Aging affects different human muscles in various ways. An image analysis of the histomorphometric characteristics of fiber types in human masseter and vastus lateralis muscles from young adults and the very old

This study is an attempt to objectively evaluate age-related changes in hum an muscles by use of histomorphometric methods, Aging in humans induces dra matic transformations in the skeletal muscles but little is known as to whe ther or not the aging processes per se may affect all muscles equally. In t his study aging of two human muscles with different functions, origin and n erve supply is compared. Sections were cut from masseter and vastus lateralis muscles obtained from young adults aged 18-24 years and from the very old aged 90-102 years. Musc le fiber types were classified with the traditional myofibrillar ATPase sta ining. Various histomorphometric parameters of the different fiber types in human masseter and vastus lateralis muscle sections were obtained by image analyses to evaluate the age-related changes in the muscle fibers. The fol lowing variables were calculated: the number of each fiber type per photogr aphed area; the area of each fiber and two indicators for the shape of the muscle fibers. In the aging muscles there was no relative preferential loss of a fiber type. High numbers of intermediate ATPase-stained fibers (IM fi bers) were found in some old vastus muscles but were only sporadic in young vastus muscles. However, there was no change in the percentage distributio n of intermediate ATPase-stained fibers when young and very old human masse ter muscles were compared. Incubation of the sections with antimyosin antib odies showed that the IM fibers in old masseter and old vastus contained di fferent myosin heavy chains, Thus ATPase activity and anti-myosin staining displayed a somewhat different pattern of fiber type distribution. The main changes in the shape and area indicated that type I fibers in the masseter became more circular while in the vastus they decreased significantly in s ize. The type II fibers in the vastus became very small and deviated signif icantly from circularity whereas the type II fibers in the masseter only ex hibited a decrease in the size of the fibers. Histomorphometric measurement s show that aging affects different human muscles in various ways.