Two components of the pineal organ in the mink (Mustela vison): their structural similarity to submammalian pineal complexes and calcification

Abstract

The pineal complex in the mink (Mustela vison) consists of a larger ventral and a smaller dorsal pineal. Both organs contain pinealocytes, neurons, glial cells, nerve fibers and synapses in an organization characteristic of nervous tissue. The cellular elements are arranged circularly around strait lumina. These lumina correspond to the photoreceptor spaces of submammalian pineals. A 9 + 0-type cilium marks the receptory pole of the pinealocytes which may form an inner-segment-like dendrite terminal in the pineal lumina. The cilia correspond to outer segments which form photoreceptor membrane multiplications in the pineal of submammalians and in certain insectivorous and mustelid mammals (bat, hedgehog, ferret). Axonal processes of the pinealocytes contain synaptic ribbons and terminate on intrapineal neurons of both organs. This pattern represents a neural efferentation of the pineal nervous tissue. The axonal processes of pinealocytes also form neurohormonal endings which pierce the perivascular limiting glial membrane in the ventral as well as in the dorsal pineal. The upper pineal ("epipineal") of the mink may correspond to the parapineal, frontal, or parietal organs of submammalian pineal complexes. Both pineals are encapsulated by the meningeal tissue of the brain stem. Afferent vasomotor axons of the meninges innervate smooth muscle cells of pineal arterioles. There are corpora arenacea in the pineal arachnoid and in the pineal nervous tissue, primarily in the ventral pineal. The localization of calcium ions detected around the membrane of pineal cells by pyroantimonate cytochemistry suggests membrane activity as the source of the calcium ions. The accumulation of calcium by the pinealocytes may be due to their neurosensory character. The mink is the first animal described to have both intrapineal and meningeal concrements like the human pineal.