Expression of differentiation markers during fetal skin development in humans: immunohistochemical studies on the precursor proteins forming the cornified cell envelope.

The cornified cell envelope is formed during the terminal differentiation of epidermis through cross-linking of specific proteins by transglutaminases. The specific arrangement of individual protein in the cornified cell envelope and participation of individual protein in the cornified cell envelope at different regions of skin, i.e., palm, foreskin, lips, etc. are not clearly understood. In order to understand the pattern and expression schedule of each individual precursor protein during the differentiation and formation of cornified cell envelope, the expression of precursor proteins in developing human fetal skins from the first to the third trimester were examined by immunohistochemical studies. Involucrin was found in the periderm and intermediate layer from 14 wk estimated gestational age, while loricrin and small proline-rich protein 1 were found in the periderm from 16 wk estimated gestational age. Filaggrin and trichohyalin that are absent in the adult cornified cell envelope were found in the granular and horny layers from 24 wk estimated gestational age. The precursor proteins except trichohyalin did not change their patterns after the onset of initial expression during development. Trichohyalin was transiently expressed in the granular and horny layers of the epidermis from 24 wk estimated gestational age with peak expression at 27 wk estimated gestational age, but was not detected in adult skin. In hair follicles, trichohyalin expression was stable without change from 20 wk estimated gestational age. These findings suggest that fetal skin may have different sets of barriers from the second trimester; the immature cornified cell envelope is formed in the early second trimester and the mature cornified cell envelope is formed in the late second or early third trimester when filaggrin and trichohyalin appear.