Morphological modifications during long-term survival of Meckel's cartilage hypertrophic chondrocytes transplanted in the mouse spleen.

To examine the ultimate fate of hypertrophic chondrocytes, Meckel's cartilage bars from 18-day-old mouse embryos were transplanted into isogenic mouse spleen for 3, 7, 14 and 21 days and observed at the light and electron microscopic levels. The midportions of these Meckel's cartilage bars were used as explants; they were characterized by many hypertrophic chondrocytes containing euchromatic round nuclei, a large amount of glycogen particles, and some vacuoles. Grafted cartilage adapted well to the splenic tissue, showing intense metachromasia around the territorial matrix. Ultrastructural observations indicated that the number of large vacuoles and glycogen aggregates in the hypertrophic cells became markedly reduced with grafting time, whereas the Golgi apparatus and rough endoplasmic reticulum were well-developed. Needle-like crystals showing initial apatite deposition appeared in association with matrix vesicles; these proliferated as time elapsed after transplantation. On day 14 after transplantation, cells displaying such various structural features as pyknotic nuclei, large vacuoles, and cytoplasmic shrinkage were noted in addition to intact hypertrophic chondrocytes. Following resorption of the calcified cartilage by multinucleated giant cells, many osteoblasts appeared along the border of the calcified matrix. Some remaining hypertrophic cells in the calcified matrix had transformed into osteocyte-like cells. On day 21, the resorbed area of the calcified cartilage was invaded by many blood vessels. Hypertrophic chondrocytes, now exposed from cellular lacunae, and the osteocyte-like cells in the calcified matrix displayed involutional changes. The present study showed that, although the hypertrophic chondrocytes in Meckel's cartilage essentially underwent regressive changes, they retained the ability to stimulate endochondral ossification within the microenvironment of the spleen. In addition, some of these cells were transformed into osteocyte-like cells.