The present review of cutaneous sensory receptors begins with a consideration of free nerve endings (FNEs) that can be considered as sensory terminals evidencing the least structural specialization of the axon and associated cells. Using the criteria established by KRUGER et al (1981), FNEs of both A delta and C fibers can be identified on the basis of ultrastructural characteristics that include an intimate relationship between axons and the associated epithelium, the lack of a complete Schwann cell investment, the accumulation of numerous vesicles and other cytoplasmic organelles, and for A delta terminals a 1:1 relationship between axon and investing Schwann cell. Using these criteria, the so-called genital end bulbs of the human glans penis are merely a skein of FNEs based on the ultrastructural study of HALATA and MUNGER (1986).
Hair follicles of most species studied to date (the exception being the rabbit and to some extent the guinea pig) are multiply innervated with lanceolate, Ruffini and FNEs. The lanceolate terminals are the rapidly adapting terminals that are numerous in guard hairs. Ruffini terminals of hairs resemble those of the periodontal ligament or joint capsules and both are remarkably similar to Golgi tendon organs in terms of ultrastructural characteristics. The key ultrastructural characteristic is the encircling of collagen bundles by axons and associated Schwann and connective tissue cells.
Axons frequently enter the epidermis either to terminate as FNEs or become associated with Merkel cells in glabrous skin at the base of the papillary ridges or in clusters of Merkel cells in hairy skin in touch domes or Haarscheiben. Merkel cells have clusters of apparent secretory granules polarized toward the axon and the axon is typically a slowly adapting mechanoreceptor. The function of the granules is not known.
Pacinian corpuscles are the largest of the corpuscular receptors of the dermis and are characterized by an elaborate inner core of stacks of numerous thin lamellae arranged in a bilaterally symmetrical manner. Based on the fact that the lamellae are coupled with gap junctions and the outer core lamellae isolated by numerous tight junctions, the authors have proposed that the unique ionic environment may be in part responsible for the remarkable sensitivity of Pacinian corpuscles (MUNGER and IDE, 1987).
Meissner corpuscles are a typical corpuscular receptor of murine (IDE, 1976, 1977), marsupial and primate glabrous skin (MUNGER, 1971). The axons typically weave back and forth between stacks of lamellae. Simple corpuscles are common in many mammals especially in the glabrous snout skin (MUNGER, 1965, HALATA and MUNGER, 1983). The lamellae around the axon are typically arranged circumferentially around the axon.
The authors have previously noted that axons in mechanoreceptors including lanceolate terminals, Pacinian and Meissner corpuscles, and muscle spindles frequently have axonal spines projecting from the elongated or Y-axis of the axon (MUNGER and IDE, 1987). Axonal spines typically are filled with filaments and lack other cytoplasmic organelles. The common presence of axonal spines provided the basis for the speculation that they may be involved in mechano-electric transduction.

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The distribution of the perikarya of endogenous digitalis-like substance (EDLS)-containing neurons in the rat hypothalamus was studied by immunohistochemistry using digoxin and digitoxin antiserum. In addition, the possible coexistence of EDLS and posterior lobe hormones was examined using an immunohistochemical double-staining technique. Digoxin-like immunoreactive neurons were demonstrated in the paraventricular and supraoptic nuclei and other hypothalamic areas. However, digitoxin-like immunoreactive neurons were not detected in the hypothalamus. A portion of the digoxin-like immunoreactive neurons showed immunoreactivity for vasopressin or oxytocin. From these results, it is suggested that these digoxin-like immunoreactive neurons correspond to “EDLS-producing neurons, ” and that EDLS seems to act not only as a natriuretic substance but also as a neurohormone or neurotransmitter.

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Ciliary processes, mainly iridial processes, of the adult rabbit eye were examined by electron microscopy. Intraepithelial nerve fibers were recognized in the ciliary epithelium. These were single, naked unmyelinated nerve fibers found between pigmented epitheliocytes but not between non-pigmented epitheliocytes. They displayed a beaded structure and contained in their dilated portions many clear vesicles, a few cored vesicles and filamentous mitochondria. Their possible functional significance is discussed.

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A simple drying method for biological materials for scanning electron microscopy was developed. Fixed specimens were immersed in t-butyl alcohol after dehydration through a graded series of ethanol. When the room temperature fell below the melting point of t-butyl alcohol (25.5°C), liquidized alcohol obtained by warming was used. Specimens in the alcohol were then frozen in a refrigerator. They were placed in the bell jar of a vacuum evaporator and simply evacuated with a rotary pump. The samples were completely dried within 1h after the frozen alcohol was sublimated in the vacuum. When examined by scanning electron microscopy (SEM), both surface and intracellular structures were demonstrated in three-dimension without any significant drying artifacts. Careful comparison of the results indicated that the SEM images obtained by this method were either superior or equal to those obtained by the critical point drying method.

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The blood vascular bed of the rat pineal gland was reproduced through a low viscosity methacrylate casting medium and observed with a scanning electron microscope. The pineal gland was located in front of the confluens sinuum which was formed by the confluence of the left and right transverse sinuses and the superior sagittal and great cerebral veins. The pineal gland was found to contain a rich vascular network of freely anastomosing capillaries. This network received a few afferent arteries from the posterior cerebral arteries while emitting ten to fifteen efferent veins. Most of these efferent veins drained into the upper segment of the great cerebral vein; the remaining one or two efferent veins continued directly into the confluens sinuum. A marked constriction, probably representing a venous valve or valve-like projection; was observed in the opening area of the great cerebral vein. Circular constrictions, probably representing sphincters, were imprinted in the efferent vessels. No direct vascular connection was noted between the pineal gland and the nervous tissues or between the pineal gland and the choroid plexuses of the third and lateral ventricles.

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The three-dimensional architecture of the autonomic nerve terminals in the rat exocrine pancreas was investigated by scanning electron microscopy using the HCl-digestion method as well as by transmission electron microscopy.
Unmyelinated nerves presumed to be autonomic in nature were found in networks covering the outer layers of arterioles and their capillary extensions, with nerve fibers often leaving the capillaries to surround acini in the interacinar spaces. Schwann cells formed scaffolds for axons of the networks. No other distinct type of cells such as the so-called interstitial cells of Cajal were found to be associated with the formation of the networks. Although nerve fibers of the networks were locally in close association with the walls of the blood vessels or with the bases of acinar cells, no specialized axonal contacts with these tissues were found. However, local swellings, presumably varicosities, were observed by scanning electron microscopy on the surface of nerves. These findings suggest that the networks of unmyelinated nerves represent terminal apparatuses of the autonomic nerves in the pancreas.
Schwann cells in the terminal networks were characterized by an unusual abundance of cell organelles. These “terminal Schwann cells” well correspond in location and reticular extension to the “interstitial cells of Cajal” as demonstrated by silver impregnation and vital methylene blue staining. The occurrence of well developed Golgi apparatuses, rough endoplasmic reticulum, and numerous ribosomes suggests that the cells are specialized Schwann cells which most likely require high levels of cellular activity in order to maintain their elaborate cytoplasmic processes extending along the terminal networks, and also to sustain the specific functions of axon terminals.

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The distribution of vasopressin (AVP)-containing or oxytocin (OXT)-containing neurons in the rat hypothalamus which project to the posterior pituitary was revealed by the combination of retrograde tracing of horseradish peroxidase (HRP) and immunohistochemistry. The majority of magnocellular neurons labeled with HRP were located in some of the hypothalamic nuclei, including the supraoptic nucleus and paraventricular nucleus. Many of these neurons were also immunostained by anti-AVP or anti-OXT. On the other hand, some of the immunostained neurons were not labeled with HRP in the dorso-medial and the most caudal parts of the paraventricular nucleus. These data confirmed previous reports concerning the distribution of AVP- or OXT-neurons projecting to the posterior pituitary, as a more direct visualization of both the neuropeptides and the retrogradely transported HRP in the same tissue section was attained. In addition, some of the HRP-labeled perikarya which seemed to have direct contact with the ventricular lumen were occasionally seen; its functional significance is discussed.

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Calcitonin gene-related peptide (CGRP)-like immunoreactivity has been reported to occur in both varicose and non-varicose nerve fibers among and around principal ganglion cells in the celiac ganglion of rats. The present electron microscopic immunohistochemistry revealed a small number of myelinated nerve fibers immunoreactive for CGRP, although most of the immunoreactive fibers were unmyelinated.
In the fetal development, CGRP-immunoreactive nerve fibers were first revealed at day 18 of gestation and thereafter gradually increased in number. Both the density and distribution of the immunoreactive fibers within the ganglion similar to those in adult rats were attained by postnatal day 14. This developmental change was compared with that of enkephalin-immunoreactive nerve fibers in the same ganglion.
In electron microscopy, the immunoreactive fibers with varicosities were characterized by abundant small clear vesicles mixed with large granular vesicles. These formed numerous axo-dendritic and several axo-somatic synapses with the principal ganglion cells, whereby the immunoreactive fibers were presynaptic.
The immunoreactive material was localized in the core of the large granular vesicles as well as in the axoplasm. On the other hand, the immunoreactive fibers without varicosities were characterized by neurofilaments, neurotubules and small mitochondria; small clear or large granular vesicles were rarely found in the immunoreactive fibers. They lay in no direct apposition to adjacent neuronal elements and were, therefore, regarded either as fibers passing through the ganglion or as preterminal portions of the CGRP-immunoreactive varicose terminals.

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The intermediate filaments in mouse taste bud cells were studied by immunocytochemistry using antikeratin antibodies, and by conventional electron microscopy. Taste bud cells (types I, II, and III) possessed less densely aggregated bundles of intermediate filaments than the surrounding epithelial cells. Type III cells, however, contained more densely aggregated bundles than type I or II cells. Basal cells in the taste buds showed aggregations of filaments as dense as those seen in the epithelial cells, although their bundles were more slender than those of the epithelial cells. The antibodies to keratins from the bovine muzzle and human stratum corneum stained all types of the taste bud cells as well as the surrounding epithelial cells. PKK2 antibody reacted with the surrounding epithelial cells, but did not react with the taste bud cells.
These results show that keratins are present in both taste bud and surrounding epithelial cells, although the keratin subtype differs between those cells. This finding has led us to the supposition that all cell types comprising the taste buds—including type III (receptor) cells—originate from the epithelial cells surrounding the taste buds. It is also suggested that both keratin subtypes and aggregation patterns of intermediate filament bundles change during differentiation from surrounding epithelial cells to taste bud cells, and from basal cells in the taste buds to types I, II, or III cells.

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The endocrine cells and nerves in the islet and the gut of the arctic lamprey Lampetra japonica were examined immunocytochemically by using antisera against brain-gut peptides and amine.
The cellular composition of the islets as reported by previous researchers in European species of the lamprey was confirmed in the present study. The islet consisted exclusively of insulin immunoreactive cells in the larvae (ammocoetes), whereas in the adult somatostatin immunoreactive cells were added to the insulin immunoreactive cells; the gut epithelium in the adult was now devoid of somatostatin cells.
In the gut of the lamprey, the endocrine cells—which were flask-shaped with a cytoplasmic process extended to the lumen—were classified into three types in the larvae, but were represented by a single type in the adult. In the larval lamprey, the first type was immunoreactive for somatostatin, the second one for gastrin/cholecystokinin (CCK) and the third cell type was immunoreactive for glucagon, pancreatic polypeptide and FMRFamide, simultaneously. In the gut of the adult lamprey, the single type of endocrine cell reacted simultaneously to C-terminal specific anti-glucagon serum, N-terminal specific anti-glucagon serum, anti-bovine PP serum, anti-neuropeptide Y serum and anti-FMRFamide serum. These cells occurred most frequently in the upper intestine, their distribution decreasing from the middle to the lower intestine.
Two types of peptide containing nerves were identified in the islet and the gut of the larval and adult lamprey. The first type of neurons (perikarya and fibers) was immunoreactive for serotonin and calcitonin gene. related peptide (CGRP), and was located in the mucous and muscular layer of the intestine and in the islet. The second type of neurons contained both serotonin- and gastrin releasing peptide (GRP)-like immunoreactivities and was scattered exclusively in the muscular layer of the gut. In larval and adult lampreys, a few serotonin/CGRP immunoreactive nerve cell bodies and beaded fibers were found in the connective tissue around the islet cell cords. These nerve fibers were sometimes closely apposed to the blood capillaries and to the islet cells. These findings indicate that a neuroendocrine correlation comparable with that in mammals may have been established in the islet of this most primitive vertebrate.

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A predominant population of mucosal nerves in the mammalian gastric body has been known to contain large amounts of gastrin-releasing peptide (GRP). The present immunohistochemical study demonstrates the depletion of GRP immunoreactivity from nerves in the oxyntic mucosa of rats with ulcers induced by restraint plus water immersion. Depletion of GRP occurred in a major part of the nerves after 3h exposure to the stress, and after 6h exposure only a few nerve fibers could be recognized. In contrast, GRP fibers in the pyloric mucosa did not decrease significantly in number in any of the stressed rats. Since the depletion of GRP immunoreactivity preceded mucosal erosion in the gastric body, the possibility is proposed that GRP released from the nerves may be related to stress-related ulceration in the stomach.

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