Pax7 Shows Higher Satellite Cell Frequencies and Concentrations within Intrafusal Fibers of Muscle Spindles

Abstract

(ZY-R) S U M M A R Y Intrafusal fibers within muscle spindles make up a small subpopulation of muscle fibers. These proprioceptive fibers differ from most extrafusal fibers because, even in maturity, their diameters remain small, and they retain expression of developmental myosins. Although both extrafusal and intrafusal fibers contain satellite cells (SCs), comparatively little is known about intrafusal SCs. Analyzing chicken fast-phasic posterior (PLD) and slow-tonic anterior (ALD) latissimus dorsi muscles, we show that SCs of both intrafusal and extrafusal fibers express Pax7. We further test the hypotheses that intrafusal fibers display parameters reflective of extrafusal immaturity. These hypotheses are that intrafusal fibers contain (a) higher SC frequencies (number of SC nuclei/all nuclei within basal lamina) and concentrations (closer together) and (b) smaller myonuclear domains than do adjacent extrafusal fibers. IHC techniques were applied to PLD and ALD muscles excised at 30 and 138 days posthatch. The hypotheses were validated, suggesting that intrafusal fibers have greater capacities for growth, regeneration, and repair than do adjacent extrafusal fibers. During maturation, extrafusal and intrafusal fibers show similar trends of decreasing SC frequencies and concentrations and increases in myonuclear domains. Thus, extrafusal and intrafusal fibers alike should exhibit reduced capacities for growth, regeneration, and repair during maturation. (J Histochem Cytochem 56:831-840, 2008)

Pax7-expressing satellite cells are indispensable for adult skeletal muscle regeneration

“…These possibilities merit further investigations, particularly for optimising stem cell-based therapeutic approaches in the context of acute myopathic diseases. We note also that spindle myofibres have higher numbers of satellite cells and they maintain embryonic gene expression (Kirkpatrick et al, 2008;Zammit et al, 2004). It would be of interest to examine satellite cell ablation in these myofibres in the future.…”

The Skeletal Muscle Satellite Cell

“…Intrafusal myofibers make up a small subpopulation of the muscle myofibers, whereas the bulk of skeletal muscle myofibers are termed extrafusal myofibers. The diameter of intrafusal myofibers remains small from an early postnatal age, and they retain expression of developmental myosins as well as expression of LacZ in myofiber nuclei of Myf5 nLacZ/+ knock-in mice (Kozeka and Ontell 1981;Walro and Kucera 1999;Zammit, Carvajal et al 2004;Kirkpatrick et al 2008;Kirkpatrick et al 2009;Osterlund et al 2011). It is unknown whether satellite cells associated with intrafusal versus extrafusal myofibers are functionally unique and/or derived from distinct myogenic populations.…”

Histological study of postnatal development of mouse tongues

“…The mechanisms underlying myogenesis in vertebrate organisms have been extensively investigated, and numerous factors, including interleukin (IL)-4, IL-6, MyoD, Myf5, Pax3 and Pax-7, have important roles in muscle development and tongue muscle satellite cell activity (16)(17)(18). However, the exact molecular mechanisms of muscle remodeling have remained elusive.…”