SUMMERS, A. P.; KOOB, T.J.: The evolution of tendon

Tendons in tetrapods have been well characterized and form the basis for comparison to the more basal goups. In a functional sense tendon has the same evolutionary history as muscle because even in the most basal invertebrates there is a junction between muscle and the tissue acted on by the muscle. From a morphological point of view the origin of tendons is more complex. There are clear tendon analogs (apodemes) in arthropods, and in the more basal vertebrates, the cepholochordates and urochordates, muscles are segmented and therefore divided by tendinous sheaths. However, the earliest appearance of linearly arrayed tendon as we know it from mammals is in the craniates. In the hagfishes (agnathans) there is the first example of a well-defined, linear tendon. This tendon connects the tongue and tooth plates to the large retractor muscle. There are short, thin tendons in lampreys as well as tendinous sheets. Cartilaginous fishes possess both linear and sheet-like tendons. They also have an analog to mammalian meniscus in a tendon that experiences appreciable lateral loading. Bony fishes have linear tendons including many examples of tendons that ossify (sesamoids). In order to follow the evolutionary history of tendon we need a clearer idea of what distiguishes tendon from other connective tissue.