The development of the carotid body in both fetuses from 13 to 30 days of gestation and newborn rabbits was studied by light and electron microscopy. The carotid body anlage first appeared as a cellular aggregation close to the ventral wall of the third branchial artery on the 14th day of gestation. Type-I cells were recognized as such by the presence of densecored vesicles in the cytoplasm on the 16th day of gestation, whereas cells destined to develop into Type-II cells became clearly distinguishable by their typical relationship to the Type-I cells on the 20th day of gestation. Afferent and efferent synapses as well as reciprocal-like synapses between Type-I cells and nerve endings were also observed in perinatal fetuses. Although cell differentiation was almost finished by birth, Type-I cells and their innervation did not seem to be fully matured.
Thus, it was concluded that ultrastructurally, the rabbit carotid body during the fetal and perinatal periods was relatively inactive.

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Nerve terminals in the human periodontal ligament were investigated by immunohistochemistry for neurofilament protein (NFP) and glia-specific S-100 protein. The human periodontal ligament was found to be innervated by NFP-immunoreactive nerve fibers, and contained free and specialized nerve endings; essentially all these nerve fibers were associated with S-100 protein-immunoreactive elements. Free nerve endings showing tree-like ramifications were frequently observed along the whole length of the tooth. The specialized nerve endings consisting of thick nervous elements were classified into four types as follows: Ruffini-like endings which were composed of expanded nerve terminals arranged in a dendritic fashion were found mainly around the root apex; coiled nerve endings were located in the mid-region of the periodontal ligament; spindle-shaped types, and expanded nerve endings, both rarely found near the root apex. Immunostaining for S-100 protein showed that no lamellated nerve terminals were found in the human periodontal ligament.
The region-specific distribution of various nerve terminals demonstrated in this study seems to be suited to effectively receive mechanical stimuli to the teeth from various directions.

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The purpose of this study was to correlate the contributions by derivatives of the neural crest with the development of the epithelial primordium of the thymus. The monoclonal antibody E/C8 was used to localize derivatives of the neural crest in chick embryos. Neural crest was ablated by microcautery of neural folds. Evaluation of thymic development was carried out on serial sections of embryos sacrificed on the sixth day of incubation. The size of the epithelial thymic primordium was smaller in experimental animals than in shams. E/C8-immunoreactivity was concentrated around the periphery of the primordium. It was determined, by quantifying reaction product using the Core-SCAN computer color analysis program, that the amount of immunoreactivity was decreased after ablation of neural crest. Statistical analysis showed that the quantity of reaction product was positively and significantly correlated with the size of the thymic primordium. It is concluded that mesenchymal derivatives of the neural crest, through participation in the early development of the epithelial primordium, play an important role in thymic development, and therefore with development of the immune system.

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Six morphologically distinct types of neuromuscular junctions were identified by scanning electron microscopy in the rat extraocular muscles: two diffuse and four focal types.
The diffuse junctions, spreading out extensively over the muscle fiber surface, were characterized by two types of varicose swellings (or terminal varicosities) of nerve endings. One type consisted of several ramifying nerve endings and shallow postsynaptic depressions with poorly-developed junctional folds. The other type consisted of a single axon and formed many synaptic contacts along the long axis of a muscle fiber. Round synaptic depressions facing the varicosities contained several junctional folds. The focal junctions, confined to an oval area on the muscle fiber surface, were characterized by the complexity and variability of their subneural apparatuses. Four different types of apparatuses, i. e., focal junctions, were found: 1) an apparatus consisting of labyrinthine gutters with numerous slit-like junctional folds, 2) apparatuses consisting of a large number (more than 20) of cup-like depressions with either a small number of pit-like junctional folds or 3) numerous slit-like ones, and 4) an apparatus consisting of a small number (about 10) of cup-like depressions with a few junctional folds.
The findings indicate that the rat extraocular muscles contain six different types of muscle fibers.

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In regenerating barbels of Heteropneustes fossilis, membrane-enclosed collagen fibrils, some within lysosomes, are found in fibroblasts, in old connective tissue at the wound site and also in the fibroblasts of the newly-formed core of the regenerate. The implication is that tissue remodelling can involve fibroblasts in phagocytosis of recently formed collagen fibrils. Cytological evidence suggests that individual fibroblasts are capable of synthesis and phagocytosis simultaneously.

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The ultrastructure of granulopoiesis in the head-kidney of the sea bass (Dicentrarchus labrax L.), an euryhaline seawater teleost, was studied. The heterophilic, acidophilic and basophilic series consisting of promyelocyte, myelocyte, metamyelocyte and mature granulocyte stages are described. The granulopoietic pattern was similar to those of higher vertebrates but dissimilar to those of some other fish, suggesting that the vertebrate granulopoietic scheme was probably conserved during the evolution of at least some teleost species. Very scarce presumptive blast cells were present. The heterophilic series showed three types of granules that might correspond to nucleated, azurophil and specific granules which have been described in neutrophils from mammals and human bone marrow. Neither a crystalloid nor a dense core was found in the granules of acidophilic granulocytes, where an occasional light zone could be seen. The scarce cells of the basophilic series showed similar characteristics to those described in other fish species.

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The external sulcus cells in the lateral wall of the cochlear duct insert cell cords into the connective tissue. To examine the shape, arrangement, and distribution of the sulcus cell cords in the cochlear duct in adult mice, the external sulcus cells were removed by chemical maceration methods, and the connective tissue exposed. The holes for the insertion of the sulcus cell cords along the entire course of the cochlear duct could thus be observed by scanning electron microscopy.
A zone perforated with the holes was seen in the lateral region of the area between the stria vascularis and the lateral edge of the basilar membrane, this zone extending from the basal end of the hook to the helicotrema. In the hook and basal half turn, the holes possessed large fusiform openings and branched in the connective tissue toward the outer bony wall. In the apical turn, the holes displayed small round openings and were shallow, having a small number of branchings. The width of the perforated zone, the population density of the openings of the holes and the area proportion of the openings per unit surface area in the lateral wall decreased from the base to the apex of the cochlear duct.
The regional differences in the organization of the sulcus cell cords suggest that the external sulcus cells function in close relation to regional auditory functions in the cochlear duct.

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Histological features of the tooth plate of lungfish were investigated by light and electron microscopy and contact microradiography with primary reference to the fine structure and formation of petrodentine, one of the principal constituents of the tooth plates. Petrodentine is a highly mineralized tissue noticeably analogous to that of enameloid, and is deposited intermittently in a proximal direction by the sole participation of mesenchymal petroblasts. The petroblasts appear very likely to have prominent biphasic functions of secreting the petrodentine matrix and of eliminating the matrix by resorption. The petrodentine thus may be capable of attaining hypermineralization with the participation mesenchymal cells alone. Mineral crystals constituting the petrodentine are large, hexagonal or similar but irregularly shaped columns. Initially these crystals appeared extremely thin, with curled ones frequently noted. The petrodentine of lungfish tooth plates is considered to be structurally distinct from the pleromin (Kosmin) of holocephalian tooth plates because of the difference between their constituent crystals.

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The mucosal capillaries of the proximal intestine in a freshwater teleost (Prochilodus scrofa) are lined by an endothelial sheet with several fenestrated or pored domes, which are observed both in the thinner and thicker cytoplasmic extensions of the endothelium. These domes sometimes communicate between the inner and the outer side of the capillary wall, forming a transendothelial channel-like structure with internal diaphragms. The pored domes seem to be a frequent structure in fishes. Their ultrastructure in the proximal segment of the intestine, where the flow of substances through the endothelium is intense, is described.

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Examination by scanning electron microscopy revealed differences between neuromuscular junctions in the muscle fibers of the zebra finch (bird) and rat.
The neuromuscular junctions between the anterior and posterior latissimus dorsi muscles of the zebra finch were compared. The junctions of the former, exclusively slow tonic fibers, were small and numerous along the long axis of a single muscle fiber. The synaptic depressions per junction were few. The junctions of the latter, exclusively fast twitch fibers, were large and consisted of more synaptic depressions than the former. Junctional folds were occasionally found in some depressions.
The neuromuscular junctions between the extensor digitorum longus and soleus muscles of the rat were also compared. The former consisted almost entirely of fast twitch muscle fibers, whereas the latter consisted of both slow twitch fibers (75%) and fast twitch fibers (25%). The junctions in the extensor digitorum longus muscle were almost all labyrinthine gutters containing exclusively slit-like junctional folds. In the soleus muscle, two types of junctions were observed. One type was similar to that of the extensor digitorum longus muscle; the other was characterized by labyrinthine gutters containing sparse, narrow slit-like and pit-like junctional folds.
We suggest from these structural differences of the subneural apparatuses that the junction of the fast twitch muscle is characterized by the subneural apparatus containing numerous slit-like junctional folds, and that of the slow twitch muscle fiber characterized by the apparatus containing sparse, narrow slit-like and pit-like junctional folds.

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Epididymal ligation in the mouse induced unusual cells, called peculiar pale epithelial cells (here-after, pale cells), specifically in the epithelium of the corpus epididymidis (ABE et al., 1982a, b). These pale cells had vacuoles with long microvilli on their inner surface. In this study, we found that the vacuoles of the pale cells occurred in normal mice and contained epididymal specific glycoprotein, sialoglycoprotein of 54, 000 dalton (SGP54). This was elucidated by indirect immunofluorescence and avidin biotin complex techniques using monoclonal antibody T21 which specifically recognizes SGP54. These immunoreactive pale cells occurred in the distal caput and proximal corpus of the epididymidis. The relationship between the pale cell and SGP54 is discussed.

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A monoclonal antibody T21 specifically recognizes the mouse epididymal sialoglycoprotein of 54, 000 dalton (SGP54). The localization of SGP54 was studied in the epididymal duct of germ cell-free WBB6F₁W/W^v mutant mice (W/W^v mice) by avidin biotin complex (ABC) immunohistochemistry using T21. None of the testis cells showed immunoreaction. No spermatozoa were present in the epididymal duct lumen. The duct luminal fluid was stained weakly in the proximal corpus epididymidis, and strongly in the cauda epididymidis. Degenerated cells appeared in the duct lumen. The degenerated cells located at the corpus epididymidis showed strong immunostaining in the cytoplasmic region, while the degenerated cells located at the cauda epididymidis showed weak immunostaining. Immunoreaction was also detected between and on microvilli along the epididymis, the intensity being very strong at the distal caput and proximal corpus epididymidis. Invaginations and coated vesicles at the luminal surface of the principal cells were frequently immunostained at the corpus epididymidis. Giant inclusions frequently occurred in the principal cells of the distal caput and corpus epididymidis, with these being very intensely immunostained. These inclusions are ultrastructurally confirmed to be giant multivesicular bodies reported by ABE et al. (1984) in the mouse with the efferent duct cutting. These results suggest that the majority of excess SGP54 are absorbed by the principal cells at the distal caput to corpus epididymidis and catalyzed in the giant multivesicular bodies.

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