reported that acute stress promotes the release of norepinephrine from sympathetic nerve terminals innervating the HF bulge, which drives the rapid proliferation and differentiation of quiescent MSCs by binding to the ADRB2 on the surface of MSCs, leading to permanent depletion of MSCs at the HF, which is considered to be the main reason of hair graying.64 Here, ADRB2 is linked to hydrops fetalis.