The mechanisms of intestinal absorption were reviewed and discussed from the viewpoint of fine morphology of the small-intestinal epithelium. After reviewing the fine structural specializations of intestinal absorptive cells, particular emphasis was given to the morphological aspects of the first entry of gut contents into the intestinal epithelium.
1. The absorption of gut contents into the absorptive cells involves two kinds of mechanisms: one is the transport across the plasma membrane of the microvilli, by which mainly micromolecules are absorbed into the cells, and the other is the transport by endocytosis at the base of the microvilli, by which macromolecules can be taken up into the cells.
2. Freeze-fracture replica studies on the microvillous membranes suggest the existence of transmembrane channels formed by assembling of three or more protein subunits, through which hydrophilic micromolecules may be transported into the cytoplasm of absorptive cells. Since disaccharidases are known to be localized in the microvillous membranes as a complex composed of two or three disaccharidase molecules, the possibility is suggested that some of the protein subunits forming the transmembrane channels might be members of disaccharidases, and thus, the sugar absorption across the microvillous membranes may be closely coupled with the terminal digestion by disaccharidases.
3. The endocytotic uptake of macromolecules such as horseradish peroxidase or ferritin is seen as a common function of the absorptive cells in the small intestine of suckling mammals or stomachless fishes, but not common in the normal small intestine of adult mammals. The absorptive cells in the adult, however, have a potential ability of endocytotic uptake of substances which might be enhanced under some pathological conditions like malabsorption syndrome or in some experimental conditions such as ligation or organ culture of the small intestine.
4. The intercellular junction between adjacent epithelial cells is not permeable to such macromolecules as intact proteins, but permeable to micromolecules such as ionic lanthanum. This characteristic permeability of the tight junction may well be reflected in the structural organization of intramembranous junctional particles which is revealed by freeze-fracturing of the tight junctional region. Thus, the intercellular transport of intestinal contents is usually restricted, if any, to very small amounts of micromolecules.

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The three-dimensional structure of the Golgi complex in rat intestinal epithelium and its morphological alteration during fat absorption were studied by the Osmium-DMSO-Osmium method.
1. The Golgi complex consisted of a Golgi stack which was a parallel array of flattened cisternae, numerous small vesicles of similar size, condensing vacuoles which expanded from the periphery of the cisterna, and the plexus of anastomotic tubules associated with many granules of various sizes.
2. During fat absorption, the condensing vacuoles of the Golgi complex were dilated and many lipid droplets appeared within the lumen of the condensing vacuoles, while the length and number of the parallel arrayed cisternae decreased remarkably. At 60min after fat administration the membrane alteration reached its maximum. The Golgi complex consisted of only vacuoles containing lipid droplets at this time.
3. The lipid droplets in the vacuoles were manifested with a colored scanning electron micrograph which was prepared by superimposition of the secondary electron image and the backscattered electron image from a given view field of the specimen, projected in two different colors.

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The retinae from rats of various ages were stained using the weak silver carbonate method of del Rio-Hortega. In newborn and early postnatal (3-day-old) rats, silver-impregnated round and amoeboidic cells displaying thick stout pseudopodial processes were distributed in the nerve fiber and ganglion cell layers. In 10-day-old rats, the cell bodies were rod-shaped bearing pseudopodia or fusiform showing slender and branching processes. The majority of the cells were found in the ganglion cell layer with occasional ones in the inner plexiform layer. By 20 days of age, the cells became smaller and exhibited long branching processes. They were located predominantly in the inner plexiform or bipolar cell layer. In the adult animals, all the silver-impregnated cells appeared small and flattened with long branching processes. Based on their morphological features, it was concluded that they were in fact amoeboid microglia and microglial cells present in the developing retina which probably function as macrophages as described in the brain tissues.

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In order to elucidate the mechanism of dentin sensitivity, the ultrastructure, distribution and organization of nerve fibers in the pulpodentinal border zone in the teeth of young human subjects were investigated by means of silver impregnation and electron microscopy.
The nerve fibers in this zone were classified into four types by the location of their terminals and pattern of their ramification: the marginal pulpal nerve fibers, the simple predentinal nerve fibers, the complex predentinal nerve fibers and the dentinal nerve fibers. The nerve fibers reached no further than 100μm from the odontoblast-predentinal border.
The nerve fibers terminated exclusively as free endings, and they were thought to conduct the sense of pain from the corresponding zone. The endings of the predentinal or dentinal nerve fibers were mostly located adjacent to the odontoblastic processes, and this appeared to be a reasonable position for these endings to actively receive the changes in the shape of the processes. The odontoblastic process and the nerve ending associated with it can be considered to be functionally a mechanoreceptive complex. In this sense these free nerve endings might be placed in a group different from the free nerve endings in the inner pulp. The mechanoreceptive complex probably plays a central role in the mechanism of dentin sensitivity.
It is suggested that stimuli to dentin first produce the deformation or movement of the odontoblastic processes; these mechanical changes are transmitted to the nerve endings, and the dentin sensitivity occurs.

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Localization and ultrastructural appearance of immunoreactive LHRH neurons were studied in 2-, 8- and 21-day-old rats. The lower half of the diencephalon including the septo-preoptic area was serially cut at 10μm in the frontal plane, and all sections stained with an anti-LHRH serum were examined light microscopically. Although the antiserum could not demonstrate LHRH-immunoreactive perikarya in adult rats, they were stained in these young rats. The immunoreactive perikarya were scattered in the following regions: nucleus tractus diagonalis, medial septum, medial preoptic area, lateral septum, lateral preoptic area, tuberculum olfactorium, suprachiasmatic area, supraoptic area, pericommissural area, anterior commissure, vascular organ of the lamina terminalis (OVLT), lateral hypothalamic region and lateral tuberal region. The first three regions contained many cells but others showed only sporadical cells. The cells were most frequently observed in 8-day-old rats. Most of the cells appeared to be bipolar showing two immunoreactive processes. Electron microscope observation was made using the sections prepared by a pre-embedding technique. In the immunoreactive perikarya, secretory granules, ribosomal particles, and the particles on the outer surface of endoplasmic reticulum appeared to be immunoreactive, but the Golgi area was immunonegative. The secretory granules appeared along the cytoplasmic membrane but were surprisingly few in number. By contrast, the immunoreactive nerve terminals in the median eminence and in the OVLT contained many granules. These findings suggest that LHRH produced in the perikarya is prompted to be transported and stored in the axonal terminals.

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The development of the lamellar cells of mouse digital corpuscles (Meissner corpuscle) was studied by light and electron microscopic histochemistry for cholinesterase (ChE). The materials used were the hind limbs taken from fetuses at 14, 17 and 20 days of gestation, and from young mice at 1, 5, 7, 15 and 20 days after birth. Embryonal Schwann cells had non-specific ChE activity in the cisternae of the rough endoplasmic reticulum and the nuclear envelope, suggesting that they had the ability of synthesizing the enzyme. After birth, such ability gradually decreased and by five days of age non-specific ChE activity was no longer demonstrable in Schwann cells at the time when myelin sheath formation began. However, Schwann cells which were associated with the axonal tips penetrating into the epidermis still had an intense non-specific ChE activity. Such Schwann cells surrounded the axons in gradually increasing numbers of cytoplasmic processes, which later became the lamellae around the axon terminals; thus by 20 days after birth they had differentiated into mature lamellar cells of Meissner corpuscles. These lamellar cells had, as in the embryonal Schwann cells, an intense ChE activity in the cytoplasm. These findings indicate that the lamellar cell is a specialized form of Schwann cell which still retains the embryonal characteristics for synthesizing non-specific ChE.

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In their transmission electron microscope (TEM) study of the rabbit kidney, ELIAS et al. (1964, 1966) reported an occurrence of “podocytic membranes” which were attenuated parts of the body of the podocytes covering the processes and pedicels of podocytes and forming closed spaces, “subpodocytic lacunae” (Fig. 1, 2).
In this scanning electron microscope (SEM) study, we demonstrate the three-dimensional structure of rabbit podocytes which largely confirms the ELIAS' diagram. However, the “podocytic membranes” are not as in ELIAS' view, attenuated portions of the body of the podocytes, but correspond to their primary processes which are flattened into plates in this species. The “podocytic membranes” do not represent a second filtration barrier for the primary urine, which ELIAS proposed.
On the glomerular endothelium, we found domed microprojections consisting of a pored cytoplasmic sheet not only in the attenuated portions of the endothelium or areolae fenestratae, but also in the nuclear portion of the endothelial cells. This hitherto unknown distribution of pored cytoplasm necessitates revision of the previous view which has interpreted the significance of the endothelial pores only from the aspect of substance transport through the capillary wall.

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Human pancreatic A-, B- and D-cell clonal strains, named JHPG-1, JHPI-1 and JHPS-1, were established successfully from adult and fetal pancreata by the single cell plating feeder layer method using a modified Rose's chamber. This is the first time that insulin-, glucagon- and somatostatin-producing clonal strains have been separated into continuous clonal cell lines.
The cultured cells are epithelial in nature, free of fibroblast contamination, and can be cloned. Under the phase-contrast microscope, the B-cell clone (JHPI-1) was generally oval or round in shape, the A-cell clone (JHPG-1) was bipolar, and the D-cell clone (JHPS-1) was nerve-like with cytoplasmic processes. By the use of immunocytochemical techniques, insulin-, glucagon- and somatostatin-like immunoreactivities were detected in each clone respectively. By radioimmunoassay it was revealed that each clone produced a single pancreatic hormone. The B-cell clone especially, was found to secrete insulin amply and continuously, for over 150 days. The glucagon release responses by the A-cell clone to insulin and glucose were also studied.
The clonal strains obtained in this study provide useful systems for the investigation of the cell-biological aspects of human islet cells in vitro.

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