The microstructural and ultrastructural examination of zebrafish (Danio rerio) spermatozoa was performed to detect the structure, geometrical feature and morphological parameters. Milt samples from 30 zebrafish were examined by use of phase contrast microscopy (PCM) and transmission electron microscopy (TEM). The zebrafish spermatozoa measured 32.79±1.97 μm in total length and had a non-acrosomal spherical head, a short cone-shaped midpiece and a flagellum. Heads were lopsided and almost occupied by nucleus with homogeneous dense chromatin, except for a few small electro-lucent vacuoles. The shallow nuclear fossa was mid-laterally located on the nuclear surface and contained the proximal centriole, which was tilted to one side of nucleus and placed at a 125^o angle against distal centriole. In the midpiece, the cytoplasmic sheath showed asymmetric organization with round or bended mitochondria gathered at one side, while at the thinnest place, was only little cytoplasm. The flagellum, 29.32±1.98 μm in length and 242.56±19.27 nm in width, was eccentrically positioned in relation to the nucleus. It had a “9×2 + 2” pattern axoneme, however, there was lack of central microtubules at transition between flagellum and distal centriole. In addition, a few vesicles appeared between the axoneme and the cytoplasmic membrane. Based on the observations, the schematic organization of zebrafish spermatozoon was drawn. The zebrafish spermatozoa possessed some structural features that were specific to cyprinid family, however, they exhibited interspecific differences.

Key words: Cyprinidae, Danio rerio, Morphology, Sperm, Transmission electron microscope