The Study Of Muscle Morphology And Muscle Fiber Type Of Flexor And Extensor Digitorum Muscle Groups In Rabbit Leg At Different Ages

Objective: The experiment was designed to explore the morphological characteristics offlexor and extensor digitorum muscle groups in rabbit leg during postnatal developmentand aging and to explore the composition of skeletal muscle fiber type at different ages.Furthermore, this study is to confirm the transformation of three muscle fiber types inflexor and extensor digitorum muscle groups during postnatal development and aging.Methods: We studied fifty-five healthy rabbits (regardless sex). Rabbits were dividedinto11groups.Each group included five rabbits. Aging of the groups are2-3days,2weeks,4weeks,8weeks,12weeks,16weeks,20weeks,24weeks,12months,24months and36months respectively. The extensor digitorum longus, extensor halluis longus, flexordigitorum superficialis and flexor digitorum profundus were taken and weighed.NADH-TR staining was used to investigate the histochemical characteristics of the musclefibers and an image analysis system was used to measure the change in muscle fibers, thecomposition of muscle fiber types, the diameter and cross-sectional area of muscle fibers.Results:(1) The weight in flexor and extensor digitorum muscle groups gradually increase withaging. The weight of extensor digitorum longus and extensor halluis longus are3.00±0.07g and0.62±0.04g respectively. However, in the24months group, the weight of theextensor digitorum muscle group is the greatest. The weight of the flexor digitorumsuperficialis and flexor digitorum profundus are4.83±0.04g and3.65±0.06g respectively,while the weight of the flexor muscle group being the greatest in the12months group. Theweight of the extensor digitorum longus and extensor halluis longus in the36-mouthdecline1.67%and6.45%respectively when compared with the24-mouth group. However,the weight of the flexor digitorum superficialis and flexor digitorum profundus decline21.53%and20.55%respectively when compared with the12-mouth group, The weight ofthe flexor digitorum superficialis and flexor digitorum profundus had a significant decrease(p＜0.05).(2) Specimens were stained by NADH-TR and then examined.The type I muscle fiberswere observed in the2-3days group, the type I and type II muscle fibers were observed inthe2weeks group and type I, type IIA and type IIB muscle fibers were distinguished in the 4weeks group. The muscle fiber types changed with aging. In the24months group, thetype I muscle fibers of the extensor digitorum longus, extensor halluis longus, flexordigitorum superficialis and flexor digitorum profundus are29.97%,21.68%,27.97%and14.38%respectively. At36months, the type I muscle fibers in the four muscles increased0.66%,16.07%,1.17%,18.39%respectively. The extensor halluis longus and flexordigitorum profundus had a significant difference (p＜0.05). In the24months group, thetype IIA muscle fibers of the extensor digitorum longus, extensor halluis longus, flexordigitorum superficialis and flexor digitorum profundus are22.50%,32.20%,21.11%and32.16%respectively. At36months, the type IIA muscle fibers of the extensor digitorumlongus, extensor halluis longus and flexor digitorum profundus increased25.86%,25.98%and15.70%respectively, but the flexor digitorum superficialis declined0.43%. Theextensor digitorum longus, extensor halluis longus and flexor digitorum profundus had asignificant difference (p＜0.05). In the24months group, the type IIB muscle fibers of theextensor digitorum longus, extensor halluis longus, flexor digitorum superficialis andflexor digitorum profundus are47.53%,46.12%,50.92%and53.46%respectively. At thetime of36months, the type IIB muscle fibers in the four skeletal muscles declined16.94%,33.50%,0.47%,17.27%respectively. The type IIB muscle fibers had a significant decreasein extensor digitorum longus, extensor halluis longus and flexor digitorum profundus (p＜0.05).(3) The muscle fiber diameter and cross-sectional area of the flexor and extensor digitorummuscle groups increased with aging, which is the greatest in the12months group. Take themuscle fiber diameter as an example: in the extensor digitorum longus, the type I, type IIAand type IIB are82.77±4.03μm,87.43±1.80μm and124.96±2.49μm respectively. The typeI, type IIA and type IIB of the extensor halluis longus are107.10±8.12μm,95.14±2.63μmand122.63±6.71μm respectively. In the flexor digitorum superficialis, the type I, type IIAand type IIB are83.77±1.73μm,91.96±7.06μm and103.02±2.36μm respectively. In theflexor digitorum profundus, the type I, type IIA and type IIB are71.68±1.42μm,81.63±3.22μm and88.83±3.37μm respectively. The muscle fiber diameter declined withaging. Comparing the muscle fiber diameter of the36months group with12months group,the type I, type IIA and type IIB of the extensor digitorum longus declined3.62%,13.54%and18.87%respectively, type IIA and type IIB had a significant difference (p＜0.05); thetype I, type IIA and type IIB of the extensor halluis longus declined21.54%,3.66%and 23.86%respectively, type I and type IIB had a significant difference(p＜0.05); the type I,type IIA and type IIB of the flexor digitorum superficialis declined15.85%,21.85%, and28.78%respectively. However, the three muscle fiber types had a significant difference(p＜0.05); the type I, type IIAand type IIB of the flexor digitorum profundus declined4.63%,23.63%and11.71%respectively, the three muscle fiber types had a significantdifference(p＜0.05).Conclusion:(1) Muscle weight of the extensor digitorum muscle group is the greatest in the24monthsgroup and decreased in the36months group. Muscle weight of the extensor digitorummuscle group is the greatest in the12months group, but began to decline in the24monthsgroup. Moreover, muscle weight of the flexor digitorum muscle group declined earlier andin larger scale than the extensor digitorum muscle group. The percentage of the decline ofthe muscle weight in the heavier muscles were less than that of the lighter, the flexormuscle weight decreased more than the extensors.(2) In the4weeks group of rabbit, the type I, type IIA and type IIB can be distinguishedaccurately. In the36month group, the composition ratio of type II decreased, especiallythe type IIB. However, type I presents a rising tendency, a small angular shape andclustering of muscle fiber types could be observed.(3) The muscle fiber diameter and cross-sectional area of the flexor and extensor digitorummuscle groups increased with aging and is the greatest in the12months group. Fiberdiameter and cross-sectional area of type IIB are the greatest in the three muscle fiber types.In the36months group, the fiber diameter and the cross-sectional area of type II havesharply fallen, especially in type IIB.