Postnatal changes in nerve cells and myelinated fibers in the pineal gland of cotton rats (Sigmodon hispidus). A light microscopic study

Light and electron microscopic immunocytochemical study on the innervation of the pineal gland of the tree shrew (Tupaia glis), with special reference to peptidergic synaptic junctions with pinealocytes

Conventional and immunocytochemical, light- and electron-microscopic studies on the innervation of the pineal gland of the tree shrew (Tupaia glis) were made. Neuropeptide Y (NPY)-immunoreactive fibers, which were abundantly distributed in the gland, disappeared almost completely after superior cervical ganglionectomy, suggesting that these fibers are mostly postganglionic sympathetic fibers. By contrast, tyrosine hydroxylase (TH)-immunoreactive fibers, which were less numerous than NPY-fibers, remained in considerable numbers in ganglionectomized animals, indicating the innervation of TH-positive fibers from extrasympathetic sources. Bundles of substance P (SP)- or calcitonin gene-related peptide (CGRP)-immunoreactive fibers, entering the gland at its distal end, were left intact after ganglionectomy. SP-fibers were numerous, but CGRP-fibers were scarce in the gland. SP-immunoreactive fibers were myelinated and nonmyelinated, and were regarded as peripheral fibers because of the presence of a Schwann cell sheath. NPY- and SP-immunoreactive fibers and endings were mainly localized in the pineal parenchyma. NPY-immunoreactive endings synapsed frequently, and SP-positive ones did less frequently, with the cell bodies of pinealocytes. The results suggest that NPY and SP directly control the activity of pinealocytes. Sections stained for myelin showed that thick and less thick bundles of myelinated fibers entered the gland by way of the habenular and posterior commissures, respectively. Under the electron microscope, the bundles were found to contain also unmyelinated fibers. A considerable number of nerve endings synapsing with the cell bodies of pinealocytes remained in ganglionectomized animals; these endings were not immunoreactive for TH or SP. Such synaptic endings may be the terminals of commissural fibers.

Peptidergic peripheral nervous systems in the mammalian pineal gland

The distribution and density of tyrosine hydroxylase (TH) and neuropeptide Y (NPY)-immunoreactive, sympathetic fibers and calcitonin gene-related peptide (CGRP)-, substance P (SP)-, and vasoactive intestinal polypeptide (VIP)-immunoreactive, non-sympathetic fibers in the pineal gland, the effects of superior cervical ganglionectomy (SCGX) on these fibers, and the location of their terminals in the pineal gland were compared between rodents and non-rodents. A dense network of TH/NPY-positive fibers is present all over the pineal gland. A less dense network of CGRP/SP- or VIP-positive fibers occurs in the whole pineal gland of non-rodents, but these fibers are usually confined to the superficial pineal gland in rodents. After SCGX, some TH/NPY-fibers remain only in the deep pineal gland in rodents, whereas considerable numbers of these fibers persist throughout the gland in non-rodents. Thus, the remaining fibers, probably originating from the brain, may be more numerous in non-rodents. Since CGRP-, SP- or VIP-immunoreactive fibers in the pineal capsule can be traced to those in the gland, and since these fibers are ensheathed by Schwann cells, it is concluded that these fibers belong to the peripheral nervous system. However, the existence of SP-positive central fibers cannot be denied in some species. In the superficial pineal gland of rodents, sympathetic terminals are mostly localized in perivascular spaces, whereas the parenchymal innervation by sympathetic fibers in the pineal gland is more dense in non-rodents than in rodents. Synapses between sympathetic nerve terminals and pinealocytes occur occasionally in non-rodents, but only rarely in the superficial pineal gland of rodents. The occurrence of the synapses may depend on the frequency of intraparenchymal sympathetic terminals.

Regional differences in the pineal gland of the cotton rat, Sigmodon hispidus: light microscopic, electron microscopic, and immunohistochemical observations

Light microscopic, electron microscopic and immunohistochemical observations of the various portions of the pineal gland of the cotton rat (Sigmodon hispidus) were made. The volume of the proximal half occupied about 30% of the whole organ, and pinealocytes were slightly smaller in size in the proximal portion than elsewhere. The distal and intermediate portions contained few interstitial cells and numerous astrocytes, but the proximal portion lacked interstitial cells and had more abundant astrocytes than elsewhere. Astrocytes, which were immunoreactive for glial fibrillary acidic protein, mainly lined the pericapillary spaces in the distal and intermediate portions, but in the proximal portion these cells often surrounded isolated or groups of pinealocytes. In the distal and intermediate portions, abundant sympathetic fibers and less numerous non-sympathetic, peptidergic fibers were mainly localized in the pericapillary spaces; these fibers were sparsely distributed in the parenchyma close to interstitial cells or astrocytes. In the proximal portion, non-sympathetic fibers were scarce and sympathetic fibers were distributed abundantly and almost exclusively in the parenchyma. Most of the sympathetic fibers were adjacent to astrocytes and, occasionally, made specialized contact with them. Fenestrae in the capillary endothelium were numerous in the distal portion but absent in the proximal portion. Thus, marked differences in structure existed between the distal and proximal portions of the pineal gland of the cotton rat suggesting that both portions are functionally dissimilar. In addition, the present study indicates that the proximal portion of the cotton rat was well developed and showed morphological features similar to the deeply situated pineal glands of other mammals.

Sprouting of non-sympathetic myelinated and unmyelinated fibres in response to chronic sympathetic denervation in the pineal gland of the Chinese hamster, Cricetulus griseus

We have examined the effects of chronic sympathetic denervation on non-sympathetic myelinated and unmyelinated fibres in the superficial pineal gland of the Chinese hamster (Cricetulus griseus), using LM, EM and immunohistochemistry. The results suggest that non-sympathetic, myelinated and unmyelinated fibres enter the superficial pineal gland at its distal portion by way of the nervi conarii, and that these fibres are immunoreactive for calcitonin gene-related peptide or substance P. Non-sympathetic, myelinated and unmyelinated fibres in the superficial pineal gland increased in number following chronic superior cervical ganglionectomy. The number of unmyelinated fibres in the nervi conarii also increased in ganglionectomized animals. Thus, the numerical increase of calcitonin gene-related peptide or substance P fibres found in the superficial pineal gland after long-term sympathectomy may be due to sprouting of these fibres. It is speculated that the growth of non-sympathetic, myelinated and unmyelinated fibres and myelination of the former fibres occurring after sympathectomy are caused by nerve growth factor-related mechanisms.