POSTEMBEDDING IMMUNOCYTOCHEMISTRY FOR ADHESIVE PROTEINS AND CLATHRIN IN LR WHITE-EMBEDDED AND LR GOLD-EMBEDDED HUMAN PLATELETS

Two hydrophilic acrylic resins, LR White and LR Gold, were tested for their suitability for post-embedding immunocytochemistry on human plat elets. Both resins gave satisfying results with regard to ultrastructu ral preservation and retention of antigenicity for von Willebrand fact or, fibrinogen, fibronectin and clathrin. Embedding in LR White and po lymerization at 50-degrees-C without accelerator proved to be the best method because it provided clear negative membrane contrast, good imm unocytochemical localization of antigens and the entire procedure requ ired a comparatively short time. Label for von Willebrand factor was f ound in typically eccentric location in alpha-granules, but also in th e form of small caps directly outside the granule membranes, beneath t he outer plasma membrane and in the cytoplasm. With the exception of t he eccentric location within granules, labelling for fibrinogen and fi bronectin revealed very similar results. Label for clathrin appeared i n the form of small gold clusters attached to alpha-granules, in the c ytoplasm and beneath membranes of the open canalicular system. Togethe r with the appearance of coated vesicles attached to granules and to t he membranes of the open canalicular system in araldite-embedded mater ial, these results further strengthen the concept that adhesive protei ns are taken up from the plasma and stored in granules. Moreover, the persistence of the clathrin coat on vesicles attached to storage granu les may also indicate that receptors for adhesive proteins are recycle d.