Embryologic, Histologic, and Anatomic Aspects
Elastic fibers are synthesized by fibrocytes (Fig. 1.80), those fibers imparting to skin the property of returning to its original shape after the effects of forces that deform it. The fibers are capable of extension to approximately twice their length at rest. In contrast to collagen bundles, elastic fibers are wavy, as well as branching (Fig. 1.81). Elastic fibers in the papillary dermis are present either as bundles of microfibrils, oxytalan fibers, or as cross-linked elastin on one hand or as elaunin fibers on the other. In the reticular dermis, elastic tissue appears mainly as elastin. Normal elastic fibers, which constitute only about 3% of the dry weight of the dermis, appear to be fragmented in sections examined by conventional microscopy, but that is an illusion resulting from a blade of a microtome having cut across elastic fibers as they crisscross the dermis on journeys marked by curves continuously. Elastic fibers are absent from scarring processes such as scars, keloids, and dermatofibromas and are decreased greatly, or are absent altogether, in anetodermas, a feature that accounts in considerable measure for the lax, bulging skin characteristic of that condition. Hereditary defects of elastic fibers, such as cutis laxa and pseudoxanthoma elasticum, display loose skin that does not return, in normal fashion, to its original shape after stretching. Neutrophils sprinkled along the course of elastic fibers in the dermis pour out lysosomal enzymes that destroy those fibers and result in the appearance of slack skin in the expression of cutis laxa that is acquired, rather than congenital.
An elastic fibril, here present in the vicinity of a fibrocyte, has a well-developed, rough-surfaced endoplasmic reticulum. In the inset, an elastic fibril can be noted to consist of microfibrils and homogenous elastin. (x22,500; inset, x41,750) (Courtesy of Ken Hashimoto, M.D.)
Elastic fibers in the papillary dermis are thin and, in the main, oriented somewhat perpendicular to the epidermis, whereas elastic fibers in the reticular dermis are thicker and tend to be arranged mostly parallel to the skin surface.