The distribution of melanin in skin determined in vivo

There continues to be a need for objective, noninvasive methods to measure melanin concentration in vivo in human skin, independent of the confounding chromophore, haemoglobin. Existing methods are limited by a lack of specificity and inability to resolve the spatial distribution of these chromophores.

To validate and calibrate the measurement of eumelanin in vivo using SIAscopic techniques, relating this with histologically and analytically determined eumelanin concentrations in nonsun-exposed skin from subjects of Fitzpatrick skin types I-VI.

Observations were made in five subjects from each of the Fitzpatrick skin types I-VI using chromophore mapping by contact and noncontact SIAscopy and other noninvasive spectrophotometric means. Measurements were performed on the inner aspect of both upper arms. Subsequently two 4 mm punch biopsies were taken from the inner upper arm, one per arm after injection of local anaesthesia. One biopsy was fixed in formalin and processed for histology; specifically, sections were stained for melanin using a silver staining technique and the amount of melanin was graded microscopically. The other biopsy was subjected to an analytical assay to yield precise quantitative measures of melanin. The correlation between the different methods of melanin measurement was determined.

Clear, significant correlations were obtained between contact and noncontact SIAscope-derived eumelanin values and actual eumelanin tissue content (determined both histologically and analytically), across the full range of Fitzpatrick skin types. There was no correlation between SIAscope-derived eumelanin and haemoglobin values, indicating efficient separation of the two chromophores.

New contact and noncontact chromophore SIAscopic mapping techniques provide robust, rapid noninvasive measures of the concentration and spatial distribution of eumelanin in vivo, independent of haemoglobin, which correspond to true tissue values for this chromophore.