Fig. 1

The histomorphological features and germ cell developmental patterns of gonads from Amur sturgeons during early sex differentiation. The gonads were at sex-undifferentiated stages (A, B 5 months after hatching (M); C 6 M). The surface of the gonad tissue was covered with a large amount of fat with continuous or discontinuous distribution. Ovarian differentiation was first recognizable by 8 M, which contained a folded or invaginated epithelium (IE) with germ cells (oogonia) underneath (D). The invaginated epithelium deepened and contained a few clusters of oogonia at 12 M (E) and primary growth oocytes at 24 M (F). Testis differentiation features were evident with a smooth epithelium (SE) with germ cells (spermatogonia) by 8 M (G), containing a few clusters of spermatogonia by 12 M (H), and presenting typically obvious seminal lobules at 24 M (I). BV, blood vessels; F, fat; GC, gonocyte; IE, invaginated epithelium; SE, smooth epithelium; OG, oogonia; SG, spermatogonia; O, cluster of oogonia; S, cluster of spermatogonia; PG, primary growth oocyte; SL, seminal lobules; SC, Sertoli cell; LC, Leydig cell; The gonadal tissues were stained with hematoxylin and eosin (HE staining). Scale bar = 50 μm