Determinants in the morphogenesis of muscle tendon insertions.

The factors that normally determine the location and insertion of a muscle were explored in human experiments of nature with early problems in morphogenesis. Monozygotic conjoined twins for whom there could be no genetic determinants for muscle attachments at the sites of juncture served as one model; these attachments had to follow general principles of morphogenesis. A second type of problem involved absence of bone that presumably antedated muscle and tendon development (e.g., genetically determined radial aplasia). A third category included mechanical alteration of early limb position that may have occurred prior to the development of muscle attachments (e.g., early amnion rupture sequence). The dissection findings from all three types imply a general hierachy of muscle tendon attachments. Tendons appear to attach preferentially to bone. In the absence of the bone to which they would normally attach, they will attach to the next closest bone. If no such bone is available, tendons may attach to other tendons; and if no tendons is available, occasionally they will attach to the fascia of another muscle. If there is no connective tissue attachment site, there will be no muscle, implying a need for function in the development and preservation of muscle.