Amenorrhea is the absence of a menstrual period. Primary amenorrhea is when a young woman has not yet had a period by age 16. Secondary amenorrhea describes someone who used to have a regular period but then it stopped for at least three months (this can include pregnancy).
Researchers at Kumamoto University, Japan generated mice lacking the estrogen receptor beta (ERβ) gene, both fiber-specific and muscle stem cell-specific, which resulted in abnormalities in the growth and regeneration of skeletal muscle in female mice.