Early gonadal development and sexual differentiation in muskellunge (Esox masquinongy)
Primordial germ cells (PGCs) were first identified in muskellunge (Esox masquinongy) of 14 mm total length (TL) 3 weeks post fertilization. At 32 mm TL, gonad strings were complete and formed a typical gonad shape in cross section. Blood vessels were first found in the gonads with Crossmon staining at 46 mm TL. Some of the PGCs underwent mitotic division at this stage. The ovarian sac started to develop in a fish of 82 mm TL, while the germ cells were still considered to be undifferentiated. In a fish of 138 mm TL, female gonads could be clearly identified from the ovarian sac and groups of oogonia, whereas in another type of gonad, the morphology of undifferentiated gonads was maintained. Germ cells became numerous in both sexes at 211 mm TL. Female gonads contained lobes with germ cells, including oogonia, early-prophase oocytes, and large oocytes. Spermatogonia and cells undergoing mitosis were observed in the testis. Ovaries in a fish of 250 mm TL were at the early stage of perinucleolus (early diplotene). Our observations indicate that in muskellunge (i) the PGCs remained in a resting state for up to 8 weeks post fertilization, (ii) gametogenesis occurred earlier in females than in males, (iii) the gonads developed from an undifferentiated stage directly into an ovary or testis, and (iv) the somatic elements in the gonads differentiated prior to the germ cells.