It is becoming increasingly evident that the in vitro induction of vertebral chondrogenesis may not be due to the acquisition of a new metabolic pattern, but to the stabilization of one that is already existing. The sclerotome regioll of embryonic chick somites will undergo chondrogenesis in vitro as a response to various stimuli, the most effective being the embryonic notochord. Before stimulation, the somites have all of the metabolic machinery necessary for chondrogenesis, as evidenced by the fact that they can synthesize chondromucoprotein. They cannot, however, accumulate matrix except under special circumstances, e.g., when they are stimulated by a n "inducer" such as the notochord (induced cartilage) or when they are permitted to express their chondrogenic bias under suitable culture conditions, such as in enriched nutrient medium (spontaneous cartilage).
A study of chondroitin sulfate synthesis in these tissues has shown that the embryonic somites utilize glucosamine differently when compared to cartilage tissue. Analysis of the metabolic steps between glucosamine and UDP-N-acetylgalactosamine indicates the possibility of a metabolic "block," which would prevent the efficient transformation of N-acetylglucosamine-1-P to UDP-N-acetylglucosamine.