Fig. 2. Different androgen actions on limb and perineal muscles.

(A) Action of supra-physiological levels of androgen on limb muscles in adults. Androgens can induce limb muscle hypertrophy. Although myofiber hyperplasia via androgens is still controversial, some reports have suggested that androgens can induce hyperplasia by the activation of muscle stem cells. (B) Physiological or supra-physiological androgen action on perineal muscles at puberty or adulthood, respectively. During puberty, increased levels of androgens can induce muscle hypertrophy and hyperplasia. Moreover, in adults, androgens can activate and increase muscle stem cell numbers in a dose-dependent manner. (C) Schematic diagram of the conversion of proliferative juvenile muscle stem cells into quiescent adult muscle stem cells via the androgen-Mib1-Notch signaling axis in limb muscles during puberty (modified from Kim et al., 2016).