Postcapillary venules in rabbit tonsils were studied by scanning (SEM) and transmission electron microscopy (TEM).
Tonsils were perfused with Ringer solution and a glutaraldehyde fixative and blood cells were drained away leaving the migrating lymphocytes in the vascular wall. Specimens were freeze-cracked and the luminal surface of the venules was exposed for examination by SEM.
The postcapillary venules under the SEM were lined by endothelial cells conspicuously and irregularly bulging into the lumen. The endothelial cell possessed columnar marginal processes, which were interdigitated at the base of the endothelium with those of adjacent cells, while they often formed intercellular bridges on the luminal surface. The endothelium thus was more intricate in construction than previously regarded.
Migrating lymphocytes observed on the endothelium by SEM were located on the endothelial cell boundary region or plugged between the adjacent endothelial cells. Lymphocytes incorporated into the endothelial wall were found in the intercellular space by TEM. Careful analysis of SEM and TEM images led to the conclusion that they pass through the intercellular space in the endothelial wall.
Migrating lymphocytes on the endothelium were all small lymphocytes and classified into three types on the basis of their surface architecture (occurrence of each type given in parenthesis): “villous” lymphocytes (12.5%), “relatively smooth” lymphocytes (59.1%), and “smooth” lymphocytes (28.4%). “Relatively smooth” and “smooth” lymphocytes formed the great majority of the migrating lymphocytes and they were tentatively identified as T lymphocytes.

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The cytodifferentiation of the theca cell in the mouse ovary was studied using electron microscopy. There are only fibroblast-like cells around the primordial follicle. Three different cell types are recognized around the secondary and Graafian follicles; those are fibroblast-like cells, theca gland cells (steroid-secreting cells), and transitional cells (partially or incompletely differentiated theca cells). The fibroblast-like cell has well developed rough endoplasmic reticulum, moderately developed Golgi apparatus, rod-shaped mitochondria with lamellar cristae, a few lipid droplets, and free ribosomes in their cytoplasm. A large, ellipsoidal nucleus, abundant lipid droplets, mitochondria with tubular cristae, and smooth endoplasmic reticulum are seen in the theca gland cell. The transitional cell, which is considered to be an intermediate form between the fibroblast and the theca gland cell, has a round or oval nucleus, many lipid droplets, mitochondria with tubular or lameller cristae and a small amount of smooth endoplasmic reticulum. As to the cytodifferentiation of the theca cell, an appearance of lipid droplets, a structural change of the mitochondrial cristae and an appearance of the elements of smooth endoplasmic reticulum are considered to be important signs. Even after the differentiation of the theca gland cell and also after the administration of PMS (pregnant mare serum gonadotropin) or HCG (human chorionic gonadotropin), many fibroblasts exist in the theca folliculi of the Graafian follicle, which might be without any ability to differentiate into theca gland cells.

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Localization of ³H-dopamine-derived radioactivity in the chromaffin cells of the mouse adrenal medulla was studied by means of light and electron microscopic autoradiography. Twenty μCi/gbw of the isotope 3, 4-dihydroxy (ring-G-³H) phenylethylamine hydrochloride was injected intraperitoneally in seven mice, which were then perfusionfixed from the left ventricle of the heart with 2.5% glutaraldehyde from 15min to 24hrs after injection. Following the post-osmication, dehydration through an ethanol series and Epon embedding, sections of pieces of the adrenal gland and liver were processed for autoradiography using the dipping method.
Both A and N cells of the adrenal medulla showed maximum radioactivity at 30min after injection of the isotope. The radioactivity on the A cells was remarkably higher than that on the N cells at 15 to 30min after the injection. However, both types of chromaffin cells contained a similar amount of radioactivity at later time points.
It was demonstrated that at all time points examined, both A and N cells were highest in the ³H-dopamine-derived radioactivity in the zone immediately beneath the cortico-medullary junction, whereas the radioactivity in the central portion of the adrenal medulla was always lower. No difference was demonstrated between the ultrastructure of the heavily labeled chromaffin cells and that of the lightly labeled cells.
No regional difference in the ³H-dopamine-derived radioactivity was demonstrated in the liver lobules.
No zonal distribution of radioactivity was demonstrated in autoradiograms of the mouse adrenal medulla prepared after the ³H-leucine (L-leucine-4, 5-³H) injection.
These results are consistent with the idea that the chromaffin cells in the zone immediately beneath the cortico-medullary junction of the mouse adrenal gland have a greater dopamine-handling capacity than those in the central portion of the adrenal medulla and that this phenomenon is characteristic of dopamine.

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The “fat-storing cell” discovered in the liver by ITO (1951) has been considered as specific to this organ. In the present study, the three light microscopic criteria of the cell, i. e., (1) occurrence of multilocular fat droplets in Sudan III staining, (2) characteristic fluorescence for the occurrence of vitamin A in the cell, (3) stainability in Kupffer's gold impregnation, were examined in the liver and other organs of the rat. Cells filling all these three criteria were found in the lamina propria of the stomach and small intestine, in the subpleural and perivascular connective tissue and alveolar septa of the lung, and in the red marrow of the spleen. All these cells emit markedly increased fluorescence for vitamin A after this vitamin has been administered to the animal. Sudanophilic cells of the same morphological features, though not confirmed by fluorescence and gold impregnation as yet, were demonstrated also in the lymph nodes, thymus, bone marrow and the sites of lymphocyte infiltration in the gut, trachea and liver.
The present study indicates that cells either identical with or closely related to the Ito cells of the liver are distributed widely in the tissues of mesenchymal origin, especially in the reticular tissues. Discussion was made concerning the possible role of this vitamin A uptake cell in the organism. This paper also preliminarily reports the wide distribution of the same cells in the human body.

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Hepatic sinusoids of human liver samples obtained by surgical or needle biopsy were examined under the scanning and transmission electron microscopes (SEM and TEM). The surgically obtained tissues were perfused, by the use of a needle and syringe, with Ringer solution and successively with glutaraldehyde solution through the cut ends of blood vessels, while the needle biopsy pieces were perfused by directly puncturing the parenchyme.
Endothelial cells of hepatic sinusoids possessed thin and flat cytoplasm with two types of fenestrations which have already been demonstrated in laboratory animals: smaller ones (less than 0.1μm) occurring in clusters to form “sieve plates” and larger ones (0.5-2μm) intermingling among the former.
Kupffer cells were definitely distinguishable from the endothelial cells under the SEM and no graduations were found between either cell type. The Kupffer cell possessed voluminous cytoplasm covered with numerous filopodia, microvillous processes and lamellipodia.
In the space of Disse examined under the TEM, there were a number of attenuated cytoplasmic processes. Because of their contents in fat droplets, caveolae and microfilaments, they were identified as the processes of fat storing cells of Ito. A discontinuous basement membrane-like material was also recognized in the space of Disse.

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The ultrastructural changes in the intestinal absorptive cells of the rat during the absorption of triglycerides, particularly medium chain triglyceride (MCT), were studied by electron microscopy. In the absorptive cell of rats fed with MCT, the agranular endoplasmic reticulum in tubular form was remarkably proliferated throughout the cytoplasm as compared with that of fasting rats. The granular endoplasmic reticulum was mostly transformed into the same tubulo-vesicular form as the agranular endoplasmic reticulum. Chylomicra, which were consistently observed in the endoplasmic reticulum of the intestinal absorptive cell of rats fed with long chain triglyceride (LCT), did not appear in that of MCT-fed rats, although small lipid particles were noticed within it. The Golgi lamellae decreased in number and length. During the absorption of MCT, the central lacteal contained some lipid particles which were smaller in size than the chylomicron which appeared in the LCT-fed rats. These evidences might suggest that the majority of MCT administered was transported through the absorptive cell without reesterification in the endoplasmic reticulum into the portal vein system, and only a minor part of the MCT given was transported via the central lacteal after reesterification.

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