Ultrastructural features of folliculo-stellate cells of the anterior lobe of the pituitary gland were described in three rodent species (the mouse, guinea pig, golden hamster). These cells are agranular and form the lining of tiny follicles projecting microvilli. Long cytoplasmic expanding processes are intermingled with granular cells of various morphofunctional types without special relationships to one cell type or another. In the guinea pig, the abundance of intermediate filaments appears as a notable feature in the cytoplasm of the folliculo-stellate cells. The results are compared with those previously published.

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Ferric chloride, when boiled with hydrazine hydrate and cacodylic acid, is converted into a fine cationic iron (ferric hydroxide) colloid which consists of 0.5-1.5nm electrondense granules, and gives a distinct Prussian blue reaction. This colloid allows light and electron microscopic detection of ionized anionic sites in tissues at a wide pH range of 0.8-7.6. It is smaller in size and more stable, and assures longer and greater staining of tissues, especially at low pH levels, than the iron colloid prepared with sodium or ammonium cacodylate by SENO and his associates (1983, 1984, 1985). Some light and electron micrographs of the rat kidney, spleen and other organs stained with our colloid are presented as examples. These micrographs confirm that the glomerular podocyte end-foot surface facing the Bowman's capsular space is strongly negative-charged. They also show that almost all lymphoid cells around the arteries in the splenic white pulp and thymic cortex contain strongly negative-charged nuclei and that the distal convoluted and collecting urinary tubules are more negative-charged than the proximal convoluted tubules.

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Formation of lymph follicles in draining popliteal lymph nodes was examined in 8-week-old, male C57B1/6 mice which had been injected in the rear footpad with any one of eleven test substances including thymus-dependent and thymus-independent antigens, and killed after 6-14 days.
HGG (10-100μg), MGG, tetanus and diphtheria toxoids (2-10Lf) and influenza HA vaccine (35CCA) induced germinal centers in association with existing follicles, but failed to produce new follicles in draining nodes. KLH (10-100μg), SRBC (1×10⁸) and formalin-killed pertussis organisms (5×10⁸) induced germinal centers in existing follicles and also produced new follicles which soon developed germinal centers. Levan and PVP (10-100μg) induced neither germinal centers nor new follicles. Ferritin (100μg) virtually failed to induce germinal centers but produced a significant number of new primary follicles.
In further experiments, artificially aggregated substances were examined with regards to their ability of inducing lymph follicle formation in draining nodes. Precipitated proteins such as alum-precipitated PHA, HGG, tetanus and diphtheria toxoids, “Sepharose”-PHA and “Sepharose”-HGG induced a significant number of new follicles.
These observations suggest that efficient follicle formation is associated with particulate and high-molecular-weight antigens which are liable to be phagocytized, whereas soluble, poorly phagocytized antigens tend to be inefficient. Soluble proteins may be effective if given in precipitated form. Thymic dependency appears to be irrelevant. The present results point to a possible participation of macrophages in the mechanism of follicle formation.

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Using antisera to urotensins I and II (UI and UII), in the carp, Cyprinus carpio, three types of caudal neurosecretory neurons were identified: those with both UI- and UII-immunoreactivities, those with only UI-immunoreactivity and those with only UII-immunoreactivity. The last type of neurons exceeded the other types in number, while neurons immunoreactive with both UI and UII antisera were relatively few. The axons of neurons of these three types terminated around the capillaries in the urophysis. In the cat shark, Heterodontus japonicus and the swell shark, Cephaloscyllium umbratile, two types of neurons were identified: those with both UI- and UII-immunoreactivities and those with only UII-immunoreactivity. Neurons of the former type were greater in number than the latter. The axons of neurons of both types terminated in the neurohemal areas.

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Microcirculation of the pancreas in the rat and rabbit with special reference to the islets was studied by scanning electron microscopy (SEM) of vascular corrosion casts, light microscopy (LM) of India ink-injected/cleared tissues, and intravital microscopy of in situ organs. The following observations were made:
1) Approximately 10-20% of the total terminal arterioles supplied the islets, while the remainder directly supplied the exocrine pancreas.
2) The vas afferens of the islets divided into sinusoidal capillaries with frequent U-shaped turns in the cortical A and D cell area of the islets, and their secondary branches supplied the core B cell area. Intravital microscopy confirmed that blood irrigated the cortex of the islets first and the core portion second.
3) All islets observed possessed insulo-acinar portal vessels.
4) About 60% of the islets in the rat possessed emissary veins leading directly into the systemic circulation, while in the rabbit, less than 5% of islets possessed emissary venules of small diameter. Thus, the well-developed emissary veins of the islets seemed characteristic of the rat, as compared with the rabbit and several other mammals examined previously.
5) The insulo-acinar portal system seems to represent a short vascular route through which islet secretions are transported in high concentrations to the exocrine pancreas, there to exert their actions. The emissary veins of the islet seem to serve for the quick conveyance of insular secretions into general circulation.
6) It is suggested that the pancreatic lobule is made up of subdivisions or microcirculatory units, each of which is supplied centrally by the insulo-acinar portal system, while peripherally the unit also receives direct branches of intralobular arterioles. The veins run the periphery of the unit.
7) The occurrence of sphincters in the vas afferens and the emissary veins of the islets is suggested as being involved in the regulation of the islet blood flow.

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A combined light and electron microscopic study revealed that there are conspicuous aggregations of smooth muscle cells in several hemopoietic organs and tissues such as in the Leydig (esophageal) and epigonal organs, diencephalic choroid plexus and perihypophyseal connective tissue sheath of the stingray, Dasyatis akajei. These cells were gathered in bundles of varying caliber or arranged concentrically around a central focus, but neither innervation nor gap junction was found. In some of the concentrically arranged cells, signs of degeneration were noticed. The possible origin of these structures is discussed in relation to the vascularity of the loose connective tissue.

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The purpose of this study was to examine whether the basal laminae of Schwann cells in allografts could survive immunological rejection and serve as a conduit for regenerating nerves, as in the case of autogenic nerve grafts. Allografts of nerves were carried out using sciatic nerves of mice after the grafts had been repetitively frozen to kill their Schwann cells. Two mouse strains, C57BL/6N and C3H/HeN, were used, as they are known to differ in major histocompatibility complex. The mid-portion of the grafted nerve segments was examined by electron microscopy. In addition, the toe pad skin and lumbrical muscles were examined for determining whether regenerating nerves reinnervate sensory end organs and motor endplates.
The process of nerve regeneration in the allograft was the same as that seen in the autograft. Cells in the graft disintegrafted into cell debris and were phagocytized by macrophages, whereas the basal laminae of Schwann cells were not removed by macrophages, remaining in the form of tubes or scaffolds. Regenerating nerve fibers grew out through such basal lamina scaffolds, keeping in contact with the inner surface. Digital sensory corpuscles and motor endplates of the operated side were well reinnervated. The results indicate that the basal laminae of Schwann cells of the allograft may survive and serve as a conduit for regenerating axons in the same way as in the case of an autograft.

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The chicken livers were electron microscopically observed under a normal condition and after an intravenous India ink perfusion. Kupffer cells and sinusoidal endothelial cells commenced to endocytose India ink particles in the earliest stages (15min) after purfusion. The attachment of the particles to the cell surface occurred only in the Kupffer cells, which actively took up the particles with coated caveolae. In the endothelial cells the particles were ingested by pinocytosis in the perikaryon and deposited in the macropinocytic vacuoles of Wisse. In Kupffer and endothelial cells, the particles were stored most abundantly at 30min and 4hr after perfusion, respectively. At 48hr, the vacuoles containing the particles were decreased in number and size, while mitotic figures were revealed in the Kupffer cells. Ito cells occasionally ingested in later stages (4 and 48hr) a few carbon particles; they underwent no mitotic division.
Extrasinusoidal macrophages scattered in the parenchyme and phagocytic reticular cells (macrophages) in the lymphoid tissues exhibited phagocytic activity only in later stages (1-4hr). In 48hr after the perfusion both cells began to store amounts of the particles in their vacuoles. This delayed phagocytic activity may be ascribed to the location of the cells separated from the sinusoid by the endothelium. Some solitary macrophages projecting a long process into the sinusoid took up the particles at earlier stages. At 48hr, several mitotic divisions took place in the phagocytic reticular cells of the lymphoid tissue, while no mitotic divisions were found in the solitary macrophages in the parenchyme. After the ink perfusion, migration of solitary macrophages into the sinusoid was accelerated and images indicating their transformation into Kupffer cells were frequently detected. It was concluded that the Kupffer cells maintain their necessary number not only by their self-proliferation but also by replenishment from the extrasinusoidal solitary macrophages scattered in the hepatic parenchyme, which in turn are replenished from the phagocytic reticular cells in the lymphoid tissue capable of mitotic proliferation.

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Sinoatrial and atrioventricular nodal cells as well as Purkinje and working myocardial cells isolated enzymatically from rabbit hearts were examined by transmission electron microscopy. The cardiac cell fine structure remained well organized in the storage solution after isolation. In normal Tyrode solution, the nodal cells changed from spindle shaped to spherical, whereas the shapes of Purkinje and working myocardial cells remained unchanged. The nodal cell fine structure became disorganized with respect to its sarcomeric arrangement in normal Tyrode solution. This was assumed to result from a combination of several factors seen in the nodal cells: the loss of the normal anchoring of the myofibrils at the previous intercalated discs, breakdown of the Z bands and the alteration of the integrity of the intermediate filaments. Also, extensive restoration of plasma membrane damaged by the isolation procedure was observed in this solution. This might correspond to the retention of the normal physiological properties of the plasma membrane of the nodal cells despite gross morphological changes undergone in normal Tyrode solution.

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The immunocytochemical localization of S-100 protein was examined in the hearts of adult guinea-pigs. In addition to Schwann cells, adipose cells and chondrocytes, fibroblast-like cells densely distributed in the cardiac skeleton, all of the four valves and tendinous chordae were immunoreactive for S-100 protein. The S-100-positive fibroblast-like cells were conspicuously rounded in shape and extended their thin, thread-like cell processes in opposite directions; the cytoplasm was restricted to perinuclear region. In these morphological features, the cells were distinctly different from ordinary fibroblasts which were immunonegative for S-100 protein. Electron microscopic observation revealed that the S-100-like immunoreactivity was distributed throughout the cytoplasmic matrix in the cell body and processes, but absent from the nuclei and cell organelles. The significance of the S-100 immunopositive fibroblast-like cells was discussed in relation to cartilaginous tissues, which have the potential to form in the region of the cardiac skeleton and valves under both normal and abnormal conditions.

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This study deals with the fine structural and three dimensional aspects of the early morphogenetic movements of the thyroid primordium in rat embryos. By scanning electron microscopy, the cells of the presumtive thyroid area show peculiar cauliflower-like structures at day 10 of gestation prior to the starting of its invagination. These structures increase in number as the invagination proceeds. Each cauliflower-like structure consists of a large apical cytoplasmic process with many microvilli and a few small bleb-like protrusions. This phenomenon is then followed by the decrease and ultimate disappearance of the microvilli and bleb-like protrusions, while many large spherical bodies, possibly derived from the bleb-like protrusions and cell debris, appear on the epithelium of the thyroid primordium, especially around the marginal zone of the primordial invagination which soon closes. These observations suggest that the gross morphogenetic movement of the thyroid primordium is due partly to the changes of cell shapes in this area. Transmission electron microscopy reveals conspicuous bundles of microfilaments about 6nm in diameter located in the apical cytoplasm and converging on well-developed junctional complexes during the invagination process. The appearance of the cytoplasmic processes and bleb-like protrusions appearing in the cells of this region is believed to indicate a role in the invagination of the thyroid primordium and the closure of the starting point of the invagination.

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The present study demonstrates the applicability of backscattered electron images to the scanning electron microscope (SEM) observation of various biological specimens stained with heavy metals. Techniques of specimen preparation for backscatter imaging were introduced and results obtained by several staining methods were shown: silver staining for argyrophile fibers (ISHII's modification of Bielschowsky-Maresch's stain), Golgi staining for intracellular canaliculi of parietal cells, osmium-staining of intracellular lipid droplets, periodic acid-thiosemicarbazide-osmium staining for the detection of periodatereactive mucosubstances and polysaccharides, and enzyme immunohistochemistry for bioactive peptides.
Observation of stereo-pairs of backscatter images augments information on the three-dimensional localization of stained substances. Moreover, pairs of secondary emission and backscatter images of the same area clearly visualize the correlation between the surface structure and stained substances. Colored images by superimposition of secondary emission and backscatter information are also useful for an understanding of the orientation of the stained substances in three-dimensional structures. It is suggested that backscatter imaging can be widely used for various topochemical studies in three-dimensional extensions of biological specimens.

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