Structure, innervation, and age‐associated changes of mouse forearm muscles

Abstract

In spite of a decline in muscle strength with age, the cause of the overall decrease in motor performance in aged mammals, including rodents, is incompletely understood. To add clarity, the gross organization, innervation, histochemical fiber types, and age-associated changes are described for mouse forearm muscles used in a variety of motor functions. The anterior (flexor) and posterior (extensor) forearm compartments have the same arrangement of muscles and gross pattern of innervation as the rat. Two primary histochemical fiber types, fast/oxidative/glycolytic (FOG) and fast/glycolytic (FG), with characteristic histochemical staining patterns were observed in all forearm muscles. Additionally, there was a small population of slow/oxidative (SO) fibers confined to the deep region of a single muscle, the flexor carpi ulnaris (FCU). Between 18 and 26 months the FCU muscle displayed fibers with morphological features distinct from earlier ages. Fibers displayed a greater variation in size, a loss of their uniform polygonal shape, and a dramatic increase in clumps of subsarcolemmal mitochondria, lysosomes, and lipofuscin granules. Many of the fibers had a distinctly atrophic, angular shape consistent with recent denervation. Morphometric analyses of the FCU's source of innervation, the ulnar nerve and one of its ventral roots (C8), were consistent with the denervation-like changes in the muscle fibers. Although, there was no net loss of myelinated axons between 4 and 26 months of age, there was a significant increase in the density of degenerating cells in both the ulnar nerve and ventral root C8.

Microscopic anatomy of brachial plexus branches in Wistar rats

“…The present study is the first to document such differences over the short interval of 3 weeks: between the ages of 4 and 7 weeks, which is indicative of the intense maturation of the peripheral nervous system during this life period in the rat. The similarity in number of myelinated fibers between different aged members of the same species is well documented (Carry et al, 1993;Schellens et al, 1993;Hashizume and Kanda, 1995;Jeronimo et al, 2005). In man (Stevens et al, 1973;Jacobs and Love, 1985;Ouvrier et al, 1987;Schellens et al, 1993), mice (Carry et al, 1993), and rats (Jeronimo et al, 2005), myelinated fiber density diminishes with age.…”

“…The complexity of the forelimb nerves and their branches offers various experimental opportunities, since selective injuries can be easily applied to motor or sensory branches compared to hind limb nerves (Bontioti et al, 2003). Further, besides locomotion, grooming and feeding movements can be evaluated (Carry et al, 1993). Most injuries to human peripheral nerves affect the upper extremities, which constitutes another reason why an experimental model of nerve injury in the forelimb is useful (Bontioti et al, 2003).…”

Prolonged absence of myostatin reduces sarcopenia

“…Sarcopenia is the profound loss of skeletal muscle mass and strength (Brooks and Faulkner, 1994) associated with normal ageing in mammals. This atrophy has previously been attributed to various factors including a decrease in muscle fiber number, an alteration in the fiber type, atrophy of existing fibers (Alnaqeeb and Goldspink, 1987), loss of motor innervation (Carry et al, 1993), and defective muscle regeneration (Grounds, 1998) which involves muscle satellite cells (Schultz and Jaryszak, 1985;Schultz and McCormick, 1994). Aged muscle appears restricted in its ability to promote satellite cell activation, proliferation and differentiation, therefore, impairing its ability to regenerate (Welle, 2002).…”

The Aging Myocardium: Roles of Mitochondrial Damage and Lysosomal Degradation

“…The degree of age-related changes is higher in cardiac myocytes 18,19 than in skeletal muscle fibers, 14,20 which is in accordance with higher oxygen consumption (and consequent ROS-induced damage) and poorer regenerative potential of the myocardium versus skeletal muscles.…”