Glycosaminoglycan synthesis by the early embryonic chick heart

Glycosaminoglycans of embryonic chick hearts labeled in situ were characterized by means of labeled precursor incorporation, electrophoretic mobility, sensitivity to testicular hyaluronidase, elution characteristics from CPC-cellulose columns, and hexosamine content. During the initial period of overt cardiac muscle differentiation (approximately stage 10) chondroitin sulfates are not detectable but an undersulfated component is present. Chondroitin sulfate synthesis appears shortly after overt muscle differentiation. Hyaluronate is present both during and after overt myocardial differentiation. Although epimerization of ³H-glucosamine-derived labeled UDP-N-acetyl-d-glucosamine occurs (determined by recovery of incorporated labeled galactosamine), label does not appear in chondroitin sulfate. ³H-Glucosamine is thus a relatively specific precursor for unsulfated glycosaminoglycans, a fact that we exploited in demonstrating their distribution radioautographically. Glycosaminoglycan synthesis was also examined in hearts labeled (a) in isolated organ culture, (b) in situ but exposed directly to the medium by removal of the splanchnopleure. In both cases fully sulfated chondroitin sulfate and chondroitin are not synthesized. Hearts make only hyaluronate and undersulfated chondroitin sulfate.