Immune cell trafficking in the secondary lymphoid organs is crucial for an effective immune response. Recirculating T cells constantly patrol not only secondary lymphoid organs but also the whole peripheral organs. Thoracic duct lymphocytes represent an ideal cell source for analyzing T cell trafficking: high endothelial venules (HEVs) allow recirculating lymphocytes to transmigrate from the blood directly, and recirculating T cells form a cluster with dendritic cells (DCs) to survey antigen invasions even in a steady state. This cluster becomes an actual site for the antigen presentation when DCs have captured antigens. On activation, effector and memory T cells differentiate into several subsets that have different trafficking molecules and patterns. DCs also migrate actively in a manner depending upon their maturational stages. Danger signals induce the recruitment of several DC precursor subsets with different trafficking patterns and functions. In this review, we describe general and specialized structures of the secondary lymphoid organs for the trafficking of T cells and DCs by a multicolor immunoenzyme staining technique. The lymph nodes, spleen, and Peyer's patches of rats were selected as the major representatives. In vivo trafficking of subsets of T cells and DCs within these organs under steady or emergency states are shown and discussed, and unsolved questions and future prospects are also considered.

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The three-dimensional architecture of enamel prisms at early stages of enamel formation and its spatial relationship to the Hunter-Schreger bands were examined in canine tooth germs by light and electron microscopy. In serial semithin sections of demineralized tooth germs tangential to the enamel-dentin junction, a straight row of enamel prisms was depicted along the longitudinal tooth axis at the level of the enamel-dentin junction and then their three-dimensional arrangement was reconstructed using computer software. The spatial arrangement of the groups of enamel rods oriented in specific sideward directions was also reconstructed in deep layers of the enamel.
Initially, all enamel prisms were parallel to perpendicular toward the enamel-dentin junction, but at 10μm from the enamel-dentin junction, some small specks, or groups of enamel prisms--tilting to the right or the left--emerged as small islands. In each speck of enamel prism, the inclined prisms were uniformly oriented in a sideward direction and gradually expanded their boundary until merging with the neighboring specks inclined in the same direction. Consequently, at 50μm from the enamel-dentin junction, the group of enamel prisms oriented either to the right or the left formed alternately arranged horizontal belt-like zones, corresponding to the parazone or the diazone of the Hunter-Schreger bands.
Reversed images of scanning electron-micrographs of the exposed surfaces of the developing enamel revealed round and bulb-like profiles of Tomes' processes at early amelogenesis and its changes into a characteristic structure combined with flat secretory and enclosing nonsecretory faces that dictated the orientation of corresponding enamel prisms.
The results suggest that the groups of enamel prisms oriented in sideward directions first appear as small island-like specks near the enamel-dentin junction, which later merge and form alternating horizontal belt-like zones as a consequence of morphological changes of the Tomes' processes. However, the mechanisms whereby the functional grouping of secretory ameloblasts with similarly oriented Tomes' processes is induced are yet to be determined.

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The three-dimensional ultrastructure of over 1000 mitochondria in human Leydig cells (from twelve sexually mature patients) was examined by high resolution scanning electron microscopy (HRSEM) of osmium-macerated specimens, as well as by transmission electron microscopy of conventional ultrathin sections. The stereo-pair imaging of the osmium-macerated specimens by HRSEM is also very useful for investigating the three-dimensional structure of cytoplasmic membranous organelles with great clarity. The mitochondria, which mainly are elongated (although some are ovate), possess cristae that are almost exclusively tubular and that occasionally display constrictions and terminal bulbules. Lamelliform cristae are quite rare. Occasionally, the tubular cristae are joined together to form a simple network. Classic crista junctions could not be identified with certainty, although the base of the tubular cristae might correspond functionally to such junctions. As a whole, in line with the identical and common embryological origin of adrenal cortex and gonads, mitochondria of human Leydig cell closely resemble those of steroidogenic cells of human suprarenal cortex treated by the same maceration method.

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The liver from a 6-day-old chick embryo was transplanted on the chorioallantoic membrane of a 9-day-old chick embryo to observe the process of liver regeneration histologically. When a piece of the liver was implanted on the chorioallantoic membrane, only cells in the superficial zone of the graft adhering to the chorioallantoic membrane survived. Eventually, these surviving cells in the superficial zone proliferated with hematopoiesis, resulting in the formation of clusters of blood cells surrounded by the hepatocytes (or hepatic parenchymal cells). Semi-thin serial sections showed that these clusters of blood cells were confined to the space formed by hepatocytes. Furthermore, structures similar to the hepatic cord, sinusoid, central vein, and bile duct appeared in the reconstructed liver eleven days after transplantation. Meanwhile, when a pellet of the dissociated liver cells was transplanted onto the chorioallantoic membrane, two types of liver like structures were reconstructed: one was clusters of hepatocytes accompanied by sinusoids after hematopoiesis, and the other was a simple accumulation of hepatocytes without any sinusoids or hematopoiesis. The sinusoids found in the former type became clear following the connection between the space in the transplant and vessels of the chorioallantoic membrane. These findings indicated that the reconstructed liver was primarily produced by the accumulation of hepatocytes accompanied by hematopoietic cells, followed by the formation of sinusoidal spaces. We therefore consider that hematopoiesis is important for liver regeneration with a normal structure. Transplantation of the liver in the chorioallantoic membrane could be also useful for research into liver regeneration.

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The microvascular anatomy of the large intestine of the adult South African Clawed Toad, Xenopus laevis (Daudin), was studied by scanning electron microscopy (SEM) of vascular corrosion casts (VCCs) and correlative light microscopy. Observations showed the large intestine to be supplied by the haemorrhoidal artery and the posterior mesenteric artery and drain via the posterior haemorrhoidal vein into either the left or right posterior abdominal vein. Both arteries and veins showed a bipinnate supply/draining pattern with branches running circumferentially. Vessels embraced the gut wall while arteries and veins in most cases alternated along the gut length. Many short terminal arterioles arose from the circumferential arteries at almost acute angles and capillarized after a short distance. Capillary lengths were short and continued into numerous postcapillary venules which merged either in a leaf vein-like formation or in a rosette-like formation with up to four draining sites per supplying arteriole. The microvasculature was found to be well adapted 1) to sustain blood flow under different amounts of feces in the gut and 2) to provide optimal conditions for the resorption of water and salts from the gut lumen into the blood vascular system by the high number of venules and their conspiciouos rosette-like and leaf vein-like patterns.

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