Fig. 4. Impaired hair cycle and follicle development in hEGFR^KI/KI mice. Histological sections through the skin of control (A,C,E) and hEGFR^KI/KI mice (B,D,F) at different stages of the hair cycle. In control skins at 18 days (end of catagen/telogen) the hair follicles are confined to the upper dermal layers (A, arrows), whereas in hEGFR^KI/KI skins, hair follicles accumulate in the subdermal fat tissue (brackets in B,D,F), as they do not progress from anagen to telogen (B). Arrows in B indicate hair follicles that start to be enlarged in mutant skins. (C) At 1 month (late anagen/early catagen) control follicles are distributed in the upper dermal and subdermal fat layers and display a normal structure (arrows). (D) Mutant follicles in the subdermal fat tissue are severely hyperplastic (arrows) at this stage. (E) At 3 months (telogen), control follicles are confined to the upper dermal layers (arrows). (F) In mutant skins, only a few aberrant follicles are present in the upper dermis (arrowheads) and most of the follicles stuck in the subdermal fat tissue have degenerated (arrows). Severe fibrosis with infiltration of inflammatory cells can be detected in the subdermal fat tissue of hEGFR^KI/KI skins as evidenced by the strong eosin (pink) staining. (G) A normal mouse hair cycle profile with the position where the hair cycle is blocked in hEGFR^KI/KI mice. A, anagen; C, catagen; T, telogen; d, days; m, months. All sections are stained with Haematoxylin and Eosin.

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