The talpid² chick limb has weak polarizing activity and can respond to retinoic acid and polarizing zone signal

Abstract

The tuZpi8 @a2) chick mutant has wide, polydactylous wings and legs. TuZpid2 limb cartilages have abnormal morphology and a very subtle anteroposterior polarity. Specifically, posterior ta2 limb structures are identifiable, while more anterior cartilages are less distinctive. Here, we investigate the development of anteroposterior limb pattern in the tu2 embryo. We show that tu2 posterior limb bud mesoderm is capable of respecifying the anteroposterior axis of a normal wing. However, the average duplication obtained after grafting a tu2 polarizing region was significantly less than the average duplication formed after a graft of normal wing bud polarizing zone. Thus, polarizing activity appears to be weak in tu2. Grafts of normal polarizing zone to the posterior edge of tu2 wing buds had no effect on the tu2 phenotype. This result suggests that a weakly functioning polarizing signal does not account for the altered anteroposterior polarity in tu2 limbs, and that normal polarizing zone activity is not sufficient for formation of normal limb bud cartilages. We demonstrated that transmission of a polarizing signal through the tu2 limb mesoderm was normal. In addition, tu2 anterior border mesoderm had no polarizing activity. We also assessed the ability of tu2 limb bud mesoderm to respond to a polarizing signal. Either normal polarizing zone tissue or a bead containing retinoic acid was placed at the anterior edge of tu2 wing buds at stages 18-23. Both polarizing zone and retinoic acid caused respecification of the tu2 wing anteroposterior axis. The result was that a tu2 ulna replaced the radius, and the most posterior digit was duplicated anteriorly. Limb cartilages with normal morphology never formed. When a bead containing retinoic acid was placed at the posterior edge of tu2 wing buds, there was no effect on anteroposterior pattern. However, beads with retinoic acid always caused a reduction in the number of tu2 wing digits which formed, whether the beads were placed at the anterior or posterior edge of the developing tu2 limb.

Expression of genes encoding bone morphogenetic proteins and sonic hedgehog in talpid (ta3) limb buds: Their relationships in the signalling cascade involved in limb patterning

“…The tu3 limb phenotype resembles the phenotype of another mutant tu2 (Dvorak and Fallon, 1992). Both are polydactylous, with digits that lack obvious a-p patterning and that are partially fused across the a-p axis.…”

“…In the tu2 mutant, polarizing activity is undetectable in the anterior mesenchyme in contrast to the tu3 mutant. Furthermore, in tu2 limbs some of the cartilage elements such as the radius and ulna are distinguishable, and following a polarizing region graft into the anterior mesenchyme of a tu2 limb bud, some degree of re-patterning is evident as the radius is respecified to form the ulna (Dvorak and Fallon, 1992). In the tu2 mutant both grafts of polarizing region and application of retinoic acid result in the reduction in the number of digits that develop, whereas in tu3 mutant a reduction in the number of digits was only observed with retinoic acid.…”

Talpid2 limb bud mesoderm does not express GHox‐8 and has an altered expression pattern of GHox‐7

“…Talpid2 limbs have subtle anteroposterior polarity (Dvorak and Fallon, 19911, and ta2 limb buds form mirror-image duplicated limbs when a polarizing zone or retinoic acid is grafted to the anterior limb bud border (Dvorak and Fallon, 1992). A subtle gradient of anteroposterior GHox-7 expression, which should reflect the subtle anteroposterior polarity in tu2 limbs, was not detectable in tu2 limb buds.…”

“…While tu2 limb buds have a polarizing zone, it is weaker than a normal polarizing zone in its ability to induce supernumerary outgrowths when grafted to the anterior border of a normal limb bud (Dvorak and Fallon, 1992). Because it has been proposed that the polarizing zone inhibits expression of GHox-7 and GHox-8, and the tu2 polarizing zone should be less efficient at inhibition, GHox-7 and GHox-8 expression should increase in tu2 limb bud mesoderm.…”

John F. Fallon, PhD: Fifty years of excellence in limb research and counting

“…• Beginning in the early 1990s, John's laboratory defined many aspects of the molecular hierarchy of pattern formation in the developing limb (Coelho et al, 1991;Dvorak and Fallon, 1992;Krabbenhoft and Fallon, 1992;Ros et al, 1992Ros et al, , 1993Riley et al, 1993;Savage et al, 1993). His laboratory was among the first to demonstrate that FGF is the critical limb bud outgrowth signal, a finding that has led to many subsequent discoveries of the nature of this signaling pathway (Savage et al, 1993;Fallon et al, 1994).…”