In order to determine whether chondrocytes are stable as to type during limb regeneration and whether any cells from limb muscle become chondrocytes in the regenerate, labeled cartilage and muscle were transplanted into limbs which were then allowed to regenerate. Transplanted cells were identified after redifferentiation in the regenerate by three kinds of label: H3-thymidine detected by radioautography; triploidy as judged by the presence of three nucleoli per nucleus; or a combination of the two.
The results of this investigation indicate that limb chondrocytes are intrinsically stable with respect to cell type, for grafts of both triploid and H3-thymidine labeled chondrocytes gave rise to morphologically dedifferentiated blastema cells which almost exclusively differentiated into chondrocytes. The small percentage of labeled noncartilage cells observed could have been the result of metaplasia, although it is more likely that all or most of them arose as spontaneous polyploids or as contaminants of the original grafts. Isotope dilution in H3-thymidine labeled chondrocytes indicated that they had usually divided at least five times. Hence, the basis for the intrinsic stability of chondrocytes must be heritable.
Chondrocytes from the coracoid, scapula, and visceral arch cartilages also contributed cells to cartilage of limb regenerates, and the available evidence indicates that cells released from these cartilages are similarly stable as to type. These cartilages did not dedifferentiate and participate in limb regeneration to the same extent as did limb cartilage.
Results from transplanted limb muscle, labeled by all three methods used in this study, indicate that certain cells of muscle can become chondrocytes during limb regeneration. The exact cellular origin of these chondrocytes, however, is uncertain, for muscle is a mixture of cell types.