The cellular origins of corticotropin and melanotropin as revealed by immunochemical staining

The cellular origins of corticotropin and melanotropin were studied by observing the response of the rat hypophysis to adrenalectomy and cortisol administration when stained immunochemically with peroxidase‐labeled antibody following application of antiserums to β 1–24 ‐corticotropin, β P 17–39 ‐corticotropin, human β‐melanotropin, and α‐melanotropin. The presence of corticotropin in the pars intermedia is indicated since this lobe, in addition to corticotropic cells of the pars distalis, stained with antiserum to β P 17–39 ‐corticotropin. However, the concentration of corticotropin is less in the pars intermedia than in corticotropic cells since at higher dilutions of antiserum the latter cells stained intensely while the pars intermedia was unstained. Also, corticotropic cells exhibited striking hypertrophy after adrenalectomy and regression after cortisol administration, while morphological and tinctorial properties of the pars intermedia were minimally altered. These observations suggest that the pars distalis is primarily responsible for supplying the corticotropin required to meet the changing requirements of the body. With antiserum to human β‐melanotropin, the pars intermedia stained intensely. Since the antiserum was ineffective after absorption the staining reaction appears to be specific for β‐melanotropin. Except for a rare cell, the pars distalis did not stain. Thus β‐melanotropin is restricted primarily to the pars intermedia.