Ionic calcium reservoirs in mammalian epidermis: Ultrastructural localization by ion-capture cytochemistry

Abstract

Although calcium ions have been shown to regulate the differentiation of keratinocytes in vitro, the role of divalent cations in vivo is not known. Prior attempts to localize divalent cations in epithelial tissues have been impeded by a lack of specificity of ultrastructural techniques, as well as translocation of precipitates within tissues. The availability of an improved cytochemical method (oxalate-pyroantimonate technique) has facilitated more precise, reliable localization of calcium. When this technique (± 10 mM EGTA) was applied to neonatal mouse epidermis, Ca++-containing precipitates localized primarily within the cytosol, mitochondria, and nuclear chromatin of some basal and spinous cells, suggesting a possible relationship of Ca++ with the cell cycle. In the lower granular layer, progressively more Ca++ precipitates appeared intercellularly, with the only intracellular Ca++ localized within mitochondria and lamellar bodies (limiting membranes and discs). The most apical granular cells always demonstrated dense extracellular deposits, and high intracellular Ca++, free in the cytosol. The extruded contents of lamellar bodies, at the granular-cornified layer interface, also demonstrated significant amounts of Ca++-containing precipitates between the lamellar discs. Although some corneocytes in the lower stratum corneum demonstrated intracellular precipitates, most were devoid of Ca++. The striking intercellular Ca++ accumulation in the mid granular layer, coupled with Ca++ influx in the upper granular layer, supports the view that changes in intracellular Ca++ may regulate epidermal differentiation. Finally, the association of Ca++ with lamellar body disc membranes and contents suggest that divalent cations may contribute to both lamellar body secretion and to the formation of intercorneocyte membrane bilayers.