In the extremely preterm infant, high transepidermal water loss (TEWL) can result in severe dehydration. TEWL has been attributed to the structural properties of the epidermis but might also be influenced by mechanisms that facilitate water transport. To investigate whether aquaporins (AQP) may be involved in the extreme losses of water through immature skin, we examined the presence and cellular distributions of AQP-1 and AQP-3 in embryonic and adult rat skin by immunohistochemistry. The expression of AQP mRNA in skin was analyzed with the use of semiquantitative reverse transcription-PCR. In rat pups of different embryonic (E) and postnatal (P) ages (days), TEWL and skin hydration were measured. AQP-1 was detected in dermal capillaries, and AQP-3 was abundant in basal epidermal layers. Both AQP displayed several times higher expression in embryonic than in adult skin. TEWL was highest at embryonic day 18 (E18) (133 Ϯ 18 g/m 2 h) and lower at E20 (25 Ϯ 1 g/m 2 h) and P4 (9 Ϯ 2 g/m 2 h). Skin hydration measured as skin electrical capacitance paralleled TEWL, being highest in fetal skin (794 Ϯ 15 pF at E18) and decreasing to 109 Ϯ 11 pF at E20 and to 0 Ϯ 0 pF at P4. We conclude that, as in infants, water loss through the skin of rats decreases markedly with maturation during the perinatal period. The expression and cellular localization of the AQP are such that they might influence skin hydration and water transport and contribute to the high losses of water through the immature skin. (Pediatr Res 53: 558-565, 2003) Abbreviations TEWL, transepidermal water loss AQP, aquaporin E, embryonic P, postnatal RT, reverse transcription SEC, skin electrical capacitance Evaporation from the skin constitutes the major route of water and heat loss in extremely preterm infants early after birth (1). These losses may result in hypothermia, dehydration, and hyperosmolality, the last of which is associated with an increased risk for cerebral bleeding (2, 3). Previous observations in very preterm infants indicate that their very high loss of water from the skin, measured as transepidermal water loss (TEWL) (4, 5), is a consequence of their poorly developed epidermis (6, 7). The formation of an effective epidermal barrier, limiting water permeability, takes place in late gestation (8) and has mainly been attributed to the structural maturation of the stratum corneum and its constituents (9, 10).Aquaporin (AQP) water channels have been shown to be extensively distributed throughout different tissues and to allow rapid and regulated transcellular water transport (11,12). Only a few studies on AQP have included skin analyses. AQP-1 and -3 have been identified in adult rat skin by immunohistochemistry (13) and by RNase protection assay (14), providing support for the speculation that AQP might play a role in transepidermal water transport (15). To understand better the mechanism underlying the high TEWL in preterm infants, we investigated the pattern of distribution and gene expression of AQP-1 and -3 in rat skin at different stages of...

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