Casts of small intestinal blood vessels and lymphatics in the rabbit were made with methacrylate and observed under a scanning electron microscope (SEM). In casting the lymphatics, a specially diluted low viscosity medium was injected intraparenchymally into the intestinal submucosa. This parenchymal injection allowed good reproduction of fine lymphatics, including the initial lacteals in the villi. The central lacteals were completely surrounded externally by the subepithelial blood capillary networks of the intestinal villi. Individual villi in the lower intestine contained only one central lacteal that drained through a thin lymphatic in the glandular layer into the submucosal lymphatic plexus. Villi in the upper small intestine were broader than those in the lower small intestine, and contained two to five lacteals. They anastomosed with each other at the villous base and formed a markedly expanded sinus.
The cast submucosal lymphatics frequently showed constrictions strongly suggestive of valves. It was constantly observed that well-developed lymphatics in the submucosa ran in pairs and held an arteriole or artery between them. This close association of the lymphatics with arteries suggests that arterial vasomotion might provide an important hydrodynamic energy source for lymph formation and transport.

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The fine structure in the ampullary region of the vas deferens of the rabbit was observed by scanning and transmission electron microscopy, with emphasis on the occurrence of epithelial spermiophagy. The epithelial cells were cuboidal or low columnar and contained moderately developed organelles. These cells, like luminal macrophages, were found to be not only involved in spermiophagy but also capable of actively taking up latex beads administrated intraluminally. Taking into consideration the previous findings in some other species, the epithelial spermiophagy seems to be a common event in the vas deferens of mammals.

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Spleens of normal structure were obtained in surgery on nine patients with gastric cancer. The freeze-cracked surfaces of the organ as well as the vascular casts of methacrylate resin were examined by scanning electron microscopy.
Penicillar arteries were confirmed to terminate in the cords of Billroth, representing the open circulation.
Labyrinthine channels of arterial capillaries were found in restricted regions in the red pulp neighboring pulp arteries and veins. They characteristically possessed in their lumen spanning trabecullae covered with endothelial cells. In some places, the flat endothelium of a channel continued to the lattice-like endothelium of a thin sinus, representing closed circulation. The occurrence and distribution of the arteriolar labyrinths were confirmed by SEM observation of the vascular casts. Their continuation to the thin sinuses was also demonstrated in the casts.
The present study evidences that in the human spleen, specialized arteriolar terminals provide a closed circulation route in restricted regions, besides the hitherto known, predominant route of open circulation.

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Stereomorphic profiles of three types of spermatogonia and their topographical distribution in the basal portion of the seminiferous epithelium of adolescent or adult rats were studied mainly with the scanning electron microscope. According to their cytological features, three types of spermatogonia (large-sized and long fusiform or polygonal; mediumsized and elliptical; small-sized and round) were discerned in both groups of rats. The basal areas of the three spermatogonia were relatively smaller in the adolescent rats than in the adult rats. The type of spermatogonia in the adolescent rats varied somewhat depending upon the diameter of the seminiferous tubules, but such a discrimination could not be made in the adult rats. In the adolescent rats, small cells were less frequently observed than large and medium cells. This ratio of appearance was less marked in the adult rats.
Clones from the same spermatogonial cells showed similar topography in both groups of rats as follows. The clones of a few medium cells were arranged singularly (As) or in pairs (Apr), but the clones of most medium cells appeared to form single cords (Aal) through intercellular bridges, while those of the small cells formed an open polygonal network as a large syncytium.

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The relative frequency and topographical distribution of proventricular endocrine cells were examined immunohistochemically in seven species of birds: common finch, pigeon, quail, chicken, duck, gull and kite. Gastrin releasing peptide (GRP), somatostatin-, avian pancreatic polypeptide (APP)-, glucagon-, 5-hydroxytryptamine (5-HT)- and neurotensin-immunoreactive cells were observed in this study. GRP- and somatostatin-immunoreactive cells were found in all species examined. All six kinds of immunoreactive cells were found with varying frequency in the pigeon, quail and gull, but not all immunoreactives were found in the other species examined. Species differences with regard to the relative frequency and topographical distribution of proventricular endocrine cells were observed.

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The fate of Kupffer cells in mice for 3 days following injection with polystyrene latex particles (0.2 and 2.0μm in diameter) was studied by electron microscopy. Kupffer cells took up the latex particles by pinocytosis as well as by phagocytosis. The particles ingested were in contact or fused with lysosomes in the cell. Two days after the final injection, most Kupffer cells were already stuffed with the particles. Within one month, cell clumps or cell aggregates, which could be called granuloma, were formed in the liver connective tissue space, i. e., Disse spaces, interlobular connective tissue spaces and subperitoneal connective tissue spaces. They were mostly composed of cells laden with numerous latex particles. The large granulomas were 80-120μm in diameter. In each granuloma, endogenous peroxidase was localized in the cisternae of the nuclear envelope and of the rough endoplasmic reticulum of most cells, and in the cytoplasmic granules of some other cells. The former is known to be a Kupffer cell, and the latter, a monocyte. The granuloma further contained a few intermediate cells, showing peroxidase activity in the cisternae of the nuclear envelope and of the rough endoplasmic reticulum, and also in the cytoplasmic granules, phagocytic cells without peroxidase activity, granulocytes, and plasma cells. Hepatic sinusoidal endothelial cells, attenuated in shape, also took up latex particles of a 0.2μm diameter and rarely with a 2.0μm diameter in their cytoplasm. Some Kupffer cells in the granuloma filled with numerous latex particles were labeled with ³H-thymidine (2.5mCi in total dose) after subcutaneous injection ten times for 80hr. In the animals 8 months after the injection of the latex particles, numerous large granulomas were distributed throughout the liver in the interlobular or subperitoneal connective tissue space. The formation of the granuloma of Kupffer cells is considered to play a great role in the disposing of foreign materials from the functional liver parenchymatous tissue.

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The gastric mucosa of adult mice was observed by electron microscopy, and the following findings were obtained. Surface mucous cells mostly undergo degeneration in situ before extrusion from the mucosal surface. Degenerating cells exhibit low electron density of the cytoplasmic matrix and interchromatin region of the nucleus. Some vacuoles can be seen in the cytoplasm. The rough endoplasmic reticulum and Golgi complex retain their normal configurations. Mitochondria are condensed. Lysosomes increase in number, and acid phosphatase activity is restricted within them. Massive excytotic release of mucus is seen at the cell apex. The basolateral plasmalemma seems intact until the latest stage of extrusion. At the tight and gap junctions, the outer leaflets of apposing plasmalemmas remain fused. On the other hand, microfilaments and tonofilaments are dissociated from the intermediate junctions and desmosomes, respectively, during degeneration.
Massive discharge of mucus and well preserved basolateral plasmalemma of the degenerating cell may restrict the back-diffusion of gastric juice into the mucosa to a minimum level during the degeneration and extrusion processes.

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