Six digit stumps (of total 48 examined) show regenerative response. The most dramatic two are presented in Figure 4. The remaining four are presented here. wpa: week post-amputation, P1: phalange 1, P2: phalange 2, P3: phalange 3, s: sesamoid bone. (a) An uncut digit, shown for a comparison. Magnified is the P2/P3 joint area to highlight key morphological markers: the knobby epiphyseal cap of P2 and the sesamoid bone embedded in the tendon on the flexor side of P2. (b) Digit stumps from control mice show either bone stump histolysis (top and middle, phenotype 1) and no visible changes in bone stump (bottom, phenotype 2). (c–f) Digit stumps from treated mice that show regenerative response. (c) In this digit, the amputation removed all P3 by a cut through the joint. At 7 wpa, the P2 stump is reduced, but recovered the epiphyseal-like end (red dashed line)—marked by solid curved shape, as opposed to irregularly shaped histolyzing bone. (d) In this digit, the amputation removed a significant portion of P2 and the sesamoid bone. The P2 stump does not regain an epiphyseal end (the end is concave and irregular). However, the sesamoid bone is reformed, as identified by its location on the flexor side of P2 and wingnut shape (Wirtschafter and Tsujimura, 1961) under the microscope. The recovery of sesamoid bone is non-trivial, as digit sesamoids form in juxtaposition to the condensing phalange, detaching from the phalange by formation of a cartilaginous joint (Eyal et al., 2019). (e) In this digit, the amputation removed a significant portion of P2 and the sesamoid bone. At 7 wpa, the P2 stump appears to be reforming an epiphyseal, rounded end (red dashed line). There is a small bone distal to P2, whose curvature articulates with the P2 end, but there are not enough morphological characters to identify the bone. (f) In this digit, the amputation removed the epiphyseal cap of P2 and the sesamoid bone. The P2 stump appears to have lost some mass, but reforms an epiphyseal-like end (red dashed line). There is an additional small bone located where the sesamoid bone should be, but lacks sufficient morphological characters to identify.