Numerical densities of myonuclei and satellite cells in muscle fiber types in the aging human thyroarytenoid muscle: an immunohistochemical and stereological study using confocal laser scanning microscopy

Glutamine-stimulated in vitro hypertrophy is preserved in muscle cells from older women

Age-related loss of muscle mass may result from reduced protein synthesis stimulation in response to anabolic stimuli, such as amino acid (AA) supplementation. The exact etiology of anabolic resistance to AA remains unclear. Therefore, the aim of this study was to investigate the anabolic response [cell size, protein synthesis and mechanistic target of rapamycin (mTOR) pathway] to the AA glutamine (a strong anabolic AA highly present in skeletal muscle) in myotubes obtained from 8 young (YW; 21-35 yrs) and 8 older (OW; 65-70 yrs) healthy women. This in vitro model of human primary myogenic cells explores the intrinsic behavior of muscle cells, while excluding potential influences of external factors. We showed that despite lower muscle mass, strength and cardiorespiratory fitness in OW compared to YW, myotube size (myotube diameter and area) and protein synthesis were not altered in OW, and glutamine-induced myotube hypertrophy and protein synthesis were preserved in OW. Apart from a lower glutamine-induced increase in P70S6 kinase phosphorylation in OW, no significant differences in other components of the mTOR pathway were observed between groups. Altogether, our data support the idea that the intrinsic capacity of muscle cells to respond to glutamine stimulation is preserved in healthy older women.

Age-Associated Methylation Suppresses SPRY1, Leading to a Failure of Re-quiescence and Loss of the Reserve Stem Cell Pool in Elderly Muscle

The molecular mechanisms by which aging affects stem cell number and function are poorly understood. Murine data have implicated cellular senescence in the loss of muscle stem cells with aging. Here, using human cells and by carrying out experiments within a strictly pre-senescent division count, we demonstrate an impaired capacity for stem cell self-renewal in elderly muscle. We link aging to an increased methylation of the SPRY1 gene, a known regulator of muscle stem cell quiescence. Replenishment of the reserve cell pool was modulated experimentally by demethylation or siRNA knockdown of SPRY1. We propose that suppression of SPRY1 by age-associated methylation in humans inhibits the replenishment of the muscle stem cell pool, contributing to a decreased regenerative response in old age. We further show that aging does not affect muscle stem cell senescence in humans.

A muscle stem cell for every muscle: variability of satellite cell biology among different muscle groups

The human body contains approximately 640 individual skeletal muscles. Despite the fact that all of these muscles are composed of striated muscle tissue, the biology of these muscles and their associated muscle stem cell populations are quite diverse. Skeletal muscles are affected differentially by various muscular dystrophies (MDs), such that certain genetic mutations specifically alter muscle function in only a subset of muscles. Additionally, defective muscle stem cells have been implicated in the pathology of some MDs. The biology of muscle stem cells varies depending on the muscles with which they are associated. Here we review the biology of skeletal muscle stem cell populations of eight different muscle groups. Understanding the biological variation of skeletal muscles and their resident stem cells could provide valuable insight into mechanisms underlying the susceptibility of certain muscles to myopathic disease.

Changes in Communication between Muscle Stem Cells and their Environment with Aging

Aging is associated with both muscle weakness and a loss of muscle mass, contributing towards overall frailty in the elderly. Aging skeletal muscle is also characterised by a decreasing efficiency in repair and regeneration, together with a decline in the number of adult stem cells. Commensurate with this are general changes in whole body endocrine signalling, in local muscle secretory environment, as well as in intrinsic properties of the stem cells themselves. The present review discusses the various mechanisms that may be implicated in these age-associated changes, focusing on aspects of cell-cell communication and long-distance signalling factors, such as levels of circulating growth hormone, IL-6, IGF1, sex hormones, and inflammatory cytokines. Changes in the local environment are also discussed, implicating IL-6, IL-4, FGF-2, as well as other myokines, and processes that lead to thickening of the extra-cellular matrix. These factors, involved primarily in communication, can also modulate the intrinsic properties of muscle stem cells, including reduced DNA accessibility and repression of specific genes by methylation. Finally we discuss the decrease in the stem cell pool, particularly the failure of elderly myoblasts to re-quiesce after activation, and the consequences of all these changes on general muscle homeostasis.

Confocal stereology: an efficient tool for measurement of microscopic structures

Quantitative measurements of geometric forms or counting of objects in microscopic specimens is an essential tool in studies of microstructure. Confocal stereology represents a contemporary approach to the evaluation of microscopic structures by using a combination of stereological methods and confocal microscopy. 3-D images acquired by confocal microscopy can be used for the estimation of geometrical characteristics of microscopic structures by stereological methods, based on the evaluation of optical sections within a thick slice and using computer-generated virtual test probes. Such methods can be used for estimating volume, number, surface area and length using relevant spatial probes, which are generated by specific software. The interactions of the probes with the structure under study are interactively evaluated. An overview of the methods of confocal stereology developed during the past 30 years is presented. Their advantages and pitfalls in comparison with other methods for measurement of geometrical characteristics of microscopic structures are discussed.

Effects of aging on thyroarytenoid muscle regeneration

OBJECTIVES/HYPOTHESIS

Regenerative properties of age-associated changes in the intrinsic laryngeal muscles following injury are unclear. The purpose of this study was to investigate the regenerative properties of the thyroarytenoid (TA) muscle in an aging rat model. The hypothesis was that following myotoxic injury, old animals would exhibit a decrease in mitotic activities of muscle satellite cells when compared with younger rats, suggesting reduced regenerative potential in the aging rat TA muscle.

STUDY DESIGN

Animal group comparison.

METHODS

Regeneration responses following injury to the TA muscle were examined in 18 young adult, middle-aged, and old Fischer 344/Brown Norway rats. TA muscle fiber cross-sectional area (CSA), satellite cell mitosis (number/fiber), and regeneration index (CSA injured side/CSA noninjured side) were measured and compared across age groups.

RESULTS

Young adult animals had a significantly higher regeneration index than the middle-aged and old groups. Within the lateral region of the TA muscle (LTA), the regeneration index was significantly higher in the young adult animals than in the middle-aged and old animals. The regeneration index of the medial TA was significantly higher than the LTA across all age groups.

CONCLUSIONS

The regenerative capacity of the TA muscle is impaired with increasing age.

Effect of aging on blood flow in rat larynx

OBJECTIVE/HYPOTHESIS

Age-associated muscular changes and fatigue have been shown to affect phonatory function. Reductions in blood flow with aging could translate to reductions in oxidative capacity within laryngeal muscles and increased fatigability. We tested the hypothesis that there would be increased capillary red blood cell (RBC) velocity and a reduction of capillary density in the thyroarytenoid (TA) muscle of senescent rats.

STUDY DESIGN/METHODS

Ten male Fisher 344/Brown Norway rats in two age groups were used: young adult (9 mo) and old (28-30 mo). Sixteen additional young and old rats were used in a fluorescent microsphere experiment that examined blood flow rates before and after a surgical manipulation. With use of a specially equipped intravital microscope, in vivo measurements of capillary geometry and flow were obtained, including RBC velocity, capillary density, tortuosity, and number of branch points.

RESULTS

There was an age-related reduction in capillary surface area as evidenced by reduced lineal density of capillaries. In addition, reduced RBC transit time was suggested by the reduction in branch points found with age. There was no change in RBC velocity with aging. The surgical method used to expose the TA muscle for blood flow recordings did not significantly affect resultant blood flow measurements.

CONCLUSIONS

We developed a method to evaluate in vivo laryngeal microvasculature. We found age-related changes in microvascular geometry within the TA muscle of the rat that could affect blood flow to this critical muscle of phonation and airway protection. These microvascular changes could contribute to age-related laryngeal dysfunction.

Muscle fiber and satellite cell apoptosis in the aging human thyroarytenoid muscle: a stereological study with confocal laser scanning microscopy

OBJECTIVE

This study determines the role of changes in numerical densities of apoptotic myonuclei and satellite cells in age-related remodeling of the thyroarytenoid muscle (TA).

DESIGN

Changes in numerical densities of apoptotic myonuclei and satellite cells were estimated for the entire TA by using stereological techniques.

RESULTS

There was an increase in N(V apoptotic myonucleus, fiber type) (P < 0.05) and in the percentage of apoptotic myonuclei (P < 0.05) in type 1 but not in type 2 muscle fibers. There was also an increase in N(V apoptotic satellite cell, fiber type) (P < 0.05) and in the percentage of apoptotic satellite cells (P < 0.05) in type 1 fibers but not in type 2 fibers.

CONCLUSION

Apoptosis may contribute to age-related fiber loss and atrophy in the TA.

SIGNIFICANCE

Therapeutic techniques based on decreasing the frequency of apoptosis may block age-related fiber loss and atrophy in the TA.