Collagen synthesis in the developing chick heart

We have surveyed the amount and types of collagen synthesized in two regions of the chick heart during embryonic development. Cardiac tissues from successive periods of development were labeled with 3H-proline in short-term organ culture. The fraction of incorporated label present as collagen was estimated by comparison of TCA-soluble radioactivity before and after digestion with purified bacterial collagenase. This measure of collagen synthesis varied only slightly with the length of the labeling period and agreed with values obtained by labeling in ovo. In the developing outflow tract, the fraction of label present as collagen increased sharply during the period of truncal septation (5-9 days), rising from initial values of 6% at 3 days of incubation to a plateau of about 25% (10-19 days). In ventricular myocardium, this fraction rose gradually from 3 to 20% between 3 and 19 days of incubation. The types of collagen synthesized in developing heart were analyzed by limited pepsin digestion and acrylamide gel electrophoresis, using collagens synthesized in tendon, cartilage, skin, and lens capsule for comparison. The types of collagen synthesized in both cardiac regions changed in similar manner during development. During the first week of cardiac function, a substantial but progressively smaller fraction of total collagen synthesized was identified as Type IV. Synthesis of Type I collagen increased sharply during this period and predominated during the second half of development. Type III collagen was synthesized in trace amounts by the middle of development and constituted approximately one-sixth of total collagen synthesis just before hatching. Minor amounts of collagen identified as Type B collagen were synthesized throughout the latter two-thirds of development.