After ³H-thymidine autoradiography was introduced to the study of cellular kinetics, the epithelial cells of the gastrointestinal mucosa have been examined extensively with regard to their kinetic behavior. After reviewing the knowledge on the kinetics of different epithelial cells in the stomach and small and large intestine, this article concentrates on the origin, differentiation and turnover of gut endocrine cells which have been under much controversy.
Autoradiographic and other investigations of EC cells support the view that these cells are not neuroectodermal in origin, but differentiate from an immature type of gut epithelial cells. We investigated the kinetics of the antral G cells in the hamster, G cells and S cells in the rat duodenum by the combined utilization of ³H-thymidine autoradiography with immunohistochemistry, and confirmed that these cells are derived from the same precursor cells as the other mucosal cells of the gastrointestinal tract. The half-life of the antral G cells was estimated at 10-15 days, while the turnover time of the G cells and S cells in the duodenum were calculated at 3-4 days and 2 days, respectively. These data also support the belief that gut endocrine cells originate from the endoderm.
Histopathological alterations such as antral gastritis and intestinal metaplasia, as well as vagal denervation, were shown to influence the population of the antral G cells in man and the hamster. This supports the view that intrinsic and extrinsic stimuli might alter the kinetics of gut endocrine cells.

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The immature part of the testis in the European fire salamander, Salamandra salamandra, consists of cephalo-caudally orientated zones of primary spermatogonia, secondary spermatogonia, primary spermatocytes, secondary spermatocytes and spermatids associated with follicle cells. Germ cells and follicle cells are arranged in lobules separated from each other by a vascularized connective tissue. Primordial germ cells are dispersed throughout the testicular stroma, preferably near the efferent ductules. They possess a multilobated nucleus, irregularly arranged mitochondria, nuage material, and a few lipid droplets. Primary spermatogonia are characterized by a more spherical nucleus, a coronal distribution of cell organelles—mainly mitochondria—small amounts of smooth endoplasmic reticulum, and lipid droplets. Secondary spermatogonia exhibit an irregular distribution of cell organelles with increased amounts of vesicular profiles of smooth endoplasmic reticulum; they are highly interdigitated and connected by intercellular bridges. In this stage degenerative processes may take place. Primary spermatocytes are provided with basal bodies and peripheral microtubules. Secondary spermatocytes and spermatids will be described elsewhere. Follicle cells and their projections surround each germinal cyst. Their nucleated parts are commonly localized in the central part of the lobule. Follicle cells and lobule boundary cells are very similar to fibrocytes. Already in the primary spermatocyte zone, both cell types show ultrastructural features indicating a setting in glandular activity.

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The fine structure of lamellated nerve endings found in the rat gingiva (Sprague Dowly) was examined by electron microscopy.
The lamellar corpuscles are localized in the subepithelial portion of the dermal papillae and are circular or oval in shape.
The lamellar cells surrounding the nerve terminals demonstrate several modifications in their configuration due to the variable number of cytoplasmic sheets. All the sheets are characterized by the presence of numerous caveolae, small mitochondria, a few cisterns of rough endoplasmic reticulum, microfilaments and microtubules.
Serial sections clearly showed that the nerve terminal is subdivided into two or four branches, each of which terminates at different levels of the corpuscle.
The laminae branch several times, ultimately connecting with those of neighboring laminae through desmosomes, and are separated from similar processes by an interlaminar substance. The nerve endings contain many mitochondria, neurotubules and small clear vesicles. Cytoplasmic protrusions extend from the nerve terminal into the clefts between the lamellar cells. Small clear vesicles are found in the basal area of the protrusion. The interlaminar spaces are occupied by an amorphous material though collagenous fibrils are conspicuous in some regions.

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Immunoperoxidase histochemical investigations on pig hypophysis show that CRF immunoreactive nerve terminals are distributed around the blood vessels in the posterior lobe, particularly in its proximal part adjacent to the intermediate lobe. CRF immunoreactive nerve fibers are also seen in the intermediate lobe. The distribution of the CRF-containing nerve in the posterior lobe was found to be similar to that of the vasopressin-containing nerve terminals. The functional significance of the presence of CRF-containing nerve fibers representing a new component of the hypothalamo-hypophyseal neurosecretory system is discussed.

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Dissociated cerebella of newborn rats were cultured in vitro as a monolayer for a long period, and ependymal cell differentiation was investigated by scanning and transmission electron microscopy. Cells with actively beating cilia were recognized on the 3rd day, and gradually increased in number thereafter. At nine months of culturing, a number of areas appeared, some of which consisted of several hundreds of ciliated cells. Cultured ependymal cells showed a striking resemblance in their fine structure to the intact ependyma. They were always found to grow upon the feltwork of astrocytic processes which served as a subependymal structure, as observed in the intact ependyma. It is possible that astrocytes may play some role in ependymal differentiation. Cultured ependymal cells are helpful in understanding the control mechanism for the metachronal cilial movement.

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The topography and ultrastructure of LHRH and somatostatin nerve fibers in the median eminence in male rats were investigated with special reference to the relation between the fiber terminals and the perivascular space of the portal capillaries. Vibratome sections taken from the rostral and preinfundibular portions of the median eminence were immunostained with anti-LHRH or anti-somatostatin serum before plastic embedding for electron microscopy. In the external layer of the median eminence, the LHRH fibers did not directly contact the perivascular space, although they were in close proximity to the external surface of the median eminence. On the contrary, somatostatin fibers divided into branches in the external layer of the median eminence and terminated in direct contact with the perivascular space in the median eminence. LHRH fiber terminals contain immunoreactive granules, whereas somatostatin fibers are characterized by the presence of synaptic vesicle-like structures, vesicle ghosts and immunoreactive granules in their terminals. These topographical and ultrastructural differences might be regarded as a morphological manifestation of different mechanisms in the release of neurohormones.

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The distribution and organization of odontoblast processes in young human dentin was examined with a scanning electron microscope. By applying the HCl-collagenase method, the extracellular matrix of dentin was almost completely removed, thereby exposing the odontoblast processes and their branches to direct observation. The odontoblast processes are located close to the dentinoenamel junction. In the middle and outer zones of dentin the processes bear numerous branches. Some of these appeared to bridge the space between the processes and connect them.

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The distribution of C cells in the thyroid glands of eight young adult monkeys (4 Macaca fuscata and 4 Macaca irus) was investigated by complete serial sections utilizing the immunoperoxidase method with anti-human calcitonin and anti-dog C-thyroglobulin antisera. The secretory granules of monkey C cells were strongly immunoreactive to both antisera. A markedly uneven distribution of C cells was found. The C cells were distributed in a small area restricted to the dorsomedial portion of the lobe along the longitudinal axis below the parathyroid IV, while the remaining larger portions were devoid of C cells. In the areas where the C cells were concentrated, the cells were scattered, surrounding a large portion or even the whole of the epithelium lining follicles and often forming multiple clusters among the follicles. In these places the C cells exceeded the number of follicular cells.

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The neutrophilic granulocyte series in the human liver obtained from 109 embryos between 28 to 49 days after ovulation and 76 fetuses between 8 to 22 weeks of gestation was investigated by light and electron microscopy.
Hemopoietic cells, considered progenitors of the granulocyte series, and undifferentiated cells first appeared in the intercellular spaces of mesenchymal cells around the ductus venosus of an embryo of 14 mm crown rump length (estimated age: 40 days after ovulation). The early hepatic myeloid progenitor cells which appeared by 50 days of gestation (early stage of hepatic granulopoiesis) differed in ultrastructure from the late hepatic myeloid progenitor cells which first occurred after 50 days of gestation (late stage of hepatic granulopoiesis).
These findings indicate that, morphologically, two kinds of hemopoietic stem cells exist in the human embryonic and fetal liver, and that the hepatic granulocyte series in the early stage differs in its process of maturation from that in the late stage.
Endogenous peroxidase activity was demonstrated in small granules of the late hepatic myeloid progenitor cells which appeared lymphoid in ultrastructure.

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Nerve segments approximately 6-7mm long were excised from the predegenerated sciatic nerves of mice, and treated 5 times by repetitive freezing and thawing to kill the Schwann cells. Such treated nerve segments were grafted into the original place, being in contact with the proximal stump of the sciatic nerve. The animals were sacrificed 2, 3, 5, 7 and 10 days, 2, 3, 5 and 8 weeks after the grafting. The grafts were examined at the middle level, i. e., about 3-4mm distal to the proximal end of the graft, by light and electron microscopy.
Within 2-3 days after the grafting, the dead Schwann cells were disintegrated into fragments and gradually phagocytized by macrophages. Howere, the basal laminae of the Schwann cells remained as empty tubes (basal lamina scaffolds). The notable finding was that the regenerating axons always grew through these basal lamina scaffolds. New Schwann cells seemed to migrate along these axons from the proximal stumps. The number of axons growing through the basal lamina scaffolds gradually increased with time. These axons were surrounded in a bundle by Schwann cells. About 1 week after the grafting, axons began to be segregated into smaller bundles by Schwann cells. Axons with a relatively large diameter (about 2μm) tended to be sorted out and surrounded by their own Schwann cells. The myelination began about 2 weeks after the grafting on such large diameter axons. The basal lamina scaffolds, through which the regenerating axons had grown, were gradually disintegrated into fragments by the expansive forces due to the increase in number and volume of the regenerating axons and Schwann cells. Groups of axons, which had been derived from the same basal lamina scaffolds, were enclosed with the cells resembling perineurial epithelial cells. These perineurial epithelial cells proliferated and further separated groups of axons into smaller ones or even into single axons. The number of myelinated axons increased with the advancement of regeneration.
These results show that the basal lamina scaffolds of Schwann cells serve as efficient conduits for the elongation, maintenance and maturation of regenerating axons.

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The localization of the gastro-entero-pancreatic (GEP) endocrine cells in the flatfish, Paralichtys olivaceus was examined using immunocytochemical methods.
The Brockmann body of the flatfish included a large principal islet and a smaller islet. Three types of endocrine cells, i. e., insulin-, glucagon- and somatostatin-immunoreactive cells were found in both islets. Pancreatic polypeptide (PP)-immunoreactive cells were restricted to the periphery of the smaller islet.
In the digestive tract, somatostatin cells occurred only in the stomach. The pyloric appendages contained cells reactive simultaneously to cholecystokinin (CCK) antiserum and to gastrin antiserum, whereas the middle portion of the intestine contained cells reactive only to the CCK antiserum. The intestinal endocrine cells showing N-terminal glucagon-immunoreactivity were also reactive to a glicentin antiserum.

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