Endothrix refers to dermatophyte infections of the hair that invade the hair shaft and internalize into the hair cell. This is in contrast to exothrix (ectothrix), where a dermatophyte infection remains confined to the hair surface. Using an ultraviolet Wood's lamp, endothrix infections will not fluoresce whereas some exothrix infections may fluoresce bright green or yellow-green.
Book: James , William . Timothy Berger . Dirk Elston . Andrews' Diseases of the Skin Clinical Dermatology, Tenth Edition . Saunders/Elsevier . 2006 . 299 . 0-7216-2921-0.