Type II collagen is transiently expressed during avian cardiac valve morphogenesis

We present new evidence of the temporal and spatial expression of type II collagen in the embryonic chick heart during the very early stages of its development. In particular, we emphasize the distribution of its mRNA and protein during valve formation. Type II collagen as well as several other fibrillar collagens (types I, III, and V) are present in stage 18 endocardial cushion mesenchymal cells. At stage 23, alpha 1 (II) collagen transcripts and the cognate polypeptide colocalize in the atrioventricular valves. As development proceeds, the relative abundance of alpha 1 (II) collagen transcripts decreases during the stages studied (stages 22 to 45; day 3.5 to day 19) as assayed by RNA blotting of extracts of whole hearts. Type II collagen protein was immunologically undetectable in stage 38 (day 12) hearts, although collagens I, III, and V persisted and localize in the valve regions, in the endothelial lining of the heart, and in the epicardium. In keeping with other observations of type II collagen expression in non-chondrogenic regions of a variety of vertebrate embryos, the avian heart also exhibits transient type II collagen expression.