A discourse on human hair fibers and reflections on the conservation of drug molecules.

A gross discourse on human hair fibers and their formation is presented stressing the various interdisciplinary aspects, such as the morphological, biological, structural and biochemical data considered to be important in the field of hair analysis. An attempt is made to explain the incorporation of drug molecules during hair fiber formation by using the classical concepts of drug absorption based on lipoid theory and the pH-partition hypothesis as well as a modern biological approach on the permeability of cell membranes. In addition to the physiochemical considerations of the transport properties of a particular drug molecule such as a) the lipophilicity, which determines permeability through the membrane, b) the pKa value, c) the plasma protein binding and d) the molecular size and shape of the drug molecule, drug absorption is thought to be limited by the surface area and the residence time in the hair bulb. The thermodynamic approach according to the Kedem-Katchalsky equations seems even more satisfying. When the principles of biological transport across cell membranes are applied to the cell populations present in the hair root, a hypothesis of extracellular and intracellular drug localizations results. It is speculated that the cell membrane complex (CMC) and the melanin granules present the main sources of incorporated drug molecules within the keratinized hair fibers.