Figure 1
Histological features of the ten categories of P. margaritifera gonad. At the early stage, gonadic tubules, surrounded by abundant connective tissue, are less bulky and contain mostly spermatogonia and spermatocytes, and oogonia and oocytes in early development (â–º), in males and females, respectively. At the intermediate stage, tubules are large and the connective tissue is less abundant. In males, spermatogonia and spermatocytes form a ring at the periphery of tubules, while sperm occupy the central part. In females, oocytes in early development are still numerous (â–º) and those at the end of vitellogenesis begin to accumulate. At the mature stage, tubules are turgid, spermatozoids fill the entire lumen and spermatogonia and spermatocytes are reduced in number in males. In females, tubules contain only mature oocytes or those close to maturity. Regression stage is characterized by the retraction of tubules, which will then contain some residual spermatozoids in males, and some degenerating oocytes in females. Inversion category characterizes gonads in sexual inversion and thus presents both types of germ cells, male and female. And undetermined gonads are those which have no gametes and cannot therefore be associated with a sex. Ov: oocytes; Spc: spermatocytes; Spg: spermatogonia; Spz: Spermatozoid, and T: Connective tissue.