Pineal Cell Biology and Innervation

Abstract

In comparative terms, pinealocytes, supporting (glial) cells, and neurons form the pineal parenchyma which is separated from the adjacent connective tissue layer (capsule) and the capillaries by a basal lamina. Pinealocytes display striking variation among different classes of vertebrates. Based on structural and ultrastructural criteria they have been divided into three main categories: true pineal photoreceptors, modified pineal photoreceptors, and pinealocytes sensu stricto (Oksche 1965, 1971; Collin 1971; Collin and Oksche 1981; Korf and Oksche 1986; Korf 1994; Figs. 2, 8). Since the pioneering work of Dodt and Heerd (1962) the direct light sensitivity of true pineal photoreceptors has been firmly established. The fact that modified photoreceptor cells are also capable of perceiving light stimuli has been known since Deguchi’s discovery in 1981 that, in the chicken, melatonin production is regulated by light perceived in the pineal organ. The direct light sensitivity has been lost in the mammalian pinealocyte. Nevertheless, all pinealocyte types appear to be closely related and can be classified as cells of the receptor line. This intimate relationship has been substantiated by immunocytochemical and biochemical studies showing that neuroendocrine pinealocytes of mammals express “photoreceptor-specific” molecules which otherwise are synthesized only by functional (retinal and pineal) photoreceptor cells (Korf et al. 1985a,b, 1986a,b, 1992; Huang et al. 1992; Kramm et al. 1993; Figs. 9, 10). As holds true for primary sensory cells, all types of pinealocytes belong to the neuronal cell lineage because they express a variety of markers typical of neurons and neuroendocrine cells (neurofilament, synaptobrevin). Pinealocytes can thus be considered as specialized neurons.