Inhibition of membrane bone formation by vitamin A in the embryonic chick mandible.

Embryonic chick mandibles (Hamburger and Hamilton [HH] stages 17-25) were cultured in the presence of various concentrations of vitamin A to determine the effect of hypervitaminosis A on membrane bone formation. In normal development, the mandible differentiates a centrally located Meckel's cartilage surrounded by membrane bones. Mandibles cultured without added vitamin A differentiated normally, though the timing of differentiation was retarded from that in ovo. Treatment with vitamin A interfered with skeletogenesis to varying degrees depending upon the initial age of the explant and the concentration of vitamin A. At low concentrations of vitamin A (1 microgram/ml), neither cartilage nor membrane bone formed in young explants (HH stage 17), whereas cartilage formed in 78% and membrane bone in 11% of older explants (HH stage 25). Higher concentrations of vitamin A (2-4 micrograms/ml) inhibited membrane bone formation in all explants, and 4 micrograms/ml of vitamin A inhibited chondrogenesis in most (88%) of the older explants. To determine whether tissue interactions influence this effect of vitamin A on skeletogenesis, mandibular mesenchyme was separated from its epithelium and treated with vitamin A. Under normal culture conditions, isolated mesenchyme (HH stage 25) differentiated both cartilage and membrane bone. Hypervitaminosis A inhibited membrane bone formation in the isolated mesenchyme at all levels tested (1-4 micrograms/ml) and inhibited chondrogenesis at levels 2-4 micrograms/ml. Hence, vitamin A can act directly upon the mesenchyme to inhibit both membrane bone formation and chondrogenesis, but its action is mitigated by the presence of the epithelium.