Cerebrospinal fluid (CSF)-contacting neurons are located periventricularly or inside the brain ventricles; they contact the CSF via their dendrites, perikarya or axons. Most of the CSF-contacting nerve cells send dendritic processes into the ventricular cavity where they form ciliated terminals. These ciliated dendritic endings resemble those of known sensory cells, yet their role is still unknown.
There are two types of CSF-contacting dendritic terminals. One bears solitary 9×2+0 cilia; it is present in different hypothalamic regions such as the paraventricular organ and the vascular sac. The magnocellular neurosecretory nuclei also contain CSF-contacting neurons, which probably furnish information about the parameters of the CSF for the regulatory function of the hypothalamo-hypophyseal system. CSF-contacting nerve cells of the parvocellular hypothalamic nuclei are suspected to participate in hypothalamo-adeno-hypophyseal regulation. A second type of CSF-contacting dendritic terminal bears many stereocilia and is found in the central canal of the spinal cord. This type of terminal is also supplied with a 9×2+2 kinocilium that may contact Reissner's fiber, the secretory material of the subcommissural organ. Resembling mainly mechanoreceptors, these spinal CSF-contacting neurons appear to form axon terminals of the neurosecretory type at the external circumference of the spinal cord.
Developing and/or regressing photoreceptor cells of the retina and pineal complex may display a similar dendritic structure characteristic of hypothalamic CSF-contacting neurons. Axons penetrating into the ventricles innervate the apical surface of the ependyma and/or the CSF-contacting dendritic terminals. Some bipolar neurons of the retina form so-called Landolt's clubs; these may be considered as the retinal component of the CSF-contacting neuronal system. Since in the lancelet nearly all nerve cells contact the CSF, the CSF-contacting neurons represent a specialized, but phylogenetically old cell type, a “protoneuron” in the vertebrate brain. They may be derived phylogenetically by inversion of the ciliated neurons found in the plate-like nervous system of more primitive deuterostomians.

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The influence of the sensory nerve fibers on the differentiation of the Merkel cell was examined in the denervated labial mucosa of adult rabbits. Part of the lower labial mucosa was excised following mental nerve resection. Twenty-one and 50 days later, the regenerated mucosa was examined by electron microscopy with special reference to the distribution and maturation of Merkel cells, and compared with the normal and the denervated intact mucosa.
A number of immature Merkel cells appeared in the denervated and regenerated epithelium by 21st day after the operation. The distribution density of the Merkel cells in a unit area of the mucosa did not change much but the percentage of the mature Merkel cells increased significantly until 50 days after the operation. The mature Merkel cells in the denervated and regenerated mucosa showed a normal ultrastructure, though their orientation and location were not uniform, shifting more superficially than the cells in the normal mucosa. In the controlateral intact epithelium of the denervated labial mucosa, no substantial decrease in the population density of Merkel cells was recognized, though the desquamation of Merkel cells was observed. It was conspicuous that the percentage of immature Merkel cells to the total number of the Merkel cells was significantly increased after the denervation in the intact labial mucosa.
This study suggests that Merkel cells differentiate independently of sensory nerve fibers, but the latter are requisite to maintain the former in a uniform, basal disposition in the epithelium.

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HeLa cells demonstrated a marked phagocytic activity towards degenerating cells in their own population after single thymidine blocking. The thymidine treatment caused the high frequency of cell death at the time of thymidine release, but these dead cells were cleared by phagocytosis or autolysis in 7hr after release. Then a prominent increase in the phagocytic activity of non-mitotic cells towards degenerating cells occurred in the mitosis-rich stage 10hr after the release, but this was not associated with a high frequency of cell death.
The non-mitotic cells in this stage had many microplicae and microvilli on their surface, while those in the other stages were relatively smooth in surface morphology. HeLa cells, epithelial in origin, are considered as non-professional phagocytes which retain a primitive phagocytic activity and manifest this function when necessary.

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The author demonstrated earlier that the nucleolus-like inclusion bodies in the central nervous system observed under an electron microscope are characteristically stained by the modified Holmes silver impregnation method. These inclusion bodies were then counted in the entire mouse brain; they numbered 1, 547±144 on the unilateral side of the locus coeruleus.
In the present experiment, mice were subjected to various experimental conditions or given several medicaments that might induce changes in body weight and in the number of inclusion bodies. Following dehydration, fasting, dehydration with fasting, induction of stress condition, reserpine, a-methyl-p-tyrosine, cycloheximide, cephalexin and puromycin administrations, body weight was decreased along with a decrease in the number of inclusion bodies in the locus coeruleus. After streptomycin sulfate injection, however, body weight was increased and inclusion bodies showed a moderate increase in number. Actinomycin D and mitomycin C, inhibitors of nucleic acid synthesis, brought about a definite decrease in body weight, but little if any changes in the number of inclusion bodies. Furthermore, after reserpine injection several small inclusion bodies were observed in the cytoplasm of locus coeruleus cells in a recovery stage and after inclusion bodies regained their normal number and size.
The results suggested that the inclusion bodies might have some relation to body weight, as well as to monoamine of the brain and protein synthesis.

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By light and electron microscopic autoradiography using ³H-thymidine, the renewal and development of surface mucous cells and glandular cells in the gastric body were studied in normal young adult golden hamsters.
1. The isthmus and neck regions of the gastric gland are composed of immature surface mucous cells, immature parietal cells, young parietal cells and mucous neck cells. It is presumed that the stem cell having the potency of differentiating into various kinds of cells is not present in the gastric mucosa of the adult golden hamster.
2. Immature surface mucous cells, located in the isthmus region and capable of synthesizing DNA, contain abundant free ribosomes and a small number of secretory granules. The secretory granules increase in number as the cells ascend towards the gastric surface, forming a thick granule zone in the cell apex. The cells further contain small electron lucent vesicles which are likely involved in the formation of glycocalyx.
3. The immature parietal cells in a low differentiated state, possessing the DNA synthesizing ability, are rarely found in the isthmus. They show collapsed intracellular canaliculi with long microvilli and contain numerous free ribosomes but smooth tubular elements are difficult to recognize.
4. Some of the mucous neck cells located in the isthmus to neck region and capable of synthesizing DNA, are characterized by abundant free ribosomes, a few elements of rough endoplasmic reticulum relatively well developed Golgi apparatus and a few dense secretory granules.
5. Mature surface mucous cells contain abundant secretory granules filling an apical granule zone. They are fated to be degenerated, and extruded into the gastric lumen. This is considered to be a holocrine process.
6. The middle and basal regions of the gland consist mainly of chief cells, parietal cells and endocrine cells. A few of the chief cells in the middle part of the gastric gland also have the activity of synthesizing DNA.
7. Some of the immature cells in the isthmus region, and mature glandular cells in the bottom of the gland are degenerated and phagocytosed by neighboring epithelial cells.
8. This paper also records the occurrence of smooth muscle fibers in the lamina propria, extending from the muscularis mucosae to the basal part of the degenerated surface epithelium.

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An electron microscope examination of the liver of the crab-eating monkey revealed small crystalloids occurring occasionally in the thicker portion of the cytoplasmic extension of the sinusoidal endothelium. They were uniformly encased in a membrane sac which was mostly smooth-surfaced but was at several points continuous to ribosome-studded cisternae of the RER. The crystalloids were mostly polygonal in configuration and were classified into three types. Type I crystalloids, according to the grade of the complexity of their composition, represented the simplest, or original, form and were composed purely of a compact bundle of tubules measuring about 300Å in diameter. Types II and III crystalloids were composed of tubules and an electron lucent matrix. In type II crystalloids, the tubules were embedded parallel to one another in two sets of matrix layers which crossed each other at a right angle, while in type III, the matrix layers embedding the tubules cut each other at about 70°. The crystalloids are presumed to have developed from a substance synthesized in the cisternae of the RER in the sinusoidal endothelial cell and their investigation may aid in elucidating the proteinic products of this RER-rich cell which have thus far remainedunder dispute.

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In order to elucidate the cytological features of preosteoclasts and the process of their differentiation into osteoclasts, fetal rat parietal bone was examined using light microscopy, organ culture, electron microscopy and histochemical methods.
1. Parietal bones of rat fetuses from 15 to 21 days of gestational age were examined light microscopically. A solid bone plate was found in 19 day old fetuses, but no multinucleated giant cells were observed in either the ecto- or endocranial periosteal surfaces. They were first observed at the endocranial periosteal surface in 20 day old fetuses, and increased in number in 21 day old fetuses.
2. Parietal bones of fetuses from 15 to 19 days of age were cultured and the possible occurrence of preosteoclasts prior to the appearance of osteoclasts was examined. During organ culture, eosinophilic multinucleated cells appeared in the parietal bones from 17, 18 and 19 day old fetuses, and increased in those from 19 day old fetuses.
3. Electron microscope observation of the parietal bones in 19 day old fetuses revealed moderate numbers of mononuclear cells identified as preosteoclasts (SCOTT, 1967) principally among the osteoblasts and preosteoblasts at the endocranial periosteal surface. Preosteoclasts with ill-developed cell organelles tended to be located between blood vessels and active osteoblasts, and sometimes located close to the bone surface with only the thin cytoplasmic processes of adjacent osteoblasts intervening. On the other hand, well-developed preosteoclasts tended to be located close to flattened osteoblasts and came into direct contact with the exposed mineralized bone between them. Preosteoclasts were not clustered together but were usually found in contact with adjacent osteoblasts and/or preosteoblasts. Membrane fusion between a preosteoclast and a flattened osteoblastic cell was observed. Multinucleated cells were principally preosteoclastic in appearance but some were both osteoclastic and osteoblastic. The multinucleated cells with ruffled borders identified as active osteoclasts increased in number over a particular time span.
4. The cytochemical localizations of ALPase, ALPase and peroxidase activities in the preosteoclasts resembled those in the osteoclasts but differed from the osteoblasts and preosteoblasts with respect to the ALPase activity. An intense peroxidase activity was detected only in monocytes and neither in preosteoclasts nor in osteoclasts.
These results suggest that the cytodifferentiation of preosteoclasts into osteoclasts may be induced by their direct contact to the mineralized bone surface exposed by detachment of osteoblasts, and that the detached osteoblasts may also serve as either an inducer or a constituent of the forming osteoclasts.

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Endocrine cells in the stomach, intestine and pancreas of the mink were investigated, using silver impregnation and immunohistochemical methods, and the following results were obtained.
1. The stomach of the mink possesses a well-developed acid-secreting region which occupies about 70% of the gastric mucosa
2. Half of Brunner's glands whose excretory duct empties in the most proximal duodenum are located in the duodenal submucosa with the remainder in the pyloric submucosa. The area covered by the glands is 7.5mm long in rostrocaudal direction.
3. Endocrine cells are numerous in Brunner's glands, in the pyloric gland region and in the duodenum, while they are few in the colorectum.
4. Somatostatin-immunoreactive cells are distributed throughout the whole GEP system, while gastrin-immunoreactive cells are located mainly in the pyloric gland region. Secretin-, motilin- and neurotensin-immunoreactive cells are found in the duodenum, jejunoileum and lower jejunoileum, respectively.
5. Glucagon-immunoreactive cells are located mainly in the pancreatic islet and are distributed scarcely in the fundic gland region. A few glucagon-immunoreactive cells are also found in the middle portion of the jejunoileum.
6. In addition to the somatostatin-immunoreactive cells, argentaffin, glucagon- and glicentin-immunoreactive cells in the fundic gland region and argentaffin and gastrin-immunoreactive cells in the pyloric gland region extend cytoplasmic processes along the basement membrane. This suggests a paracrine secretion of these cell types.
7. A few open type cells which are stained with Hellerström-Hellman's or Sevier-Munger's method or are reactive to the somatostatin antiserum are found in the fundic gland region.
A possible relation between the present observation of the endocrine cells and the eating habits of the mink is discussed.

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Immunocytochemical studies of the rat and pig indicate that immunoreactivity for PHI (heptacosapeptide recently isolated by TATEMOTO and MUTT, 1981, from the porcine intestine) is located in numerous nerve fibers in the ileum and colon. Myenteric and submucosal plexuses contain abundant PHI-immunoreactive fibers. Alternate staining of adjacent sections with anti-PHI and anti-VIP sera indicates that PHI-immunoreactive nerves coincide with VIP nerves. This co-existence of both peptides in the same neurons may be accounted for by their probable sharing of a common precursor.

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