Background: Mast cells (MCs), critical effector cells in allergic inflammation and innate immunity to bacteria, are located in large numbers in tissues that interface the external environment, including the skin. However, little is known about the distribution and numbers of human skin MCs.
Objectives: To assess the influence of age, sex and skin region on size and spatial distribution of MC populations in normal human skin.
Methods: Biopsies of healthy skin were obtained from 150 male and female individuals (age range 10-86 years). MCs were quantified and mapped planimetrically by histomorphometry in 15 anatomical sites (abdomen, thorax, lower and upper back, lower and upper arm, lower and upper leg, foot, hand, chin, nose, cheek, forehead, back of head).
Results: No differences in skin MC numbers or distribution were found when comparing skin obtained from male or female and from young or old individuals. At all skin sites, regardless of age or sex, MC numbers were highest in the most superficial skin layers where up to 10-fold more MCs were found as compared with the subcutis, which consistently contained the lowest numbers of MCs. Interestingly, MC numbers were highest at peripheral skin sites (maximum: chin and nose) and lowest at central skin sites (minimum: abdomen). Thus, healthy human skin exhibits a proximal/distal and a central/peripheral MC gradient and 'skin MC numbers' may vary by a factor of more than 20, depending on the skin layer and skin site analysed.
Conclusions: These findings support the recently identified role of MCs in the elicitation of protective immune responses against infectious microorganisms. One possible explanation for these unexpected findings is that skin site/layer-specific factors (e.g. the density of nerves or vessels, exposure to ultraviolet irradiation, frequency of minimal trauma) are involved in the regulation of skin MC numbers and distribution.