Analysis of the tendon cell fate

The functional integrity of the musculoskeletal system requires precise and elaborate attachment of muscles to their respective skeletal elements in order for the force generated during muscle contraction to be transmitted faithfully to the skeleton. The collagen-rich muscle attachments range from narrow bands of connective tissue in some axial muscles to the long limb tendons (Benjamin and Ralphs, 2000). Most studies of differentiation and patterning of the musculoskeletal system in vertebrates have focused on the skeletal elements, which can be easily seen in cleaned and stained preparations, and on the muscles, whose differentiation have been particularly well studied on a molecular level. However, while tendons and other muscle attachments are of equal functional importance, surprisingly little is known about their genesis. To date, most developmental studies of tendon formation have concentrated on the limb tendons, and in particular, on the morphogenesis of the distal autopod tendons (reviewed by Benjamin and Ralphs, 2000). The patterning of specific distal tendons is preceded by an extracellular matrix (ECM) scaffold that is present in the hand plate from stage 27 onwards (Hurle et al., 1989). This scaffold is found on both the dorsal and ventral sides of the limb and consists of a ‘mesenchymal lamina’ (ML), which is parallel to and in contact with the ectodermal basement membrane. A complex fibrillar system extends in a dorsoventral orientation from the ML and serves as a scaffold for the forming tendons (Hurle et al., 1990). During these stages, a number of tendon-specific genetic markers have been described, all of which are expressed in a broad sub-ectodermal layer, in cells above and below the developing cartilage. These include EphA4 (Patel et al., 1996), TGFβ2 (Merino et al., 1998), follistatin (D’Souza and Patel, 1999), Six1 and Six2 (Oliver et al., 1995), and Eya1 and Eya2 (Xu et al., 1997). In the case of the Six and Eya genes, distinct family members are expressed in the forming dorsal extensor and ventral flexor tendons. As the morphogenesis of the tendons proceeds, these genes exhibit complex and dynamic changes in their expression patterns, ultimately resolving to specific regions of the tendons, for example, follistatin is found near the insertion of the tendon, while EphA4 becomes localized to the body of the tendon (D’Souza and Patel, 1999). 3855 Development 128, 3855-3866 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV2676