Transmission electron micrograph of the thiamine pyrophosphatase reaction of the Golgi apparatus in rat epididymis. In this staining method, osmium builds up in the innermost (concave) side of the Golgi cisternae. This is the opposite result of staining with collidine-buffered osmium, where osmium builds up on the outermost (convex) side of the Golgi cisternae.