an anterior darkely stained nucleus and a small posterior part drawn into a tail (flagellum).