Limb Development (use notes) Flashcards
What are four stages of of pattern formation of cells?
- morphogenic field is made up 2. Signals to “organizers” are established within positional information 3. recorded position based on cell 4. cells differentiate based on signals
When forelimbs bud and develop they rotate… where as when hind limbs rotate…
Forelimb rotate pollex lateral and hind limbs rotate hallux medial
Arm (humerus) or thigh (femur)
stylopod
forearm (radius/ulna) or leg (tibia/fibula)
Zeugopod
hand or foot
autopod
What causes limb field formation in the first month of pregnancy?
LPM expresses Fgf10 which causes mitosis and bulging under the ectoderm. This LPM will for the CT, and somites will recruit from hypaxial myotome to become muscle
How do limbs know where to form?
Hox genes direct limb formation. Humans (tetrapods) have 4 limbs, so 4 hox clusters from HoxA-HoxD.
What are functions of Hox 4, Hox 5, and HoxC9?
Hox 4 and Hox 5 are expressed in forelimbs which induces Tbx5 expression. HoxC9 is expressed in hind limbs causing expression of Pitx1/Tbx4.
Tbx5
defines the forelimb field
Defines the hind limb field
Tbx4
Hox genes, RA, and Beta catenin all induce Tbx4 and Tbx5 which induce
Fgf10 in both fore and hind limbs.
What does Fgf8 do?
creates positive feedback loop to Fgf10 to cause more outgrowth
Pitx1 + Tbx4
Hind limb growth
Pitx1
Hind limb phenotype
In the first month, what is the failed development of one or more limbs?
Amelia, which is caused by defect in Wnt3 and blocks beta catenin induction of Tbx4 and Tbx5
In the second month what allows for outgrowth?
FGF8 expression leads to formation of apical ectodermal ridge which is at the dorsal/ventral boundary.
What is congenital duplication of the foot and sometimes leg?
Diplopodia which is caused by duplicated AER
Region of mitotic mesodermal cells beneath AER?
Progress Zone which is where limb outgrowth continues right before the begin to differentiate to forelimbs
True/False: earlier cells produce distal structures and later cells produce proximal structures
FALSE: earlier cells produce proximal, later produce distal
True/False: In the two signal model, signals from body wall assign proximal fate and signal from AER assigns distal fate
TRUE
True/False: Close exposure to FGFs and AER allows cells to differentiate, and removal from progress zone allows cells to undifferentiate in response to proximal and distal signals
False: the closer to FGFs and AER the more undifferentiated and removing the progress zone allows cells to differentiate to proximal and distal signals.
What creates the AP axis?
The zone of polarizing activity or ZPA. It creates the posterior region of limbs
… is expressed in the ZPA
Sonic Hedgehog which is used to create digits. Digits 5 are made of Shh expressing ZPA cells, digits 3/4 are made of that don’t express ZPA anymore, Digit 2 is made of cells exposed to Shh, and digit 1 is no Shh
What is the presence of supernumerary digits?
Polydactyly which is point mutation in ZPA regulatory sequence
How does ZPA and AER interact?
Originally AER and LPM are used to cause early outgrowth. Then ZPA and AER maintain distal outgrowth by expressing FGF4 and Shh (feedback loop)
What is an autosomal recessive disorder with congenital amputation of the distal limbs and aplasia of the hands and feet?
Acheiropodia which is caused by deletion of ZRS leading to lack of ZPA/AER maintenance
Which genes determine dorsal patterning vs ventral patterning?
Dorsal is Wnt7a, Lmx-1,and rFng and ventral is BMP and En1. Between expression of these genes is where the AER is
… are expressed in apoptotic zones
BMPs
…are expressed in AER to promote cell survival and antagonize BMP
FGFs
What is the condition where two or more digits are fused?
syndactyly
Hoxa and Hoxd regulate … growth. Ex. Investigated in Club foot
skeletal growth
How does skeleton form?
Mesenchyme condenses in limb and increased expression of gap junctions. This formed an early chondrocyte which then goes through endochondral ossification. Secondary ossification replaces cartilage in the epiphysis (growth plate)
What is type 2 vs type X collagen?
Type 2 is Hyaline which is cartilage and type X is calcified. During the ossification of type 2 it becomes X than type one when perichondrium becomes periosteum
What cells make up growth plate?
resting which are cartilage progenitors (type 2), proliferating, hypertrophic which are the collagen type X, apoptotic layer, and ossification into type 1
Region of cell density induced by Wnt14 and Gdf5 cell death?
Joint, and Gdf5 forms the cavity of the joint
What is shortening if the hands and feet due to shortened bones?
Brachydactyly caused by altered Gdf5 signaling through Bmpr1B
How does musculature form?
Myoblasts migrate from dermamyotome because of HGF/Scatter expression
What forms musculotendinous junctions?
derived from LPM, induced by FGF, organized by TGFB, and consolidated by Scleraxis
What forms vasculature?
from mesoderm aorta or angiogenesis and spreads from meshwork under AER
What forms neural invasion?
axons guided by pattern signals. In forelimb, cranial nerves target lateral dermatome, in hind limb cranial nerves target medial dermatome.
