Seasonal morpho-histological changes of testes

a) Resting phase or early immature phase:

• The testes are thin, slender, translucent, and pale in color.
• Histologically, seminiferous tubules are small in size and full of spermatogonia.

b) late immature phase:

• The morphological appearance is similar to the previous phase, except that there is a slight increase in the weight and volume of the testes.
• Histologically, slow mitotic activity is seen, and the spermatogonia starts dividing.

c) Maturing phase:

• There is an increase in the weight and volume of the testes, which look more vascular and opaque.
• Histologically, intense spermatogenesis is seen during the later part of this phase.
• Spermatogonia decreases in number and numerous primary and secondary spermatocytes are visible.

d) Mature phase:

• During this period, the tests show a marked increase in weight and volume.
• They are turgid and pink in color.
• Milt oozes out on pressing the abdomen.
• Histologically, the seminiferous tubules are larger and full of sperm.
• Spermatogonia are few, and all stages of spermatogenesis can be seen in various lobules.

e) Spent phase:

• The testes become flaccid due to excessive discharge of sperm.
• The weight and volume are considerably reduced and the testes again become thin, slender, and translucent.
• After a brief period of rest, the testes start the cycle again.
• The spermatogonia are the only germ cells during the resting phase, but are present throughout the year, although their number is reduced during the spawning period.
• These are known as ‘resting germ cells’ and are believed to give rise to the next generation of sex cells.