Cytokeratin （CK） 8 mRNA expression in developing and degenerating rat prostate was studied using in situ hybridization with an antisense RNA-probe. It was found that : 1) the CK 8 antisense probe was accumulated only within prostatic epithelial cells ; 2) after castration

CK 8 mRNA signals in ventral prostute（VP） sections were significantly increased

and elevated CK 8 mRNA expression persisted even long after prostate involution was complete; and 3) during prostate development

the strongest CK 8 mRNA staining was found in the early neonatal prostatic epithelia which were composed mainly of prostatic stem cells. Thereafter

a shift of CK 8 mRNA staining to peripheral regions and decreased overall CK 8 mRNA levels were noted. These data indicate that excessive expression of CK 8 mRNA is a characteristic of prostatic stem cells

and CK molecules are excellent markers for determing the hierarchical pathway of cell differentiation in prostate epithelium.