[Steroids and their receptors at the ultrastructural level. Problems posed by immunocytochemistry]

At the ultrastructural level, steroids and their receptors can be detected into their target cells by immunocytochemistry done on sections of anterior pituitary obtained by cryoultramicrotomy. An antigenic determinant is thus shown on fixed and frozen tissue with a monoclonal or polyclonal antibody. Tissue preparation is a compromise between preservation of ultrastructural features and the availability of the antigen. Specificity requires several controls knowledge of crossreaction of the anti-steroid or anti-receptor serum, immunological and biochemical analysis of the antigen, a check of the immunocytochemical reaction itself, specific absorbtion tests, modification to the immunocytochemical reaction in various experimental conditions. Specificity is also modulated by the dilution of the anti-steroid or anti-receptor serum as well as other experimental and physiological variations. Finally a hormone detected in its target cell may not be bound to its specific binding sites. Specific binding is characterized by: tissue and cell specificity, limited number or sites, hormonal specificity and a high affinity. Only thus is it possible to localise by immunocytochemistry the receptor or the hormone bound to its binding site.