The stabilization of cartilage properties in the cartilage-forming mesenchyme of the embryonic chick limb.

Experiments have been done to determine the time in the development of the embryonic chick limb when the cells in the cartilage-forming area become "stabilized." The particular criterion for stability used in this work was the ability of the cells to resist the infiuences in the limb which cause some limb mesenchyme cells to form cartilage and other limb mesenchyme cells to form soft tissue. Blocks of cartilage-forming mesenchyme were transplanted from the cartilage-forming area of one limb to the prospective soft tissue area of a second limb. The host limb was permitted to develop for 48 hours, and then examined to determine if the implanted cells had formed cartilage outside of the normal cartilage pattern of the host limb. The embryos used as a source of the implanted blocks varied in stage from stage 22 to stage 27, the hosts also varied in stage from stage 22 to stage 27. It was found that a block of cartilage-forming mesenchyme generalIy would conform with the host limb pattern if the donor war stage 24 or younger and the host was stage 24 or younger. A block of cartilage-forming mesenchyme generally did not conform with the host limb pattern if the donor was stage 25 or older whatever the stage of the host. We conclude that the cartilage-forming cells become stabilized by this criterion between stage 24 and stage 25. It has been observed that the cartilageforming cells of the embryonic chick limb remain labile until surprisingly late in limb development (Searls, '67). The experiments described in that paper had been done to investigate the role of cell migration in the development of the limb. Blocks of mesoderm from the cartilage-forming areas of stage 23 (Hamburger and Hamilton, '51) and stage 24 embryonic chick limbs were implanted into the dorsal surface of the right wings of embryos growing in ouo. After a period of further development, the host limbs were examined to discover the fate of the implanted cells. No evidence of cell migration out of or through the implanted blocks could be detected. It was further observed that the implanted blocks became integrated into the host limb and that the implanted cells differentiated according to the host pattern, forming cartilage in the cartilageforming area of the host limb and soft tissue in the soft tissue area of the host limb. This lability of cartilage-forming mesenchyme from stage 24 limb buds was unexpected because extracellular metachromatic material appears in the cartilageJ. EXP. ZOOL., 170: 365-376. forming area of the limb during stage and