Filipin-sterol complexes in the plasma membrane of zebrafish spermatozoa

The presence and distribution of filipin-sterol complexes in the plasma membrane of zebrafish (Brachydanio rerio) sperm was investigated. The zebrafish sperm plasma membrane, treated with freeze-fracture techniques, is seen to contain a multitude of intramembranous particles that, in a specific region of the posterior part of the sperm head, are organized into unusual particle arrays that appear as simple hexagons or parallelograms. The polyene antibiotic filipin forms complexes with 3-beta-OH sterols to produce characteristic protrusions and pits in membranes that are readily observable in freeze-fracture replicas. Numerous filipin-sterol complexes were found to populate the sperm plasma membrane, and the complexes exhibited variability in their distribution in different sperm. This appears to be the first illustrated example of an acrosomeless sperm that exhibits a high concentration of filipin-sterol complexes. In contrast, the unique grating formed by the intramembranous particles as well as variable amounts of membrane surrounding the unusual particle arrays were always free of the filipin-sterol complexes. Thus, while cholesterol appears to be present in the plasma membrane of the zebrafish sperm, it is not apparent in the highly differentiated region of the membrane based on the observed distribution of the filipin-sterol complexes.