Structure and ultrastructure of the pigmented cells in the adult dog pineal gland.

The light and electron microscopic features of pigmented cells in the adult dog pineal gland have been described. The presence of pigmented cells was a constant characteristic of the dog pineal gland, though wide variations in the amount of pigment could be found among different animals. Conversely, the localisation of pigmented cells was very constant on the basal surface of the proximal region of the pineal gland. Frequently, clusters of pigmented cells were seen in the posterior commissure and the neighbour meningeal spaces, near the pigmented pineal zone. The pineal pigment has been identified as melanin according to its morphological features and histochemical properties. Several types of granules were identified ultrastructurally, apparently corresponding to different stages of a maturation process. The pigmented cells were identified as a special type of pinealocyte according to their ultrastructural features.