In this study, results are reported from the analyses of vastus lateralis muscle biopsy samples obtained from a subset (n = 90) of 125 previously phenotyped, highly active male and female cyclists aged 55-79 years in regard to age. We then subsequently attempted to uncover associations between the findings in muscle and in vivo physiological functions. Muscle fibre type and composition (ATPase histochemistry), size (morphometry), capillary density (immunohistochemistry) and mitochondrial protein content (Western blot) in relation to age were determined in the biopsy specimens. Aside from an age-related change in capillary density in males (r = -.299; p = .02), no other parameter measured in the muscle samples showed an association with age. However, in males type I fibres and capillarity (p < .05) were significantly associated with training volume, maximal oxygen uptake, oxygen uptake kinetics and ventilatory threshold. In females, the only association observed was between capillarity and training volume (p < .05). In males, both type II fibre proportion and area (p < .05) were associated with peak power during sprint cycling and with maximal rate of torque development during a maximal voluntary isometric contraction. Mitochondrial protein content was not associated with any cardiorespiratory parameter in either males or females (p > .05). We conclude in this highly active cohort, selected to mitigate most of the effects of inactivity, that there is little evidence of age-related changes in the properties of VL muscle across the age range studied. By contrast, some of these muscle characteristics were correlated with in vivo physiological indices.

Supported by the U.K. National Institute of Health Research (NIHR) (Z.A.P.), the Batchworth Charitable Trust, the Moulton Charitable Foundation, and the NIHR University College London Hospitals Biomedical Research Centre (UCLH BRC) (A.S.M.), the Research Councils UK (S.D.H.), the NIHR Clinical Research Facility and BRC at Guy’s and St. Thomas’ National Health Service (NHS) Foundation Trust (GSST) and King’s College London (N.H. and B.C.), and by University College London and UCLH BRC (H.E.M., Z.A.P., and A.S.M.). The NIHR doctorate fellowship (2010–2014) underpinned the core patient population study on which this follow-up was built, as did funding from the Batchworth Charitable Trust, Moulton Foundation, and European Society of intensive Care Medicine. Additional funding was received from the Whittington Hospital NHS Trust and the European Society of Intensive Care Medicine. Author Contributions: Concept and design—Z.A.P., P.S.S., J.M., S.D.H., N.H., and H.E.M.; data collection—Z.A.P., A.S.M., J.R., B.C., and A.R.; analysis and interpretation—Z.A.P., A.S.M., B.C., P.S.S., A.R., J.M., S.D.H., N.H., and H.E.M.; manuscript drafting and revision—Z.A.P., A.S.M., B.C., P.S.S., A.R., J.M., S.D.H., N.H., and H.E.M. Author disclosures are available with the text of this letter at www.atsjournals.org.

Critical Care Medicine www.ccmjournal.org 1603 Research (core study funding). The National Institute of Health Research, the European Society of Intensive Care Medicine, Guy’s & St Thomas’ and King’s College London NIHR Comprehensive BRC and the Whittington Hospital NHS Trust had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; or decision to submit the manuscript for publication. The remaining authors have disclosed that they do not have any potential conflicts of interest. For information regarding this article, E-mail: Zudin_amilka_puthucheary@ nuhs.edu.sg

A rapid and early loss of skeletal muscle mass underlies the physical disability that is common amongst survivors of critical illness (CI). The functional capacity of skeletal muscle depends on its quantity as well as quality, which may be adversely affected. Our main objectives were to characterise changes in muscle echogenicity, pennation angle and fascial characteristics that occur early in CI, and to relate these to histologically defined myofibre necrosis and fascial pathology. Subjects comprised a subgroup of patients recruited to the Musculoskeletal Ultrasound in CI: Longitudinal Evaluation (MUSCLE) study. Comparisons were made between sequential Vastus Lateralis (VL) biopsy specimens and ultrasound assessment of Rectus Femoris (RF) echogenicity. Change in RF pennation angle was measured. In 30 patients, change in muscle echogenicity was greater in patients who developed muscle necrosis than in those who did not [8.2% (95%CI −5.3 to 21.7) versus −15.0% (95% CI −28.9 to −1.09), p = 0.016]. The AUROC for prediction of myofibre necrosis was 0.74 (95% CI 0.565–0.919, p = 0.024) increasing to 0.85 (95% CI 0.703 −0.995, p = 0.003) with the removal of those with potential iatrogenic muscle damage. Fasciitis was observed in 18 out of 30 biopsies (60%) and was dominated by macrophages by day 7 or day 10. Mean pennation angle decreased from 7.6° ± 4.0 to 5.5° ± 2.1 (p = 0.01) over the first 10 days of CI. Myofibre necrosis and fascial inflammation can be detected noninvasively using ultrasound in CI. Fasciitis precedes and frequently accompanies muscle necrosis and is dominated by macrophages in the late acute phase. Rapid decreases in pennation angle are seen. These findings may have functional implications for survivors of critical illness.

The repair and regeneration of skeletal muscle requires the action of satellite cells, which are the resident muscle stem cells. These can be isolated from human muscle biopsy samples using enzymatic digestion and their myogenic properties studied in culture. Quantitatively, the two main adherent cell types obtained from enzymatic digestion are: (i) the satellite cells (termed myogenic cells or muscle precursor cells), identified initially as CD56+ and later as CD56+/desmin+ cells and (ii) muscle-derived fibroblasts, identified as CD56– and TE-7+. Fibroblasts proliferate very efficiently in culture and in mixed cell populations these cells may overrun myogenic cells to dominate the culture. The isolation and purification of different cell types from human muscle is thus an important methodological consideration when trying to investigate the innate behavior of either cell type in culture. Here we describe a system of sorting based on the gentle enzymatic digestion of cells using collagenase and dispase followed by magnetic activated cell sorting (MACS) which gives both a high purity (>95% myogenic cells) and good yield (~2.8 x 106 ± 8.87 x 105 cells/g tissue after 7 days in vitro) for experiments in culture. This approach is based on incubating the mixed muscle-derived cell population with magnetic microbeads beads conjugated to an antibody against CD56 and then passing cells though a magnetic field. CD56+ cells bound to microbeads are retained by the field whereas CD56– cells pass unimpeded through the column. Cell suspensions from any stage of the sorting process can be plated and cultured. Following a given intervention, cell morphology, and the expression and localization of proteins including nuclear transcription factors can be quantified using immunofluorescent labeling with specific antibodies and an image processing and analysis package.

Objectives: A rapid and early loss of skeletal muscle mass underlies the physical disability common amongst survivors of critical illness. However, skeletal muscle function depends not only on its quantity but its quality, which may be adversely affected. We set out to characterise the changes in macroscopic muscle echogenicity and fascial characteristics that occur early in critical illness, and to relate these to microscopic histologically defined myofibre necrosis and fascial pathology.Design and Setting: Prospective two center observational study.Patients: Thirty subjects comprising a subgroup of patients recruited to the Musculoskeletal Ultrasound in Critical Illness: Longitudinal Evaluation (MUSCLE) study.Measurements and Main Results: Comparisons were made between sequential Vastus Lateralis histological specimens and ultrasound assessment of Rectus Femoris echogenicity. Change in muscle echogenicity was greater in patients who developed muscle necrosis (n = 15) than in those who did not (8.2% [95% CI, -5.3 to 21.7] vs -15.0% [95% CI, -28.9 to -1.09]; p = 0.016). The area under receiver operator curve for ultrasound echogenicity's prediction of myofiber necrosis was 0.74 (95% CI, 0.565 to 0.919; p = 0.024) increasing to 0.85 (95% CI, 0.703 to -0.995; p = 0.003) with the removal of those with potential iatrogenic muscle damage. Fasciitis was observed in 18 of 30 biopsies (60%).Conclusions: Myofiber necrosis and fascial inflammation can be detected noninvasively using ultrasound in the critically ill. Fasciitis precedes and frequently accompanies muscle necrosis. These findings may have functional implications for survivors of critical illness.

IMPORTANCE Survivors of critical illness demonstrate skeletal muscle wasting with associated functional impairment.OBJECTIVE To perform a comprehensive prospective characterization of skeletal muscle wasting, defining the pathogenic roles of altered protein synthesis and breakdown.DESIGN, SETTING, AND PARTICIPANTS Sixty-three critically ill patients (59% male; mean age: 54.7 years [95% CI, 50.0-59.6 years]) with an Acute Physiology and Chronic Health Evaluation II score of 23.5 (95% CI, 21.9-25.2) were prospectively recruited within 24 hours following intensive care unit (ICU) admission from August 2009 to April 2011 at a university teaching and a community hospital in England. Patients were recruited if older than 18 years and were anticipated to be intubated for longer than 48 hours, to spend more than 7 days in critical care, and to survive ICU stay.MAIN OUTCOMES AND MEASURES Muscle loss was determined through serial ultrasound measurement of the rectus femoris cross-sectional area (CSA) on days 1, 3, 7, and 10. In a subset of patients, the fiber CSA area was quantified along with the ratio of protein to DNA on days 1 and 7. Histopathological analysis was performed. In addition, muscle protein synthesis, breakdown rates, and respective signaling pathways were characterized.RESULTS There were significant reductions in the rectus femoris CSA observed at day 10 (-17.7%[95% CI, -25.9% to 8.1%]; P < .001). In the 28 patients assessed by all 3 measurement methods on days 1 and 7, the rectus femoris CSA decreased by 10.3%(95% CI, 6.1% to 14.5%), the fiber CSA by 17.5%(95% CI, 5.8% to 29.3%), and the ratio of protein to DNA by 29.5%(95% CI, 13.4% to 45.6%). Decrease in the rectus femoris CSA was greater in patients who experienced multiorgan failure by day 7 (-15.7%; 95% CI, -27.7% to 11.4%) compared with single organ failure (-3.0%; 95% CI, -5.3% to 2.1%) (P < .001), even by day 3 (-8.7%[95% CI, -59.3% to 50.6%] vs -1.8%[95% CI, -12.3% to 10.5%], respectively; P = .03). Myofiber necrosis occurred in 20 of 37 patients (54.1%). Protein synthesis measured by the muscle protein fractional synthetic rate was depressed in patients on day 1 (0.035%/hour; 95% CI, 0.023% to 0.047%/hour) compared with rates observed in fasted healthy controls (0.039%/hour; 95% CI, 0.029% to 0.048%/hour) (P = .57) and increased by day 7 (0.076%[95% CI, 0.032%-0.120%/hour]; P = .03) to rates associated with fed controls (0.065%/hour [95% CI, 0.049% to 0.080%/hour]; P = .30), independent of nutritional load. Leg protein breakdown remained elevated throughout the study (8.5 [95% CI, 4.7 to 12.3] to 10.6 [95% CI, 6.8 to 14.4] mu mol of phenylalanine/min/ideal body weight x 100; P = .40). The pattern of intracellular signaling supported increased breakdown (n = 9, r = -0.83, P = .005) and decreased synthesis (n = 9, r = -0.69, P = .04).CONCLUSIONS AND RELEVANCE Among these critically ill patients, muscle wasting occurred early and rapidly during the first week of critical illness and was more severe among those with multiorgan failure compared with single organ failure. These findings may provide insights into skeletal muscle wasting in critical illness.

Introduction Muscle wasting occurs early and rapidly in critically ill patients. It results from a decrease in muscle protein synthesis and an increase in its breakdown, with muscle necrosis a common associated finding. The drivers for such atrophy and necrosis remain poorly understood, as do the related regulatory pathways. We hypothesised that systemic and intracellular cytokines play a role in this process. Methods The UK-MUSCLE study prospectively studied the wasting response (change in Rectus Femoris cross sectional area (RFCSA) using serial ultrasound) in critically ill patients admitted to the ICU. Cytokine profiles (high sensitivity cytokine chip array, Randox, Ireland) were analysed in serum samples from 62 of these patients (days 1 and 7) and in contemporaneous Vastus Lateralis biopsies of 35 patients. Tumour Necrosis Factor (TNF)-α, TNF receptor (TNFR) 1 and 2, interleukin (IL)1a, il1b, IL-2, IL-4, IL-6, IL-8, IL-10, Vascular Endothelial growth Factor (VEGF), Interferon (IFN)-γ, Monocyte Chemoattractant Protein-1(MCP-1) and Epidermal Growth Factor were assayed. Muscle necrosis was determined by hematoxylin and eosin staining of the Vastus Lateralis biopsies. Results Intramuscular TNFR1 concentrations increased over 7 days (0.83 ± 1.1 µg/L to 2.07 ± 2.65 µg/L; p = 0.042), as did intramuscular interleukin-10 (22.69 ± 26.5 ng/L to 59.8 ± 80.0 ng/L; p = 0.005). Increases in serum IL-1a (0.52 ± 0.26 ng/L to 0.57 ± 0.28 ng/L, p = 0.03), VEGF (166.86 ± 231.7 ng/L to 246.6 ± 236.7 ng/L, p < 0.001) and MCP-1 (886.8 ± 685.0 ng/L to 386.49 ± 469.7 ng/L, p < 0.001) were seen as well as a decrease in IL-6 (322.2 ± 422.7 ng/L to 78.55 ± 184.4 ng/L, p < 0.001), il-10 (22.73 ± 51.1 ng/L to 7.61 ± 15.3 ng/L, p = 0.03), IFN- γ (4.76 ± 11.0 ng/L to 1.28 ± 2.02 ng/L, p = 0.02) and MCP-1 (886.8 ± 685.0 ng/L to 386.49 ± 469.7 ng/L, p < 0.001). Neither myonecrosis nor change in RFCSA was related to that in intramuscular cytokines by linear and logistical-regression analysis, using 10% loss as a cut off. Loss in RFCSA over 10 days was very weakly correlated with serum TNFR1 concentration on days 1 (r2=0.12; p < 0.01) and 7 (r2=0.09; p = 0.02) and with serum IL-10 concentration (r2=0.19; p < 0.01). Myofibre necrosis was unrelated to serum cytokine profile. Discussion Soluble TNFR1 is associated with the degree of muscle wasting in critical illness. This relationship may be causal as TNF1R signalling leads to activation of nuclear factor kappa beta and apoptosis. Whilst no evidence was seen for intramuscular inflammation, increased intramuscular IL-10 may be protective, in its anti-inflammatory role.

Objective: Type I myosins are molecular motors necessary for glucose transport in the cytoplasm and initiation of transcription in the nucleus. Two of these, MYO1H and MYO1C, are paralogs which may be important in the development of malocclusion. The objective of this study was to investigate their gene expression in the masseter muscle of malocclusion subjects. Two functionally related proteins known to contribute to malocclusion were also investigated: KAT6B (a chromatin remodelling epigenetic enzyme which is activated by MYO1C) and RUNX2 (a transcription factor regulating osteogenesis which is activated by KAT6B).Design: Masseter muscle samples and malocclusion classifications were obtained from orthognathic surgery subjects. Muscle was sectioned and immunostained to determine fibre type properties. RNA was isolated from the remaining sample to determine expression levels for the four genes by TaqMan (R) RT-PCR. Fibre type properties, gene expression quantities and malocclusion classification were compared.Results: There were very significant associations (P <0.0000001) between MYO1C and KAT6B expressions. There were also significant associations (P < 0.005) between RUNX2 expression and masseter muscle type II fibre properties. Very few significant associations were identified between MYO1C and masseter muscle fibre type properties.Conclusions: The relationship between MYO1C and KAT6B suggests that the two are interacting in chromatin remodelling for gene expression. This is the nuclear myosin1 (NM1) function of MYO1C. A surprising finding is the relationship between RUNX2 and type II masseter muscle fibres, since RUNX2 expression in mature muscle was previously unknown. Further investigations are necessary to elucidate the role of RUNX2 in adult masseter muscle. (C) 2014 Elsevier Ltd. All rights reserved.

In many fish, both production of new muscle fibres and neurogenesis continue into juvenile life. To test the hypothesis that new motoneurons are produced to supply the expanding muscle target we used the seabream (Sparus aurata), which shows a many-fold increase in the number of fibres in lateral muscle during posthatching juvenile development. A motor nerve branch innervating a segment of epaxial lateral white muscle was identified, and the type and number of its axons were measured in fish of several larval and post-larval ages. Contrary to expectation, total axon number was greatest in the larval fish (114.3±22.6); unmyelinated axons were found only in the larval nerves, and the number of myelinated axons increased only modestly over the ages examined, from 58.5±12.4 in larval fish to 77.5±7.3 in post-larval juveniles. We conclude that in seabream the larval nerve still includes axons of motoneurons destined to die during the normal developmental phase of targetdependence in addition to those axons which will survive into juvenile life, and that the definitive number of motoneurons is already present in the larval fish before the main increase in muscle fibre number occurs.

The aim of this work is to review the relationship between the function of the masseter muscle and the occurrence of malocclusions. An analysis was made of the masseter muscle samples from subjects who underwent mandibular osteotomies. The size and proportion of type-II fibers (fast) decreases as facial height increases. Patients with mandibular asymmetry have more type-II fibers on the side of their deviation. The insulin-like growth factor and myostatin are expressed differently depending on the sex and fiber diameter. These differences in the distribution of fiber types and gene expression of this growth factor may be involved in long-term postoperative stability and require additional investigations. Muscle strength and bone length are two genetically determined factors in facial growth. Myosin 1H (MYOH1) is associated with prognathia in Caucasians. As future objectives, we propose to characterize genetic variations using "Genome Wide Association Studies" data and their relationships with malocclusions.

SummaryMuscle samples were taken from the gluteus, semitendinosus and longissimus muscles of a captive cheetah immediately after euthanasia. Fibres were "skinned" to remove all membranes leaving the contractile filament array intact and functional. Segments of skinned fibres from these cheetah muscles and from rabbit psoas muscle were activated at 20°C by a temperature jump protocol. Step and ramp length changes were imposed after active stress had developed. The stiffness of the non-contractile ends of the fibres (series elastic component) was measured at two different stress values in each fibre; stiffness was strongly dependent on stress. Using these stiffness values, the speed of shortening of the contractile component was evaluated, and hence the power it was producing. Fibres were analysed for myosin heavy chain content using gel electrophoresis, and identified as either slow (Type I) or fast (Type II). The power output of cheetah Type II fibre segments was 92.5 ± 4.3 W kg-1 (mean ±s.e., 14 fibres) during shortening at relative stress 0.15 (=stress during shortening/isometric stress). For rabbit psoas fibre segments (presumably Type IIX) the corresponding value was significantly higher (P<0.001), 119.7 ± 6.2 W kg-1 (mean ±s.e.,7 fibres). These values are our best estimates of the maximum power output under the conditions used here. Thus the contractile filament power from cheetah was less than that of rabbit when maximally activated at 20°C, and does not account for the superior locomotor performance of the cheetah.

Introduction: Genetic influences on the development of malocclusion include heritable effects on both masticatory muscles and jaw skeletal morphology. Beyond genetic variations, however, the characteristics of muscle and bone are also influenced by epigenetic mechanisms that produce differences in gene expression. We studied 2 enzymes known to change gene expressions through histone modifications, chromatin-modifying histone acetyltransferase KAT6B and deacetylase HDAC4, to determine their associations with musculoskeletal variations in jaw deformation malocclusions. Methods: Samples of masseter muscle were obtained from subjects undergoing orthognathic surgery from 6 malocclusion classes based on skeletal sagittal and vertical dysplasia. The muscles were characterized for fiber type properties by immunohistochemistry, and their total RNA was isolated for gene expression studies by microarray analysis and quantitative real-time polymerase chain reaction. Results: Gene expressions for fast isoforms of myosins and contractile regulatory proteins and for KAT6B and HDAC4 were severalfold greater in masseter muscles from a patient with a deepbite compared with one with an open bite, and genes related to exercise and activity did not differ substantially. In the total population, expressions of HDAC4 (P = 0.03) and KAT6B (P = 0.004) were significantly greater in subjects with sagittal Class III than in Class II malocclusion, whereas HDAC4 tended to correlate negatively with slow myosin type I and positively with fast myosin gene, especially type IIX. Conclusions: These data support other published reports of epigenetic regulation in the determination of skeletal muscle fiber phenotypes and bone growth. Further investigations are needed to elucidate how this regulatory model might apply to musculoskeletal development and malocclusion.

Zudin A. Puthucheary, MRCP; Jaikitry Rawal, MRCS; Mark McPhail, PhD; Bronwen Connolly, BSc; Gamunu Ratnayake, MRCP; Pearl Chan, MBBS; Nicholas S. Hopkinson, PhD; Rahul Phadke, FRCPath; Tracy Dew, MSc; Paul S. Sidhu, PhD; Cristiana Velloso, PhD; John Seymour, PhD; Chibeza C. Agley, MSc; Anna Selby, PhD; Marie Limb, PhD; Lindsay M. Edwards, PhD; Kenneth Smith, PhD; Anthea Rowlerson, PhD; Michael John Rennie, PhD; John Moxham, PhD; Stephen D. R. Harridge, PhD; Nicholas Hart, PhD; Hugh E. Montgomery, MD

We characterised the adherent cell types isolated from human skeletal muscle by enzymatic digestion, and demonstrated that even at 72 hours after isolation these cultures consisted predominantly of myogenic cells (CD56(+), desmin(+)) and fibroblasts (TE-7(+), collagen VI(+), PDGFRα(+), vimentin(+), fibronectin(+)). To evaluate the behaviour of the cell types obtained, we optimised a double immuno-magnetic cell-sorting method for the separation of myogenic cells from fibroblasts. This procedure gave purities of >96% for myogenic (CD56(+), desmin(+)) cells. The CD56(-) fraction obtained from the first sort was highly enriched in TE-7(+) fibroblasts. Using quantitative analysis of immunofluorescent staining for lipid content, lineage markers and transcription factors, we tested if the purified cell populations could differentiate into adipocytes in response to treatment with either fatty acids or adipocyte-inducing medium. Both treatments caused the fibroblasts to differentiate into adipocytes, as shown by loss of intracellular TE-7, upregulation of the adipogenic transcription factors PPARγ and C/EBPα, and adoption of a lipid-laden adipocyte morphology. By contrast, myogenic cells did not undergo adipogenesis and showed differential regulation of PPARγ and C/EBPα in response to these adipogenic treatments. Our results show that human skeletal muscle fibroblasts are at least bipotent progenitors that can remain as extracellular-matrix-producing cells or differentiate into adipocytes.

Background Critical illness survivors demonstrate skeletal muscle wasting with associated functional impairment. We prospectively characterised this process, and defined the pathogenic roles of altered protein synthesis and degradation. Methods Critically ill patients (n=63, 59% male, age 54.7±18.0 years, APACHE II score 23.5±6.5) were recruited <24 hours following Intensive Care Unit (ICU) admission. Muscle loss trajectory was determined through serial ultrasound measurement of rectus femoris cross-sectional area (RFCSA) and, in a subset (n=28), quantification of myofibre area (FibreCSA) and protein/DNA ratio in vastus lateralis biopsies. Histopathological analysis was also performed. Muscle protein synthesis and breakdown rates were determined ([1,2–13C2]Leucine incorporation and D5-Phenylalanine dilution, n=11), and respective signalling pathways examined (Luminex technology and Western Blotting, n=33). Results (i) RFCSA decreased significantly, (–17.7±12.1% [p<0.001]), but underestimated muscle loss determined by either FibreCSA (–10.3±10.9% vs.–17.5±30.2%, p=0.31), or assessment of protein/DNA ratio (–10.3±10.9% vs. –29.5±41.5%, p=0.03). (ii) Fall in RFCSA was greater in multi- than single-organ failure (–21.5±10.5% vs –7.2±9.7% respectively, p<0.0001), even by day 3 (–8.7±16.3% vs –1.8±9.6%, p<0.01). Those suffering ≥ 4 organ were worst affected (–26.3±12.0% vs –19.5±9.4% for 2–3 organ failure, p<0.01). (iii) Histopathological myofibre necrosis occurred in >50% (17/33) of subjects. (iv) Protein synthesis was depressed, to levels observed in fasted controls (0.035±0.018%/hr vs. 0.039±0.011%/hr respectively, p= 0.57). Synthesis rates increased by day 7 (0.076±0.066%/hr, p=0.07) to levels associated with healthy fed controls (0.065+0.018%/hr, p0.30). These effects were independent of nutritional calorie and protein load received. (v) Protein breakdown remained elevated throughout (8.5±5.7 to 10.6±5.7 mmol phe/min/IBW, p=0.4). (vi) Principal component analysis of patterns of intracellular signalling suggested an orchestrated programme of increased breakdown (r –0.83, p0.005) and depressed synthesis (r–0.69, p0.041). Conclusions Skeletal muscle wasting (defined for the first time by three independent measures) (1) occurs early and rapidly in critical illness and (2) is greatest in those with multi-organ failure. Suppression of protein synthesis and increases in catabolism (isotope uptake and intracellular signalling data) were shown, for the first time, to underpin these changes. Importantly, these overall effects appear independent of feeding status, and also to be commonly associated with (previously unrecognised) myonecrosis. ZAP is a National Institute of Health Research Research Fellow, and has received funding from the European Society of Intensive Care Medicine, Guys and St Thomas Comprehensive Biomedical Research Centre and the Whittington Hospital NHS Trust.

Purpose: We identified masseter muscle fiber type property differences in subjects with dentofacial deformities.Patients and Methods: Samples of masseter muscle were collected from 139 young adults during mandibular osteotomy procedures to assess mean fiber areas and percent tissue occupancies for the 4 fiber types that comprise the muscle. Subjects were classified into 1 of 6 malocclusion groups based on the presence of a skeletal Class II or III sagittal dimension malocclusion and either a skeletal open, deep, or normal bite vertical dimension malocclusion. In a subpopulation, relative quantities of the muscle growth factors IGF-I and GDF-8 gene expression were quantified by real-time polymerase chain reaction.Results: Fiber properties were not different in the sagittal malocclusion groups, but were very different in the vertical malocclusion groups (P < .0004). There were significant mean fiber area differences for type II (P <= .0004) and type neonatal-atrial (P = .001) fiber types and for fiber percent occupancy differences for both type I-II hybrid fibers and type II fibers (P <= .0004). Growth factor expression differed by gender for IGF-I (P = .02) and GDF-8 (P < .01). The ratio of IGF-I: GDF-8 expression associates with type I and II mean fiber areas.Conclusion: Fiber type properties are very closely associated with variations in vertical growth of the face, with statistical significance for overall comparisons at P <= .0004. An increase in masseter muscle type II fiber mean fiber areas and percent tissue occupancies is inversely related to increases in vertical facial dimension. (C) 2012 American Association of Oral and Maxillofacial Surgeons J Oral Maxillofac Surg 70: 440-448, 2012