Limb Development - 18 Flashcards
Why have salamanders been used to study limb development? (3)
- They are able to withstand transplantation techniques.
- Their limbs are accessible due to the lack of extraembryonic membranes.
- They are unusual among vertebrates in being able to regenerate lost limbs.
Limb Disc
thickened area on the side of the salamander that will give rise to the limb, it is the primordium for the limb
Morphogenetic Field
peripheral region around the disc, can give rise to the limb if the disc is removed. If the limb field is removed, no limb will form.
if you remove half the limb disc…
the Remaining half will form a complete limb
If you Vertically divide the disc into two halves and physically separate the two halves:…
Each half will form a normal limb
If you Fuse two discs together…
A single normal limb will form.
initial initiators of limb development
*Tbx5 and Tbx4
Expression of Tbx5
initiates future forelimb
Expression of Tbx4 and Pitx-1
initiates future hindlimb
Expression of Gli3 and Hand 2
fix AP axis: Gli-3 (anterior) and Hand-2 (posterior)
Paraxial mesoderm signals
via retinoic acid based on a Hox code
Three axes define the vertebrate limb
dorsoventral axis
anterior-posterior axis
Proximodistal axis
Early research on establishment of axes showed:
two pics
Hoxb8
- Determines position of ZPA via its highest concentration
- Expression is induced by retinoic acid
Sonic hedgehog
- Signal produced by ZPA
- Also maintains structure and function of AER
Induces expression of gremlin:
- Inhibits BMP-2 which inhibits FGF-4 in AER
- Inhibits Gli-3 in posterior part of limb bud
Gli-3 (in anterior part of limb bud):
Inhibits expression of shhin anterior part of limb bud
Advantages of using chick embryos in limb development research:
Apical ectodermal ridge (AER):
Limb ectoderm in birds:
- Consists of two layers:
- Outer, continuous flattened periderm
- Inner simple columnar layer
- Separated by basal lamina from limb bud mesoderm
AER is located at the border between dorsal ectoderm and ventral ectoderm
- Dorsal ectoderm expresses radical fringe.
- Ventral ectoderm expresses Engrailed-1 (En-1)
In amphibians the thickenings of the lateral plate mesoderm and the formation of a mass of mesenchymal tissue under the epithelium may coincide closely with
the positions of the two pairs of limbs
In amniotes the thickenings and mesenchyme gatherings are continuous throughout the whole length of the body in the form of ____. The intermediate part later ___.
horizontal ridges (Wolffian ridges)
disappears
Apical Ectrodermal Ridge Function and Signaling
- Interacts with underlying limb bud mesoderm and promotes continued outgrowth of limb bud.
- Outgrowth-promoting signal = FGFs (FGF-8, FGF-4, FGF-9, FGF-17) and Wnts.
- Cells in posterior part of limb bud are under the influence of retinoic acid and differentiate into proximal limb components.
Limb Bud Mesoderm
- Involved in determination of actual limb morphology
- Removal of AER results in cessation of further outgrowth of the limb bud
What signal do cells in distal limb bud express?
Msx-1
In what is Hoxdgene expression in chick limb bud involved?
Involved in pattern formation in the proximodistallimb axis
What are the elements of typical tetrapod limbs?
Stylopod, proximal zeugopod, distal zeugopod, proxiaml autopod, distal autopod
Stylopod
- single proximal element
- forms humerus or femur
Proximal Zeugopod
- two parallel elements
- forms radius/ulna or tibia/fibula
Distal Zeugopod
- 12 elements
- forms carpals/tarsals
Proxiaml autopod
- five elements
- forms metatarsals/metacarpals
Distal Autopod
- variable number of elements
- form phalanges
The mesenchyme portion of the limb bud is determined when?
early—before closure of the neural tube
*If the limb bud mesenchyme is transplanted under the epidermis of the flank or head, a limb will form.
The structure of the developing limb in every case is determined by…
the nature of the mesodermal components
*Experimentally it is possible to combine the leg bud mesoderm with the epidermis of a wing bud and vice versa.
Effects of limb mesoderm on overlying ectoderm is an example of
instructive induction
*Normal wing bud mesoderm combined with mutant AER ectoderm fails to produce an AER
Saunders, Zwillig, and others experimented by removing the AER from a chick wing bud. What was the result?
Further outgrowth of the limb mesoderm ceases
If one grafts extra AER over limb mesoderm:
Additional limb forms
Mesoderm produces ___ that functions to ___.
Mesoderm produces an apical ectodermal maintenance factor (AEMF) that maintains the AER in a healthy state
AER maintains the mesodermal cells below it in a ___ and ___ state.4
What happens to limb growth?
AER maintains the mesodermal cells below it in a labile and mitotic state.
- Limb bud continues to grow distally.
- Various regions of the limb begin to appear in a proximal-distal direction.
Interactions between AER and Limb Mesoderm
One theory suggests that presumptive regions are present from the earliest stages but that the more distal regions are compressed into a narrow band a few cells deep. •Removal of the AER stops outgrowth of the presumptive regions within the narrow distal band. •As the limb bud lengthens, cells are left behind the distal labile band and their positional value is frozen by the number of mitotic divisions they have undergone. •Cells undergoing the fewest mitotic divisions are destined to form the more proximal regions of the limb.
Shaping of Limb via Apoptosis
Necrotic zones of programmed cell death develop within the limb bud mesoderm and will “carve” the shape of the limb.
- If cells from the potential necrotic zone are removed and cultured in vitro, the cells will die on cue.
- This suggests that apoptosis is a mechanism for morphogenesis.
- Apoptotic zones have been found in most tetrapods, including humans.
- Specific zones: •Posterior necrotic zone (area that will become the axilla) •Areas between future digits (not as active in animals such as ducks that have webbed feet in the adults).
Limb bud mesenchyme consists of cells derived from _____. To what will it give rise and to where will they migrate?
lateral plate mesoderm
- This mesenchyme will give rise to appendicular skeletal elements, connective tissue, and some blood vessels.
- Note that mesenchymal cells destined to form limb musculature will migrate into the limb bud from the somiticventral dermomyotomes.
Where do initial mesenchymal condensations (for appendicular skeleton) form?
Form in the proximal part of the limb bud, restricted to the formation of cartilage in the central core
Cascade, lots of factors wnt etc
What protein initiators are important to the joint formation pathway?
Noggin
BMP
What protein initiators are important to limb muscle formation?
Pax-3
Cadherin
Wnt-6
Tcf-4
After skeletal element condensations form, myogenic cells condense into two muscle masses called…
mass of flexor muscles
mass of extensor muscles
Name the three plexes of higher vertebrates
cervical
brachial
lumbar
The upper limb in humans is supplied by what spinal cord levels?
C5-T1 (the fifth, sixth, seventh, and eighth cervical levels and the first thoracic level)
Motor axons originate from spinal cord during the ___ week.
5th
If the forelimb rudiment of a salamander embryo is removed so that the limb fails to develop:
- The nerves of the brachial plexus remain smaller (thinner) than they would have been if the limb were still there.
- Spinal ganglia three, four, and five (associated with the brachial plexus in the salamander) are also smaller. The number of cells in each ganglion may be reduced by as much as 50 percent.
If an additional limb rudiment is transplanted to the flank of a salamander embryo:
- The local spinal nerves supply the innervationto the transplanted limb and these local nerves increase in thickness.
- Ganglia associated with these nerves increase in size; the increase in number of cells may be as much as 40 percent.
If the forelimb rudiment of an amphibian embryo, prior to the outgrowth of nerves, is cut out and transplanted to a position very near the original one:
The brachial nerves will deviate from their normal paths and will be deflected in the direction of the transplanted limb. If the distance of the transplanted limb from the original position is not too great, the brachial nerves will penetrate into the limb and ramify. The limb becomes fully functional.
If an additional limb is transplanted into the immediate vicinity of the host limb:
The brachial nerves will develop branches running out to the additional limb and will supply it.
If the normal path of the nerves is blocked by some obstacle:
- The outgrowing nerves may avoid the obstacle, go around it, and still reach their normal destination.
- Hamburger inserted a piece of mica into a frog embryo between the spinal cord and the region where the hindlimbrudiments were to develop. The nerves formed loops around the mica plate and still reached the hindlimbrudiments.
If the limb rudiment transplant is placed farther away from the normal limb site, or if the obstacle between the spinal cord and the limb rudiment is too great:
- The nerve fibers fail to be attracted to the limb.
- If the limb rudiment is placed on the flank of the embryo, it will attract the local spinal nerves. These nerves will grow into the limb, but they cannot provide for the normal limb function.
- It appears that only the areas of the spinal cord from which the nerves of the brachial and lumbar plexuses originate possess the properties necessary for controlling the functioning of the limbs.
What happens if you swap fore limb and hind limb buds?
The fore-and hindlimbcenters are interchangeable. You can transplant forelimb buds in place of hindlimbbuds
What happens if you place limb buds on the head?
Limbs transplanted to the head may be supplied by fibers of the cranial nerves, and then they can be seen to move synchronously with the respiratory movements of the jaws and gills.
What was the result of an eye being transplanted into the side of an embryo after the forelimb rudiment was removed
- The brachial nerves were deflected from their normal path and grew out toward the transplanted eye.
- Having approached the eye, however, the brachial nerves failed to penetrate into the eye and establish an actual connection with it.
- It would seem then that the attraction of the outgrowing nerves to peripheral organs seems to be unspecific to a very high degree; possibly any growing mass of tissue will attract a nerve that is sufficiently near to it. Connections between a nerve and the end organ can be made only if the two correspond to each other.
Early vasculature arises from what?
endothelial cells from aorta, cardinal veins, and angioblasts
Initial limb vasculature
- Fine capillary network
- Eventually specific channels begin to form within the meshwork
- Results in a central artery
- Capillary beds connect central artery to marginal sinus just below AER.
- Blood in marginal sinus drains via peripheral venous channels.
