White Robert B, Biérinx Anne-Sophie, Gnocchi Viola F, Zammit Peter S
King's College London, Randall Division of Cell and Molecular Biophysics, Guy's Campus, London SE1 1UL, UK.
BMC Dev Biol. 2010 Feb 22;10:21. doi: 10.1186/1471-213X-10-21.
Postnatal growth in mouse is rapid, with total skeletal muscle mass increasing several-fold in the first few weeks. Muscle growth can be achieved by either an increase in muscle fibre number or an increase in the size of individual myofibres, or a combination of both. Where myofibre hypertrophy during growth requires the addition of new myonuclei, these are supplied by muscle satellite cells, the resident stem cells of skeletal muscle.
Here, we report on the dynamics of postnatal myofibre growth in the mouse extensor digitorum longus (EDL) muscle, which is essentially composed of fast type II fibres in adult. We found that there was no net gain in myofibre number in the EDL between P7 and P56 (adulthood). However, myofibre cross-sectional area increased by 7.6-fold, and length by 1.9-fold between these ages, resulting in an increase in total myofibre volume of 14.1-fold: showing the extent of myofibre hypertrophy during the postnatal period. To determine how the number of myonuclei changes during this period of intense muscle fibre hypertrophy, we used two complementary mouse models: 3F-nlacZ-E mice express nlacZ only in myonuclei, while Myf5nlacZ/+ mice have beta-galactosidase activity in satellite cells. There was a approximately 5-fold increase in myonuclear number per myofibre between P3 and P21. Thus myofibre hypertrophy is initially accompanied by a significant addition of myonuclei. Despite this, the estimated myonuclear domain still doubled between P7 and P21 to 9.2 x 103 microm3. There was no further addition of myonuclei from P21, but myofibre volume continued to increase, resulting in an estimated approximately 3-fold expansion of the myonuclear domain to 26.5 x 103 microm3 by P56. We also used our two mouse models to determine the number of satellite cells per myofibre during postnatal growth. Satellite cell number in EDL was initially approximately 14 satellite cells per myofibre at P7, but then fell to reach the adult level of approximately 5 by P21.
Postnatal fast muscle fibre type growth is divided into distinct phases in mouse EDL: myofibre hypertrophy is initially supported by a rapid increase in the number of myonuclei, but nuclear addition stops around P21. Since the significant myofibre hypertrophy from P21 to adulthood occurs without the net addition of new myonuclei, a considerable expansion of the myonuclear domain results. Satellite cell numbers are initially stable, but then decrease to reach the adult level by P21. Thus the adult number of both myonuclei and satellite cells is already established by three weeks of postnatal growth in mouse.
小鼠出生后的生长迅速，在最初几周内骨骼肌总量会增加数倍。肌肉生长可通过增加肌纤维数量、增大单个肌纤维尺寸或两者结合来实现。生长过程中肌纤维肥大需要添加新的肌核，这些肌核由肌肉卫星细胞提供，肌肉卫星细胞是骨骼肌的驻留干细胞。
在此，我们报告了小鼠趾长伸肌（EDL）出生后肌纤维生长的动态变化，成年EDL肌主要由快肌II型纤维组成。我们发现，在出生后7天（P7）至56天（成年）之间，EDL肌的肌纤维数量没有净增加。然而，在这两个年龄段之间，肌纤维横截面积增加了7.6倍，长度增加了1.9倍，导致肌纤维总体积增加了14.1倍：这显示了出生后阶段肌纤维肥大的程度。为了确定在这个强烈的肌纤维肥大期肌核数量如何变化，我们使用了两种互补的小鼠模型：3F-nlacZ-E小鼠仅在肌核中表达nlacZ，而Myf5nlacZ/+小鼠在卫星细胞中有β-半乳糖苷酶活性。在出生后3天（P3）至21天之间，每个肌纤维的肌核数量增加了约5倍。因此，肌纤维肥大最初伴随着大量肌核的添加。尽管如此，在出生后7天至21天之间，估计的肌核域仍增加了一倍，达到9.2×10³立方微米。从出生后21天起没有进一步添加肌核，但肌纤维体积继续增加，到出生后56天时，估计肌核域扩大了约3倍，达到26.5×10³立方微米。我们还使用这两种小鼠模型来确定出生后生长期间每个肌纤维的卫星细胞数量。EDL肌中卫星细胞数量在出生后7天最初约为每个肌纤维14个卫星细胞，但到出生后21天降至成年水平，约为5个。
小鼠EDL肌出生后快肌纤维类型的生长分为不同阶段：肌纤维肥大最初由肌核数量的快速增加支持，但在出生后21天左右停止添加核。由于从出生后21天到成年期显著的肌纤维肥大是在没有净添加新肌核的情况下发生的，导致肌核域大幅扩大。卫星细胞数量最初稳定，但随后减少，到出生后21天达到成年水平。因此，在小鼠出生后三周的生长过程中，成年期的肌核和卫星细胞数量就已确定。
