Cytological localization of alpha-MSH, ACTH and beta-endorphin in the pars intermedia of the cichlid teleost Sarotherodon mossambicus.

The pars intermedia of S. mossambicus contains two different endocrine-cell types. The predominant cell type is lead-haematoxyline-positive and assumed to synthesize MSH and related peptides. The second cell type is PAS positive and its function and product(s) are unknown. Staining of light-microscopic and ultrathin sections with antisera against alpha-MSH, ACTH 1--24 and human beta-endorphin revealed that only the lead-haematoxyline-positive cells of the pars intermedia react with these antisera, and that the secretory granules of these cells contain compounds that were immunoreactive to all three antisera. These findings are in line with the hypothesis that alpha-MSH, ACTH and endorphins are derived from the same precursor molecule. No specific reaction with one of the antisera could be detected in the PAS positive cells.