Figure 1.
Relationship between core cartilage patterning network and “bare bones” framework for limb development.
(A) Chicken embryo at 5 days of development with right fore-limb bud accentuated by staining of the embryonic flank with Eosin. (B) Schematic representation of core mechanism of chondrogenic pattern formation, comprising activator subnetwork (A, green circle), inhibitory subnetwork (I, red circle) and adhesive, matricellular and ECM molecules induced by A (violet). The molecules in the violet circle promote precartilage condensation, which induces chondrogenesis. Subnetwork A has positively autoregulatory properties; it also induces I, which in turn inhibits A. (C) Schematic representation of “bare bones” limb development model in a 2D template corresponding to a 5-day chicken wing bud. The template is divided into a LALI (lateral autoactivation-lateral inhibition) zone in which the reaction-diffusion process defined by A and I operates. Action of A within the LALI zone is suppressed by a diffusible signal (FGF, red-to-pink gradient), with its source at the AER (apical ectodermal ridge) at the distal end (right). At the low end of the FGF gradient a portion of the LALI zone, termed the active zone, is permissive for the production of the cartilage-promoting molecules shown in (B) (violet). Cartilage (blue) forms in cells that leave the LALI zone due to elongation of the limb bud.
Figure 2.
Simulation of chicken wing development.
(Left) Developmental progression of chicken forelimb between days 3 and 7 of development (indicated by the corresponding Hamburger-Hamilton stages). Early cartilage, including precartilage condensations, shown in light blue; definitive cartilage shown in darker blue. Based on ref. [23]. (Right) A sequence of snapshots from the simulation of normal limb development. See File S1 for the “standard” set of parameters used. Time is in arbitrary units, but can be used for comparison between different simulations.
Figure 3.
Dependence of the number of stripes on the kinetic parameters and
during the first phase of normal development.
During this period (0≤T≤1.4, T represents the time), the LALI zone shrinks in the P-D direction (see File S3). One stripe is formed for in the range 1500–1750 and
in the range 4.6–4.7, two stripes are formed when
is in the range 1000–1250 and
is in the range 4.6–4.8, and three stripes are formed for
from 1750–2000 and
from 4.8–4.9.
Figure 4.
(Left two columns) Drawings of AER removal experiments, based on Saunders's study [40]. Top images show an intact chicken wing bud at an early stage of development and the limb skeleton that it generates. Middle images show a wing bud at the same early stage with the AER removed, and the resulting limb skeleton, which attains a normal size but is truncated beginning at the elbow. Bottom images show a later stage wing bud whose AER has been removed. The resulting skeleton is truncated from the wrist onward. (Right column) Simulations of limb development using standard parameters. Top: AER (i.e., the source of suppressive FGF morphogen) left intact; normal development results. Middle: AER deleted early during the simulation. Bottom: AER deleted later during the simulation. All simulated limbs were allowed to develop for the same time.
Figure 5.
Simulations of effect of limb bud distal expansion.
ZPA graft (left) expanded chicken wing bud that results from anterior graft of an ectopic zone of polarizing activity from the proximal posterior region of another wing bud (normal limb profile at this stage shown in red); (center) resulting cartilage skeleton, with mirror-image duplication; (right) end-stage of simulation with distal expansion corresponding to that shown on left. Shh−/−, Gli3−/− (left) expanded mouse forelimb bud in embryos null for both Shh and Gli3 (normal limb profile at this stage shown in red); (center) resulting skeleton, with supernumerary digits [61]; (right) end-stage of simulation with distal expansion corresponding to that shown on left. talpid2 (left) expanded wing bud of chicken embryo homozygous for talpid2 mutation; (center) cartilage skeleton formed from such a limb bud later during development; (right) end-stage of simulation with distal expansion corresponding to that shown on left. Dogfish (left) shape of the pectoral fin-bud in an embryo of the dogfish Scyliorhinus torazame; (center) cartilaginous fin skeleton formed from such a limb bud [5]; (right) end-stage of simulation using a limb bud contour like that shown on left. In each of these simulations reaction parameter values different from the standard ones were used (see File S4 for details).
Figure 6.
Relation between A-P width and number of stripes.
(a) Simulation outcomes. Here and
. The final time is
. Color legend: black corresponds to 5.5, white to 0.0; (b) graphical representation.
Figure 7.
Simulation of fossil limb skeletons.
A selection of limb skeletal patterns from fossil specimens (left), were simulated by employing hypothetical developmental scenarios. The end-stages of the simulations of two lobe-finned fish, Sauripterus and Eusthenopteron, and two forms that are transitional between those organisms and amphibians, Panderichthys and Tiktaalik, are shown on the right. Snapshots of the full hypothetical sequence for the ichthyosaur Brachypterygius are shown in File S5, as are details of the simulations. Drawing of Brachypterygius paddle adapted from [63]; drawings of the fossil fish fins based on [62].