Osteoclasts of rat parietal bones were cultured and observed under the transmission electron microscope. The processes forming the ruffled border were studied three-dimensionally by a reconstruction method. After the observation of one plane (A-plane), serial ultrathin sections were made at right angles to the A-plane (B-plane). The processes of the ruffled border in the B-plane were reconstructed, based on the A-plane features, with Nikon Cosmozone 2S.
The three-dimensional structure of the ruffled border of the osteoclasts was composed of two kinds of processes: one finger-like and the other plate-like. Both finger-like and plate-like processes branched from the stem processes projecting directly from the cell body. The long plate-like processes branched into secondary plates, extending perpendicularly to the bone surface.

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The blood vascular bed of the rat thyroid gland was reproduced by injection of a methacrylate casting medium and observed with a scanning electron microscope. The rat thyroid gland received the superior and inferior thyroid arteries and emitted the superior and inferior thyroid veins. Anastomosis between the interlobular arteries or between the interlobular veins was frequently observed in the thyroid gland. The arteriovenous anastomosis was rarely observed between the terminal branches of the lobular arteries and veins.
The thyroid blood vascular bed was divided into lobular units which consisted of basket-like capillary networks surrounding the thyroid follicles; a small lobular unit consisted of a few networks, whereas a large one of fifty or more networks. Sizes and forms of the networks varied widely in each case. However, the networks in the superficial layers of the lateral parts of the thyroid gland were typically most developed.
Regardless of its size and form, each network always received a proper afferent vessel from the lobular artery and issued a proper efferent vessel continuous with the lobular vein, though it was sometimes provided with an accessory afferent or efferent vessel. Only occasionally were the adjacent networks fused with each other or connected by transfollicular capillaries. Thus, the present data suggest that each follicular capillary network is a fairly independent functional-unit in the thyroid microcirculation. The capillaries of the network were sinusoidal in nature and sometimes protruded fine projections which indicated the neogenesis of capillaries.
The blood vascular bed of the newborn rat thyroid gland was not always differentiated into basket-like capillary networks.

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The distribution of fibronectin and 140Kd fibronectin-receptor was examined in the truncus arteriosus of the chick embryonic heart during aortico-pulmonary (AP) septation by immunohistochemistry.
In 4-day-old chick embryos, immunoreactivity of both anti-fibronectin and anti-140Kd fibronectin-receptor was seen in the primordia of the AP septum. In 5-day-old embryos, cells stained with anti-140Kd fibronectin-receptor antibody were rarely found in the AP septum, though strong immunoreactivity of the anti-fibronectin antibody was observed in the AP septum. In 6-day-old embryos, at a late stage of septation, cells stained with anti-140Kd fibronectin-receptor antibody were not found in the AP septum, and immunoreactivity of the anti-fibronectin antibody had decreased. In the cushion tissue, immunoreactivity of the anti-fibronectin antibody was seen, but that of the anti-140Kd fibronectin-receptor antibody was recognizable only on a few cushion cells in the 4-day-old embryos.
These findings suggest that fibronectin is involved in the formation of the AP septum primordia, that fibronectin does not promote the development of the AP septum, and that the migration mechanism of cushion cells is different from that of neural crest cells.

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The ontogenesis of the mouse gastric mucosa was studied by carbohydrate histochemistry and ³H-thymidine autoradiography. Surface mucous cells and glandular cells were identified from day 16 of gestation. Sugar residues in the mucin of surface mucous cells seem to undergo no major changes throughout the period under study, since secretory granules of the cells were positive in periodic acid-Schiff (PAS) and galactose oxidase-Schiff (GOS) reactions and consistently bound certain lectins.
Chief cells and mucous neck cells are not separated until the third postnatal week, though primitive chief cells are present during earlier developmental stages. Secretory granules of primitive chief cells shared positive PAS and GOS reactions with mucous neck cells and bound similar lectins, but the intensity was generally weaker. The granules of primitive chief cells were also stained by Bowie's solution which exclusively stained zymogen granules in chief cells in adults. These results suggest that secretory granules of primitive chief cells contain a complex carbohydrate similar to mucin in mucous neck cells, but with a lower sugar/protein ratio.
It is concluded by studies using ³H-thymidine autoradiography combined with carbohydrate histochemistry that, though immature surface mucous cells, primitive chief cells and mucous neck cells actively proliferate, chief cells rarely undergo mitosis.

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The development of the small intestine in rat fetuses was studied by light and electron microscopy with special reference to morphological events during the stage of obliteration. Prior to this stage, the epithelium lining the intestinal lumen was simple, tall and columnar and consisted of undifferentiated cells mutually joined by the adluminal junctional complex.
At the stage of obliteration, the epithelium appeared stratified and the intestinal lumen was identified as a narrow irregular slit. In addition to the junctional complex surrounding the intestinal lumen, secondary junctional complexes were scattered in the more basal parts of the epithelium. The intra-epithelial cavity was formed within the macula occludens of the secondary junctional complex and became larger by division of the surrounding cells.
Since the small intestinal epithelium was essentially simple columnar, mitosis occurred facing the lumen. At the stage of obliteration when the number of epithelial cells was rapidly increasing, mitosis occurred facing not only the main intestinal lumen but also the intraepithelial cavity.
As the intra-epithelial cavities fused with the main intestinal lumen, mesenchyme invaded to form villi and the epithelium returned to a simple columnar form. Consequently, the intestinal lumen was enlarged and villi were formed. At the same time, absorptive cells with a brush border appeared in the villous epithelium and immature goblet cells were occasionally seen.

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The morphogenesis of the epithelial rests of Malassez in rat molars was studied by light and electron microscopy. The epithelial islands immediately after disintegration of the Hertwig's root sheath consisted of a few irregularly shaped cells with scanty rough endoplasmic reticulum, which were surrounded in portions by disrupted basal lamina. Close topographic relationships between the epithelial cells and mesenchymal cells of the dental sac were often observed in this period. After thirty days of age, the epithelial cells started to form ovally shaped epithelial islands consisting of four or five cells, but collagen fibrils still remained in the intercellular spaces. Vacuoles containing collagen fibrils were occasionally observed in the cytoplasm of the epithelial cells during this period in association with an acid phosphatase positive reaction. In more mature rat molars, the epithelial islands were characterized by an increased number of epithelial cells, only rarely observed collagen fibrils in the cytoplasm and the intercellular spaces, and an almost complete lining of basal lamina. The results indicate that the phagocytosis of collagen fibrils by the epithelial cells is a crucial step in the formation of the epithelial rests. The possibility is suggested that interactions with mesenchymal cells of the dental sac and regeneration of the basal lamina may also contribute to the epithelial rest formation.

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A single intraperitoneal injection of 10mg estriol (E₃) rapidly induced a transient decrease in the serum albumin (ALB) level in 3 to 5 days in adult mice. At a dose of 5mg, E₃ showed a remarkable threshold effect on the decline of serum ALB. By immunohistochemistry, two types of ALB-positive cells were found in the liver. Type 1 cells were stained intensely and almost homogeneously throughout all or nearly all their entire cytoplasm, and type 2 cells showed only granular or vesicular cytoplasmic deposits. After administration of 10mg E₃, the type 1 cells diminished rapidly and markedly in direct proportion to the fall in serum ALB from the centro- and mediolobular zones, where they had been normally localized either singly or in small groups. Simultaneously, the granular or vesicular deposits that had been normally stained very weakly in the type 2 cells in the centro- and mediolobular zones became intensely stained along with cells in the perilobular zone. Alpha-fetoprotein (AFP)-containing cells in the liver appeared in maximal numbers at 5 to 7 days after E₃ administration. These serological and cytological changes after E₃ adiministration are here discussed on the basis of a possible inverse relationship between ALB and AFP gene expression.

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