Stable, position-related responses to retinoic acid by chick limb-bud mesenchymal cells in serum-free cultures.

Retinoic acid (RA) has dramatic effects on limb-skeletal patterning in vivo and may well play a pivotal role in normal limb morphogenesis. RA's effects on the expression of pattern-related genes in the developing limb are probably mediated by cytoplasmic RA-binding proteins and nuclear RA-receptors. Little is known, however, about how RA modifies specific cellular behaviors required for skeletal morphogenesis. Earlier studies supported a role for regional differences in RA concentration in generating the region-specific cell behaviors that lead to pattern formation. The present study explores the possibility that position-related, cell-autonomous differences in the way limb mesenchymal cells respond to RA might have a role in generating pattern-related cell behavior. Mesenchymal cells from different proximodistal regions of stage 21-22 and 23-24 chick wing-buds were grown in chemically defined medium and exposed to 5 or 50 ng/ml of RA for 4 days in high-density microtiter cultures. The effects of RA on chondrogenesis in these cultures clearly differed depending on the limb region from which the cells were isolated. Regional differences in RA's effects on growth over 4 days in these cultures were less striking. The region-dependent responses of these cells to RA proved relatively stable in culture despite ongoing cytodifferentiation. This serum-free culture model will be useful in exploring the mechanisms underlying the region-dependent responsiveness of these cells to RA.