A mature epidermis is an effective barrier which prevents dehydration from the loss of body water, poisoning from the absorption of noxious substances, and systemic infection from invading surface microorganisms. The epidermal barrier resides within the most superficial layer of the skin, the stratum corneum. In utero the fetus has no need for a skin barrier, so the stratum corneum does not start to develop until around 24 weeks' gestation. After 24 weeks there is a steady increase in the number of epidermal cell layers and in epidermal thickness, although it is not until around 34 weeks' gestation that a well-defined stratum corneum has completely developed. A weak epidermal barrier is, therefore, present is very preterm infants (<30 weeks' gestation) during the first 2-3 weeks of life and if the skin is damaged by trauma or disease.
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