Automatic morphometric analysis of skeletal muscle fibers in the aging man.

A qualitative and quantitative analysis of M. vastus lateralis fibers from 40 male sedentary subjects, ranging in age from 30 to 89 years, was carried out by light and electron microscopy and by an automatic Interactive Image Analysis System. Biopsies for enzyme histochemical and ultrastructural studies were taken from subjects subdivided into four age groups: 30-50, 60-70, 71-80, and 81-89 years. Attention was focused on the fiber type size and distribution, the size and the amount of mitochondria, and the amount of lipid droplets. Main changes observed in the four age groups are indicative of a sequence of events within senescent skeletal muscle fibers. With increasing age the enzyme histochemical reactions reveal changes in fiber type distribution characterized by decrease in muscle fiber diameter and by type I fiber predominance. Type II fiber atrophy is consistent. Type I fiber predominance seen in older subjects could be related to a selective decrease of type II fibers with age. It also suggests a possible conversion of type II fibers to type I fibers. Lipid droplet percentage per fiber area increases, while mitochondrial size and mitochondrial percentage per fiber area decrease with age. It is possible that energy requirements decline with age and that the decrease in mitochondrial size and percentage represents a response to a reduced metabolic demand.