Ultrastructural study of unencapsulated vertebrate mechanoreceptor terminals facilitated by double staining and resectioning of thick plastic sections.

A set of techniques for localization of unencapsulated sensory receptor terminals in plastic-embedded tissue is described. The tissue is pinned flat in fixative and flat-embedded in a small amount of medium-soft Epon. Pieces of the specimen are remounted so that tangential thick sections (2-3 micron) can be cut. These sections are stained with a mixture of 15 ml of 0.5% toluidine blue, 1% sodium borate; 10 ml of 2% p-phenylenediamine, and 5 ml of acetone. Stained sections are examined with a light microscope and brown-stained sensory receptor regions are located. Sections to be examined in the electron microscope are remounted by inverting a capsule of medium-hard Epon over them, and polymerized blocks are removed from the slide after heating it. Thin sections cut from remounted thick sections are stained with uranyl acetate and lead citrate. This procedure has been used successfully to locate aortic baroreceptors and very small apparent nerve endings in the atrium of the rat as well as stretch receptors in dog trachealis muscle. All of these receptor endings stain brown and are surrounded by light blue collagen and darker blue Schwann cells.