Elastic fibers play a major role in the formation of stretch marks. Indeed, they present an abnormal, fragmented ultrastructure, where their density is higher than normal. Elastic fibers acquire a similar appearance as they age. Collagen fibers are arranged parallel to the skin surface and are oriented perpendicular to the axis of the stretch mark, thus in the direction of the mechanical stress to which the skin is subjected.
Changes in the size, number and arrangement of collagen fibers would correspond to a biological response to the mechanical stress caused on the skin by the tension of the underlying muscle.
At the cellular level, fibroblasts are modified within stretch marks; they present a globular rather than stellar shape. They are inactive and lose all signs of fibrillary secretion. In recent stretch marks, the fibroblasts acquire a more contractile character corresponding to that of old fibroblasts. This is a reaction process to excessive stretching of the skin and underlying tissue. This process becomes inactive over time, when the lesions take on a scar-like appearance.