Regeneration of TA muscle in Per1^−/−, Per2^−/−, and Per1^−/−:Per2^−/− mice. (A) H&E staining of day 4.5 TA muscle sections. TA muscle was injured with barium chloride at ZT14 on day 0, and EdU was injected i.p. 96 h later for F and G. The muscle was harvested 12 h later at day 4.5. Bar, 100 µm. (B) Size distribution of H&E-stained myofibers containing centrally located nuclei on day 4.5. The minimal Feret’s diameter in each myofiber was measured. n = 8 mice in each group, including 4 males and 4 females, in B and C. (C) Average of the minimal Feret’s diameters of myofibers with centrally located nuclei on day 4.5. (D) Immunofluorescence staining of TA muscle with antibodies against eMHC and laminin (showing the border of each myofiber) on day 4.5. DNA was counterstained with DAPI. Bar, 100 µm. (E) Average of the minimal Feret’s diameters of eMHC⁺ areas on day 4.5. n = 4 mice. (F) Immunofluorescence staining of TA muscle sections with the MyoD antibody and an EdU kit. Bar, 25 µm. (G) Frequency of positive cells for EdU uptake and MyoD staining shown in F. n = 4 mice. (H) Sirius red staining of days 7 and 14 after injury and uninjured TA muscle. Bar, 200 µm. (I) The area percentage of Sirius red⁺ fibrosis in the groups shown in H. (J–L) Differentiation index (J), fusion index (K), and the frequency of EdU uptake (L) during the differentiation of primary myoblasts prepared from WT and Per KO mice. Data represent biological triplicates with n = 80–130 nuclei in each replicate. Data are presented as mean + SEM. *, P < 0.05; **, P < 0.01; and ***, P < 0.001 with Student’s t test compared with WT.