Electron microscopic study of pineal innervation in lacertilians

Abstract

Summary The innervation of the pineal organ of Lacerta viridis, Lacerta muralis and Tarentola mauritanica was investigated by electron microscopy. Non-myelinated and myelinated nerve fibers, both running in pericapillary spaces, were observed. The non-myelinated fibers course mostly in bundles lined by a basement lamina. The fibers are embedded in the cytoplasm of lemmocytes. Their terminals show three types of vesicles: (1) agranular vesicles varying in diameter between 300 and 700A, (2) granular vesicles of the same size, and (3) granular vesicles measuring 800–1670Ain diameter. Although the numerical distribution of these vesicles is variable, the number of the large type granular vesicles is constantly less than the numbers of agranular and small granular vesicles. Synaptic connections between nerve endings and basal processes of pineal epithelial cells were not observed. The morphological characteristics of the non-myelinated nerve fibers and their terminals are strikingly similar to those of the orthosympathetic postganglionic axons innervating the mammalian pineal organ. The myelinated fibers which do not contain granular vesicles run either singly or in small groups. Their number is much smaller than that of the non-myelinated autonomic fibers. With every single myelinated fiber many lemmocytes are associated which did not produce the myelin sheath. These fibers are considered to be sensory pineal fibers, earlier described, which originate from the rare intraepitheial nerve cell bodies and constitute the sensory pineal nerve. In comparison with the pineal photoreceptor cells in most Anamniota, those in the lacertilian pineal have lost some of their typical photosensory cytological characteristics. On the other hand, they contain secretory granules varying in size but being generally larger than the largest granular vesicles in the terminals of the autonomic nerve fibers. Therefore, these elements have been termed secretory rudimentary photoreceptor cells. Earlier findings suggest that an indoleamine, probably serotonin, is stored in the secretory granules. The sensory function of the lacertilian pineal organ is regressive whereas its secretory function is of much more importance. Probably, the autonomic afferent pineal innervation in lizards, now for the first time ultramicroscopically demonstrated, is involved in the regulation of the synthesis, secretion and release of chemical compounds at the level of the secretory rudimentary photoreceptor cells which can be considered the phylogenetic forerunners of the mammalian pinealocytes.