Markers for Ca⁺⁺ -induced terminal differentiation of keratinocytes in vitro under defined conditions

Differentiation of normal human keratinocytes (NHK) grown in vitro as a monolayer to confluency can be triggered with an acute increase in concentration of extracellular Ca⁺⁺ . Over several days, induced by Ca⁺⁺ , the cells form pseudostratified sheets that somewhat resemble the basic organization of the intact skin. This experimental system is widely used in studies of keratinocyte biology and skin pathology. However, expression pattern of the genes considered as markers for cells in specific layers of epidermis in vivo does not always match the specific pattern observed in vitro and might lead to misinterpretation of data. Here, we demonstrate that among 18 markers of terminally differentiated keratinocytes of stratum granulosum (SG) and stratum corneum (SC) in vivo, only four (CDSN, KPRP, LCE1C and SPRR4) have reproduced their expression pattern in vitro. Our data suggest that findings based on two-dimensional (2D) Ca⁺⁺ -induced terminal differentiation of NHK in vitro should be subjected to additional scrutiny before conclusions could be made and, if possible, verified in other experimental system that might more faithfully represent the in vivo microenvironment.