Human hair histogenesis for the mitochondrial DNA forensic scientist

Analysis of mitochondrial DNA (mtDNA) sequence from human hairs has proven to be a valuable complement to traditional hair comparison microscopy in forensic cases when nuclear DNA typing is not possible. However, while much is known about the specialties of hair biology and mtDNA sequence analysis, there has been little correlation of individual information. Hair microscopy and hair embryogenesis are subjects that are sometimes unfamiliar to the forensic DNA scientist. The continual growth and replacement of human hairs involves complex cellular transformation and regeneration events. In turn, the analysis of mtDNA sequence data can involve complex questions of interpretation (e.g., heteroplasmy and the sequence variation it may cause within an individual, or between related individuals. In this paper we review the details of hair developmental histology, including the migration of mitochondria in the growing hair, and the related interpretation issues regarding the analysis of mtDNA data in hair. Macroscopic and microscopic hair specimen classifications are provided as a possible guide to help forensic scientists better associate mtDNA sequence heteroplasmy data with the physical characteristics of a hair. These same hair specimen classifications may also be useful when evaluating the relative success in sequencing different types and/or forms of human hairs. The ultimate goal of this review is to bring the hair microscopist and forensic DNA scientist closer together, as the use of mtDNA sequence analysis continues to expand.