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. Author manuscript; available in PMC: 2016 Jun 1.
Published in final edited form as: J Biol Rhythms. 2015 Jan 13;30(3):163–182. doi: 10.1177/0748730414563537

Figure 1. The complex anatomical organization of skin and its circadian clocks.

Figure 1

The schematic drawing depicts the three principle layers of the skin: the keratinocyte-containing epidermis (brown), the fibroblast-rich dermis (purple) and the fat cell-containing adipose tissue (yellow). The interfollicular epidermis is a stratified squamous epithelium containing stem/progenitor cells in the basal layer (BL) and post-mitotic differentiated cells in the suprabasal layers (SBL). The top layer of the epidermis, the stratum corneum (SC), is composed of dead cells with a tough cell envelope that is sealed by an impermeable intercellular lipid layer. Also, highlighted are hair follicles with the associated lipid-rich sebaceous glands, as well as sweat glands; both are keratinocyte-containing mini organs which develop as outgrowths from the epidermis. The skin is also inhabited by melanocytes, melanin-producing pigment cells that confer color to hair and skin. In mice, melanocytes are primarily found in hair follicles while in humans melanocytes are also prominent at the epidermal-dermal junction. In addition, several types of resident and transient immune cells are found within the skin. These include Langerhans cells and dendritic cells (DCs), antigen-presenting cells residing in the epidermis and dermis, respectively, as well as lymphocytes, macrophages and mast cells. The skin is also richly vascularized and innervated; cells within these structures likely have their own circadian clock that could modify their functions including sensory responses, heat regulation and oxygenation. The surface of the skin is populated by a large number of commensal bacteria (microbiota) that may play a role in skin function and disease. There is evidence for active circadian clock in all cell types of the skin, and it is highly likely that distinct functions are modulated in different cell types. It is also known that circadian clock activity in skin is coordinated by the suprachiasmatic nucleus, presumably through neuronal and hormonal mediators although this remains to be defined in skin.