Ultrastructural demonstration of exocytosis in the pineal gland

Granular vesicles are present in pinealocytes and in rudimentary photoreceptor cells of many vertebrates, sometimes in large amounts. Their dense cores have been shown to store proteinaceous compounds, but the way they are released remains speculative. The aim of this study was to demonstrate whether or not exocytosis is the mechanism by which secretory products stored within granular vesicles are released. Therefore, a method has been used allowing a clear ultrastructural study of secretory products by exocytosis, even in tissues in which this process of secretion is quite rare and/or very slow. Exocytotic figures have been clearly demonstrated in the three species studied: golden hamster, snake, and parakeet. Nevertheless, they were never commonly observed as it was the case in neurohypophysis, even in such animals as the parakeet and snake, in which granular vesicles are very numerous. The possible reasons of this observation are discussed.