Recently, 2 new important concepts have been introduced in human trichology. They are exogen/teloptosis and kenogen. As it is customary when new concepts have to be accepted, some perplexity and confusion originated. We will try to clarify what exogen/teloptosis and kenogen mean and what importance they may gain in hair physiology and pathology.
Exogen is a term coined by Stenn1 and indicates the moment in which the club hair is shed from the follicle that is already occupied by a new terminal anagen hair. In Stenn's view, exogen, as the term suggests, is a new phase of the hair cycle. Its duration, however, is too short for deserving the privilege of a "phase" and, as Piérard-Franchimont and Piérard2 remarked, the term might rather suggest a biological exogenous process opposite to an endogenous one. They proposed instead the term teloptosis derived from the Greek τϵλος πτωση (falling off) to indicate the same phenomenon.
Rebora A, Guarrera M. Teloptosis and Kenogen: Two New Concepts in Human Trichology. Arch Dermatol. 2004;140(5):619-620. doi:10.1001/archderm.140.5.619