Patterns of Expression of Trichocytic and Epithelial Cytokeratins in Mammalian Tissues

The study examined the expression patterns of trichocytic (T) and epithelial (E) cytokeratins in developing human hair and nail tissues during different fetal stages. It was found that precursor cells to trichocytes, initially expressing only E cytokeratins, began expressing T cytokeratins in the late bulbous-peg stage of hair development. In hair follicles, T cytokeratins appeared prominently in maturing trichocytes, while E cytokeratins were present in lower hair matrix cells. In nail development, T cytokeratins were first detected in the lunula region of week 12 and 13 fetuses, later extending to the nail matrix and nail-bed epithelium. Coexpression of T and E cytokeratins was observed in certain cells, with a more complex pattern in nail structures compared to hair follicles. The study concluded that trichocytic differentiation begins in cells expressing E cytokeratins, with a more abrupt transition to T cytokeratins in hair follicles and a gradual, complex coexpression in nail development.