Spermatozoon ultrastructure of Platichthys flesus.

The ultrastructure of the spermatozoon of Platichthys flesus (flounder) has been determined. The cell is of the primitive type and lacks an acrosome. The bullet-shaped, homogeneously electron-dense nucleus has a deep, caudal articular fossa containing proximal and distal centrioles. Both centrioles are embedded in pericentriolar material which is intimately associated with the nuclear envelope. Around the proximal centriole the material is organised into a dense ring which bears nine fibres associated with the triplets, and a large fibrous body, all connected to the nuclear envelope. Connections exist also between the pericentriolar material of the proximal and distal centrioles. The material around the distal centriole is organised into a complex collar. The anterior surfaces of eight spherical mitochondria lie within shallow depressions of the nuclear caudal surface. They form a ring surrounding the posterior opening of the articular fossa and the proximal region of the 9 + 2 flagellar axoneme.