Apical ectodermal ridge and proximal-distal limb outgrowth
The apical ectodermal ridge (AER) is a thickened strip of tissue at the tip of the growing limb, and its growth-promoting signals keep the limb lengthening outwards.
Remember
The ridge at the tip keeps the growing limb lengthening, and the earlier its signal is lost, the shorter the limb will be.
Memory aid
count the bones, one, then two, then many
Use it to recall the order of the stylopod, the zeugopod and the autopod.
A limb bud (the early form of the limb) lengthens from its tip, and the apical ectodermal ridge sits at the far, or distal, rim of the bud. The ridge is a signalling ectoderm, while the mesoderm forms the core of the bud. It does not turn into bone, because the humerus and femur come from that mesoderm core, the tissue the ridge keeps growing.
The ridge works through fibroblast growth factor (FGF), a growth-promoting messenger protein. Its signals keep the mesenchyme beneath it, the immature connective tissue at the tip, dividing, so the bud keeps lengthening along the proximal-distal axis, the line from the body outwards to the fingertips.
As the bud lengthens, its parts appear in order. The stylopod comes first and holds a single long bone, the humerus of the arm or the femur of the thigh. The zeugopod follows with paired bones: the radius and ulna in the forearm, or the tibia and fibula in the lower leg. The autopod comes last and is the hand or foot, with many bones.
The time at which the ridge signal is lost decides the level at which the limb stops. If the signal is lost early, outgrowth arrests when only proximal structures have been established, so the limb stops early and is truncated, left with only proximal parts. Later loss still allows the more proximal and intermediate parts to form but removes structures that still needed continued distal growth.
Growth and identity are related, but a limb needs both growth and positional information. Other signals tell the tissue whether it occupies anterior, posterior, dorsal or ventral territory, but those positional signals cannot make up for distal growth that has already stopped.
Key facts
- Structure
- The apical ectodermal ridge is a thickened strip of ectoderm along the distal rim of the limb bud.
- Signal
- Its fibroblast growth factor signals keep the mesenchyme beneath it dividing, so the limb lengthens along the proximal-distal axis.
- Order of parts
- The stylopod forms first, then the zeugopod, then the autopod: one bone, then two bones, then many bones.
- Timing rule
- The earlier the ridge signal is lost, the more proximal and severe the truncation.
- Missing autopod
- A limb with a normal stylopod and zeugopod but a missing autopod suggests that the ridge signal was lost late, after the proximal and middle parts were laid down.
- Positional signals
- Positional signals cannot make up for distal growth that has already stopped.
Exam traps
- The apical ectodermal ridge does not turn into bone, because it is a signalling tissue at the far rim of the limb bud; the humerus and femur come from the mesoderm core that the ridge keeps growing.
- Positional signals cannot replace the ridge, because they tell cells their position but do not sustain distal outgrowth once the ridge fails early, so the limb is truncated.
- The more severely truncated limb is not the one that lost the ridge signal later, because proximal structures form first: the earlier the signal is lost, the more severe the truncation.
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