Fig. 1. Fetal digit tip formation. (A) An Alcian Blue stained stage 11 digit (E14.5), showing the chondrogenic structure of the digit at the time of amputation. tp, terminal phalanx. (B-F, K-L) In situ hybridization of sagittal sections of neonatal digits. Left side of each image is dorsal; top is distal. (H-J) In situ hybridization of frontal sections of the E14.5 digits. The top of each image is distal. (B) Msx1 transcripts are localized to the loose connective tissue subjacent to the nail organ (n) and surrounding the dorsal region of the terminal phalanx in the P7 digit. (C) Msx2 transcripts are localized in the nail bed and nail matrix of the P7 digit. (D) Bmp4 transcripts are expressed in dorsal loose connective tissue cells beneath the nail bed in the P2 digit. (E) Ihh is expressed at the distal tip of terminal phalanx in the newborn digit. (F) Hoxc13 is expressed in the nail bed and nail matrix of the newborn digit. (G) Whole-mount in situ hybridization of Msx1 in the stage 11 autopod. (H) Msx1 is expressed in the apical mesenchymal cells surrounding the forming terminal phalanx. The line indicates the amputation level that elicits a regeneration response. Digit amputation at a level proximal to this line does not elicit a regeneration response. (I) Msx2 is expressed in the apical epidermis and in mesenchymal cells subjacent to the epidermis. (J) Bmp4 is expressed in apical mesenchymal cells in a domain similar to that of Msx1. (K) Ihh is expressed in digit tip cells, initiating endochondral ossification of the terminal phalanx (arrow). (L) The nail organ marker, Hoxc13, is expressed in the distal epidermis associated with presumptive nail tissue (arrow).