Age of the zebrafish is appr. 6w and of the fathead minnow appr. 9w; both images have equal magnification. Fathead minnow sections were kindly provided by Dr. Grace Panter. H&E staining difference of testis development between fathead minnow (Pimephales promelas) and zebrafish

The differentiating testis of zebrafish (upper image) is characterized by clustering of spermatogenic cells and progression of these clustered cells to meiosis. In the developing testis of fathead minnow, on the contrary, there are only occasional indications of progression to meiosis. This testis is predominated by proliferating spermatogonia, which are organized as early tubules; such a tubular organization appears only at a later stage of development of the zebrafish testis. Preliminary lumen are visible in both species.

Age of the zebrafish is appr. 6w and of the fathead minnow appr. 9w; both images have equal magnification. Fathead minnow sections were kindly provided by Dr. Grace Panter. H&E staining detail of early fathead minnow testis

This detail image shows a clearly circumscribed tubule with its central lumen. The image is predominated by spermatogonia, which are grouped in small clusters. These cells have a high mitotic activity, illustrated by dividing cells in metaphase and anaphase, respectively. Sertoli cells can also be distinguished by their (mostly) triangular shape and position at the basement membrane.