Peritubular myoid cells, surrounding the seminiferous tubules in the testis, have been found in all mammalian species, but their organization in the pentubular interstitial tissue varies by species. In laboratory rodents, including rats, hamsters and mice, only one layer of myoid cells is seen in the testis. The cells in these animals are joined by junctional complexes as are epithelial cells. On the other hand, several cellular layers exist in the lamina propria of the seminiferous tubule in the human and some other animals. Myoid cells contain abundant actin filaments which are distributed in the cells in a species-specific manner. In the rat, the filaments within one myoid cell run both longitudinally and circularly to the long axis of the seminiferous tubule, exhibiting a lattice-work pattern. The arrangement of the actin filaments in the cells changes during postnatal development, and the disruption of spermatogenesis, such as cryptorchidism, seems to affect further the arrangement of the filaments. Other cytoskeletal proteins, including myosin, desmin/vimentin and α-actinin, are also found in the cells. Myoid cells have been shown to be contractile, involved in the transport of spermatozoa and testicular fluid in the tubule. Several substances (prostaglandins, oxytocin, TGFβ, NO/cGMP) have been suggested to affect the contraction of the cell, though the mechanisms of the contraction are still unknown. Recent in vitro studies have demonstrated that the cells secrete a number of substances including extracellular matrix components (fibronectin, type I and IV collagens, proteoglycans) and growth factors (PModS, TGFβ, IGF-I, activin-A). Some of these substances are known to affect the Sertoli cell function. Furthermore, it has been reported that myoid cells contain androgen receptors and are involved in retinol processing. Considering all this, it is evident that peritubular myoid cells not only provide structural integrity to the tubule but also take part in the regulation of spermatogenesis and the testicular function. Their precise roles, however, remain to be solved.

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Regeneration followed partial amputation of the tail fin in three representative species of teleosts. A systematic study, using histochemical methods, radioautography and transmission electron microscopy, disclosed the essentials of the natural history of regeneration of the fin soft-rays. At about 24 hours, epidermal cells had completely covered the cut edge. By the second day an apical epidermal cap was established. Beneath this cap a blastema was formed by about three days after amputation. A collagenous lepidotrichial matrix had begun to develop by 4 days after excision. Actinotrichia were first detected in the regenerates at 5 days. During these processes, the basal lamina of the epidermis played an important morphogenetic role. The cells responsible for the regenerate underwent regressive changes after form and function had been restored.

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The inner sublayer (P-layer) of the circular muscle coat in the canine proximal colon has been known to produce spontaneous mechanical contractions associated with characteristic electrical activities called slow waves. We recorded the mechanical activities of tissue preparations from this P-layer. Normal Krebs solution (K⁺; 6mM) was used as the perfusate. Elevation of extracellular K⁺ concentrations in the range of 12mM and 36mM induced intensified phasic contractions. Administration of an NO-synthase inhibitor, N_ω-nitro-arginine methyl ester (L-NAME, 50μM), enhanced both the spontaneous mechanical rhythms and high extracellular K⁺-induced contractions. Administration of the substrate for NO synthases, L-arginine (400μM) remarkably suppressed the effects of L-NAME on the amplitude of the spontaneous rhythms and on responses to extracellular high K⁺.
Histological structures of nerves in the P-layer were investigated by an NADPH (nicotinamide adenine dinucleotide phosphate)-diaphorase technique and by the immunohistochemistry of NO-synthases, since NO-producing (nitrinergic) nerves usually, if not always, show a histochemical NADPH-diaphorase positive reaction in formaldehyde-fixed specimens, and since features of ganglia and nerve strands in the outer subdivision of the submucosal plexus (plexus submucosus externus; or so-called Henle's plexus) together with the delicate network of nerve terminal varicosities within the P-layer were clearly visualized by this method. The topographical arrangement of nitrinergic nerves supported the view that they produce nitric oxide (NO), being one of the major chemical mediators of the neural control of the spontaneous rhythms in the P-layer.

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Light microscopic data on the innervation of taste buds in the soft palate of the rat obtained by methylene blue (MB) supravital staining are presented here for the first time. The taste pore including subpopulations of taste bud cells and nerve fibers were stained. Intragemmal nerve fibers were detected ascending up to the taste pore; this suggests the possibility of additional paracellular pathways in taste transduction directly via nerve fibers. Moreover, relatively large bulbous or laminar enlargements of the intragemmal nerve fibers were found. The branches of these nerve fibers, showing minute spiny protuberances, appeared to anastomose. Some perigemmal nerve fibers were also seen in close apposition to the taste pore. The basal plexus in the vicinity of the taste buds showed enlargements which resembled the Merkel's discs, but were significantly smaller. These structures did not occur in the subgemmal part of the basal plexus. Regarding the oxygen-dependency of the staining reaction, it was speculated that both MB and oxygen were attracted by iron-containing enzymes. Therefore, the strong affinities for MB shown by subpopulations of nerve fibers and taste bud cells were explained by their high requirements for oxygen. Furthermore due to its selectivity for intraepithelial nerve fibers and sensory cells, the method applied here is well-suited for routine use in light microscopy.

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Recently, we showed that capsaicin induced the degeneration of not only glomerular CI terminals but also of non-glomerular CI terminals making presynaptic contact with interneuronal soma. Studies of the nature of interneurons making contact by non-glomerular CI terminals should provide important information to facilitate our understanding of the processing of nociceptive impulses in the substantia gelatinosa. The most likely candidate molecule involved in this process in these interneurons is γ-aminobutylic acid (GABA). Therefore, ultrastructural relationships between nonglomerular CI terminals and GABAergic interneuronal soma in the mouse substatia gelatinosa were examined by an immunocytochemical method using an antibody to GABA. Terminals with the same profiles as the CI terminals, i. e., slender, sinuous and scalloped terminals filled with clear synaptic vesicles, were found to make synaptic contacts with GABA-immunoreactive somata. Thus, nociceptive primary afferents are suggested to modulate pain transmission by themselves via GABAergic neurons in the substantia gelatinosa.

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The three-dimensional organization of the lymphatics in the dog stomach was studied by scanning electron microscopy of corrosion casts, which were made by direct parenchymal injection of low-viscosity Mercox-resin into the mucosa and the muscular layers. Although the organization of lymphatics in the stomach has been studied by a variety of methods, the origin of the lymphatics and their connection in each layer has not been clearly defined. In this study, using dog stomach because of its structural similarity to the human stomach, we defined the lymphatic structure in all gastric layers, with lymphatics absent in the upper two-thirds of the lamina propria mucosae. They were first encountered at the deepest level of the lamina propria, immediately above the lamina muscularis mucosae. These lymphatics were composed of single-layered irregular meshes. Slender lymphatics arising from this network passed through the lamina muscular is mucosae and drained into the lymphatic plexus, which was composed of thicker lymphatics at the uppermost layer of the submucosa. Lymphatic valves were frequently seen in this plexus. From this plexus, slender connecting lymphatics with valves extended straight downward without lateral communications and drained into the lymphatic plexus at the deepest layer of the submucosa. This latter plexus, composed of large-caliber lymphatics, issued flattened lymphatics which formed a three-dimensional network in the muscular layer. The subserosal lymphatics were composed of thick lymphatics with tortuous courses and drained into the efferent lymphatics.

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Structure and vitamin A-storage of the mesangial cells in the lamprey kidney are documented using the gold chloride method, fluorescence histochemistry and electron microscopy. Abundant lipid droplets in the cytoplasm of the mesangial cells release intense, but quickly fading vitamin A-fluorescence under a fluorescence microscope and react with gold chloride. Characteristic tubular invaginations of the plasma membrane which usually occur in the lamprey smooth muscle cells are observed in these cells. We discuss similarities of the lamprey mesangial cells with hepatic stellate cells in the lamprey and mammals, such as vitamin A-storage, blood flow-regulation and proliferative potential. To the best of our knowledge, this is the first report on and proof of vitamin A-storage in mesangial cells, providing evidence for a vitamin A-storing cell lineage in vertebrates.

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This study aims to investigate the occurrence and nature of dark neurons in the central nervous system under physiological conditions. Mouse brain tissues were perfusion-fixed with paraformaldehyde or glutaraldehyde at 4h intervals during one day (3:00, 7:00, 11:00, 15:00, 19:00, 23:00). Paraffin sections were stained with the cationic colloidal iron method, and counterstained with nuclear fast red or carbolthionin.
The dark neurons were readily distinguishable as their shrunken cell bodies stained densely with nuclear fast red or thionin. Some of the dark cells were coated with perineuronal sulfated proteoglycans; this coat, which formed a smoothly extended meshwork in light cells, presented spicule-like forms in the dark cells.
The occurrence of dark cells in the retrosplenial cortex varied by the time of day: the incidence of the dark neurons was low (10-15%) at 11:00, 15:00 and 23:00, while it was significantly high (50-60%) at 3:00 and 19:00. Previous authors have ascribed the occurrence of dark neurons either to artifacts due to inappropriate fixation or to pathological damage. However, the present study strongly suggests that this type of neuron occurs under physiological conditions as reversible changes, and vary over a day, showing distinct peaks. These peaks occurred coincidentally while the mice were awake. Such morphological changes may be involved in the neuronal activation and exhaustion. Our view is consistent with the hypothesis (TEWARI and BOURNE, 1963) that the neurons take such dark profiles at certain stages of neurosecretion.

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Perivascular accumulation of leukocytes in the lung was induced by intratracheal administration of sheep erythrocyte antigen to primed mice. The route of migration of intravascular leukocytes to the perivascular space in the lung, in particular from arteries, and the structure of lymphatic vessels among the aggregated leukocytes were examined by transmission electron microscopy. Leukocytes—lymphocytes, granulocytes, monocytes and macrophages—were demonstrated to adhere to the endothelial surface and to migrate between endothelial cells to reach the internal elastic lamina of arteries. Becoming conspicuously constricted, the leukocytes penetrate through this elastic lamina. They further migrate through the smooth muscle layer to the interstitium, passing through the external elastic laminar region. At 2 days after antigen administration, dilated lymphatic vessels containing large numbers of leukocytes in the lumen and bearing endothelial gaps open to the interstitium began to be seen. The lymphatic walls were more convoluted and richer in pinocytotic vesicles than those prior to antigen challenge.
This study confirms the light microscopic findings by CURTIS et al. (1990) that arteries, besides veins, venules and capillaries, may represent a major route of inflamatory cell entry into the lung parenchyme in an acute and vigorous immune response. In addition, lymphatic vessels were suggested to be newly formed for the transport of fluid and immune cells from the sites of inflammation in the lung.

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