Recently acquired knowledge about the Merkel cell is reviewed, and unresolved problems on the development and function of the cell are discussed. Concerning its developmental origin, the epidermal origin hypothesis has become prevalent since the discovery of cytokeratin polypeptides in it. While the significance of this cell is still unclear, the following functions are presumed: 1) the release of neurotransmitters or neuromodulators to the SAI mechanosensory nerve terminals; 2) the attraction and trophic action on peripheral nerve fibers; 3) the stimulation of the proliferation of keratinocytes leading to the three-dimensional development of the epidermal anlagen; 4) the maintenance of the normal differentiation of keratinocytes; and 5) the release of bioactive substances to subepidermal structures. These multiple functions imply a possibility that one Merkel cell sequentially performs all these different roles during its life span. However, it is also possible that Merkel cells consist of several functionally different subpopulations.

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The recent debate on the nature of the cytoplasmic matrix is reviewed on the basis of results obtained by electron microscopy of embedment-free materials, i. e., the critical point-dried whole mount-cell method, the polyethylene glycol (PEG)-embedding and subsequent de-embedding section method, and the freeze-etching method. Fine structural images obtained by these methods are carefully evaluated and close correspondence with electron microscopy regardless of these methods is demonstrated. Especially, ‘novel’ filamentous sturctures —the microtrabecular strands and the cross-linkers— correspond well to each other; they are structures which have been included in epoxy sections by conventional methods, but have been obscured simply because of a similar property of electron scattering between the filamentous structures and epoxy resin. Although this correspondence seems to support the existence of the microtrabeculae, the electron microscopy of serum albumin, when processed by the PEG-method, also exhibits filamentous networks resembling the microtrabecular lattice. This, together with the finding on the centrifugation of in situ cells, strongly suggests a possibility that most, if not all, microtrabecular strands and cross-linkers in cells without pretreatment by detergents do not represent actual in situ structures.

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A qualitative and quantitative ultrastructural study (volume density of the nucleus and cell countings) on age-related changes occurring in neocerebellar oligodendrocytes of the granular layer was carried out in female albino rats aged 2-24 months. Taking into account the cytological features, two types of oligodendrocytes could be discerned, designated as Type I and Type II. Observations showed that, from 18 months onwards, the appearance of bundles of nuclear filaments running throughout the euchromatin areas are common occurrence in Type II oligodendrocytes. The lowest and highest mean volume density of the nucleus is achieved, respectively, at 2 months (50.67%) and at 21 months (64.93%); the analysis of variance of the parameter with ageing displays a most statistically significant result. There is also a positive and linear correlation between the volume density and age. The percentage of Type I oligodendrocytes (out of the total of counted oligodendrocytes) predominates from 2 to 15 months; after being even at 18 months, the percentage of Type II oligodendrocytes preponderates at 21 and 24 months. Although oligodendrocytes are capable of undergoing mitosis, it is concluded that they are prone to morphological changes with ageing, a warning that the physiology of oligodendrocytes may be eventually affected.

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Liposome-entrapped dichloromethylene diphosphonate (Cl₂MDP) was injected locally into the thymus of adult rats. This treatment, which has been found to deplete resident macrophages in other organs, also reduced the number of thymic macrophages. The depletion was evident in the cortex and the corticomedullary zone at 5, 13 and 23 days after Cl₂MDP treatment, while the decrease in medullary macrophages only became significant 13 and 23 days after injection. However, the maximal reduction of macrophages was evident 13 days after this treatment. Thirty-five days after Cl₂MDP treatment, macrophages repopulated the thymus. Our results suggest that the cortical and cortico-medullary macrophages have a high phagocytic activity consistent with their predominant role as scavengers of the large numbers of thymocytes that die under physiological conditions. It is concluded that liposome-mediated depletion of thymic macrophages can serve as an experimental model to study the roles of these cells.

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The epithelial cells of the human parotid main excretory duct (Stensen) were studied by transmission (TEM) and scanning (SEM) electron microscopy through a variety of procedures that allowed the visualization of their three-dimensional microanatomy. Stensen's duct in humans is lined, in its distal portion, with a pseudostratified epithelium with tall principal cells and smaller basal cells, while the epithelium becomes progressively stratified cylindrically toward the oral stoma. Goblet cells are scattered among the other epithelial cells. The principal cells exhibit, on their lateral surfaces, numerous flattened laminar folds probably involved in transporting processes. A welldeveloped smooth endoplasmic reticulum intermingled with mitochondria occupies the cellular apices. Some vesicles are recognized on the cytoplasmic surfaces off the apical and lateral plasmalemma when cytoplasmic organelles are removed. All these features are interpreted as being involved in the process of endocytosis. In both TEM and SEM, the principal cells show a relevant number of irregular apical protrusions that may represent a kind of apocrine secretion. Thus, with regard to function, the human Stensen's duct seems to modify the composition of saliva by processes of resorption and secretion, the latter coming from goblet cells as well. The basal cells have a surface microanatomy completely different from that of principal cells. They exhibit, in fact, only sparse microvillosities and smooth areas on their lateral aspect, while their stromal surface is greatly augmented by irregular thin ramified processes. The role of basal cells is also discussed.

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Previous studies have demonstrated that the epithelium of the respiratory portion of rat nasal mucosa is amply supplied by nerve fibers with immunoreactivities for calcitonin gene-related peptide (CGRP) and substance P (SP), these fibers most likely acting as sensory mediators in the mucosa. The present study demonstrates that some intraepithelial fibers contain a VIP-immunoreactivity whose occurrence in these nerves has previously been neglected. The present study further aims to confirm the occurrence of NO-producing intraepithelial nerve fibers in the rat nasal mucosa and to examine its colocalization with CGRP and with VIP.
Double staining methods were used to evaluate the colocalization of NADPH-diaphorase reactivity and CGRP- or VIP-immunoreactivity. The reactivity for NADPH-diaphorase and that for CGRP coexisted in only a small part, if any, of the nerve fibers distributed at the basal portion of the epithelium. In the perpendicularly and obliquely oriented transepithelial nerve fibers, both reactivities were clearly demonstrated to be separated in different fibers.
VIP immunoreactivity was also present in a part of the intraepithelial nerve fibers of the nasal mucosa, and their entire population was shown to be positive for NADPH-diaphorase.
The NADPH-diaphorase-positive reaction was displayed in only a small population of neurons in the trigeminal ganglion, whereas it was seen in numerous neurons in sphenopalatine ganglion, being colocalized with VIP.

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Human superior and inferior venae cavae at the orifices to hearts obtained from two cadavers were histologically examined with regard to the distribution of cardiac muscle fibers in their walls. The superior vena cava contained cardiac muscle fibers together with smooth muscle fibers. The cardiac muscle fibers were distributed uninterruptedly from the atrium to the root of the azygos vein, covering a length of 45mm. Cardiac myocytes were present outside the smooth muscle and coursed in bundles longitudinally, obliquely, or circularly. Cardiac myocytes occupied one to two thirds of the wall thickness, but decreased in amount toward the periphery. The inferior vena cava also contained both cardiac and smooth muscle fibers. The cardiac muscle fibers extended continuously, covering a distance of 18mm from the atrium to a level just under the diaphragm. Their fibers were bundled, running circularly or obliquely, and being more abundant in the anterior wall than in the posterior. From these findings, the venae cavae close to the atrium are histologically regarded as an extension of the atrium. The venae cavae close to the atrium probably contract together with the atrium and work as a functional valve that contributes to the pumping mechanism of the heart.

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Casts of the lymphatics of the canine small intestine were made by direct injection of a low viscosity methacrylate resin into a large lymphatic in the submucosal layer, and observed under a scanning electron microscope. The lymphatics started with rod-like central lacteals in the villi of the jejunum and leaf-like ones in those of the ileum. The bases of the lacteals were connected by slender lymphatics forming a three-dimensional network, tentatively called the “superficial lamina propria lymphatic plexus”. From this plexus, a few straight branches descended through the lamina propria to drain into a well developed “deep lamina propria lymphatic plexus”, which was extended two-dimensionally closely above the lamina muscularis mucosae. From this plexus, a few short lymphatics extended and penetrated the muscularis mucosae and drained into the “superficial submucosal lymphatic plexus”, a coarse mesh work of thick lymphatics. From this plexus, a few slender lymphatics descended to drain into a second lymphatic plexus, called the “deep submucosal lymphatic plexus”, which extended two-dimensionally on the circular muscle layer. This deep submucosal plexus was a coarse network of thick knotty lymphatics. A large collecting lymphatic was occasionally seen running through the mesh. The tunica muscularis contained tubular lymphatics extending horizontally parallel to the muscle fiber, both circular and longitudinal.

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A study of the immunocytochemical localization of acidic fibroblast growth factor (aFGF) in the rat digestive system using a polyclonal anti-aFGF antiserum revealed immunoreactive cells in the periphery of the islets of Langerhans and in the mucosal epithelia of the intestine, from the jejunum through the colon. In the pancreatic islet, light microscopic immunostaining of consecutive sections with anti-aFGF and anti-glucagon antisera as well as pre-embedding immuno-electron microscopy demonstrated that aFGF-like immunoreactivity belongs to the A cell. In the intestinal epithelium, the aFGF-immunoreactive cell was simultaneously reactive to anti-glucagon antiserum and was ultrastructurally identified as the L-cell, which is known to produce glucagon-like immunoreactants derived from proglucagon. Post-embedding immuno-electron microscopy further revealed that the immunoreactivity is localized in the secretory granules of A- and L-cells. These results suggest that an aFGF-like immunoreactant occurs in the populations of gasrto-entero-pancreatic endocrine cells that express proglucagon.

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Anionic sites in the rat sciatic nerve were studied by light and electron microscopy using a fine-granular cationic colloidal iron staining method (MURAKAMI et al., 1986). The axon, as well as the endoneurium, the epineurium and the basement membrane of Schwann cells, were all confirmed to react strongly to the cationic colloidal iron even at a pH value of 1.0-2.0. Prior hyaluronidase digestion decreased the colloidal stain of the epineurium; chondroitinase ABC weakened that of the endoneurium and the basement membrane of Schwann cells. However, as axons retained stainability with cationic colloidal iron even after combined digestion with hyaluronidase, chondroitinase ABC, heparitinase and keratanase, the authors consider sulfated glycoconjugates and not those substances which are digestible with such common enzymes. The acid groups ionized at pH 1.0 are most likely sulfate groups. Methylation deprived the axon of the reactivity to cationic colloidal iron staining, and even subsequent saponification could not recover this reactivity to its full extent. All these suggested the presence of sulfate groups. In the axon, electron microscopy revealed a deposition of colloidal iron on the external surfaces of microtubules and neurofilaments in the axoplasm and of very fine filaments connecting them. This highly negatively charged intra-axonal network could also serve toward a supportive function in maintaining the spatial distribution of microtubules either mechanically or through electrostatic repulsion or, possibly, serve as an intra-axonal cation exchange reservoir.

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