Eventually, we're going to have really tall super mice with all the experiments that go on involving mice.
Lengthening of mouse hindlimbs with joint loading
"For devising clinical approaches to treating limb length discrepancies, strategies that will generate differential longitudinal growth need to be improved. This report addresses the following question: does knee loading increase bone length of the loaded hindlimb? Knee loading has been shown to induce anabolic responses on the periosteal and endosteal surfaces, but its effects on longitudinal bone growth have not yet been examined. In the present studies, loads were applied to the left hindlimb (5-min bouts at 0.5 N[at 5Hz) of C57/BL/6 mice (21 mice, ~8 weeks old). Compared to the contralateral and age-matched control groups, knee loading increased the length of the femur by 2.3 and 3.5%, together with the tibia by 2.3 and 3.7% (all P < 0.001), respectively. In accordance with the length measurements, knee loading elevated BMD and BMC in both the femur and the tibia. Histological analysis of the proximal tibia revealed that the loaded growth plate elevated its height by 19.5% (P < 0.001) and the cross-sectional area by 30.7% (P < 0.05). Particularly in the hypertrophic zone, knee loading increased the number of chondrocytes (P < 0.01) as well as their cellular height (P < 0.001) along the length of the tibia. Taken together, this study demonstrates for the first time the potential effectiveness of knee loading in adjusting limb length discrepancy."
"The total length increase in the growth plate was more than the sum of the increases in the proliferative and hypertrophic zones, indicating that other regions such as the resting and calcifying zones were also affected"
According to Determining optimal loading frequencies for bone formation on joints using Finite Element Methods, "the maximum bone formation rate was observed with loading frequencies at 5 – 10 Hz in the tibia and 15 – 20 Hz in the femur". "We hypothesized that most effective induction of bone formation is achieved when knee loading is applied at a lowest natural frequency of each of the tibia and femur." "The critical results indicated are that the natural frequency for the femur is 23.574 Hz and 12.085 Hz for the tibia."
Unfortunately, the mice in this study were 8 weeks old, however, it has been reported to work on mice of up to 16 weeks old. Much of the benefit occurred from increasing the number of chondrocytes and the cellular height of those chondrocytes(See the LSJL growth plates). Knee loading provides a lateral load to the knee(0.5N which is pretty small but remember mice are small as well). The benefit seems to be as a result of the increase in hydrostatic pressure(which promotes chondrogenic differentiation) rather than a result of microcracks in the osteons increasing height.
The boundary of the growth plate cells also changed indicating that new stem cells were differentiating. If the growth plate did not involve independent cells then one would except the boundaries between zones to remain constant despite an acceleration of growth.
It's not the knee that's being loaded(the patella) but rather it's the epiphysis of the tibia and femur that's being loaded. Within the epiphysis of long bones there is bone marrow which contains mesenchymal stem cells. By laterally loading the end of the epiphysis you are distracting the epiphysis, increasing hydrostatic pressure, and causing microfractures in the trabecular bone in the epiphysis of the long bone. The increase in hydrostatic pressure is the most likely height increasing effect.
Another possibility is that lateral synovial joint loading causes a distraction force in the cortical bone but the very low loads used makes it unlikely that the distraction would be sufficient to cause the mice to grow taller.