The capacity of the dental pulp to form calcified tissue was examined in papilla cells dissociated from first molar tooth germs of the neonatal mouse and isografted in the spleen for up to 7 days. To obtain papilla cell populations without odontoblasts, pulpal mesenchyme was isolated mechanically from the enamel organ after 0.1% trypsin treatment and rolled on a membrane filter. On day 3 after transplantation, the grafted papilla cells had changed into large, spindle-shaped cells, and initial calcification with needle-like crystals began in association with the collagenous matrix surrounding those cells. On day 7 after transplantation, the spindle cells transformed into odontoblast-like cells containing well-developed secretory organelles, and irregular, but nontubular, calcified tissues were commonly observed surrounding the extracellular collagenous matrix. The calcified tissue matrix with cellular inclusions displayed a structure similar to that of osteodentin. During this period, an intense positive reaction for alkaline phosphatase (ALPase) activity was demonstrated along the cell membranes of the odontoblast-like cells aligned at the periphery of forming calcified tissue. Enzymatic activity could not be detected on the cells incorporated completely into osteodentin-like matrix. The present results show that the papilla cell population transplanted into the spleen formed osteodentin-like material, thus demonstrating the capacity of papilla cells to produce calcified tissue.
Summary. We found that when the midsection of Meckel's cartilage bars obtained from mice on the eighteenth day of gestation were grafted into isogenic mouse spleen, chondrocytes induced an endochondral calcification. Concurrent with the onset of calcification throughout Meckel's cartilage matrix, periodic banded thick collagen fibrils and matrix vesicles were observed around the chondrocytes.Although most of the chondrocytes prior to grafting were hypertrophic cells, they survived for seven days in the splenic tissue and had well-developed secretory organelles. The cells which were surrounded by calcified matrix were relatively small, spherical, and showed a morphology closely resembling that of osteocytes.These findings suggest that the life span of hypertrophic chondrocytes is influenced by the microenvironment of the spleen.Meckel's cartilage, known as a fetal organ, is initially formed as the main support of the mandible. It is divided into three distinct subdivisions by the regional transformation of the cartilage (BHASKER et al., 1953; FROMMER and MARGOLIES, 1971). Among these regions, the rostral process (distal portion) and auricular end (proximal portion) of the Meckel's cartilage have been thought to undergo endochondraltype ossification, but not the midsection (BHASKER et al., 1953; GORET-NICAISE and PILET, 1983; RICHMAN and DIEWRT, 1988).The hypertrophic chondrocytes in the midsection of the normal Meckel's cartilage degenerate and are replaced by invading cells accompanied by membrane bone formation around the cartilage. Recently, it has been proposed that some of the hypertrophic cells following morphological changes transform into osteogenic cells (HOLTROP, 1972;SHIMOMURA and RAY, 1973;RICHMAN and DIEWERT, 1988;YOSHIOKA and YAGI, 1988). Thus, there is still some confusion regarding the ultimate fate of the terminal chondrocytes.The present study examined morphologically the fate of the hypertrophic chondrocytes in the middle area of Meckel's cartilage grafted into the spleen. MATERIALS AND METHODSMeckel's cartilage bars obtained from mouse (ddy strain) embryos on the eighteenth day of gestation (day of vaginal plug =day 0) were used because cartilage in this stage contains a large number of hypertrophic chondrocytes in the absence of degenerated chondrocytes.The mandibular arches containing Meckel's cartilage were dissected aseptically and placed in Hanks' balanced salt solution supplemented with Kanamycin (1 mg/ml). After they were immersed in 10 mM EDTA in magnesium-calcium free-phosphate buffer for 10 min to remove excessive cellular elements of the connective tissue from Meckel's cartilage bars, the Meckel's cartilage was removed mechanically. These spicules were immediately transferred to fresh Hanks' solution and one-third were cut away using scissors. The midsection was used for grafting because it was known that this region does not have endochondral calcification.These explants were grafted into the spleen of isogenic adult mice as previously reported (ISHIZEKI et al., 1987). Briefly, us...
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