Figure 3.

Cleavage of Muscle Bundle Is a Step in the Morphogenesis of Only Some Muscles

Consecutively staged dorsal views of four E13.5 forelimb zeugopods indicate the progressive cleavage of the single extensor carpi radialis (ECR) bundle into two distinct ECR longus and ECR brevis muscles. Muscles (blue/purple) are stained by whole-mount immunohistochemistry to detect Myosin.

(A) In the most immature specimen, cleavage of the single extensor carpi radialis (ECR) bundle has started at the distal end (black arrow).

(B) Cleavage extends from distal to proximal and is almost complete.

(C) Separation of the single bundle into two discrete units is complete at the proximal end of the bundles (black arrows).

(D) At the end of the cleavage event, two distinct ECR longus and ECR brevis bundles are formed. Scale bar, 200 μm.