Sertoli cells in the freshwater pearl mussel Margaritifera laevis (Bivalvia: Margaritiferidae): A histological and ultrastructural study.

•Sertoli cells include a large number of glycogen granules, electron-dense globules, lipid droplets, and sperm morulae.•Electron-dense globules and lipid droplets are independent structures, and no relation between them could be demonstrated.•Sperm morulae are derived from spermatogonia engulfed by the Sertoli cells.•Spermatogenesis occurs via two pathways, i.e., the typical spermatogenesis and that accompanied by the formation of sperm morulae.•Sperm spheres are formed by the gathering of spermatozoa around large lipid droplets.