The principal cells in the initial segment of the mouse epididymal duct were observed by light and electron microscopy over a period from 6h to 4 weeks after efferent duct cutting. These were compared with the cells in Segment II (the segment next to the initial segment), in order to understand the functional differentiation of both segments. In the initial segment, a large number of the principal cells were degenerated and disappeared within 2 days after the operation. The remaining epithelial cells rapidly decreased in height as the degeneration proceeded and the principal cells were light- and electron-microscopically transformed into the cells similar to the principal cells in Segment II. The principal cells in Segment II showed no changes after efferent duct cutting. The findings suggest that the principal cells in the initial segment have a secretory potentiality of Segment II cells, which is normally latent but becomes manifest after the efferent duct interruption that removes the absorptive function of the testicular fluid.

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Follicular dendritic cells (FDCs) in the lymph follicles extend many dendritic cytoplasmic processes and are embedded among abundant lymphoid cells of the germinal centers, making it difficult to visualize their profiles and clearly identify their precursors.
In rats pre-immunized with horseradish peroxidase (HRP), three cell types revealed a positive HRP reaction in the germinal center of the lymph node. One type was the typical FDC with remarkable cytoplasmic processes showing complicated invaginations. The second type comprised cells which were associated with reticular fibers enclosed by their cytoplasmic processes. These cells possessed fewer cytoplasmic processes than the typical FDCs, but their cytoplasmic invaginations were remarkable. Cells of the third type were attached to the outside of capillaries. Some of them surrounded reticular fibers with their cytoplasmic processes, while others extended complicated cytoplasmic processes like those of the typical FDCs.
These observations support a new view that FDCs belong to the reticular cell population. Reticular cells, which generally are considered to be a mere supportive structure in lymph nodes thus are presumed to have the potential to transform into FDCs, thereby playing a role in the immune response.

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The immunolocalization of carbonic anhydrase (CA) isozymes in bovine salivary glands and stomach were first investigated in order to discuss their biologic functions. In parotid glands, CA-II was located in serous acinar cells, whereas CA-III, in the duct segments. In contrast, a strong reaction was shown for both CA-II and CA-III in the duct segments of the submandibular gland, especially CA-III was selectively located in basal cells of the interlobular ductal epithelium; however, these glands essentially lacked CA-I. On the other hand, epithelial cells of the rumen, reticulum and omasum showed cytoplasmic reaction for CA-I, II and III in all layers of the epithelium, except the stratum coroneum. The parietal cells in the abomasal epithelium were more intensely stained for CA-II, but not for CA-I and CA-III. Immunolocalization of CA isozymes in serous cells in the parotid gland indicates their primary function in secreting macromolecules, whereas localization of CA in striated and excretory ducts in the parotid and submandibular gland suggests their traditional function in fluid and electrolytic transport. The biologic function of CA isozymes in the ruminal, reticular and omasal mucosa are postulated to influence the absorption and excretion of volatile fatty acid and NH₄; the abomasal parietal cell is considered to be involved in ion transport.

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The present study describes neuronal degeneration and its accompanying non-neuronal cellular reaction in the hypoglossal nucleus following an intraneural injection of Ricinus communis agglutinin-60 (RCA-60) into the hypoglossal nerve. The first noticeable structural changes were observed in neurons in hamsters killed 3 days after the RCA injection. Drastic alterations occurred in the period extending from the 5th to the 15th postoperative day. Two forms of neuronal degeneration were observed: light and dark types. In the light type, masses of free ribosomes were observed; other changes included the dilation of Golgi saccules and the presence of abnormal mitochondria. In the dark type of degeneration, the cells became condensed with vacuoles in their cytoplasm. Axon terminals presynaptic to the degenerating cells during this period appeared to be normal. A massive influx of mononuclear leucocytes by diapedesis occurred at the large venules. Some of the infiltrated cells were clearly lymphocytes, while others were monocytes which became indistinguishable from indigenous microglia once they were in the neuropil. Neural macrophages, most probably derived both from microglia and the infiltrated monocytes, were engaged in the phagocytosis of neuronal debris. A remarkable finding in the present study was the wide-spread occurrence of dark axon terminals in the neuropil in longer surviving animals (90 and 120 days). The structural alterations, e. g., clumping and swelling of some of the synaptic vesicles in the enhanced cytoplasmic density, suggest that these were undergoing atrophic changes resulting from the long period of dysfunction following the death of postsynaptic neurons induced by RCA.

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The sites of intra-embryonic haemopoiesis of the chick embryo (stages 15-26) preceding the initiation of hepatic haemopoiesis was investigated by means of serial sections stained with azan liquid. Hitherto undescribed blood islands of large size and clear outline were found in the neck and lateral body fold of the embryo. They consisted of dense aggregations of erythroblasts and thin endothelial cells surrounding them, and could be clearly identified by azan staining. In the neck, the blood islands were located near the branchyal arteries at stages 15-18, while in the lateral fold they were seen mainly at stages 17-19.
Examination of serial sections indicated that the blood islands extended in a cephalocaudal direction, assuming a rod shape. The largest blood island was found in the lateral fold, measuring 50μm in diameter and 900μm in length.
Besides these islands, occasional sites of haemopoiesis were found in the mesonephros, mesentery, dorsal regions of the trunk, head and allantois after stage 18. As the blood islands found in this study alternated their localization sites according to developmental stage, they were considered to be temporary haemopoietic tissues rather than persistent ones.

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The differentiation of two types of T-lymphocyte accessory cells, i. e., interdigitating reticulum cells and Langerhans cells, was studied immunocytochemically and ultrastructurally on cutaneous lesions from patients with mycosis fungoides, a neoplasm of mature T-lymphocytes. In such a condition the lymphoid infiltrate creates, adjacent to the epidermis, a microenvironment in the dermis similar to that of T-cell areas of lymphoid organs. Immunocytochemistry revealed that CD11c+CD1a-putative monocytic cells co-exist with CD11c+CD1a+putative mature accessory cells. By electron microscopy, large numbers of interdigitating reticulum cells in the dermal infiltrate and Langerhans cells in the epidermis were found. Furthermore, monocytes were frequently observed, at times with cells showing intermediate features between monocytes and interdigitating reticulum cells on the one hand and Langerhans cells on the other. In the absence of proliferative phenomena of the above cells, it is conceivable that both interdigitating reticulum cells and Langerhans cells originate from locally migrated monocytes. A possible role of the local tissue microenvironment-namely the T-lymphoid microenvironment for interdigitating reticulum cells and the epidermal microenvironment for Langerhans cells-in inducing the differentiation of monocytes into the two kinds of accessory cells is proposed.

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In an attempt to identify regenerating axons in the central nervous system, a partial transection of the dorsal funiculus in the rat spinal cord was carried out with a pair of microdissection scissors, and a nylon thread loop was inserted into the lesion to demarcate the severed tissue. Nerve regeneration through the demarcated lesion was observed 4-20 days after the operation by light and electron microscopy.
In the early stage, many naked axons appeared from the caudal part of the lesion, and some of these further extended into the demarcated space. They contained an accumulation of mitochondria, smooth-surfaced endoplasmic reticulum and vesicles in the axoplasm; this axoplasmic feature indicated that they were regenerating axons. They gradually increased in number, and took highly irregular courses exhibiting various fluctuations in diameter throughout their lengths. Immature Schwann cells as well as glial cells including oligodendrocytes and astrocytes appeared in close association with these regenerating axons. Oligodendrocytes eventually formed thin myelin sheaths.
On the other hand, naked axons were present deflecting outside the thread loop; they showed no axoplasmic characteristics as described above. These axons could be regarded as uninjured ones merely undergoing demyelination due to the surgery.
Thus, regenerating axons were clearly distinguished from merely demyelinated ones, and some of them were shown to grow through the traumatic lesion in the dorsal funiculus of the rat spinal cord.

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Ovaries were obtained from golden hamsters during proestrus and immersion-fixed for the immunocytochemical localization of albumin by the peroxidase-antiperoxidase method. Another group of ovaries were obtained from immature perfusion-fixed animals which had been stimulated for 48 or 96h by gonadotrophins. Perfusion-fixed tissues were processed for the ultrastructural albumin localization using the protein A-gold technique. At the light microscopical level, albumin positive cells were seen more often in the corona radiata and the inner layer of the granulosa, but seldom in the outer layer. Thecal and interstitial gland cells revealed albumin reactivity as did the intra- and extravascular spaces. Using the electron microscope, enriched gold grains signalizing a positive albumin response were detected over lipid droplets of granulosal, thecal and luteal cells. Though labeled coated vesicles were absent, the following functional significance is proposed for this study: precursors of steroidogenesis or steroids may be transferred as ligands of albumin into or within the cell after a receptor-mediated uptake of the albumin ligand complex.

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The three-dimensional construction of the thin limb epithelium was demonstrated in all four segments previously determined by transmission electron microscopy. Special reference was given to the pattern and extent of both intercellular and intracellular interdigitation.
In the thin limb of short-looped nephrons, epithelial cells were simple polygonal in shape. The cells showed fine intercellular and intracellular interdigitations of basolateral microprojections.
The upper descending thin limb of long-looped nephrons was characterized by well-developed intercellular and intracellular interdigitations. Both types of interdigitations showed a double structure, as they were composed of primary lateral processes or deep basal grooves, with numerous secondary microprojections on their walls. This segment additionally revealed apical interdigitations of small laminar processes, which caused the fine undulation of luminal cell boundaries.
In the lower descending thin limb of long-looped nephrons, the intercellular and intracellular interdigitations also showed the double structure, though diminished in degree. This segment lacked the fine apical interdigitations.
The ascending thin limb was characterized by an elaborate pectineal pattern of intercellular interdigitations, and by the absence of intracellular interdigitations. The intercellular interdigitations in this segment were formed by thin primary processes oriented parallel to each other. No secondary microprojections were found on their lateral surfaces.

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The head-kidney of the sea bass is a source of erythropoietic and thrombopoietic cells. No significant lymphopoietic activity is found. Erythropoiesis, thrombopoiesis and granulopoiesis are intermingled, which suggests that only one environmental “niche” exists to modulate the lineage development. No numerous proerythroblasts are present. Erythropoiesis consists of a proerythroblast, basophilic erythroblast, polychromatophilic erythroblast, acidophilic erythroblast, young erythrocyte and old erythrocyte, the latter being nucleated and without nuclear vacuolation. The numbers of the free ribosomes and polyribosomes decrease progressively from proerythroblasts to young erythrocytes, suggesting a high synthetic activity from early on. Immature erythropoietic cells show pits and protrusions correlated to micropinocytotic vesicles, indicating ropheocytosis. The peripheral band of microtubules is the most remarkable cytoplasmic structure in proerythroblasts. Granular cytoplasmic inclusions or lysosomes were not observed in erythropoietic cells. The old erythrocyte reveals an electron-dense homogeneous cytoplasm with occasional mitochondria and a small Golgi apparatus. Thrombopoietic cells comprise both the immature and mature prothrombocyte and adult thrombocyte. Nuclear and cytoplasmic densities increase and the surface connected canalicular system develops during maturation. A marginal band of microtubules is present in the cytoplasm from prothrombocytes to mature thrombocytes. Some pseudopodial processes, dense granules and vesicles, probably indicating passive storage, are also observed.

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Mononuclear cells in the extraembryonic and intraembryonic spaces of mice were examined qualitatively and quantitatively by semithin light microscopy. At 9 and 10 days of gestation, the extraembryonic serous cavities contained a small number of mononuclear free cells. These cells had an elongated or kidney-shaped nucleus and the cell surface showed many villous projections. The cytoplasm occasionally contained small lucent vesicles but no phagocytic vacuoles. The average cell diameter was 8.4±0.9μm and N-C ratio, 0.51±0.21. Cell larger than 10μm in diameter constituted only 0.4% of the total. In vitelline vessels at 9 days, mononuclear cells bearing a close morphological resemblance to extraembryonic free cells were observed. At 12 days of gestation, extraembryonic and peritoneal cavities contained mature macrophages and a few small mononuclears which had the same morphological features as those in the extraembryonic coelom and vitelline vessels.

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The patterns of [¹⁴C] adenine ([¹⁴C] A) incorporation into DNA by proliferative cells in the kidney were studied by the autoradiographic technique. It was revealed that, after 3 daily injections of [¹⁴C] A (1μCi/g body weight each), a portion of the glomerular cells and a few fibroblastoid cells in the cortical intexstitium incorporated [¹⁴C] A into DNA to a remarkable extent. Such cells also incorporated [³H] thymidine, but to a lesser extent.
The cells which incorporate [¹⁴C] A to a particularly great extent (adenine uptake cells) also occur in other tissues. Such cells are confined to a few cell types of either the macrophage or fibroblast or reticulum cell lines. This fact suggests that the adenine uptake cells observed in the glomerulus are also of a similar cell line and most likely mesangial cells.
By immunohistochemical examination for Ia antigen, adenine uptake cells are divided into Ia-positive and Ia-negative types. The present examination showed that the major portion of adenine uptake cells in the glomerulus are Ia-negative, and it is suggested that these cells are analogous to the Ia-negative macrophages in the lung. This suggestion is supported by the fact that, in the glomerulus, colloidal carbon uptake cells (macrophage-like cells) are present in fairly large numbers. The Ia-positive cells seem to be of the same cell line as the adenine uptake cells that express Ia antigen in other tissues, such as septal fibroblasts in the lung.

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The response of neural elements to a dentin injury was morphologically investigated in rat molars by use of immunostaining for neurofilament protein (NFP). An artificially formed cavity in dentin by drilling rapidly caused the displacement of some odontoblasts into the exposed dentinal tubules, while others were detached from the predentin. The subodontoblastic nerve plexus consisting of NFP-immunoreactive nerves shifted inward together with the separated odontoblasts, while a movement of the nerves into the exposed dentinal tubules was not recognized.
The odontoblasts separated from the predentin degenerated and disappeared one day after the cavity preparation; at this time, the subodontoblastic nerve plexus underlying the drilled dentin was remarkably disrupted, presumably losing dentinal sensation of the drilled area.
Three days after the cavity preparation, the destroyed odontoblastic layer began to be repaired by newly differentiating odontoblasts; the reparative dentin was produced from 5 to 7 days onward. Numerous NFP-positive nerves, beaded in type, gathered in the odontoblastic layer in accordance with the differentiation of the new odontoblasts. The increased beaded nerve fibers were suggested to represent peptide-containing nerves.
In 10-15 days, the reparative dentin accumulated quite remarkably under the cavity area. The NFP-positive subodontoblastic nerve plexus was entirely reconstituted and also regained continuity to its surrounding plexus. The nerve fibers in the reconstituted plexus were mostly non-beaded in type as seen in the control teeth. Since dentinal tubules in the reparative dentin are not normally continuous to the primary dentinal tubules, dentinal sensation may not have been restored.

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