Striae gravidarum (SG), or stretch marks of pregnancy, begin as erythematous streaks and mature into hypopigmented atrophic bands.

In order to investigate molecular alterations that may promote atrophy of SG, we investigated dermal type I collagen fibrils, which provide human skin with support.

We obtained skin samples of recently developed, erythematous abdominal SG from pregnant women. To examine the organization of collagen fibrils, second‐harmonic generation imaging was performed using multiphoton microscopy. Immunostaining was used to determine protein expression and localization of type I procollagen, the precursor of type I collagen fibrils. Real‐time polymerase chain reaction was used to determine gene expression levels.

In control (hip) and stretched normal‐appearing perilesional abdominal skin, dermal collagen fibrils were organized as tightly packed, interwoven bundles. In SG, collagen bundles appeared markedly separated, especially in the mid‐to‐deep dermis. In the spaces separating these bundles, loosely packed wavy collagen fibrils lacking organization as bundles were present. These disorganized fibrils persisted into the postpartum period and failed to form densely packed bundles. Numerous large fibroblasts displaying type I procollagen expression were in close proximity to the disorganized fibrils, suggesting that the fibrils are newly synthesized. Supporting this possibility, immunostaining and gene expression of type I procollagen were increased throughout the dermis of SG.

Early SG display marked separation of collagen bundles and emergence of disorganized collagen fibrils that fail to form bundles. These alterations may reflect ineffective repair of collagen bundles disrupted by intense skin stretching. Persistent disruption of the collagenous extracellular matrix likely promotes formation and atrophy of SG.