Lecture notes for Friday, March 9, 2018
Limb bud development
Cartilages differentiating inside a living chicken hind-limb bud
Limb buds develop into legs, arms, wings, or fins.
But are there ways to induce salamander embryos to form three legs on each side?
(Like the Martians in "John Carter of Mars"!)
Surgically implanting otic or olfactory placodes, or plastic beads containing Fibroblast Growth Factor, will cause three legs to form on the same side.
Scientists suspect that embryos re-use the same inductive signals in different parts of the body; and inductively competent tissue is already concentrated along both flanks.
If myotomes are removed, limbs will still form, but they will lack muscles.
Limb buds develop if transplanted to other parts of the body, and to the chorioallantoic membrane of chicken embryos.
You can also split limb buds, fuse limb buds, rotate them upside down, or backwards.
If you cut an early limb bud in two, it can form 2 arms (or legs), or often it will form a branched leg (mirror images).
Fusing two limb buds can often form only one leg.
Notice the similarity to what Driesch discovered! That splitting early echinoderm embryos in two results in each half developing into a half-sized "scale model" embryo.
Several body organs have been discovered to form two organs, for example two complete hearts.
Another example is eyes, and also the nose rudiment, which can be either split into 2, or two can be fused into one.
The human uterus also forms by side to side fusion of the lower end of the oviduct. (Müllerian Duct).
[Be prepared for an exam question: Describe at least ? embryonic organs that develop by fusion of two masses of cells. List examples of organs or organisms being split into two, where each half is able to form all parts normally developed when they are not split.]
Some kinds of parasitic worms often split limb buds in frogs, resulting in 5 or 6 hind limbs!
Development of the 3 geometric axes are controlled separately in limb buds And at 3 different times, by 3 different sets of chemical signals
During a later period of development, limb buds are no longer able to regulate (change) their anterior-posterior axis, but can still regulate the dorso-ventral axis!
In effect, that means that the part that would have developed into the palm of the hand can change its fate, and develop the geometry appropriate to the back of the hand, and vice versa, but a rotated limb bud can no longer change where the thumb forms, etc.
Analogous surgical experiments were done with the inner ear (otic placode) cutting it out and grafting it back in, upside down and backwards, etc.
Before a certain stage, it "regulates" completely, in the sense that the different cells change how they develop according to their new orientations, such that the semi-circular canals, otoliths, etc. all develop at locations that are normal with respect to the rest of the body.
At a later stage of development, the dorso-ventral and medio-lateral axes are still able to regulate, but the anterior-posterior axis is irreversible.
These rotation experiments were done in salamanders, in the 1910s-30s.
The Apical Ectodermal Ridge (AER) is a thickening of the epidermis
that forms along the outer rim of the limb bud
In frog embryos, the AER is comparatively small and indistinct, and for many years, everyone believed that frogs didn't have an AER. But more careful studies found them. The scientific meeting at which this evidence was first presented was in Wales in 1971, & I was in the audience!
Surgical removal of the AER causes failure of the distal structures to develop. Cut the AER off early, then no elbow, forearm, wrist or hand. Cut the AER off later, then no wrist & hand.
This was discovered by Prof. John Saunders, who taught for many years at the State University of New York in Albany, and who visited this department a few years ago, and is one of the most stimulating scientists I have ever met.
In sections of bird AERs, the epithelial cells seem to be pinched together at their basal ends, and the ridge is as much a "pucker" as a thickening.
Sections through mouse embryo AERs show a different geometry. It is definitely a thickening, but with some indication of contraction at Both basal and apical surfaces.
Here is a photograph of an oblique, or slanted section through a chicken AER.
The developing tail fins of fish have a similar thickened ridge.
Grafting an extra AER can cause a double wing to form. A mutation in chickens causes double AERs and double wings!
But the AER doesn't itself become any these structures.
It somehow signals to them to form.
There has been much research and several theories on this subject.
Fibroblast growth factor 10 seems to be the key signal molecule.
FGF10 can replace the effect of the AER;
(If anybody has done this with snake embryos, I haven't heard about it!
Legless lizards and legless salamanders might also be subject to such experiments.
Another interesting fact is that salamanders (especially newts) are the only kinds of vertebrates that can regenerate cut off legs (more on this in the next lecture), and also are the only kind of vertebrate that don't form an AER.
However, during regeneration of salamander legs, an epithelial thickening is formed at the tip of the regenerating limb stump! This thickening is not elongated, however.
I suggest to you that some big conceptual breakthrough is waiting for you to discover!
back to syllabus