Electron microscopic observations on neuron-like cells in the ground squirrel pineal gland

Ultrastructural details of neuron-like cells as well as synaptic nerve endings in the pineal gland of the ground squirrel are described. The neuron-like cells are situated mainly in the distal portion of the gland. Since the neuron-like cells differ considerably from the pinealocytes and exhibit cytological features characteristic of nerve cells, they are presumably true neurons. The cell bodies or processes of the neuron-like cells receive many synapses. The nerve endings synapsing on these cells contain numerous small non-granulated and some large granulated vesicles but no small granulated vesicles. These synaptic nerve endings are rather abundant within the parenchyma but they are also found in perivascular spaces ensheathed completely or partially by the Schwann cell cytoplasm. Similar nerve endings occasionally make typical synaptic contacts with the cell bodies or processes of the pinealocytes. Since the adrenergic nerve endings do not form synapses on the pinealocytes in the ground squirrel, it is clear that nerve endings other than adrenergic ones also distribute within the pineal gland of this animal. These synaptic nerve endings may contribute, at least to some extent, to the innervation of the ground squirrel pineal gland, whether they are derived from the nerve fibers penetrating the pineal gland or from the neuron-like cells.