Limb Development Flashcards
What happens when half the limb disc is removed?
-remaining half will form a complete limb
What happens when the disc is vertically dived into two halves and physically separated into two halves?
-each half will form a normal limb
What happens when two discs are fused together?
-a single limb will form
What initiates limb development?
-par axial mesoderm signals via RA based on a Hox code
What initiates forelimb growth?
-Tbx5
What initiates hindlimb growth?
-Tbx4 and Pitx-1
What does the expression of Gli3 and Hand2 do?
-fixes anterior-posterior axis
How does ectoderm and mesoderm communicate?
- Ectoderm signals mesoderm with FGF-8
- mesoderm signals ectoderm with FGF-10
What are the three axes of the vertebrate limb?
- dorsoventral axis
- anterior-posterior axis
- proximodistal axis
What happens when a limb. Disc is transplanted to the opposite side?
- AP axis is reversed but not DV axis
- normal limb is formed, but it’s AP axis is reversed
What happens when a limb disc is rotated 180 degrees?
- both the AP axis and DV axis will be reversed
- a normal limb will form but only its AP axis is revered
What happens when flank tissue from posterior of limb disc is transplanted to an ectopic site just anterior to the limb disc?
- resulting limb consists of 2 posterior halves arranged in mirror image fashion
- Conclusion: strip of flank tissue just posterior to the limb disc organizes the posterior edge of the disc -> posterior organizing region
- RA sets up gradient across disc, part of disc with higher conc forms the posterior half of the limb
What happens when anterior part of a wing bud is treated with RA to behave like posterior organizing region?
- these cells begin expressing RNA product of Shh
- Conclusion: RA is not the morphogen but activates Shh gene which then expresses what might be the morphogen
- this is the ZPA
What is Hoxb8 responsible for?
- determines position of ZPA via highest conc
- expression is induced by RA
What does Shh do?
- signal produced by ZPA
- also maintains structures and function of AER
-induces expression of gremlin
+inhibits BMP-1 which inhibits FGF-4 in AER
+inhibits Gli-3 posterior part of the limb bud
What does Gli-3 do?
-inhibits expression of Shh in anterior part of limb bud
What does the apical ectodermal ridge do?
-consists of two layers
+outer, continuous flattened periderm
+inner simple columnar layer
-separated by basal lamina from limb bud mesoderm
Where is the AER located? What do these structures express?
- at the border between dorsal ectoderm and ventral ectoderm
- dorsal ectoderm expresses radical fringe
- ventral ectoderm expressed Engrailed-1
What does the AER do during the outgrowth. Of the limb bud?
- interacts with underlying limb d mesoderm and promotes continued outgrowth of limb bud
- outgrowth promoting signals: FGFs and Wnts
- cells in posterior part of limb bud are under influence of RA and differentiate into proximal limb components
What does the limb bud mesoderm do during the outgrowth of the limb bud?
- involved in determination of actual limb morphology
- removal of AER results in cessation of further outgrowth of the limb bud
What do the cells in the distal limb bud express?
-Msx-1
What is involved in pattern formation in the proximodistal limb axis?
-Hoxd
What is the stylopod?
- single proximal element
- forms humerus or femur
What is the proximal zeugopod?
- two parallel elements
- form radius/ulna or tibia/fibula
What is the distal zeugopod?
- twelve elements
- form carpals or tarsals
What is the proximal auto pod?
- five elements
- form metacarpals/metatarsals
What is the distal autopod?
- variable number of elements
- form phalanges
When is the mesenchyme portion of the limb bud determined?
- 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 developing limb in every case is determined by the nature of the _______________ components.
- mesodermal
- experimentally it is possible to combine the legbud mesoderm with the epidermis of a wing bud and vice versa
Effects of limb mesoderm on overlying ectoderm is an example of ______________________.
-instructive induction
What happens when the AER is removed?
-further outgrowth of the limb mesoderm ceases
What happens with an extra AER is grafted over limb mesoderm?
-additional limb forms
What is secreted from the mesoderm and maintains the AER in a healthy state?
-apical ectodermal maintenance factor
AER maintains the mesodermal cels below it in a labile and mitotic state.
- min bud continues to grow distally
- various regions of the limb bud begin to appear in a proximal-distal direction
One theory suggests the presumptive regions are present from the earliest stages but that the more distal regions __________________.
- 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
What carves the shape of the limb?
-necrotic zones of programmed cell death
What does. Limb bud mesenchyme form? Where does it come from?
- give rise to appendicular skeletal elements, connective tissue, and some blood vessels
- migrate into limb bud from sometime ventral dermomyotomes
What are initial mesenchyme condensations?
-restricted to the formation of cartilage
Describe initial cartilage formation in the limb bud.
- restricted to the central core of the limb bud due to inhibitory restraints caused buy the overlying limb bud ectoderm, which produces Wnt-7a
- mesenchymal precartilage condensations express BMP-2 and BMP-4
- BMP-3 is expressed later in cartilage
- BMPs are later restricted to perichondral cells
-cartilage cells begin to hypertrophy and to form a whirled pattern when seen in cross section
+hypertrophic cells express ihh which may induce expression of BMP-6
List the stages of joint formation.
- limb skeleton forms as continuous pre cartilaginous rods
- condensed zone of cells tranverses the rod at the point of future joint formation
- BMP is necessary to maintain cartilage development
- Noggin inhibits BMP in the region of future joint cavity
- cell necrosis occurs in the region of the joint cavity
- loose CT fills in the future joint area
- articular cartilages are formed by the skeletal elements on either side of the joint cavity
- other joint components are formed by additional mesenchymal cell condensations
What is the limb disc?
- thickened area on the side of the salamander that will give rise to the limb
- actual primordium for the limb
- peripheral region around the disc is the morphogenic field and can give rise to the limb if the disc is removed
- if removed, no limb will form
What are the events in the limb muscle formation?
- Proximal cells of limb bud produce scatter factor
- Pre muscle cells in the somite express c-met (scatter factor receptor)
- 30-100 pre muscle cells from each somite in the limb bid region will migrate to the limb bid
- Migrating pre muscle cells express Pax-3 and N-cadherin
- Differentiation of premuscle cells into muscle within the limb bud depend on Wnt-6 expressed by the limb bud ectoderm
- After skeletal element condensations form, myogenic cells condense into two muscle masses (mass for flexor and mass for extensor muscles)
List the steps in limb muscle formation.
- TCR-4 is expressed by the CT associated with the muscle masses
- CT determines morphology of individual muscles
- Common muscle masses split into definitive muscles of the limb (splitting may be due to blood vessel pattern via platelet derived growth factor, myoblasts begin to form early myotubes)
- Muscle growth occurs via addition of material from satellite cells
When do motor axons develop?
5th week
What are the only areas of the spinal cord that possess properties necessary for controlling the functioning of limbs?
Brachial and lumbar plexus
Where does early vasculaization come from?
- endothelial cells from aorta
- Cardinal veins
- angioblasts
What is the initial limb vasculature?
-fine capillary network
-eventually specific channels begin to form within the meshwork
-results in central artery
-capillary beds connect central artery to marginal sinus just below AER
-blood in marginal sinus drains via peripheral venous channels
Overlying ectoderm sinus drains via periphery venous channels
