Ultrastructural assessments of the melanosome distribution patterns and pigmentation features in human epidermal cells after UV irradiation and kojic acid treatment.

It is well-known that skin pigmentation depends, among others, on number, aggregation and distribution of melanosomes in the epidermis. Here we describe a correlative microscopy-based ultrastructural approach that investigates the spatial distribution and pigmentation features of the melanosomes within melanocytes and keratinocytes. Data obtained from control skin, ultraviolet (UV)-stimulated tissue and kojic acid-treated UV-irradiated explants are compared. We introduce original parameters for the evaluation of the aggregation and pigmentation features of the melanosomes: the aggregation and pigmentation indexes. The aggregation index evaluates the presence of clustered melanosomes when the pigmentation index expresses the electron-density level of the pigment granules. The present study demonstrates that the last parameters clearly express histological effects induced by UVB irradiation. Results indicate that UV light did not change the number of melanosomes within either melanocytes or keratinocytes, but it definitely modified the distribution patterns of the pigment granules in both cell types. It also enhanced the pigmentation state of the epidermal cells. Moreover, statistical analysis concerning keratinocytes discloses a significant decrease in the mean pigmentation index when explants exposed to UV light were treated with kojic acid. Obviously, the present numerical findings point out the relevance of the introduced parameters to characterize the pigmentation state of skin.

Characterization of the mineral content of a physical sunscreen emulsion and its distribution onto human stratum corneum.

When using physical sunscreens, protection and aesthetic aspects are directly related to particle size and mineral distribution onto skin. These investigations aimed to characterize the different mineral raw materials entering into the composition of a specific sunscreen emulsion and to assess the distribution of the sunscreeen agents at the surface of human stratum corneum. X-ray diffractometry revealed the crystallographic features of the TiO2 and ZnO crystals. Frequency distributions of crystal length and aspect ratio were determined for the zinc oxide which presented the largest particle size and varying shapes. Scanning and transmission electron microscopy disclosed structural information. The results reported in this study specify the characteristics of the oxide particles involved in the formulation and show a satisfactory mineral distribution along the irregular topography of the skin. No intracellular penetration was noted.