RETINOIC ACID RESPONSIVENESS OF CELLS AND TISSUES IN DEVELOPING FETALLIMBS EVALUATED IN A RAREHSPLACZ TRANSGENIC MOUSE MODEL

Limb morphogenesis is a complex phenomenon in which retinoids play an important role. Abnormal maternal retinoid levels from high oral doses cause fetal malformations, including abnormalities of the musculoskel etal system. Our purpose was to identify the retinoid-responsive cells in bone and cartilage during limb development by using a transgenic l ine of mice containing a reporter gene insert consisting of a retinoic acid response element linked to an Escherichia coli beta-galactosidas e gene. Transgenic fetuses from day 11.5 after conception to birth (da y 20) were analyzed histologically. Retinoid-responsive cells and tiss ues were first seen in the limb bud at 12.5 days in the webs between t he forming digits. The webs stained maximally at 14.5 days, after whic h staining intensity subsided. Staining in the muscles was detectable at 13.5 days, at a stage coinciding with myoblast fusion. Specific reg ions of perichondrium and periosteum also stained at this stage. Occas ional staining was observed in individual chondroblasts in all chondro genic regions, including hypertrophic chondroblasts and certain articu lar surfaces of developing joints. Staining of these tissues decreased in intensity in subsequent stages. Osteoclasts started to express bet a-galactosidase at 15.5 days and continued to stain into maturity. Our results indicate that specific subsets of cells respond to retinoids at specific stages in the course of normal limb development. In hypert rophic chondrocytes and cells in the webs and joints that display such a response, retinoid-induced effects may be linked to cell death that occurs in these regions. Staining in muscle, perichondrium, and perio steum may reflect retinoid-induced effects associated with cell differ entiation and growth. These results suggest that retinoids play a role in a variety of tissues, including bone and cartilage, at specific st ages during morphogenesis.