Mesenteric lymph nodes of the rat were studied by scanning electron microscopy after arterial perfusion fixation. Observation was made mainly of the high endothelial venules (HEVs), inner cortex and medulla.
1. The HEVs were distributed not only in the inner cortex, but also in the interfollicular areas in the outer cortex, extending to closely beneath the marginal sinus. Various surface structures of the HEV endothelial cells were recorded, from quite smooth to densely granular. Special attention was paid to endothelial cells with cytoplasmic ridges concentrated at the center of the cell apex. Lymphocytes were numerously attached to the luminal surface of the HEV. Many lymphocytes presumably under and after emigration through the HEV wall could be recognized.
2. Hitherto unknown labyrinths densely filled with lymphocytes were demonstrated in the inner cortex. These tubular and saccular spaces lined by attenuated cells originated from beneath the germinal centers and from around the HEVs to pour into the medullary sinuses. The lymphatic labyrinths were differentiated from the sinuses by their lack of reticular cell trabeculae and by their containing almost exclusively small lymphocytes.
3. It was proposed that lymphocytes in the blood in HEVs massively emigrate to the labyrinth, and via the medullary sinuses, enter the efferent lymphatics of the lymph node.
4. In the medullary sinuses, some lymphocytes in association with a macrophage were found to extend wing-like processes, probably reflecting their activation. Large free cells extending laminar projections, but smooth in surface otherwise, were occasionally found. Their possible identity with the veiled cells (KELLY et al., 1978) was considered.

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An electron microscope study of the corpus callosum in postnatal rats of various ages was carried out to elucidate the fate of the amoeboid microglial cells. The cells present in the corpus callosum of younger rats (3-5 days) were round and showed an eccentric nucleus with marginal chromatin. They displayed numerous lysosomal granules and vacuoles in the cytoplasm. In older animals, i. e., from 7 days onwards some of the cells became oval so that by 15-20 days of age most of the cells were elongated and branched. In the latter, the cells showed a flattened or angular nucleus with dense chromatin clumps. The cytoplasm showed fewer lysosomal granules and vacuoles which were absent in cells of 20 day old animals. Quantitative measurements showed that there was a gradual diminution in the amount of cytoplasm at the cell body of amoeboid microglial cells with age, so that by the age of 20 days the cells were reduced to less than one-third of their original size as seen in 3 day old rats. The reduction of cytoplasm at the cell body is probably because it is channelled to the cytoplasmic processes which are evident in older rats. Some cytoplasm may have been extruded and phagocytosed by companion cell types.

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The distribution and structure of the neurons containing opioid peptide-like immunoreactivity (enkephalin neurons) in the antimesenteric border of the guinea pig proximal colon were immunocytochemically investigated using an antiserum for methionine-enkephalin-Arg⁶-Gly⁷-Leu⁸ (R-0171). Whole-mount preparations of the different layers of the intestine perfusion-fixed with Bouin's fluid were immunostained by peroxidase-antiperoxidase techniques. Immunopositive nerve fibers were apparent in the longitudinal muscle layer, myenteric plexus, circular muscle layer and submucosa. Immunopositive perikarya of the ganglionic cells were found in the myenteric plexus. A Golgi-type panoramic view was obtained in the intensely-immuostained enkephalin neurons. Distinct immunoreactivity was shown in the many Dogiel type 1 neurons, characterized by short broad processes (winglets or alulae) and one long axon-like process, as well as a few type 2, characterized by several tapering processes, and type 3 neurons, characterized by dendrite-like processes. Many twig-like processes originated from the free margin of the winglet of the enkephalin neurons (wing-ramuli). A part of them entered the intramuscular fasciculus, while the rest remained inside the ganglion. There were transitional forms between these wing-ramuli and the tapering processes of the type 2 neurons or the dendrite-like processes of the type 3 neurons. The axon-like processes sent out branches (axon-ramuli) along their courses or into the intramuscular fasciculus. At the origin of these axon-ramuli, there was a nodulous or humped swelling of the axon-like process (nodulus or crista). In the myenteric ganglion, the axon-ramuli formed varicose terminals. In the guinea pig proximal colon, many axon-like processes of the enkephalin neurons ran in the oral direction. This polarity of neuronic processes may have a functional significance in the neuronal control of the antiperistalsis.

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Radial components of the optic nerve of the mouse were studied by using thin sections and freeze-fracture replicas. The investigations were performed on normal optic nerves, as well as on those undergoing Wallerian degeneration following eyeball enucleation. Normal radial components in thin sections were observed as a series of light lines composed of small electron-lucent dots situated in the interperiod lines across the myelin sheath. They are frequently found in those parts of the myelin sheath lying near the outer and inner processes of the oligodendroglia. Radial components in freeze-fracture replica were observed as a parallel array of many ridges composed of a row of particles.
The particles of radial components located in the deeper part of the myelin sheath lose their linear arrangement and fall into disorder in a relatively early post-operative period. The parallel array of rows of particles located closely beneath the outer processes of the oligodendroglia remained intact for a long period, even in a markedly distorted myelin sheath.
The present observations suggest that the radial components are resistant against the disintegration of the myelin lamellae during Wallerian degeneration.

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Intercellular spaces in the epithelium of the rabbit Peyer's patch and appendix were examined by scanning and transmission electron microscopy to elucidate their three-dimensional structure and their relationship to the reticular spaces in the underlying lymph nodule.
Two types of intercellular spaces were distinguished: regularly arranged tubular channels and irregularly winding tunnels. The tubular channels were observed around normal enterocytes on the villi or internodular folds and apical portions of nodule domes. The channel spaces were lined with a successive arrangement of belt-like intercrestal surfaces of prismatic enterocytes and variously sized processes on the crests. The processes adjoining opposed crests formed a ladder-like structure or pectinate septa between neighboring channels.
The irregularly winding tunnels were formed among processes of irregularly shaped cells corresponding to FAE cells (BOCKMAN and COOPER, 1973) or M cells (OWEN and JONES, 1974) in the nodule associated epithelium. In the appendix, the tunnels were frequently organized into two-storey spaces, the adlumenal and basal spaces, which were incompletely separated by cytoplasmic processes. These tunnels continued by pores in the basement membrane to the reticular spaces in the underlying lymph nodules. Furthermore, the tunnels and the basement membrane pores constantly contained single or grouped free cells or their processes.
The findings in the present study suggest that the tubular channels are intraepithelial compartments for the absorption of nutrients and fluid, and the irregular tunnels are an intraepithelial network of spaces for the housing of lymphoid cells coming from the underlying lymph nodule.

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Blood vascular beds of the rat pituitary intermediate lobes were reproduced by injection of low viscosity methacrylate media, and then observed with a scanniag electron microscope. Although the intermediate lobes of newborn and pubescent rat were poorly vascularized, the adult rat intermediate lobe contained numerous capillaries forming a fairly independent network whose density, however, was not so great as in the anterior and posterior lobes. The vascular network of the intermediate lobe could be divided into two parts: a superficial plexus close to the anterior lobe, and a deep one close to the posterior lobe, though the two plexuses were continuous with each other. The superficial plexus consisted of anastomosing capillaries, and the deep one of non-anastomosing capillaries with a pallisade-fashioned arrangement. The superficial plexus seemed crucially important for the blood supply of the intermediate lobe since it developed or thickened as the animals aged. The superficial plexus received its proper afferent vessels from the middle and posterior hypophyseal arteries and emitted its proper efferent vessels continuous with the sinusoidall capillaries of the anterior lobe. The capillaries of the deep plexus usually communicated with the arterial capillaries of the posterior lobe and possibly represented another afferent route to the superficial plexus. This paper, thus, strongly suggests a portal circulation from the intermediate lobe to the anterior lobe though its functional significance is unknown.

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Grandry and Herbst corpuscles of the palatine mucosa, gingival mucosa and beak skin were studied with the electron microscope.
Typical Grandry corpuscles are surrounded by thick bundles of collagen fibers and composed of a terminal axon sandwiched between Grandry cells. Occasionally, two or three nerve endings are found within one corpuscle. The Grandry cell contains numerous electrondense granules similar to those of a Merkel cell. The cell extends protrusions at the poles and the opposing faces. Desmosome-like attachments are noted between interdigitated protrusions, and between the axon and Grandry cell.
Herbst corpuscles are composed of an outer capsule, inner core and central nerve ending. The outher capsule consists of 15 to 20 concentric lamellae, while the inner core possesses 60 to 70 sheets of cytoplasmic extensions. Under the scanning electron microscope, the corpuscles appeared as an elongated oval form surrounded by dense fibrous connective tissue, and each lamella of the outer capsule was composed of a dense network of thick and thin fibrils. These were seen under the transmission electron microscope as thick fibrils about 50nm in diameter and peripheral thin fibrils about 10nm in diameter. In some portions, a periodic increase of cytoplasmic density about 20nm in width located in neighboring sheets was noted.
The terminal axons of both corpuscles contain numerous neurofilaments, slender mitochondria, neurotubules, smooth endoplasmic reticulum and vesicles. Both corpuscles are limited by several flat cells which are arranged in parallel and probably correspond to the peripeural cells of the peripheral nerve. Tight junctions were observed between these cells.

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Spheroid and flattened synaptic vesicles were isolated from the brain homogenate of guinea pigs by a modified purification method. For scanning and transmission electron microscopy, a simple dipping method of preparation was developed and used. The purest and richest fraction of synaptic vesicles was obtained from a 0.1M sucrose fraction of density gradients. The pellets of synaptic vesicles were easily resuspended without aggregate after ultracentrifugation at 40, 000rpm for 5min. The isolated synaptic vesicles were dispersed as a monolayer on the surface of a copper grid covered with Formvar membrane. Adequate contrast was obtained by metal impregnation of specimens and gold coating at magnifications as high as 100, 000 times using an accelaration voltage of 25 to 40kV. The specimens were fixed in 0.75% glutaraldehyde (0.1M phosphate buffer, pH 7.3) and then postfixed in 1% osmium tetroxide. After dipping for 1 to 2min each in tannic acid, phosphotungstic acid, lead citrate and uranyl acetate, they were dehydrated with graded ethanol and coated with gold by ion sputtering at 400 to 560 volts for 4min. The preparation method is reported on and technical problems are discussed.

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The fine structure of the stratified lamina fenestrata of the mouse glomerulus was described in detail in both transmission and scanning electron microscopy. We propose to name the extremely developed structure of the stratified lamina fenestrata as the alveolus endothelialis. It consists of numerous small irregular spaces partitioned by thin cytoplasmic processes. Individual spaces communicate with each other through the fenestra surrounded by the cytoplasmic partitions. This structure occurs not only on the basal lamina but also on the perikaryal portion of the endothelium. The constant appearance of the alveolus fenestratus without association of any pathological or degenerative changes suggests that this structure represents a certain general and fundamental feature of the fenestrated endothelial cell. Possible mechanisms for the genesis of this structure are briefly discussed.

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Human dental pulp was investigated by an immunofluorescence method using antisera against neurofilament protein (NFP) and glia-specific S-100 protein. Nerve fibers coursing through the pulp were selectively stained with the anti-NFP serum, and their Schwann sheath with the S-100 antiserum. In addition to a definitive demonstration of the subodontoblastic nerve plexus, we found another hitherto unknown nerve plexus composed of delicate fibers at the base of the odontoblastic layer. This study shows that immunohisto chemical techniques using antisera against nervous system-specific proteins are superior to previous silver impregnation methods, due to their specificity and constancy in reaction.

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