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.
References edit
James, William; Timothy Berger; Dirk Elston (2006). Andrews' Diseases of the Skin Clinical Dermatology, Tenth Edition. Saunders/Elsevier. p. 299. ISBN 0-7216-2921-0.