The mammalian pineal organ: electron microscopic studies on the fine structure of pinealocytes, glial cells and on the perivascular compartment.

Abstract

The architecture of the pineal organ (epiphysis cerebri) of cats and monkeys (Macaca mulatta and Saimiri spec.) is basically similar to that of the central nervous tissue. The parenchyma consists of two cell types: the pinealocyte, which seems to be a specialized kind of neuronal cell rather than a secretory cell. The second type is composed of glial cells, primarily filamentous astrocytes, which separate the pinealocytes from the perivascular and subarachnoid space. Pinealocytes and astrocytes form cell clusters around extensions of the extracellular space and some pinealocytes show a certain polarity between such cavities and the perivascular compartment. In the cat and Rhesus monkey this polarity is not so obvious. Pinealocytes of the cat probably do not have more than one or two cell processes, while in the monkey pineal organ a multipolar type of pinealocyte occurs. Pinealocyte processes often end with terminals inside the parenchyma, having contact with other pinealocytes or their processes. Sometimes they terminate close to the perivascular space (especially in the squirrel monkey), or they seem to enter the perivascular space. Since most such processes are still covered by thin glial cell processes, they are actually not constituents of the space. However, there are occasional processes which do penetrate this glial barrier, and hence become part of the perivascular space. These have been found in all regions of cat and monkey pineal organ. In the Rhesus and squirrel monkey, many pineal cell processes terminate with endings containing dense bodies which may be a neurohumoral product. Endings of pinealocyte processes within the parenchyma contain vesicles, “synaptic ribbons” and some mitochondria. Sympathetic nerve fibers course through the perivascular space to be distributed throughout the pineal organ. However, they contact pinealocytes and pinealocyte processes outside the perivascular space after entering the parenchyma through an interruption of the glial barrier. Nerve fibers and endings contain numerous empty and granulated vesicles and, in addition, a few larger granulated vesicles.