Figure 4.

Positional information can be altered pharmacologically and is harbored by specific cell types. (A) A regenerated limb from an axolotl that had been bathed in retinol palmitate for 15 days after amputation. A serial duplication of the upper arm, lower arm, and hand has arisen from a wrist-level amputation. (Panel A from Maden 1982; reprinted, with permission, from Springer © 1982.) (B) Schematic depicting the interpretation of the experiment in A. Retinoids proximalize blastema cells, but not positional memory in steady-state cells. Proximalized wrist blastema cells initiate regeneration from the upper arm level, appearing to violate the rule of distal transformation. (C) Connective tissue-derived blastema cells were labeled transgenically with GFP (green). When distal GFP-labeled cells were transplanted to an unlabeled, upper arm stump, the GFP-labeled descendants contributed to hand elements, but not more proximal limb elements. Thus, connective tissue lineages harbor positional memory. (D) Similar experiment as C but performed with GFP-labeled muscle lineages. Distal muscle transplanted to an upper arm stump contributes to all arm segments. Thus, muscle does not harbor positional memory. (Panels C and D from Nacu et al. 2013; reprinted, with permission, from Company of Biologists © 2013.) (E) Positional memory-encoding cell types in the axolotl limb. (Summarized from Carlson 1974, Kragl et al. 2009, and Nacu et al. 2013.)