Enlarged rete pegs with excessive accumulation of melanosomes leading to darker aging spots revealed by histomorphological measurements of internal structures of the epidermis.

Abstract

Various histological studies of facial pigmented spot sites such as solar lentigo have been reported, but few studies have used quantitative indices by histomorphometric analysis of the internal structure of pigmented spot sites using non-invasive methods. In the present study, to quantitatively elucidate morphological changes in the epidermis in male, darker-pigmented spots and female, light-pigmented spots, indices that characterize the internal structure of the epidermis in pigmented spot sites were measured using in vivo confocal laser scanning microscopy (CLSM). The darkness of pigmented spots on the cheeks of 69 women and 43 men was analysed using image analysis software. The L* value was calculated from RGB values obtained from facial images. The internal structures of pigmented spots on the cheeks of 13 subjects were observed by CLSM. Various parameters were measured using CLSM images from the surface of the stratum corneum to the bottom of the dermal papillae, including the thickness of the epidermis, melanosome content, and shape of the dermal papillae. Mean ΔL* values between pigmented spots and non-pigmented areas of male subjects were significantly increased in the 40s and 50s compared with those of female subjects. Conspicuous pigmented spots increased in the 40s in male subjects and the 50s in female subjects. In CLSM observations, significant increases in the thickness of the epidermis and melanosome content were confirmed in pigmented spots compared with surrounding non-pigmented areas. In particular, melanosome content in the male subject group with dark-coloured pigmented spots increased significantly to about eight times that of non-pigmented areas, and more than double that of the male subject group with light-coloured pigmented spots. From the measurements of quantitative parameters, morphological changes in the epidermis were clearly related to the surface colour tone of pigmented spots. Darker pigmented spot sites tended to show longer rete pegs in the epidermis. Accumulation of melanosomes in epidermal basal cells could be considered to increase with the degree of elongation of rete pegs at pigmented spot sites and, thus, induce darker pigmented spots.