The number and localization of vertebrate motor endplates on the muscle fiber, and their structure, vary according to phylum and species, and among the different fiber types in a given species.
Vertebrate skeletal muscle fibers are classified into two major groups: the twitch (fast) and the slow (tonic) fibers. The twitch fiber has straight Z-lines and a well developed T-SR system, and is singly innervated with en plaque (plate-like) type motor endplate. The twitch fibers are further subdivided into three types: the red (mitochondria-rich), intermediate (mitochondria-moderate) and white (mitochondria-poor) fibers. The motor endplate of the white fiber is large and has a complicated structure, that of the red fiber is small and less complicated, and that of the intermediate fiber possesses intermediate characteristics. The slow fiber has zigzag Z-lines and poorly developed sarcoplasmic reticulum (SR), and is multiply innervated with en grappe (grape-like) type motor endplates. The morphological features of the motor endplate in each of these fiber types of the mammalian, avian, reptilian, amphibian, and fish skeletal muscles are reviewed. Special emphasis has been placed on the three-dimensional structure of the motor endplates of the different fiber types as observed by high-resolution scanning electron microscopy.

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The head kidney of Sparus auratus (teleost) is comprised of erythropoietic, granulopoietic and lymphopoietic cells. This study describes the ultrastructure of the granulopoietic series of cells.
Heterophils, eosinophils and basophils were found at various stages of development. The most numerous cells are the neutrophils, the cells of the basophilic series being very scarce.
The most outstanding characteristics of the neutrophil series are the eccentric and slightly segmented nucleus and cytoplasm with numerous, homogeneously dense granules. The eosinophils show a lobed nucleus and two types of granules in the cytoplasm. In the basophils, the cytoplasmic granules are large with fibrillar contents.

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The distribution of spermatozoa and that as well as the staining reaction of PAS-positive material in the epididymal duct were histologically observed in 60 day-old mice and mice 12h, 24h, 2 days, and 7 days after efferent duct ligation at 60 days of age. We divided the mouse epididymal duct into five segments; Segments I, II, and III making up the head of the epididymis, Segment IV the body, and Segment V the tail. The cytoplasm of the principal cells in Segment II was PAS-positive. The lumen of the epididymal duct contained spermatozoa and PAS-positive material in the distal portion of Segment I and other segments distal to Segment I. The lumen in Segments IV and V was distended with abundant spermatozoa and PAS-positive material. After efferent duct ligation, spermatozoa disappeared from the lumen in Segments I, II, and III within 2 days, whereas they took 5 days to pass from Segment IV. As spermatozoa disappeared, the luminal material increased in intensity of the PAS-reaction in the distal portion of Segment I and initial portion of Segment II and Segment IV. Segment I became similar in appearance to Segment II 7 days after the ligation. The findings suggest that the PAS-positive material is produced in the distal portion of Segment I and Segment II and accumulates, in the absence of spermatozoa, at the production site and Segment IV, a storage site of spermatozoa; furthermore, Segment I has the capacity of exerting a Segment II-like function, which becomes distinct after blocking the entrance of the testicular fluid.

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The distribution and ultrastructure of Somatostatin-immunoreactive neurons, nerve fibers and axon terminals in the dorsal horn of rat thoracic spinal cord were studied by immunohistochemistry at both the light and electron microscopic levels. Somatostatin-immuno reactive neurons were predominantly observed in Rexed laminae I and II of the dorsal horn of spinal cord. Somatostatin-immunoreactive electron dense peroxidase material was concentrated in the Golgi apparatus and rough endoplasmic reticulum of the somatostatin-immunoreactive neurons, and was characteristically sparse in other regions of the cytoplasm. In the somatostatin-immunoreactive fibers and terminals, immunoreactive electron dense material was concentrated in the microtubules and large synaptic vesicles.

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In addition to annulospiral sensory endings and fusimotor endings, multiaxonal nerve endings consisting of a bundle of naked axons were frequently found in the muscle spindles of the adult Chinese hamster. The multiaxonal endings were mainly distributed in the equatorial region of the nuclear bag fibers, adjacent to the annulospiral sensory endings. Terminal axons composing the endings were less than 1.5μm in diameter and contained a significant number of clear synaptic vesicles and large granulated vesicles. Some axons formed synaptic contacts with each other and with the muscle cell surface. These structural features suggest that the multiaxonal endings are efferent in nature. They may possibly represent a temporal structure of fusimotor endings, or belong to the autonomic nerves.

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Nerve fibers supplying the Duvernoy's gland, a venom-secreting oral gland, of the Japanese colubrid snake, Rhabdophis tigrinus, were examined by formaldehyde-induced fluorescence (FIF) and acetylcholinesterase (AChE) histochemistry, immunohischemistry, and electron microscopy.
The innervation by the FIF fibers was rather meager and was restricted to the area around the arteries localized in the interlobular connective tissue. The AChE-reactive fibers, in contrast, were abundantly supplied all over the gland, especially to the lobule consisting of secretory units. Peptide histidine isoleucine (PHI) (1-15)-like immunoreactive fibers were also detected in the gland and proved identical to the AChE-reactive fibers. The reactivities for both AChE and PHI (1-15) were particularly prominent around the blood capillaries distributed in the lobules.
Under the electron microscope, nerve fibers were frequently seen to terminate near capillaries subjacent to the secretory unit. Those nerve terminals containing small clear vesicles and dense core granules were devoid of Schwann cell coverage on the side facing the blood capillary. These features suggest that the nerve terminals may possibly release at least a portion of their secretory contents into the blood as neurohormones.

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The intrathymic distributions of facteur thymique sérique (FTS)-containing cells and Ia-expressing cells were examined by a double immunofluorescence technique in C57BL/6 mice of various ages.
In the thymic medulla of all the mice examined, there were both FTS-immunoreactive cells and Ia-immunoreactive cells. The former cells, epithelial in nature, extended elongate cytoplasmic processes, while the latter were connected with each other by their spiny processes. The FTS-immunoreactive epithelial cells were all Ia-negative. The FTS-immunoreactive cells and Ia-immunoreactive cells were located in close proximity to each other.
The cortex of the adult mice contained no FTS-immunoreactive epithelial cells. The cortex of the newborn and old mice, however, contained scattered FTS-immunoreactive epithelial cells with processes. FTS-immunoreactive cells in the cortex of the old mice possesed Ia-immunoreactive partner cells, while those in the cortex of newborn mice did not.
In the marginal zone of the medulla of old mice, there were several cavities bounded by a partly ciliated epithelium and containing lymphocytes. The epithelial cells lining the cavities showed intense immunofluorescence for FTS.
In addition to the epithelial cells, a certain population of round cells with thin cytoplasm lacking in discernible processes immunofluoresced for FTS. Located both in the medulla and in the cortex, these round cells were postulated to be thymocytes, the target cells of FTS, binding FTS on their cell surface.

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The collagen fibrillar framework in the human and rat liver was demonstrated by a cell-maceration/scanning electron microscope (SEM) method. Maceration of fixed tissues with alkali plus water successfully removed the cellular elements, exposing collagen fibrils which measured about 60nm in diameter and were identified as such by transmission electron microscopy (TEM). The normal human liver contained 12.4mg of collagen fibrils/g of wet tissue, while rat livers contained 1.3mg of collagen fibrils/g of wet tissue. In the Glisson's sheaths were condensations of collagen fibrils which extended to the hepatic lobules. In the spaces of Disse collagen fibrils ran either solitarily or in bundles and formed sheaths for housing the sinusoids. The central veins and the sublobular veins were also surrounded by the collagen fibrillar sheaths which were continuous with those in the spaces of Disse. Between adjacent sheaths of sinusoids frequently stretched collagen fibrillar bundles which were confirmed by TEM to occur in interhepatocellular spaces continuous with the spaces of Disse. The collagen fibrillar layer of the human liver capsule was much thicker (70-100μm in thickness) than that of the rat liver (less than 5μm in thickness). The collagen fibrils of the capsule were also continuous with those in the spaces of Disse. The collagen fibrillar framework of the liver is presumed not only to mechanically support the tissue, but also to form a microenvironment for hepatocytes and cells in the Disse's space.

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Exocytotic release of neurotransmitters from nerve endings was demonstrated ultrastructurally in the taste buds of rat circumvallate papillae by stimulation of high K⁺ and Ca²⁺ Ringer perfusion and application of tannic acid-Ringer incubation (TARI) method. Omega-shaped images of large cored vesicles and small clear vesicles, indicating exocytotic release of their contents, were found only in the non-synaptic sites. Occasionally exocytosis occurred at sites facing other nerve fibers. Many coated pits were also seen, which presumably represent membrane retrieval at a later stage of exocytosis. It is likely that the taste buds receive more than one type of innervation.

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