A light and electron microscopic study of the pineal gland of the ground squirrel, Citellus tridecemlineatus.

The pineal gland of the 13-lined ground squirrel (Citellus tridecemlineatus) has been examined at the light and electron microscopic level. This gland is composed of low-density parenchymal cells interspersed among which are occasional glial, vascular and neural elements. Punctuating the glandular parenchymal mass are prominent perivascular and intercellular spaces. The parenchymal cells possess numerous mitochondria and less prominent profiles of rough and smooth-surfaced endoplasmic reticulum. Golgi apparatus, microtubules and lipid droplets of varying size and electron density constitute regular cytoplasmic features, with dense-core vesicles being present occasionally. The parenchymal cells have numberous processes. One among these in each cell extends for several micra to terminate in a bulbous expansion containing both clear and dense-core vesicles and occasional electron-dense inclusions. These bulbous terminals are found within the perivascular and intercellular spaces where they course in close proximity to both other parenchymal elements and axon terminals. Glial cells and their processes invest the pineal periphery and incompletely separate the parenchymal cells.