Fig. 1

Morphological observation of hair follicles across six developmental stages in sheep. a, b Haematoxylin-eosin (H&E) staining of skin for horizontal (magnification: × 40) and longitudinal (× 10) sections at embryonic day 65 (E65), E85, E105, E135, and postnatal day 7 (P7) and P30, respectively. The induction of hair follicles is initiated around E65 and characterized by crowded epidermal keratinocytes, hair placode (Pc) (red dashed lines) and dermal condensate (DC) (blue dashed lines). At E85, the number of primary follicles (PFs) increases significantly, and secondary follicles (SFs) start to form. At E105, PFs are regularly arranged, and the surrounding SFs, sebaceous gland (SG), outer root sheath (ORS), and inner root sheath (IRS) layers are clearly visible. At E135, the secondary-derived follicles branching from SFs appear. At P7, hair follicles mature with complete structure. The dermal papilla (DP), ORS, and IRS structures are clearly visible, and hair shafts emerge through the epidermis. At P30, the hair follicles enter into the mature anagen phase. DF, dermal fibroblast; EHG, elongated hair germ; TAC, transit amplifying cells; BSCP, bulge stem cell precursors. c Numbers of PF and SF, SF/PF ratio, body weight and length (average of three replicates; mean ± SD) of Merino sheep during hair follicles development