Theory 27/Hair follicle cycle/Termination phase
Catagen Is Programmed Retraction
The claim
Growth ends when the ratio of apoptotic signal to survival signal crosses a threshold, and the club hair forms regardless of fibre length.
01Mechanism
The lower follicle regresses by apoptosis; the ratio determines initiation.
02Prediction
Catagen timing should track the ratio.
03Refutation
Catagen tracking fibre length would remove the signal.