Effect of Force Level on Synthesis of Type III and Type I Collagen in Mouse Interparietal Suture

Abstract

Nine-week-old Swiss male white mice were divided into groups killed after time intervals of force application of six h, and one, three, five, seven, ten, 14, 21, and 28 days. Each group had 45 animals: three control, three sham-operated, and three experimental animals for each of the five force levels: 50 g, 35 g, 25 g, 15 g, and 5 g. The experimental animals had helical springs placed surgically in their calvaria for expansion of the interparietal suture. The sham-operated animals received inactive springs. Control animals were at the same age as the experimental and sham-operated animals.After death, the amount of sutural expansion was measured, and the calvaria with the implanted springs were explanted into Trowelltype organ culture dishes. [14C]-glycine was added for two h after 60 min of culture for all explants. The rate of suture expansion was directly proportional to the force value of the tensile stress, and a maximum 2.0-mm expansion was achieved for all force levels by the 28th day. Sutural collagen was solubilized by limited pepsin digestion, and radiolabeled types I and III a-chains were separated by SDS-PAGE, visualized fluorographically, and measured densitometrically. All the experimental and sham-operated animals responded with a rapid rise followed by an almost equally rapid fall in the proportion of newly-synthesized type III collagen before becoming stabilized for the rest of the experimental period at a level that was significantly higher than that of the control and sham-treated animals of the same age. The increase in the proportion of type III collagen was directly related to force level and rate of suture expansion. The peak value of the proportion of type III collagen due to the lightest force was much closer to the proportions that occur naturally in the suture during normal rapid growth and development than to those caused by the heavier forces. Thus, it appeared that light forces tended to initiate a more physiological response than heavy forces, with respect to the proportion of newly-synthesized type III collagen.