This paper reviews firstly the microvascular corrosion casting/scanning electron microscope method and secondly the microvascular organization of endocrine and digestive system as revealed by this technique. Detailed descriptions of the microvascular arrangement of the hypophysis, pineal body, thyroid gland, pancreas, adrenal gland, salivary gland, liver, stomach and small intestine are given. Various hypotheses are also proposed regarding the physiological significance of the microcirculatory patterns observed.

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Nucleolar changes during the differentiation of erythroblasts in the mouse spleen were examined quantitatively by electron microscopy. On the basis of a three-dimensional nuclear analysis, as reported previously, the erythroblasts could be classified into four types: small (S), medium (M), large (L) and extra-large (EL). Extra-large and large erythroblasts have large prominent nucleoli which are usually attached to the nuclear membrane. Medium and small erythroblasts have small nucleoli which are generally separated from the nuclear margin. The volumetric ratio of nucleoli to nucleus obtained by a point-counting method is 0.157±0.016 in EL; 0.135±0.011 in L; 0.035±0.004 in M and 0.015±0.004 in S, and the volume of nucleoli is 37.8μm³ in EL; 17.3μm³ in L; 2.3μm³ in M; 0.5μm³ in S, respectively. The number of nucleoli per nucleus is largest (3.9) in EL and smallest (0.6) in S. The nucleolar changes are discussed in relation to the developmental sequence of the erythroid series.

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It is generally known that spermatozoa aquire the capacity for fertilization during passage through the proximal region of the epididymal duct and are then stored in the distal region of the duct (BEDFORD, 1975; HAMILTON, 1975). The mouse epididymal duct has been divided into five segments (I-V) by light microscopy; Segments I, II and III constitute the head of the epididymis, Segment IV the body, and Segment V the tail; Segments I, II and III seem to belong to the proximal region, and Segments IV and V to the distal region (TAKANO, 1980). In this electron microscope study, we have examined the regional differences of the principal cells of the epididymal duct to understand the functional significance of each segment.
The principal cells decrease in height distalwards from Segments I to V. The nucleus is situated basally in the cells, with a well developed Golgi apparatus located above it. The endoplasmic reticulum in Segments I, II and III is generally vesicular, and is distributed throughout the cytoplasm. The ribosomes attached to the endoplasmic reticulum increase in number from Segments I to III. In Segments IV and V, flattened rough endoplasmic reticulum is seen in the basal cytoplasm of the cells. The multivesicular bodies are usually located in the supranuclear cytoplasm. They are large in Segment II and frequent in Segments II and IV. The dense bodies are specific in appearance for the cells in each segment. They are seen in the supranuclear cytoplasm in Segment I and in the infranuclear cytoplasm in Segments III, IV and V. Few dense bodies are found in Segment II. The apical cytoplasm contains coated and non-coated vesicles. These vesicles consist of large and small types in Segments I, II and III. They are of the small type in Segments IV and V. The luminal surface membrane has coated or non-coated invaginations between stereocilia.
The findings suggest that the principal cells have secretory and absorptive functions specific for each segment. Discussion shall be made on the possibility that the secretion of the specific epididymal substances may provide the fertilizing ability to spermatozoa and the absorption pertains to the testicular fluid, epididymal secretions and substances bound to spermatozoa.

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The mouse epididymis, consisting of a head, body and tail, was examined by electron microscopy. The principal cells of the epididymal duct in the epididymal body have rod-shaped inclusions, 0.3 to 1.0μm in diameter and 2 to 5μm in height. The inclusions contain a bundle of tubules, about 40nm in diameter, with a wall about 6nm in thickness. The tubules are regularly arranged in a hexagonal pattern. A goniometric study of the inclusions shows evidence that the wall of the tubules consists of filament-like subunits which run parallel to each other and are inclined at about 25° to the axis of the tubules, forming loose spirals. The inclusions occasionally contain material similar to that in the lysosomal bodies. These inclusions were observed in every epididymis examined, and seem to be related to the specific functions of the principal cells in the epididymal body.

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Serotonin neurons in newborn mouse and rat brainstem cultures were studied by peroxidase-antiperoxidase immunohistochemistry using a serotonin antibody. The intensity of the immunohistochemical reaction in the neuronal somata decreased gradually with time and was hardly detectable after one month of culturing. On the other hand, the immunoreactivity of the processes noted at the early stages later on became more intense and covered the full extent of the fibers. From the early stages of the cultures, the axon of each serotonin neuron formed not only a network by branching, but also a true anastomosis with the axonal networks of the other serotonin neurons.

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Basal-granulated cells (BGC) in the human duodenal bulb were observed by light and electron microscopy, and both the cell types and their population densities in the duodenal crypts and in the Brunner's glands were compared.
The number of the BGC in the Brunner's glands was much smaller than in the crypts. On the basis of their ultrastructural features, nine types of BGC, i. e. an EC cell, N cell, D cell, D₁ cell, S cell, I cell, G cell, L cell and P cell were identified in the human duodenal bulb. In the duodenal crypts, as is generally recognized, EC cells were most numerous, making up 40% of the total BGC. N cells and D cells were around 10% of the total, and S cells, I cells and L cells were less than 10%, respectively. By contrast, in Brunner's glands, D cells and small granule-containing cells such as S cells, I cells and D₁ cells were predominant, accounting for about 80% of the total BGC. EC cells and N cells were about 10% or less, respectively.
These results indicate that the Brunner's glands are definitely different from the ordinary intestinal mucosa in regard to their BGC population, and are considered to have endocrine functions mainly performed by D₁ cells, S cells and I cells.

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Megakaryocytes in the liver obtained from 185 human embryos and fetuses during the period from 28 days to 22 weeks of ovulation were investigated by light and electron microscopy.
The early hepatic megakaryoblasts and the early hepatic promegakaryocytes at stage I observed in the intercellular spaces of the hepatocytes until 10 weeks of ovulation (early stage of hepatic hemopoiesis) were larger than the late hepatic megakaryoblasts and the late hepatic promegakaryocytes at stage I observed after 10 weeks of ovulation (late stage of hepatic hemopoiesis). The chromatin of the former two cells was finely dispersed, whereas that of the latter two showed moderate central and peripheral clumping.
These findings seem to indicate that the progenitor cells of the magakaryoblasts and the hemopoietic stem cells in the liver in the early stage of hepatic hemopoiesis morphologically differ from those in the liver in the late stage, and that the megakaryocytes in the liver until 10 weeks of ovulation differ in maturation course from those after this ovulation stage.

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Corpuscular nerve endings were found in the tunica externa of the lymph heart of a turtle (Pseudemys scripta elegans). They consisted of axon terminals and surrounding cellular lamellae. Serial thin sections of one such presumable sensory corpuscle were made and reconstructed for the three-dimensional analysis of the corpuscle, with special reference to the axon terminals. It was revealed that axon terminals ramified at several points within the corpuscle and formed varicosities of various sizes ranging from 1.0 to 3.0μm in diameter in their courses. Such varicosities contained an abundance of mitochondria as well as clear and granular vesicles 500 to 800Å in diameter. Finger-like protrusions often projected from the axon terminals into the interlamellar spaces. The lamellae consisted of several thin cytoplasmic processes of modified Schwann cells. The corpuscle was enclosed in an incomplete capsule consisting of a thin layer of cellular processes. The corpuscular nerve endings may represent pressoreceptors in the wall of the lymph heart.

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The ultrastructure of the hepatic perisinusoidal spaces of the fresh-water catfish was examined by electron microscopy. The hepatic sinusoidal wall of the catfish consists of endothelial cells, Ito's fat-storing cells and smooth muscle cells, but is devoid of Kupffer cells.
The present study demonstrates the sporadic occurrence of smooth muscle cells in the perisinusoidal spaces of catfish liver. This seems to be the first report on smooth muscle cells in the perisinusoidal spaces of vertebrate livers. The location of these muscle cells indicates that they may be responsible for the regulation of the sinusoidal blood flow by their contraction, thus functioning as a sphincter.

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An electron microscope study of the Harderian gland of the snake, Rhabdophis tigrinus, revealed the occurrence of a myoid cell in the glandular body.
The myoid cell, in oval profile, is located in the perivascular space and enveloped by a basement membrane. The plasma membrane of the cell is studded with a number of vesicular caveoli. The myoid cell cytoplasm is largely occupied by myofilaments which do not form discrete bundles of myofibrils. The striations comprise the A and I bands together with Z lines, but neither an M nor an H zone is detectable. Although the transverse tubule appears to be scarce, the sarcoplasmic reticulum is well developed. No triadic configuration is observed. The cytoplasm includes a few numbers of mitochondria, sarcoplasmic reticulum, fat droplets, dense membrane-bound granules and free ribosomes. The histological characteristics of the present myoid cell are compared to those of identical cells commonly existing in the thymus.

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