This result suggests that the barrier function of the stratum corneum, the skin component that protects against the penetration of foreign substances, may also protect the skin from ultraviolet irradiation.
Kao has been among the first to investigate the cutaneous barrier function of the stratum corneum. Collaborative research by Kao and the Tokyo Women's Medical University has identified decreased levels of intercellular lipids, especially ceramides, in the stratum corneum of patients with atopic dermatitis2 .
A review of skin development, beginning in utero, is helpful to understanding current knowledge about the stratum corneum barrier.
At full gestational maturation, the epidermis has structural integrity and functions as the stratum corneum skin barrier.
The stratum corneum begins to form around hair follicles at about 14 weeks of gestational age, subsequently expanding--between gestational weeks 22 and 24--to include the epidermis between the hair follicles.
Keywords: skin, stratum corneum, hair follicles, histology, Myotis ricketti
The epidermis is made up of two layers, namely epithelial and stratum corneum. The epithelial layer is largely composed of stratified squamous epithelium and a variable amount of connective tissue containing melanocytes, Langerhan's cells and Merkel cells.
The external layer of the skin, the stratum corneum, is considered a mechanically strong and resilient structure that can withstand physical strain and stress.
Specifically, we hypothesized that fine particles are able to penetrate the stratum corneum and reach the immunologically active epidermis, and that flexing motion, as at the wrist, provides the kinetic energy necessary to move particles into the skin.
The cohesion of the cells of the stratum
corneum is maintained by cholesterol sulfate, which acts as an intercellular cement.
The ultrasound-induced bubbles disrupt the stratum
corneum's orderly lipid bilayers, creating "paths of minimal resistance," Langer explains.
CLSM images showed that micronized ZnO and Ti[O.sub.2] particles concentrated on the top of the stratum corneum and at the microrelieflines in both adults and infants after a single application of the sunscreen (Figure).
This study found that the particles did not appear to penetrate the skin beyond the first layers of the stratum corneum; this finding provides additional evidence to confirm what several others have found; namely, that physical particulate sunscreen filters such as ZnO and Ti[O.sub.2] do not penetrate into the viable epidermis.
Current studies also demonstrate that physiologic lipid replacement accelerates dermal restoration of the stratum corneum. (2) This therapeutic approach is reported to improve barrier functions such as moisture retention and protection from irritants.
However, steroids do not support the restoration of the stratum corneum. In fact, data show that steroid use is associated with a thinning of layers of the stratum corneum, deterioration of barrier homeostasis, delayed barrier recovery, and abnormal stratum corneum integrity.
