Endochondral calcification by hypertrophic chondrocytes in the Meckel's cartilage grafted into the isogenic mouse spleen.

Abstract

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...

Morphological characteristics of the life cycle of resting cartilage cells in mouse rib investigated in intrasplenic isografts

“…However, there was no evidence that they were transformed into other cell types. Regarding this point, the hypertrophic chondrocytes in the resting cartilage clearly differed from those in Meckel's cartilage, which, as we reported in our previous study (Ishizeki et al 1990a), induced endochondral ossification, and were transformed into osteocyte-like cells when 18-day embryonic Meckel's cartilages were transferred into a permissive environment. There have been similar reports showing that some chondrocytes, including chondroprogenitor cells, following morphological changes, are transformed into osteogenic cells (Holtrop 1972;Silbermann et al 1983;Richman and Diewert 1988).…”

“…The cellular differentiation of resting cartilage cells into terminal hypertrophic chondrocytes, however, has not yet been reported morphologically. Our recent study demonstrated that hypertrophic chondrocytes in mouse embryonic Meckel's cartilage grafted into spleen survived for long periods, and subsequently induced extensive endochondral ossification (Ishizeki et al 1990a). This finding involved morphological evidence which showed that the features of Meckel's cartilage, which essentially has a regressive fate, may be modulated by the intrasplenic microenvironment.…”

Meckel's cartilage chondrocytes in organ culture synthesize bone-type proteins accompanying osteocytic phenotype expression

“…However, such features of the Meckel's cartilages are not ubiquitous, and in our recent studies (Ishizeki et al 1990a(Ishizeki et al , 1992a(Ishizeki et al , 1994 we showed that when Meckel's cartilage was maintained in heterotopic sites, it expressed bone-type cellular elements and matrix calcification. It was therefore suggested that M e c k e l ' s chondrocytes have the ability to induce bone matrix-like precipitation.…”

“…Previously, we reported that when Meckel's cartilage tissue was transplanted into ectopic sites, the chondrocytes expressed an osteocytic phenotype accompanying matrix calcification (Ishizeki et al 1990a(Ishizeki et al , 1992a(Ishizeki et al , 1994. In the present study, we focused on the cellular transformation and on the matrix calcification of Meckel's cartilage in organ culture and further followed up our previous findings.…”

High-density culture of mouse Meckel's cartilage cells stimulates phenotypic conversion to osteocyte-like cells

“…Holtrop [4], Shimomura and Ray [5], and Richman and Diewert [6] suggested that differentiated chondrocytes are involved in bone formation and subsequently transform into bone-related cells . Furthermore, we previously found that when Meckel's cartilage was transplanted into isogenic mice, these cells survived for long periods and further transdifferentiated into osteocyte-like cells accompanied by bony-type matrix calcification [7][8][9] . Meckel's cartilage has regionally diverse developmental fates : the auricular and rostral ends of Meckel's cartilage develop into osseous tissue through endochondraltype ossification, while the entire middle portion is resorbed without obvious matrix calcification [6,10] .…”