Figure 5.4
Molecular validation of the basic premises in Meinhardt's Boundary Model.
a. The Boundary Model II (as revised by Meinhardt in 1983 [2808]). In this version (cf. Fig. 5.3) there are 3 new morphogens (A, B, C). Each of them arises at an interface (P/V, V/D, D/P) where region-specific molecules (P, V, D) interact after diffusing a short distance into the adjacent region. A, B, and C then react at the center to form the tip morphogen T. Finally, T diffuses to form a conical gradient that provides the radial coordinate of a polar system, while the angular coordinate is specified by the graded concentrations of A, B, and C. The gradients must overlap (not shown) in order to encode every angle uniquely.

b. According to this model, a VP fragment should duplicate because V and P will create a second source of A (A') when they heal together.

c. Hh-Dpp-Wg Model. Developing legs use a strategy like Meinhardt's model, though the first induction involves one-way (not bidirectional) diffusion (arrowheads), and there are only 2 (not 3) long-range morphogens. Cells in the Posterior Compartment (PC) make Hedgehog (Hh) -- a short-range signal. When Hh diffuses across the A/P border, cells in the ventral AC (vAC) respond by making Wingless (Wg), while dorsal (dAC) cells make Decapentaplegic (Dpp).

How Wg and Dpp travel is not known. One idea (Arc Scenario, above) is that they move in arcs to form curved gradients, with subsequent steps (e.g., specification of angular values) as in Boundary Model II [1807]. The gene Distal-less (Dll) is shown being activated above a threshold (5) in the T gradient (assumed to be 10-to-1 centrifugally). Another idea (Cloud Scenario, below) is that Wg and Dpp diffuse randomly [618, 2456], in which case they could activate downstream genes directly (rightmost arrow) where their clouds overlap (via the rule Wg + Dpp = Dll) without any intermediate tip morphogen [2813]. Indeed, one Wg target gene (Dfz3) does appear to be expressed in a parabola-shaped domain [3977]. However, defect arcs in genetic mosaics [1811] are more easily explained by the former scheme (see text). The Arc Scenario may also apply to the developing notum (cf. Fig. 6.14), where the dpp-ON stripe is at a ~30° angle relative to where its target gene (wg) is turned ON [4369].

Panels a and b are based on [2808] (p. 381, para. 2), and c is adapted from [618, 1807, 1991, 2456].