The SIX1 gene plays a critical role in hypaxial muscle differentiation in myogenesis.
2.
In mice lacking this gene, severe muscle hypoplasia affected most of the body muscles, specifically hypaxial muscles.
3.
Azizi " et al . " found that longitudinal contractions of the constant volume hypaxial muscles were compensated by an increase in the dorsoventral dimensions.
4.
The exception is " Trichosurus ", which remarkably among marsupials has shifted the hypaxial muscles from the epipubic to the pelvis, exploying a more placental-like breathing, having lost the benefits of the epipubic in regards to lung ventilation.
5.
Only placentals, and possibly the early mammaliformes " Megazostrodon " and " Erythrotherium ", lack them; in thylacines and sparassodonts, they appear to have become primarily cartilaginous and the osseous element has become strongly reduced or even absent . " Trichosurus " mimicked placentals in shifting hypaxial muscles attachment sites from the epipubic to the pelvis, losing the respiratory benefits ( see below ), but otherwise retains large epipubics.
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