Expression of low molecular mass cytokeratins in oocytes of Schistosoma mansoni.

We studied the distribution of acidic 45 kDa keratin 18 and 40/42 kDa keratin 19 in Schistosoma mansoni, a trematode of medical importance in many tropical regions. The monoclonal antibodies which were produced against the cytoskeleton of mammary carcinoma cell line BT-20 recognized cytokeratins preferentially in parasite oocytes. As has been described in mammalian oocytes, the acidic cytokeratins were present in a nonfibrillar form. The two monoclonal antibodies also recognized testicular cells. No keratin immunoreactivity could be demonstrated by immunofluorescence microscopy at the larval stage, the miracidium. In immunoblotting, the molecular mass as determined by SDS-polyacrylamide gel electrophoresis of schistosome cytokeratins was about 15 kDa higher than that of the corresponding cytokeratins recognized by the monoclonal antibodies in BT-20 cells. The results suggest that acidic low molecular mass cytokeratins in trematodes have a phylogenetically conserved major function in oocytes which is unrelated to the documented cytoskeletal role in differentiated mammalian epithelial cells.