The Ontogeny of Skin.

Significance: During gestation, fetal skin progresses from a single layer derived from ectoderm to a complex, multi-layer tissue with the stratum corneum (SC) as the outermost layer. Innate immunity is a conferred complex process involving a balance of pro- and anti-inflammatory cytokines, structural proteins, and specific antigen-presenting cells. The SC is a part of the innate immune system as an impermeable physical barrier containing anti-microbial lipids and host defense proteins. Postnatally, the epidermis continually replenishes itself, provides a protective barrier, and repairs injuries. Recent Advances: Vernix caseosa protects the fetus during gestation and facilitates development of the SC in the aqueous uterine environment. The anti-infective, hydrating, acidification, and wound-healing properties post birth provide insights for the development of strategies that facilitate SC maturation and repair in the premature infant. Critical Issues: Reduction of infant mortality is a global health priority. Premature infants have an incompetent skin barrier putting them at risk for irritant exposure, skin compromise and life-threatening infections. Effective interventions to accelerate skin barrier maturation are compelling. Future Directions: Investigations to determine the ontogeny of barrier maturation, that is, SC structure, composition, cohesiveness, permeability, susceptibility to injury, and microflora, as a function of gestational age are essential. Clinicians need to know when the premature skin barrier becomes fully competent and comparable to healthy newborn skin. This will guide the development of innovative strategies for optimizing skin barrier development.