muscle cells too. the muscle cells you have now are the same cells you had as a kid (not entirely sure how far back it goes). they dont even divide when you gain muscle, the cells just get bigger and absorb nuclei from neighboring cells to support the extra mass. the cool thing is the nuclei stick around even if you let the muscles atrophy, so theyre forever changed by having once been big. thats probably part of why its easier to get back to muscle size youve been at before.