Histological and ultrastructural investigation of early gonad development and sex differentiation in Adriatic sturgeon (Acipenser naccarii, Acipenseriformes, Chondrostei).

Gonad development and sex differentiation from embryos to 594-day-old individuals were investigated in farmed Acipenser naccarii using light and transmission electron microscopy. The migrating primordial germ cells first appear along the dorsal wall of the body cavity in embryos 1.5 days before hatching. The gonadal ridge, containing a few primary primordial germ cells (PGC-1) surrounded by enveloping cells, appears in 16-day-old larvae. At 60 days, the undifferentiated gonad is lamellar and PGC-1 multiply, producing PGC-2. In 105-day-old juveniles, a distinct germinal area with advanced PGC-2 appears on the lateral side near the mesogonium and the first blood vessels are visible. At 180 days, putative ovaries with a notched gonadal epithelium and putative testes with a smooth one appear, together with adipose tissue on the distal side. In 210-day-old juveniles, active proliferation of germ cells begins in the putative ovaries, whereas putative testes still contain only a few germ cells. The onset of meiosis and reorganization of stromal tissue occurs in ovaries of 292-day-old individuals. Ovaries with developed lamellae enclosing early oocyte clusters and follicles with perinucleolar oocytes occur at 594 days. Meiotic stages are never found, even in anastomozing tubular testes of 594-day-old individuals. Steroid producing cells are detected in the undifferentiated gonad and in the differentiated ones of both sexes. Anatomical differentiation of the gonad precedes cytological differentiation and female differentiation largely precedes that of the male. Gonad development and differentiation are also associated with structural changes of connective tissue, viz. collagen-rich areas are massive in developing testes and reduced in ovaries. (c) 2008 Wiley-Liss, Inc.