(A) Schematic of tamoxifen (TM) inductions using the endothelial-specific Cdh5-CreERT2, with hair cycle stages indicated at postnatal days. M, morphogenesis; C, catagen, T, telogen, A, anagen. (B) Skin sections from Rosa26-tdTomato reporter mice crossed with Cdh5-CreERT2, induced with tamoxifen (TM) at PD17, and sacrificed at PD35, show specific induction in the CD31+ vasculature. (C) Skin sections from Alk1flox/flox mice crossed with Cdh5-CreERT2 and induced with tamoxifen (TM) at PD17 show different hair cycle stages as indicated by morphology and staining for Ki67 at PD25 and at PD28/31. (D) Same as (C) but stained with hematoxylin and Eosin and mice were sacrificed at PD35. (E) Number of littermate control (CT) Alk1 End KO (KO) mice at specific indicated hair stage obtained from Ki67 staining/total number of mice analyzed at each age/genotype group. For instance, at PD25 we analyzed a total of 5 control (CT) mice of which three were at Tel-AnaI and two were at Ana I-II. Also see Table 1 for more detailed information on each mouse and its respective hair cycle stage.