Differential cell composition and split epidermal differentiation in human palm, sole, and hip skin

Palmoplantar skin is structurally and functionally unique, but the transcriptional programs driving this specialization are unknown. Here, we exploit bulk and single-cell RNA-sequencing of human palm, sole, and hip skin to describe the distinguishing characteristics of palmoplantar and non-palmoplantar skin while also uncovering previously unappreciated differences between palmar and plantar sites. Our approach reveals downregulation of diverse immunological processes and decreased immune cell populations in palmoplantar skin, highlighting an altered immune environment in the skin of the palms and soles. Further, we identify specific palmoplantar and non-palmoplantar fibroblast populations that appear to orchestrate key differences in cell-cell communication in palm, sole, and hip. Dedicated analysis of epidermal keratinocytes highlights major differences in basal cell fraction among the three sites and validates the presence of a more differentiated, cycling basal population. Finally, our data demonstrate the existence of two spinous keratinocyte populations that constitute two parallel, site-selective epidermal differentiation trajectories. Together, these results provide a deep characterization of the highly adapted palmoplantar skin and contribute new insights into the fundamental biology of human skin. Overall design: Bulk RNA-seq was performed using RNA extracted from 4 mm punch biopsies obtained from 15 healthy research subjects. Of the 15 subjects, 10 contributed hip and palm biopsies and 5 contributed hip and sole. We performed pairwise comparisons between the hip and palm, hip and sole, and palm and sole to identify differentially expressed genes.