The regression phase of the hair cycle (catagen) is an apoptosis-driven pro
cess accompanied by terminal differentiation, proteolysis, and matrix remod
eling. As an inhibitor of keratinocyte proliferation and inductor of kerati
nocyte apoptosis, transforming growth factor beta 1 (TGF-beta 1) has been p
roposed to play an important role in catagen regulation. This is suggested,
for example, by maximal expression of TGF-beta 1 and its receptors during
late anagen and the onset of catagen of the hair cycle. We examined the pot
ential involvement of TGF-beta 1 in catagen control. We compared the first
spontaneous entry of hair follicles into catagen between TGF-beta 1 null mi
ce and age-matched wild-type littermates, and assessed the effects of TGF-b
eta 1 injection on murine anagen hair follicles in vivo. At day 18 p.p., ha
ir follicles in TGF-beta 1 -/- mice were still in early catagen, whereas ha
ir follicles of +/+ littermates had already entered the subsequent resting
phase (telogen). TGF-beta 1 -/- mice displayed more Ki-67-positive cells an
d fewer apoptotic cells than comparable catagen follicles from +/+ mice. In
contrast, injection of TGF-beta 1 into the back skin of mice induced prema
ture catagen development. In addition, the number of proliferating follicle
keratinocytes was reduced and the number of TUNEL + cells was increased in
the TGF-beta 1-treated mice compared to controls. Double visualization of
TGF-beta type II receptor (TGFRII) and TUNEL reactivity revealed colocaliza
tion of apoptotic nuclei and TGFRII in catagen follicles. These data strong
ly support that TGF-beta 1 ranks among the elusive endogenous regulators of
catagen induction in vivo, possibly via the inhibition of keratinocyte pro
liferation and induction of apoptosis. Thus, TCF-beta RII agonists and anta
gonists may provide useful therapeutic tools for human hair growth disorder
s based on premature or retarded catagen development (effluvium, alopecia,
hirsutism).