Structural and ultrastructural characterization of zebu (Bos indicus) spermatozoa

The ultrastructure of normal, ejaculated spermatozoa of Bos indicus was studied by means of electron microscopy, being evaluated in two principal parts, the head and the tail. The head is flat, oval or paddle-shaped with a square base, which provides a concave recess for the insertion of the tail. The acrosome tightly covers the anterior two thirds of the nucleus. A distinct unilateral acrosomal bulge was observed along the apical edge of the head. The equatorial region demarcates the acrosome from the post-equatorial region that covers the caudal one third of the nucleus. The classical 9+9+2 fiber pattern which composes the axoneme was observed along three segments of the tail, namely middle, principal and terminal pieces. The axoneme is anteriorly bound by the mitochondrial helix (middle piece) and posteriorly by the fibrous helix (principal piece), except at the terminal piece. The border between the middle piece and principal piece was well defined due to the termination of the thick mitochondrial helix and the presence of the annulus. Some of the spermatozoa presented cytoplasmatic droplets, which appeared as stalk-like appendages.