Fig. 2. (A) Normal chick wing at 10 d development stained with alcian green to show skeleton laid down in cartilage. Digit pattern 234. (B) Chick wing with mirror-image duplication following a polarizing region graft to anterior margin. Digit pattern 432234.

Table 1

Tissues with Polarizing Activity

Posterior margin of limb bud, chicken, quail, mouse, snapping turtle, human (1,2,12) Hensen's node, chicken, mouse (13,14) Presumptive flank tissue, chicken (11,15) Floor plate of neural tube, chicken, mouse (14,16) Amnion, chicken (17) Mesonephros, chicken (1) Tail bud mesenchyme, chicken (18) Genital tubercle, mouse (19) Gut epithelium, chick (20)

but when the graft is left for longer, then the formation of more posterior digits, such as digit 3, results (22). Polarizing signaling can be reproduced by beads soaked in either retinoic acid or sonic hedgehog protein (23,24). Many of the tissues that have polarizing activity have been shown to be able to generate retinoic acid and to express sonic hedgehog (reviewed in ref. 5).

Limb duplications can be obtained when just the mesenchyme cells from the limb posterior margin are grafted without the normally associated ectoderm and/or apical ridge, provided that grafts are placed in contact with the apical ridge of the host limb. In the normal limb bud, apical ridge and ectoderm maintain polarizing activity of posterior mesenchyme (25). Sonic hedgehog transcripts are found in posterior mesenchyme, and the distribution of these transcripts closely follows maps of polarizing activity in developing limbs (26). The posterior part of the apical ridge expresses Fgf-4, and FGF-4 maintains sonic hedgehog expression in posterior mesenchyme (27).

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