Distribution of immunoreactive LHRH-like substance was studied by PAP method using two different anti-LHRH sera (AsI: anti-Tyr⁵-BSA-LHRH serum; AsII: anti-Gly¹⁰-BSA-LHRH serum) in the brain of several species selected from each class of vertebrates. Although LHRH-immunoreactivity was slightly stronger in AsI than AsII, LHRH-like substance was found in the median eminence of all tetrapods tested (Table 1). LHRH-positive perikarya were detected by both AsI and AsII primarily in the septo-preoptic region of the cat, Elaphe climacophora, Rana catesbeiana, Xenopus laevis, Cynopus pyrrhogaster ensicauda. However, in the guinea pig, those perikarya were detected only by AsI in the medial basal hypothalamic region in addition to the septo-preoptic region. In most of those animals, LHRH-positive fiber pathways to the median eminence were also demonstrated. Among the teleosts examined, immunoreactive products were detectable in the fiber terminals of the neurohypophysis of Anguilla japonica and Fugu niphobles, but not in the three other species (Table 1). In the former two species, however, LHRH-immunoreactivity was slightly stronger in AsI than AsII. When AsI was applied to the hypothalamic sections of the lamprey, Entospenus japonica, dense immunoreactive product was found in the nerve perikarya of the preoptic nucleus and in the fiber terminals of the neurohypophysis. However, when AsII was applied to the lamprey, only a little immunoreactive product was detectable in the same regions. No immunoreactive product was detected in the brain of the dogfish (Triakis scyllia) and the hagfish (Eptatretus burgeri) in either case of AsI or AsII.

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Liquor contacting peptidergic neurons (LCPNs) in the preoptic nucleus of the Japanese eel (Anguilla japonica), are investigated submacroscopically, light microscopically, electron microscopically (transmission and scanning) and histochemically.
LCPNs appear in 8-13per cent of all neurons constituting the preoptic nucleus and their cytoplasm contains many secretory granules stained by aldehyde-thionin or chrome hematoxylin. LCPNs have an epithelial cell-like polarity and their cytoplasmic organella shift to the supranuclear region. LCPNs are classified into three types (A, B, C) according to the liquor contacting portion of the cell:
Granular type A neuron (40-50×40-50μm²), the cell body of which is in contact with the cerebrospinal fluid (CSF), is the most common type and distributed in the ventral portion of the preoptic nucleus; this neuron is not connected with the neighboring ependymal cells by tight junctions. Bipolar type B neuron (60×30μm²), contacts the CSF with the tip of its cell process and is scattered throughout the preoptic nucleus; the cell is connected with the surrounding ependymal cells by tight junctions. Bipolar type C neuron (60×30μm²) possesses a cell process protruded into the third ventricle and is distributed in the dorsal portion of the preoptic nucleus; this also is connected with the adjacent ependymal cells by tight junctions.
Regardless of type, all LCPNs exhibit a positive acetylcholinesterase and a negative ATPase reaction. Numerous fluorescent varicosities of monoaminergic nerve terminals are closely associated with the cell bodies of the LCPN. LCPNs are likely regulated by monoaminergic fibers.

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The location of LH-RH producing neurons, the pathways and terminal distribution of their fibers in the newt, Cynops pyrrhogaster were studied immunohistochemically. The LH-RH positive perikarya are located in the nucleus of the diagonal band of Broca and the medial septal nucleus. No LH-RH immunoreactive neuronal perikarya could be detected in the diencephalon. The pathways of fibers originating from the LH-RH reactive neurons were classified into three groups: the first project to the median eminence, the second to the olfactory bulb and the third to the habenular nucleus. The latter two pathways are extrahypothalamic, similar to those in mammals. These observations are briefly discussed and compared with those in other amphibian and mammalian species.

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Merkel cells and associated nerve endings of dog snout skin as well as foot and toe pads of rats were studied with the transmission electron microscope. Normal Merkel cells are characterized by the following morphological features: 1) localization restricted to the basal layer of the epidermis, 2) adjoining afferent nerve endings containing many mitochondria, 3) specific small round granules usually accumulated in the cytoplasm on one side of the nucleus facing the junction of the nerve ending, 4) highly lobulated irregular shape of the nucleus, 5) well developed Golgi apparatus with a few immature granules situated on the side of the nucleus opposite the junction at the ending, 6) spine-like processes extending from the cell surface where no nerve endings abut, 7) desmosomes and desmosome-like membrane thickening on the cell surface both abutting the surrounding keratinocytes and the nerve endings.
The Merkel cells with nerve endings were observed and measured at various days after denervation. The experiments were carried out on rats whose sciatic nerves were transacted. The samples of skin of foot and toe pads of the denervated legs were taken almost every day from 1 to 30 days. Almost no changes were noticed after 24hrs, but within the next day the nerves retracted and were no longer observed in the epidermis. The Merkel cell granules increased in number in 1-3 days but decreased again to about a half of the value of the control at 30 days. The polarity of granule distribution showing a strong accumulation in the cytoplasm near the nerve ending became less marked after denervation. Some cells sustained severe degeneration in the later stage of experiment.
It was remarkable that the activity of Merkel cells might depend on the associated nerves, but they never completely disappeared even after a long period of denervation.

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In adult dd-mice, response of peritoneal free cells to either horseradish peroxidase (HRP) or glutaraldehyde-fixed erythrocytes was examined by electron microscopy.
As reported previously (ABE et al., 1979), peritoneal free cells are classified into three types: type I, II and III cells. Type I cells are small lymphocytes, type II cells are medium-sized mononuclear cells, and type III cells are macrophages.
The amount of HRP taken up is very large in type III cells. The cells contain numerous vacuoles filled with HRP. Type I and II cells also have vesicles containing HRP. The vesicles are fewer in type I cells than in type II.
Type III cells rapidly enclose erythrocytes by thin cytoplasmic flaps and then ingest several erythrocytes. In type III cells, a labyrinth of ramifying tubes seen in the peripheral cytoplasm may serve as a reserve of the membrane system for allowing a rapid coverage and internalization of large foreign materials. A part of type II cells attach to erythrocytes and take them by a varying depth of invagination. Type I cells do not respond to erythrocytes at all.
In conclusion, the three types of peritoneal cells can be differentiated by their response to foreign materials as well as by their cytologic features.

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The nerve fibers supplying the islets of the dog pancreas were examined by electron microscopy. Axons with swollen portions containing various types of synaptic vesicles, including a cholinergic type, two varieties of presumable aminergic type and a possible peptidergic type, rush to the pericapillary space to end there, whereas only a small part of them seem to terminate on the endocrine cells of the islet.
Schwann cell cytoplasm invests the axons and often separates these from the endocrine cells. The terminal portions filled with vesicles usually become free of the Schwann sheath on the side facing the blood capillary. Here the neuronal secretions are believed to be released into the blood stream through the pored endothelium of the capillary.
The neurosecretions, together with the islet hormones are distributed to the exocrine pancreas in high concentrations via the insulo-acinar portal system. It seems thus sufficient for the nerves to supply the islet in order to control the exocrine function of the pancreas.

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Retinal capillaries of the rabbit were treated with HCl and collagenase to visualize the pericytes attached to the stromal surface of the capillary walls under the scanning electron microscope. Two types of pericytes were distinguishable on the basis of shape and localization. One type (Type I) had a slightly bulged, fusiform cell body with a few, long, branching cytoplasmic processes, and these cells were scattered mainly in the region of the true capillary. The other type (Type II) possessed a rather flattened and polymorphic cell body from which radiated short irregular cytoplasmic processes, and these cells, which tended to overlap more or less, were present predominantly in the region of the pre- or postcapillary vessels in the capillary bed. The latter type cells were assumed to be the precursors of the smooth muscle cells.

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Some endocrine glands of the carp-funa hybrid were studied with a light microscope to elucidate their detailed structure and the possible causal factor of sterility in the males. Adult specimens of carp (Cyprinus carpio), gengoroh-buna (Carassius auratus cuvieri), and their hybrid (F₁) were examined.
The hybrid males are sterile as manifested by the failure of meiosis and seminomatous neoplasm in their testes. The hybrid females revealed well-developed ovaries, but their fertility was not tested.
The hybrid hypophysis shows an intermediate condition between the parent species in the grade of ramification of the pars nervosa into the pars intermedia. Among seven types of granular cells demonstrated in the adenohypophysis, certain degenerative and anomalous changes are recognized only in the gonadotrophs of the hybrid hypophysis, especially in the female. These changes are discussed as a possible cause of sterility. A considerable amount of aldehyde fuchsin stainable neurosecretory material occurs in the cells of the nucleus preopticus and in the pars nervosa. The nucleus lateralis tuberis exhibits a histologically healthy condition.

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The fine structural localization of peroxidase activity in the endostyle of Ciona intestinalis was studied by cytochemistry at electron microscopic level. Only zone 7 cells in this organ are positive for this enzyme activity. The reaction product is recognized in the external surface of the apical plasma membrane, and in the cisternae of the rough endoplasmic reticulum, of the nuclear envelope, and of the Golgi lamellae, and small vesicles of the Golgi region and of the apical cytoplasm. By electron microscopic autoradiography of ¹²⁵I, the main site of iodination of thyroglobulin-like protein is considered to be the apical plasma membrane region of the zone 7 cells. The possibility of iodination of protein taking place within the endostylar lumen near the zone 7 cell cannot be ruled out. The relationship between the fine-structural localization of peroxidase activity and the site of iodination of protein is discussed.

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Spinal cords of the lamprey and hagfish were fluorescence histochemically and electron microscopically examined. Ventrally to the central canal of both cyclostomes, yellow fluorescence bound to small to medium sized neurons was observed. In the lamprey only, weakly blue-green fluorescent subependymal cells were seen just beneath the central canal. In the ventral floor of their spinal cord, yellow fluorescent varicosities were observed; their density was much higher in the lamprey than in the hagfish. The lateral surface of the hagfish spinal cord was marginated by a chain of yellow fluorescent varicosities. The yellow fluorescence was microspectrofluorometrically identified as fluorescence due to 5-HT. Electron microscopically, the 5-HT neurons contained many large dense-core vesicles. The 5-HT varicosities or terminals seen in the ventral zone of both cyclostomes possessed the large dense-core vesicles and small clear synaptic vesicles, which appeared as small dense-core vesicles after KMnO₄ fixation. The terminal of the lamprey was nakedly situated on the ventral surface, while that of the hagfish was always covered by the superficial glial layer. This finding seems to favor the view that lamprey and hagfish should be divided into two different classes.

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In the preoptic, infundibular and posterior recesses of the lamprey, Lampetra japonica cerebrospinal fluid (CSF)-contacting neurons are distributed much more abundantly than in higher vertebrates. They are classified into three different types on the basis of their granule size or electron density: type 1 containing round dense granules of large size (180-230nm in diameter), type 2 containing variously shaped dense granules of medium size (150-200nm) and type 3 containing variously shaped granules of small size (100-150nm). The neurons of the third type may be further classified into two subtypes: type 3a possessing dense granules and type 3b showing less dense granules.
The intraventricular process of CSF-contacting neurons contains Golgi cisternae that are forming new secretory granules and well developed granular endoplasmic reticulum with dilated cisternae. It is suggested that secretory granules might be formed in the intraventricular process in addition to the perikaryon.
Besides the small cytoplasmic protrusions presumably involved in microapocrine secretion, intraventricular processes occupied with many empty granules are detected which might represent a process of diacrine secretion. The intraventricular processes further contain multivesicular bodies which incorporate old or overproduced secretory granules, suggesting a process (crinophagy) and large lipid droplets likely corresponding to the final digestive product.
The CSF-contacting neurons of all types are usually supplied with axo-somatic synapses on the perikaryon and subsurface cisternae are sometimes observed beneath the postsynaptic membrane. Axon terminals are also found forming synapses on the intraventricular process of type 2 and 3 neurons.

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Human lymphoid cells were studied by electron microscopy to find any morphological difference between T and B cells.
The T cell population generally contains a smaller amount of the rough endoplasmic reticulum than the B cell in various functional states, i.e. peripheral blood lymphocytes of normal individuals, mitogen-stimulated lymphoblasts and lymphoid cell lines. This may depend on greater need of the rough endoplasmic reticulum for immunoglobulin synthesis in B cells, even though T cells produce some soluble mediators.
Other differences between T and B cells are as follows: (1) Peripheral blood T lymphocytes often have clustered dense granules, while granules containing parallel tubular structures are more frequently seen in B lymphocytes. (2) Cultured T cell lines usually exhibit a higher nuclear-cytoplasmic ratio and less prominent nucleoli than B cell lines.

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Stress was induced in mice by restraining on a board by pinning their limbs followed by immersion of the hind feet in a water bath of 20°C. Fine structure of adrenal chromaffin cells was studied by light and electron microscopy. After 8 to 22hrs' stress, remarkable decrease in the number of argentaffin granules of the NA cell was demonstrated by light microscopy. By electron microscopy, the decline in the granule number was marked in A, NA and SGC cells. This degranulation was accompanied by the occurrence of numerous small cytoplasmic vesicles which often appeared empty but sometimes contained an electron-dense material. Granule-containing invaginations of plasma membrane were frequently seen and were interpreted to represent the exocytotis profiles. The idea that the secretory granule membrane and associated substances might be recovered in the form of cytoplasmic vesicles was thus supported. Exposure of the mouse to restraint plus water immersion stress proved to be a simple method for production of acute and drastic degranulation of the adrenal chromaffin cells.

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Autonomic ganglia in the pancreas may include islet cells; while islets sometimes contain a group of ganglion cells. Electron microscope observation in adult mice confirms occurrence of a structure called neuro-insular complex type I by FUJITA (1959), in which islet cells are juxtaposed to nerve cells either directly or by intercalation of a thin glial cell cytoplasm. Islet cells are equivalent to neurons in their attitude towards neurons and towards glial elements in the neuro-insular complex. There seems to be every gradation between pure ganglia, mixed forms representing the neuro-insular complexes, and pure islets. Pancreatic islets may thus be regarded as modified ganglia.

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