Cell sorting and chondrogenic aggregate formation in micromass culture.

A fundamental feature of cartilage differentiation in the developing limb is the formation of a prechondrogenic cell condensation. An apparently similar process of prechondrogenic cell aggregation occurs in micromass cultures of limb bud mesenchyme with the formation of cellular aggregates which often differentiate into cartilage nodules. We have investigated the process of aggregate formation in micromass culture using chimaeric mixtures of potentially chondrogenic and nonchondrogenic cell types. Two systems were studied: mixtures of distal and proximal limb mesenchyme cells and mixtures of distal limb cells with avian tendon fibroblasts. In both cases cultures of varying proportions of each cell type have been prepared. The results demonstrate that aggregate formation in vitro is the consequence of a cell sorting process which can involve prechondrogenic cells of widely different spatial origins within the developing limb. This contrasts with in vivo prechondrogenic condensation in which there is no evidence of cell sorting (Searls, R.L. (1967), J. Exp. Zool. 166, 39-50). However, our findings do indicate that cell surface differences occur in apparently undifferentiated limb mesenchyme. The results also suggest that mesenchymal cell aggregates must achieve a threshold size before chondrogenesis can proceed. In addition, the results show that under some culture conditions nonchondrogenic cells will form aggregates.