Mechanical cues from muscle contraction regulate TGFβ signaling and epitenon formation during embryonic tendon development.

Muscle loading is required for embryonic tendon growth; however, the underlying mechanisms that regulate tendon development downstream of mechanical cues remain unidentified. Although tendons in muscle paralysis models are structurally and functionally inferior, whether these differences arise from cell or matrix deficits remains unclear. Analysis of muscular dysgenesis embryos by atomic force microscopy showed that structural and functional deficits in paralyzed tendon arise in part from reduced proliferation and collagen fibril disorganization. Bulk and single cell transcriptional analyses reveal that both collagenous and non-collagenous extracellular matrix components, as well as cytoskeletal and actomyosin-associated proteins, are dysregulated in mdg tendons, whereas tendon markers remain unchanged. Surprisingly, we find that an arrest of TGF{beta} signaling occurs during normal embryonic tendon growth and that TGF{beta} signaling is abnormally prolonged in paralyzed embryos. We also show for the first time, that specification of the epitenon depends on muscle contraction. Together, these findings establish cell and molecular requirements for muscle contraction in embryonic tendon development. TeaserMuscle contraction is required for embryonic tendon development through regulation of TGF{beta} signaling, epitenon formation, and matrix organization.