Comparative Aspects of the Inner Root Sheath in Adult and Developing Hairs of Mammals in Relation to the Evolution of Hairs

The study investigated the inner root sheath (IRS) of hairs in various mammals, including monotremes, marsupials, and placentals, to understand hair evolution. Using electron microscopy and immunocytochemistry, researchers found that trichohyalin, a protein involved in IRS cornification, was conserved across species, indicating an ancient evolutionary origin. The IRS in monotremes was simpler, lacking a distinct Henle layer, suggesting a primitive structure. The research highlighted the IRS's role in facilitating hair detachment without damaging the epidermis, providing insights into the evolution of hair from reptilian scales. The study involved skin samples from several species, including 6 platypuses and 9 rats, and emphasized the importance of the IRS in hair morphogenesis and evolution.