2.1.1.3 Regulation of skin barrier homeostasis
Several mechanisms are involved in the regulation of skin barrier homeostasis，including skin surface pH，hydration of the stratum corneum，calcium ion gradient in the epidermis，nuclear hormone receptors and so on.
1.Skin surface pH
The normal skin surface pH is 4.5-5.5.Microbial metabolites，free fatty acid of pilosebaceous origin，lactic acid，breakdown products of epidermal filaggrin（pyrrolidone carboxylic acid and urocanic acid）contribute to the formation of acidic environment on the surface of the skin.The acidity of the skin surface is essential in preventing infection，epidermal barrier recovery and regulation of the exfoliation of keratinocytes.Changes in the pH may play a role in the pathogenesis of some skin diseases，such as irritant contact dermatitis，atopic dermatitis，ichthyosis，acne vulgaris，and Candida albicans infection.
2.Hydration of the stratum corneum
Water content of the stratum corneum is related to skin elasticity，the process of keratinocytes desquamation on the skin surface and the repair of disrupted barrier.The skin with low stratum corneum hydration shows reduced elasticity.Several enzymes that regulate keratinocytes desquamination such as serine proteases and glycosidases，need the water in the stratum corneum to perform their normal function.Hydration of the stratum corneum is also important for skin barrier recovery.Under high humidity conditions，lipid biosynthesis decreases after barrier disruption，and the barrier recovery is slower.(https://www.daowen.com)
3.Calcium ion gradient
There exists calcium ion gradient in the epidermis.In basal layer，spinous layer and stratum corneum，the calcium ion concentrations are very low，whereas the calcium ion concentration in the granular layer is the highest.Calcium ion gradient is important in the repair process after the destruction of the skin barrier，and the participation of calcium ions is also required in the formation of cell-to-cell junctions.Epidermal calcium ion gradient abnormality has clinical significance.X-linked ichthyosis and psoriasis demonstrate dysfunctions in the epidermal calcium ion gradient.
4.Nuclear hormone receptors
The function of nuclear hormone receptors is closely related to the transcription process and plays an important role in regulating epidermal development and skin homeostasis.Lipids and their metabolites bind and activate nuclear hormone receptors，and then regulate the lipids secretions and further processing of lamellar bodies.Thus，the role of nuclear hormone receptors that regulate the formation of bricks-and-mortar during epidermal development and recovery of the impaired skin barrier should not be ignored.