Collagen fibril diameter in the common carotid artery of the rat.

The diameter of collagen fibrils has been measured in electron micrographs of the tunica media and adventitia of the common carotid artery of 14 rats (19,262 counts). The fibrils of the media were measured in each of its three interlamellar spaces. The mean fibril diameters in the innermost space and the middle space are not different (29.7 and 30.7 nm). However, the diameter of the fibrils is larger in the outermost space of the media than in the inner two spaces. With a diameter of 37.4 nm, the difference from the other medial spaces is highly significant. The adventitial fibril diameter of 66.1 nm is significantly different from the values for any of the medial spaces. The only cells present in all three spaces of the tunica media of the rat common carotid artery are smooth muscle cells, which presumably synthesize similar procollagen throughout the media. The larger diameter collagen fibrils of the outer interlamellar space may be an adaptation to greater mechanical stress in the outer media, or possibly the activity of the fibroblasts in the adjoining adventitia may affect fibril growth in this space of the media.