Using a validated swine model of human
scar formation, hyperpigmented and hypopigmented
scar samples were examined for their histological and optical properties to help elucidate the mechanisms and characteristics of dyspigmentation. Full-thickness
wounds were created on the flanks of red Duroc pigs and allowed to heal. Biopsies from areas of
hyperpigmentation,
hypopigmentation, and uninjured tissue were fixed and embedded for histological examination using
Azure B and primary
antibodies to S100B, HMB45, and α-
melanocyte-stimulating hormone (α-
MSH). Spatial frequency domain imaging (SFDI) was then used to examine the optical properties of
scars.
Hyperpigmentation was first noticeable in healing
wounds around weeks 2 to 3, gradually becoming darker. There was no significant difference in S100B staining for the presence of melanocytes between hyperpigmented and hypopigmented
scar samples.
Azure B staining of
melanin was significantly greater in histological sections from hyperpigmented areas than in sections from both uninjured skin and hypopigmented
scar (P < .0001). There was significantly greater staining for α-
MSH in hyperpigmented samples compared with hypopigmented samples (P = .0121), and HMB45 staining was positive for melanocytes in hyperpigmented
scar. SFDI at a wavelength of 632 nm resulted in an absorption coefficient map correlating with visibly hyperpigmented areas of
scars. In a red Duroc model of
hypertrophic scar formation, melanocyte number is similar in hyperpigmented and hypopigmented tissues. Hyperpigmented tissues, however, show a greater amount of
melanin and α-
MSH, along with immunohistochemical evidence of stimulated melanocytes. These observations encourage further investigation of melanocyte stimulation and the inflammatory environment within a
wound that may influence melanocyte activity. Additionally, SFDI can be used to identify areas of
melanin content in mature, pigmented
scars, which may lead to its usefulness in
wounds at earlier time points before markedly apparent pigmentation abnormalities.