Human yolk sacs were studied by light, transmission and scanning electron microscopy. Twelve human embryos at Carnegie stages ranging from 13 to 20 (28-49 days of gestation) were used for this research. The series of events which occur in the yolk sac wall during its period of maximum functional activity were recorded. The endodermal epithelium consisted of a single layer of columnar cells which, through cellular proliferation, formed endodermal cords which became cavitated, thereby forming endodermal vesicles. At the peak of yolk sac activity, intercellular spaces became very large and isolated individual endodermal cells. The epithelial cells were characterized by numerous microvilli on their free surface, high pinocytotic activity and by the formation of dense cisternae. Abundant intracellular vesicles fused together to empty their contents into the endodermal vesicles. The luminal surfaces of both intracellular and endodermal vesicles presented microvilli. The endodermal cells were characterized by an abundant granular endoplasmic reticulum, a well-developed Golgi apparatus, numerous mitochondria and glycogen particles. Endodermal vesicles were normally seen opening into the vitelline cavity through an endodermal orifice. The surface of the outer mesothelium was covered by numerous lengthy microvilli which were denser here than in the endodermal layer. A mucus-like material, present on the surface of the mesothelium, showed relatively few alterations during the study period. The mesothelial cells were less rich in organelles and far less active than the endodermal cells.
The microanatomy of the endoderm supports the contention that its cells serve as absorptive structures as well as sites of protein synthesis during early embryonic development. Therefore, the endodermal vesicles could function as a pump regulating the fluid volume into the vitelline cavity, thereby avoiding the collapse of the organ due to the absorptive activity of the endodermal cells. Furthermore, mesothelial microvilli together with their mucous material harbor a layer of serous exudate and thus create a lubricated cushion designed to protect the thin mesothelium from frictional damage.

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An immunocytochemical study using a monoclonal antibody showed the presence of γ-glutamyl transpeptidase (γ-GTP) in the bile duct epithelium of the normal adult rat liver, while a slight immunoreactivity for γ-GTP could be observed on the membranes of bile canaliculi (the membranes of hepatocytes facing the lumen of bile canaliculi). In hepatocarcinogenesis induced by 3′-methyl-4-dimethylaminoazobenzene, no immunoreactivity was detected in α-fetoprotein-positive cholangiolar “oval” cells, though it was detected on the membranes of bile canaliculi. In livers with hyperplastic nodules, γ-GTP immunoreactivity was observed in clusters, with two different staining patterns. In one pattern, γ-GTP was present not only on the membranes of bile canaliculi, but also on the membranes facing the Disse's space or the lateral cell surface of hepatocytes. In the other pattern, γ-GTP immunoreactivity was seen on the membranes of the dilated bile canaliculi. In livers with developing hepatocellular carcinomas, the immunoreactivity was present on the membranes of irregularly dilated bile canaliculi in malignant cells.

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The relationaship between the course of enamel prisms and the direction of secretory face in Tomes' process of ameloblast was examined three-dimensionally in the dog tooth by light and electron microscopy, as well as by reconstruction.
Scanning electron microscopic observation on the developing enamel surface after the dissolution of the enamel organ showed numerous groups of pits with bulbous faces inclined in the same sidewards direction, while those faces in neighboring groups were inclined in an opposite direction.
Tangential semithin sections from demineralized tooth germs were serially cut from the enamel surface to the enamel-dentin junction. These sections showed numerous belt-like zones arranged perpendicular to the meridian of the tooth. A straight row of enamel prisms perpendicular to the boundaries of the belt-like zones was selected at 142μm from the enamel-dentin junction. The row of prisms and their successive Tomes' processes were reconstructed from micrographs with a personal computer. The cut ends of the enamel prisms initially perpendicular to the boundaries appeared as a sine curve, viewed from the enamel-dentin junction. In a single belt-like zone, the horizontal tilt angles of the enamel prisms towards the enamel-dentin junction tended to be largest at the center of the zone, and smallest near the boundaries. The long axis of each enamel prism was at right angles to the secretory face of the Tomes' process. The secretory faces in a single belt-like zone were inclined in the same sidewards direction and the secretory faces in neighboring zones were inclined in opposite directions.
The results suggest that the direction of the sidewards displacement in ameloblasts is related to that of the secretory face of Tomes' processes. Further, one group of ameloblasts with their secretory faces inclined in the same sidewards direction form one belt-like zone.

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Retrograde tracing was used to determine the locations of sympathetic nerve cells whose axons project to the stomach, small intestine, caecum, proximal colon, distal colon and spleen of the guinea-pig. Projections from prevertebral ganglia were organotopically arranged within and between ganglia. The cranially located coeliac ganglion provided the major input to proximal gut regions; the distal gut received more caudal input, from superior and inferior mesenteric and the hypogastric nerve ganglia. Nevertheless, minor proportions of the innervation of some target organs arose from other than the closest ganglion and the caecum had input from each of the coeliac, superior mesenteric and inferior mesenteric ganglia. Topography within a ganglion was best defined in the coeliac, in which nerve cells whose axons projected to the spleen, stomach and duodenum were preferentially laterally located, whereas most of those projecting to the proximal colon were medial. Fewer neurons projected from paravertebral—compared with prevertebral—ganglia to abdominal viscera. Projections to the stomach came from all thoracic chain ganglia, those to the duodenum and spleen from lower thoracic ganglia and those to the large intestine from lumbar chain ganglia. It is suggested that the previously reported chemical topography of nerve cells in sympathetic ganglia might be secondary to their organotopic organization.

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Wholemounts and sectioned retina from adults of two lizard species, Pogona vitticeps and Varanus gouldii, were studied by immunohistochemistry for neuropeptide Y (NPY)-like immunoreactivity. In both species the morphology of two classes of amacrine cells (types A and B) were described. Cell somata were located mostly in the inner nuclear layer (INL) but were occasionally displaced into the ganglion cell layer (GCL). In the Pogona retina, type A cells had large somata and dendritic arbor that branched in sublamina (S) 1 and 2/3 of the inner plexiform layer (IPL). Type B amacrine cells had smaller somata and dendritic arbor branching mostly in S5 of the IPL. In the Varanus retina, the levels of dendritic branching of types A and B amacrine cells in the IPL were similar to those in Pogona although branching in the middle of the IPL occurred at S3. NPY-immunoreactive cells with small somata and narrow to medium sized dendritic fields were predominant. Unclassified cells also displayed NPY-like immunoreactivity; however, their dendritic morphology could not be determined due to the faint and inconsistent staining. In transverse retinal sections three bands of NPY-like immunoreactivity were evident in the IPL of both species, to which the unclassified cells also contributed.
In both species type A cells were most numerous. Total NPY-immunoreactive cells were estimated to be 8, 600 in Pogona and 32, 860 in Varanus. In both species types A and B cells were non-uniformly distributed across the retina. The most apparent non-uniformity in distribution was observed in type A cells in Varanus. Peak cell density was found across the horizontal meridian of the retina from where cell density decreased towards the dorsal and ventral retinal margins.
The results of this study provide evidence for the presence of NPY-immunoreactive amacrine cells in the lizard retina of which two types were morphologically characterized. Cross-species comparisons were also made among NPY-immunoreactive amacrine cells, and their possible function/s discussed.

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The objectives of this study were to determine the first developmental stage when immunoreactive prolactin (PRL) cells appear in the fetal rat anterior pituitary, and then to compare their cytogenesis with that of growth hormone (GH) cells. Immunoreactive PRL cells first appeared on Day 18.5 of gestation, the same stage when GH cells were found to differentiate in the pituitary. During the fetal period, PRL cells were concentrated mainly in the anterior half of the pituitary, whereas GH cells were found predominantly in its posterior half. When estimated as the total sum of immunoreactive areas, the proportion of PRL cells remained low during fetal life in contrast to a marked increase in the GH immunoreactive area. The flip-flop (mirror) section technique revealed that, in the fetal rat, adenohypophysis PRL and GH are contained in different cells. The present study thus indicates that GH and PRL accumulation occurs in independent cells at least in early developmental stages in the rat.

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Changes in the expression of cytokeratin subunits during regeneration of the intralobular duct in partially injured rat submandibular glands were investigated. Limited parts of rat submandibular glands were injured by irradiation with YAG laser. Irradiated glands were investigated histologically and immunohistochemically using anti-cytokeratin monoclonal antibodies, RCK105 and CK19.
Irradiated areas became necrotic one day after YAG laser irradiation. At three and five days, duct-like structures and epithelial clusters began to regenerate at the periphery of the remaining lobule. Epithelial clusters without ductal spaces were situated at the terminal portion of the duct-like structures. At seven and ten days, duct-like structures were composed of cuboidal or low columnar cells, and the number of epithelial clusters decreased. Immunohistochemically, cells of intralobular ducts in normal rat submandibular glands reacted to RCK105 and CK19. At three and five days, the epithelial cells of duct-like structures were positive for both antibodies. Many cells in the epithelial clusters showed negative reaction. However, in some epithelial clusters, inner cells and cells facing narrow luminal spaces were positive for both antibodies. At seven and ten days, positive reaction for both antibodies was identified in duct-like structures.
This study showed that cells of the epithelial cluster were less mature than those of the duct-like structure, and that in the epithelial cluster, the inner cells were the first to differentiate into intralobular ductal cells.

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The pineal organ of the golden hamster consists of deep and superficial portions which are connected to each other by a stalk. The permeability of capillaries for horseradish peroxidase (HRP) injected intravenously was examined in sections of entire portions of the gland that cut either along coronal or sagittal planes. Two distinct portions of the parenchyma, i. e., dorsal major and ventral minor ones, were found in the superficial gland. Most of the capillaries in the dorsal portion were of the continuous type of endothelium, whereas those in the ventral portion were fenestrated. In the dorsal portion, HRP readily crossed the endothelium, permeated the basal lamina, flowed into the perivascular connective tissue space and intruded into the intercellular clefts of the parenchyma. In contrast, HRP was not found to penetrate through the endothelium of the capillaries in the ventral portion to reach the perivascular area, thereby leaving the intercellular clefts of the parenchyma free of HRP. In the deep gland the capillaries were exclusively of the non-fenestrated type. Intravenously injected HRP was prevented from crossing the endothelium by the tight junction. In some areas, HRP penetrated through the capillaries in the pia mater, and crossed the outer limiting membrane to reach the intercellular clefts of the parenchyma and the basal lamina of the capillaries in the peripheral region of the deep pineal gland. The junctions between endothelial cells were not penetrated by HRP. The observations indicate that the type of capillary, absence of perivascular spaces, and permeability in the deep pineal are all similar to these factors in the general brain tissue; they differ from these in the superficial pineal gland, in which the dorsal portion shows characteristics found in other endocrine glands, but the ventral zone exhibits a unique situations: the presence of a blood-pineal barrier with a pericapillary connective tissue area.

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Fine fibrils in the basilar membranes of mouse and guinea-pig cochleae were examined by transmission electron microscopy. The fibrils were 10nm in diameter and ran straight without branching. The fibrils appeared as two parallel lines in longitudinal sections, and round or polygonal in cross-sections, displaying tubular structures with a lumen. They were similar in ultrastructural details to the elastin-associated and elastin-independent microfibrils of Low (1962) in the subcutaneous tissue. It is concluded that the fibrils in the basilar membrane are the microfibrils.

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The distribution of substance P (SP)-immunoreactive nerve fibers in the rabbit carotid body was studied in combination with catecholamine autofluorescence images of sections where SP immunoreactivity was confirmed. Immunoreactivity for SP was found in nerve fibers distributed in the parenchyma of the carotid body. No glomus cells with SP immunoreactivity were observed in the carotid body. On comparing the distribution of SP-immunoreactive fibers with the catecholamine autofluorescence image in a single section, most SP fibers appeared associated with the fluorescent glomus cells, and were located around clusters of them. These results support the suggestion that SP fibers in the cat and rat carotid bodies are involved in chemosensory mechanisms. Furthermore, a survey of the present results and previous ones reported by other workers indicates that SP may be an essential neuropeptide in chemoreceptor organs in most vertebrates from amphibians on upwards evolutionally. In addition, the courses of some catecholaminergic fibers precisely agreed with those of some SP fibers. This suggests that certain sympathetic nerve fibers also contain SP.

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