ACCUMULATION OF CALCIUM IN THE STRATUM CORNEUM OF THE ROCK PIGEON (COLUMBA LIVIA): STORAGE OR SECRETION?

Abstract

In the pigeon (Columba livia), a different response of cutaneous water evaporation (CWE) is seen at different acclimation states. While in heat-acclimated pigeons the CWE at high ambient temperatures will be activated as expected, no such response to appropriate thermal stimuli is seen in cold-acclimated birds. This difference is also reflected in the function of the water barrier, which in the stratum corneum modulates the diffusion of water through the skin. In this respect, the skin of birds seems to be more dynamic than that of mammals. In mammals, the formation of this barrier by extracellular lipid lamellae derived from lamellar bodies of viable keratinocytes is a prerequisite for normal skin function. It is present in cold-acclimated pigeons, but is lacking in heat-acclimated birds. The ionic stimulus of extracellular calcium (Ca²⁺) has been claimed to mediate the formation of the mammalian-type barrier. In order to elucidate the role of Ca²⁺ in water barrier formation in avians, we studied winter-acclimatized and thermally non-challenged pigeons for their epidermal barrier structure and Ca²⁺ distribution by calcium capture cytochemistry by electron microscopy. In both groups of pigeons, we show that both the extracellular and the intracellular domains in the stratum corneum contain organized lamellar lipids. In the stratum corneum, the bulk Ca²⁺ is located in corneocytes. In thermally non-challenged birds, Ca²⁺ is localized also in the outer extracellular spaces. We may conclude that, unlike in mammals, (1) corneocytes contain massive intracellular lamellae that extend up to the outer layers of the stratum corneum, and (2) Ca²⁺ accumulates in the corneocytes. This suggests that pigeons may lose substantial amounts of Ca²⁺ via desquamation and, on the other hand, the skin may serve as a Ca²⁺ reservoir. These results indicate a different role of Ca²⁺ in the water barrier formation, as well as novel functions of the avian skin as compared to that of mammals.