Histochemical localization of progestin receptor cells in the rat thymus

Using progesterone (Po)-horseradish peroxidase (HRP) conjugate (PPC), the histochemical localization of progestin (Pi) receptor in the female rat thymus was investigated. The PPC staining was predominantly detected in the large polygonal cells with irregular cell borders that contain large clear, round or oval nuclei, 'epithelial cells', but rarely in the small round cells with dark round or oval nuclei, 'thymic lymphocytes'. The PPC-positive cells were distributed everywhere in the tissue, but found more frequently at the corticomedullary junctional area than at the other tissue areas. The stained cells tended to gather to make a cell cluster. The PPC staining within the epithelial cells was predominantly localized in the cytoplasm, but only faintly in the nucleus. The steroid specificity of Pi binding was studied by simultaneous incubation of the tissue sections with PPC and a series of unconjugated steroids such as progesterone-3-O-carboxymethyl oxime (PCMO), progesterone (Po), testosterone (T), estradiol-17 beta (E2 beta) and corticosterone (CC). The incubation studies indicated that two progestins, PCMO and Po, bind to Pi receptor in the thymus with a reasonable degree of specificity, but other nonprogestational steroids do not. The method was reproducible and showed satisfactory technical stability. The present results suggest that the effect of Pi on the thymus may be mediated by Pi receptor present in the epithelial cells.