Twice a week, I do an hour of weight training to maintain muscle strength and tone. Millions of Americans do the same, and there’s always a lot of attention paid to those upper arm muscles—the biceps and triceps. Less appreciated is another arm muscle that pumps right along during workouts: the brachialis. This muscle—located under the biceps—helps your elbow flex when you are doing all kinds of things, whether curling a 50-pound barbell or just grabbing a bag of groceries or your luggage out of the car.
Now, scientific studies of the triceps and brachialis are providing important clues about how the body’s 40 different types of limb muscles assume their distinct identities during development . In these images from the NIH-supported lab of Gabrielle Kardon at the University of Utah, Salt Lake City, you see the developing forelimb of a healthy mouse strain (top) compared to that of a mutant mouse strain with a stiff, abnormal gait (bottom).
In each strain, you see the lateral triceps and brachialis muscles (purple), other types of muscle (red) and tendons (green). However, in the healthy mouse, the lateral triceps and brachialis muscles are distinct, which gives the forelimb its flexibility; while in the mutant mouse, the two muscles are fused and indistinct, limiting the forelimb’s function.
The mice with the abnormal lateral triceps and brachialis have a mutation in a gene called Tbx3, which codes for a transcription factor that switches other genes off and on. If you follow this blog, you know that a lot of exciting research is currently focused on transcription factors, including how precise combinations of transcription factors can turn skin cells into blood stem cells or be used to make neurons.