T3 Limb Flashcards
In early experiments of salamander limb development, what is the outcome if you remove half the limb disc?
Remaining half will form a complete limb
What is the limb disc? salamander
it is the thickened area on the side of the salamander that will give rise to the limb.
What is the actual primodium for the limb? salamander
the limb disc
The peripheral region of salamander gives rise to what?
it is the region around the disc is the morphogenetic field and can give rise tot he limb if the disc is removed.
What is the outcome of the limb if the limb field is removed? salamanders
no limb will form
In early experiments of salamander limb development, what is the outcome if you vertically divide the disc into two halves and physically separate the two halves?
each half will form a normal limb
In early experiments of salamander limb development, what is the outcome if you fuse two discs together?
a single, normal limb will form
What initiate limb development?
paraxial mesoderm signals via retinoic acid based on hox code.
What are the initial initiators of limb development?
Tbx5 and Tbx4
What does the expression of Tbx5 initiate?
the future forelimb
What does the expression of Tbx4 and Pitx-1 initiate?
the future hindlimb
The expression of what two transcription factors fixes the anterior-posterior axis?
Gli-3 and Hand-2
What is associated with fixing the anterior portion of the anterior-posterior axis?
Gli-3
What is associated with fixing the posterior portion of the anterior-posterior axis?
Hand-2
What are the three axes that define the vertebrate limb?
dorsoventral, anterior-posterior, and proximodistal
Early research on establishment of the three axes did what?
- transplantation experiments allowed workers to reverse one or more axes.
- transplant a limb disc to the opposite side
- rotate a limb disc 180 degrees
- Slack in 1976 transplanted a strip of flank tissue from just posterior to the limb disc to an ectopic site just anterior to the limb disc. This causes the limb to consist of two posterior halves arranged in a mirror image fashion
- Riddle (1993) suggested that the morphogen expression by the posterior organization region might be the RNA product of the shh gene. Treatments of the anterior part with retinoic acid caused cells in this region to behave like cells in the posterior organization region. Retinoic acid was not the morphogen but was/is capable of activating shh, which may be the actual morphogen.
What is the function of signaling molecule Hoxb8?
determines the position of the zone of polarizing activity (ZPA) via its highest concentration, and its expression is induced by retinoic acid.
What is the function of signaling molecule Shh?
- it is a signal produced by ZPA
- maintains the structure and function of AER
- induces the expression of gremlin
What is the function of gremlin?
- inhibits BMP- which inhibits FGF-4 in AER
- inhibits Gli-3 in posterior part of limb bud
What is the function of signaling molecule Gli-3 (in anterior part of limb bud)?
inhibits the expression of shh in anterior part of the limb bud.
What are the advantages of using chick embryos in limb development research?
- avian limbs are usually larger than amphibian limbs
- avian embryonic limbs are accessible even though they are enclosed within extraembryonic membranes
- because the chick embryo usually lies on its left side, the right limb bud can be used in experimental manipulations while the left limb remains untouched as the control
- a limb bud from a donor embryo may be completely removed and transplanted to the CAM of a host embryo where it will continue to develop.
What is the AER?
apical ectodermal ridge, it is the limb ectoderm in birds.
What is the location of AER? What does the dorsal express? What does the ventral express?
- located at the border between dorsal ectoderm and ventral ectoderm
- dorsal expresses radical fringe
- ventral expresses engrailed-1 (En-1)
What is the role of the AER?
interacts with underlying limb bud mesoderm and promotes continued outgrowth of limb bud
What are the outgrowth promoting signals?
FGFs (8, 4, 9, 17), and Wnts
Cells in the posterior part of the limb bud are under the influence of what signaling molecule and differentiate into proximal limb components?
retinoic acid
What is the role of limb bud mesoderm?
involved in the determination of actual limb morphology
What is the result if the AER is removed?
cessation of further outgrowth of the limb bud
Cells in the distal limb bud express what?
Msx-1
Hoxd gene expression in chick limb bud is involved in pattern formation in what axis?
proximodistal limb axis
What is a stylopod?
single proximal element, forms humerus or femur
What is a proximal zeugopod?
two parallel elements, form radius/ulna or tibia/fibula
What is a distal zeugopod?
twelve elements, form carpals or tarsals
What is a proximal autopod?
five elements, form metacarpals/metatarsals
What is a distal autopod?
Variable number of elements, form phalanges
The mesenchyme portion of the limb bud is determined early - before closure of the neural tube. If the limb bud mesenchyme is transplanted under the epidermis of the flank or head, what is the result?
a limb will form
The structure of the developing limb in every case us determined by the nature of the mesodermal components. Experimentally, it is possible to combine the leg bud mesoderm with what of a wing bud and vice versa?
epidermis
Effect of limb mesoderm on overlying ectoderm is an example of what?
instructive induction. Normal wing bud mesoderm combined with mutant AER ectoderm fails to produce an AER.
What occurs if the AER is removed from a chick wing bud?
further outgrowth of the limb mesoderm ceases
What is the result if extra AER is grafted over limb mesoderm?
additional limbs form
Mesoderm produces what that maintains the AER in a healthy state?
apical ectodermal maintenance factor (AEMF)
AER maintains the mesodermal cells below it in a labile and mitotic state. What does this allow to happen?
Limb bud continues to grow distally, and various regions of the limb begin to appear in a proximal-distal direction.
Necrotic zones of programmed cell death development within the limb bud ________ and will “carve” that shape of the limb?
mesoderm
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 of what?
morphogenesis.
What does the posterior necrotic zone form?
area that will become the axilla
The limb bud mesenchyme consists of cells derived from where?
lateral plate mesoderm
Limb bud mesenchyme gives rise to what?
appendicular skeletal elements, connective tissue, and some blood vessels.
NOTE: mesenchymal cells destined to form limb musculature will migrate into the limb bud from the somitic ventral dermomyotomes.
Initial mesenchymal condensations for the appendicular skeleton form in what part of the limb bud, and these cells are restricted to the formation of what?
proximal part of the limb bud; cartilage
Cartilage formation is restricted to the central core of the limb bud due to inhibitory restrains caused by the overlying limb bud ectoderm, which produces what?
wnt7a
What does mesenchymal pre-cartilage condensations express?
BMP-2 and BMP-4
What is expressed in later cartilage?
BMP3
To what cells are BMPs later restricted to?
perichondrial cells
Cartilage cells being to ________ and form a whorled pattern when seen in cross-section.
hypertrophy
Hypertrophic cells express __________ which may induce expression of __________.
indian hedgehog (ihh); BMP-6
noggin inhibits what TF?
BMP
BMP is necessary to maintain what?
cartilage development
What forms as a continuous precartilaginous rods?
limb skeleton
What traverses the rod at the point of future joint formation?
condensed zone of cells
What fills the future joint area?
loose connective tissue
What forms the articular cartilages?
The skeletal elements on either side of the joint cavity.
What does mesenchyme cell condensations form?
Other joint components
Proximal cells of the limb bud produce what?
scatter factor
Pre-muscle cells in the somite express what?
C-met, the receptor for scatter factor
What do migrating pre-muscle cells express?
Pax 3 and N-cadherin
Differentiation of premuscle cells into muscle within the limb bud depend on what expressed by the limb bud ectoderm?
wnt-6
After skeletal element condensations form, myogenic cells condense into two muscle masses. What are they?
mass of flexor muscles and mass of extensor muscles
Tcf-4 is expressed by what?
connective tissue associated with the muscle masses
What determines the morphology of individual muscle?
connective tissue
Muscle growth occurs via addition of materials from what cells?
satellite cells
Before entering the limbs, the nerves become interconnected and form what?
networks or plexuses
What are the major plexuses in higher vertebrates?
cervical, brachial, and lumbar plexuses
Motor axons originate from the spinal cord during what week?
5th week
Motor axons grow into limb ______ and ______ muscle masses
dorsal; ventral
Motor axons originating from more medial locations in the spinal cord grow into what muscle mass?
A. medial
B. dorsal
C. ventral
ventral
Motor axons originating from more lateral locations in the spinal cord grow into what muscle mass?
A. medial
B. dorsal
C. ventral
dorsal
Spinal nerves supplying the limbs in terrestrial animals are _________ than other spinal nerves and the corresponding ganglia are _________.
larger; larger
What is the outcome if the salamander forelimb fails to develop?
- nerves of the brachial plexus remain smaller/thinner than they would if a limb had been there.
- spinal ganglia (3, 4, 5) associated with brachial plexus, are also smaller
If an additional limb rudiment is transplanted to the flank of the salamander embryo, what is the outcome?
- local spinal nerves supply the innervation to the transplanted limb limb and these local nerves increase thickness
- ganglia associated with these nerves increase in size; the increase in number of cells may be as much as 40%
What is the outcome if the forelimb rudiment of an amphibian embryo, prior to outgrowth of nerves, is cut out and transplanted to a position very near the original one?
- the brachial nerves will derivate from their normal paths and will be deflected in the direction of those transplanted limbs.
- 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 will be fully functional.
What is the outcome 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.
What is the outcome of the normal path of the nerves is blocked by some obstacle?
the outgrowing nerves may avoid the obstacle, go around it, but ultimately will reach their normal destination.
What is the outcome if the rudiment transplant is placed further away from the normal limb or obstacle too great?
nerve fibers fail to be attracted to the limb, but it may attract local nerves. limb will be non-functional
If limbs are transplanted to the head, what happens
CN innervate, but are non-functional.
If an eye was transplanted onto the side of an embryo after the forelimb rudiment was removed, what would be the outcome?
nerves would grow towards an eye, but would fail to penetrate and establish a connection with it.
Early vascularization arises from endothelial cells arising from where?
aorta, cardinal veins, and angioblasts.
What germ layer inhibits formation of blood vessels?
ectoderm
