Laser capture microscopy in a study of expression of structural proteins in the cuticle cells of human hair

The cuticle of human hair consists of several layers of flat cells that are hardened through their content of cross-linked proteins and protect the hair structure from the environment. Known proteins in the cuticle are the sulphur-rich KAP 5 and KAP10 proteins located in the exocuticle and cross-linked by disulphide bonds. Isopeptide bonds are also present and led to a proposal from amino acid analysis that the surface of cuticle cells also contains keratinocyte cell envelope proteins, loricrin, involucrin and small proline-rich proteins that contribute to the stability of the hair cuticle. Confirmation of that proposal by protein chemical methods is difficult because of the insolubility of the surface membranes. In the previous studies by other authors, involucrin was not detected in the cuticle by in situ hybridization or by immunoelectron microscopy with specific antibodies. An alternative approach was undertaken to determine whether mRNAs encoding keratinocyte envelope proteins are expressed in cuticle cells in the human hair follicle. The study utilized dissection of the cuticle, cortex and inner root sheath layers from follicles by laser capture microscopy. RNA was isolated and subjected to PCR analysis with specific primers to detect expression of mRNAs encoding cell envelope proteins. Their presence in the cuticle was not detected, and it was concluded that the proteins they encode are not produced. The structural consequences including the possibility that KAPs 5 and 10 are the prime components cross-linked by both disulphide and isopeptide bonds are discussed.