©2013 Landes Bioscience. Do not distribute. www.landesbioscience.com BioArchitecture 25 BioArchitecture 3:2, 25–37; March/April 2013; © 2013 Landes Bioscience RESEARCH PAPER RESEARCH PAPER *Correspondence to: Edna C. Hardeman; Email: e.hardeman@unsw.edu.au Submitted: 02/01/13; Revised: 05/06/13; Accepted: 05/08/13 http://dx.doi.org/10.4161/bioa.24966 Introduction The function of skeletal muscle is directly determined by its architecture; the numbers and size of muscle fibers and rela- tive contribution of fibrotic tissue and fat. Regeneration is a key process in skeletal muscle in which competition between these different cell types to populate the regenerated muscle signifi- cantly impacts muscle function. The regenerative process in skel- etal muscle is understood to be altered during aging with some reports concluding that it is impaired 1-4 and reports that it is delayed, but resolves. 5,6 The field has debated relative roles of cell- intrinsic and/or cell-extrinsic changes responsible for declining regenerative potential with increasing age and focused on the role(s) of the muscle-resident stem cell, the satellite cell. 1,2,4,7,8 Most studies of rodent muscle report a reduction in satellite cell number with increasing age which may underpin impaired regeneration. 9-11 Numerous reports have employed a wide range of assays of muscle regeneration to investigate the time-course of the process, regenerative capacity in health and disease and the temporal and spatial relationship of myogenesis to inflammation and re-innervation. However, significant variations across those While the general understanding of muscle regenerative capacity is that it declines with increasing age due to impairments in the number of muscle progenitor cells and interaction with their niche, studies vary in their model of choice, indices of myogenic repair, muscle of interest and duration of studies. We focused on the net outcome of regeneration, functional architecture, compared across three models of acute muscle injury to test the hypothesis that satellite cells maintain their capacity for efective myogenic regeneration with age. Muscle regeneration in extensor digitorum longus muscle (EDL) of young (3 mo-old), old (22 mo-old) and senescent female mice (28 mo-old) was evaluated for architectural features, iber number and central nucleation, weight, collagen and fat deposition. The 3 injury paradigms were: a myotoxin (notexin) which leaves the blood vessels and nerves intact, freezing (FI) that damages local muscle, nerve and blood vessels and denervation-devascularization (DD) which dissociates the nerves and blood vessels from the whole muscle. Histological analyses revealed successful architectural regeneration following notexin injury with negligible ibrosis and fully restored function, regardless of age. In comparison, the regenerative response to injuries that damaged the neurovascular supply (FI and DD) was less efective, but similar across the ages. The focus on net regenerative outcome demonstrated that old and senescent muscle has a robust capacity to regenerate functional architecture. Aged skeletal muscle retains the ability to fully regenerate functional architecture Antonio S.J. Lee, 1 Judy E. Anderson, 2 Josephine E. Joya, 1 Stewart I. Head, 3 Nalini Pather, 1 Anthony J. Kee, 1 Peter W. Gunning 4 and Edna C. Hardeman 1, * 1 Neuromuscular and Regenerative Medicine Unit; School of Medical Sciences; University of New South Wales; Sydney, Australia; 2 Faculty of Science; University of Manitoba; Winnipeg MB, Canada; 3 Department of Physiology; School of Medical Sciences; University of New South Wales; Sydney, Australia; 4 Oncology Research Unit; School of Medical Sciences; University of New South Wales; Sydney, Australia Keywords: skeletal muscle, aging, progenitor cells, fiber branching, contractility Abbreviations: DD, denervation-devascularization; EDL, extensor digitorum longus; FI, freeze injury; HRT, half-relaxation time; Ntx, notexin; TA, tibialis anterior; TTP, time-to-peak twitch studies, related to injury protocol, animal strain, sex and age, use of parabiotic pairs, and the preservation of the neurovascular sup- ply continue to obscure a clear understanding of whether and how aging per se, affects regeneration capacity in muscle. In the present study, we used three muscle acute injury para- digms, denervation-devascularization (DD), freeze (FI) and notexin, to evaluate the differential impact on the outcome of regeneration of functional architecture from neurovascular dam- age at different ages. We employed DD injury which requires complete re-establishment of vascular perfusion and re-innerva- tion of the muscle for successful regeneration. 12-14 By comparison, freeze injury causes localized muscle damage and destroys only local nerves and vasculature at the site of injury 15 and results in partial neurovascular damage and peri-regional fibrosis. 2 In con- trast to such “physically inclusive” injuries that impact multiple aspects of the muscle architecture, myotoxins such as notexin cause fiber damage through depolarization and sarcolemmal degradation and importantly leave intact architectural scaffolds including the basal lamina and associated nerves, neuromuscu- lar junctions, intramuscular vessels and satellite cells. 16,17 These highly effective scaffolds ensure coordination of debris removal