Tbx5 is essential for initiation of the forelimb, and its deletion in mice results in the failure of forelimb formation. Misexpression of dominant-negative forms of Tbx5 results in limb truncations,suggesting Tbx5 is also required for forelimb outgrowth. Here we show that Tbx5 is expressed throughout the limb mesenchyme in progenitors of cartilage, tendon and muscle. Using a tamoxifeninducible Cre transgenic line, we map the time frame during which Tbx5 is required for limb development. We show that deletion of Tbx5 subsequent to limb initiation does not impair limb outgrowth. Furthermore, we distinguish two distinct phases of limb development: a Tbx5-dependent limb initiation phase, followed by a Tbx5-independent limb outgrowth phase. In humans, mutations in the T-box transcription factor TBX5 are associated with the dominant disorder Holt-Oram syndrome (HOS), which is characterised by malformations in the forelimb and heart. Our results demonstrate a short temporal requirement for Tbx5 during early limb development, and suggest that the defects found in HOS arise as a result of disrupted TBX5 function during this narrow time window.

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