Limb Development (use notes)

Question 1

What are four stages of of pattern formation of cells?

Answer 1

1. 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

Question 2

When forelimbs bud and develop they rotate… where as when hind limbs rotate…

Answer 2

Forelimb rotate pollex lateral and hind limbs rotate hallux medial

Question 3

Arm (humerus) or thigh (femur)

Answer 3

stylopod

Question 4

forearm (radius/ulna) or leg (tibia/fibula)

Answer 4

Zeugopod

Question 5

hand or foot

Answer 5

autopod

Question 6

What causes limb field formation in the first month of pregnancy?

Answer 6

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

Question 7

How do limbs know where to form?

Answer 7

Hox genes direct limb formation. Humans (tetrapods) have 4 limbs, so 4 hox clusters from HoxA-HoxD.

Question 8

What are functions of Hox 4, Hox 5, and HoxC9?

Answer 8

Hox 4 and Hox 5 are expressed in forelimbs which induces Tbx5 expression. HoxC9 is expressed in hind limbs causing expression of Pitx1/Tbx4.

Question 9

Tbx5

Answer 9

defines the forelimb field

Question 10

Defines the hind limb field

Answer 10

Tbx4

Question 11

Hox genes, RA, and Beta catenin all induce Tbx4 and Tbx5 which induce

Answer 11

Fgf10 in both fore and hind limbs.

Question 12

What does Fgf8 do?

Answer 12

creates positive feedback loop to Fgf10 to cause more outgrowth

Question 13

Pitx1 + Tbx4

Answer 13

Hind limb growth

Question 14

Pitx1

Answer 14

Hind limb phenotype

Question 15

In the first month, what is the failed development of one or more limbs?

Answer 15

Amelia, which is caused by defect in Wnt3 and blocks beta catenin induction of Tbx4 and Tbx5

Question 16

In the second month what allows for outgrowth?

Answer 16

FGF8 expression leads to formation of apical ectodermal ridge which is at the dorsal/ventral boundary.

Question 17

What is congenital duplication of the foot and sometimes leg?

Answer 17

Diplopodia which is caused by duplicated AER

Question 18

Region of mitotic mesodermal cells beneath AER?

Answer 18

Progress Zone which is where limb outgrowth continues right before the begin to differentiate to forelimbs

Question 19

True/False: earlier cells produce distal structures and later cells produce proximal structures

Answer 19

FALSE: earlier cells produce proximal, later produce distal

Question 20

True/False: In the two signal model, signals from body wall assign proximal fate and signal from AER assigns distal fate

Answer 20

TRUE

Question 21

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

Answer 21

False: the closer to FGFs and AER the more undifferentiated and removing the progress zone allows cells to differentiate to proximal and distal signals.

Question 22

What creates the AP axis?

Answer 22

The zone of polarizing activity or ZPA. It creates the posterior region of limbs

Question 23

… is expressed in the ZPA

Answer 23

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

Question 24

What is the presence of supernumerary digits?

Answer 24

Polydactyly which is point mutation in ZPA regulatory sequence

Question 25

How does ZPA and AER interact?

Answer 25

Originally AER and LPM are used to cause early outgrowth. Then ZPA and AER maintain distal outgrowth by expressing FGF4 and Shh (feedback loop)

Question 26

What is an autosomal recessive disorder with congenital amputation of the distal limbs and aplasia of the hands and feet?

Answer 26

Acheiropodia which is caused by deletion of ZRS leading to lack of ZPA/AER maintenance

Question 27

Which genes determine dorsal patterning vs ventral patterning?

Answer 27

Dorsal is Wnt7a, Lmx-1,and rFng and ventral is BMP and En1. Between expression of these genes is where the AER is

Question 28

... are expressed in apoptotic zones

Answer 28

BMPs

Question 29

...are expressed in AER to promote cell survival and antagonize BMP

Answer 29

FGFs

Question 30

What is the condition where two or more digits are fused?

Answer 30

syndactyly

Question 31

Hoxa and Hoxd regulate ... growth. Ex. Investigated in Club foot

Answer 31

skeletal growth

Question 32

How does skeleton form?

Answer 32

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)

Question 33

What is type 2 vs type X collagen?

Answer 33

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

Question 34

What cells make up growth plate?

Answer 34

resting which are cartilage progenitors (type 2), proliferating, hypertrophic which are the collagen type X, apoptotic layer, and ossification into type 1

Question 35

Region of cell density induced by Wnt14 and Gdf5 cell death?

Answer 35

Joint, and Gdf5 forms the cavity of the joint

Question 36

What is shortening if the hands and feet due to shortened bones?

Answer 36

Brachydactyly caused by altered Gdf5 signaling through Bmpr1B

Question 37

How does musculature form?

Answer 37

Myoblasts migrate from dermamyotome because of HGF/Scatter expression

Question 38

What forms musculotendinous junctions?

Answer 38

derived from LPM, induced by FGF, organized by TGFB, and consolidated by Scleraxis

Question 39

What forms vasculature?

Answer 39

from mesoderm aorta or angiogenesis and spreads from meshwork under AER

Question 40

What forms neural invasion?

Answer 40

axons guided by pattern signals. In forelimb, cranial nerves target lateral dermatome, in hind limb cranial nerves target medial dermatome.