Limb Development

Question 1

When does the limb start to develop?

Answer 1

around 4-8 weeks

-4 weeks: limb bud

Question 2

Which limb develops first?

Answer 2

• fore limb

- hind limb 1 to 2 days later

Question 3

Limb development is an interaction between what?

Answer 3

Epithelial-Mesenchymal interaction:

• Ectoderm
• Mesoderm

Question 4

What do limbs arise from?

Answer 4

Lateral Plate Mesoderm

Question 5

Where does the limb know where to start development on the embryo?

Answer 5

• competent stripe

- HOX genes are expressed along the competent stripe at the point where limbs develop

Question 6

What initiates and maintains bud outgrowth?

Answer 6

• by establishing a positive feedback loop of FGF signaling comprised of FGF10 expressed in the Lateral Plate Mesoderm
• controlled by Tbx transcription factors

Question 7

What transcription factors control the formation of forelimb? Hindlimb?

Answer 7

Forelimb: Tbx5

Hindlimb: Tbx4

Question 8

What is the ultimate signal for initiation of limb outgrowth?

Answer 8

Wnt-3

Sources of signal:

• Paraxial (somatic) Mesoderm
• Intermediate Mesoderm
• HOX determines where
• Wnt determines when, regulates Tbx

Question 9

Amelia? Defective signal?

Answer 9

• failed development of one or more limbs, could be shrunken or deformed
• Meromelia: partial absence of limbs
• Tetra-amelia: failure of all 4 limbs
• linked to defect in Wnt3 signaling

Question 10

Apical Ectodermal Ridge (AER)?

Answer 10

• condensation of ectodermal cells
• creates a proliferative zone beneath, called the progress zone (0.4mm thick)
• border between dorsal and ventral
• gradient in either direction, differentiates the two

Question 11

What transcription factors are expressed in the Apical Ectodermal Ridge (AER)?

Answer 11

• Dorsal ectoderm: Wnt-7a

- Ventral ectoderm: Engrailed-1

Question 12

What happens when the AER is removed?

Answer 12

• removal results in loss of limb element
• remove early: lose radius, ulna, and hand
• remove late: lose hand

Question 13

Diplopodia?

Answer 13

• congenital anomaly in tetrapods that involves duplication of elements of the foot on the hind limb
• duplication of AER result in supernumerary limbs

Question 14

Zone of polarizing activity (ZPA)?

Answer 14

• found at distal (posterior) base of limb bud
• produces anterior and posterior limb pattern
• secretes sonic hedge hog (shh)

Question 15

Acheiropodia?

Answer 15

• autosomal recessive developmental disorder presenting with bilateral congenital amputations of upper and lower extremities and aplasia of hands and feet (hemimelia)
• lack of shh

Question 16

Polydactyly? post and preaxial?

Answer 16

• congenital physical anomaly in humans and animals having supernumerary fingers or toes
• opposite of oligodactyly (fewer fingers and toes)
• Preaxial: result from ZPA duplication (duplication of 1st or 2nd digit)
• Postaxial: addition of another digit (6th digit)

Question 17

Tetrapod body plan?

Answer 17

• stylopod: humerus
• zeugopod: ulna and radius, tibia and fibula
• autopod: wrist and fingers, ankle and toes

Question 18

Progress zone model of proximal-distal axis patterning?

Answer 18

• mesoderm at the distal tip of the limb bud constitutes a progress zone of fixed dimensions, in which cells are receiving progressively more distal positional information over time
• new layers formed in succession
• first PZ, then stylopod, then S and Z; then S,Z and A

Question 19

Early specification model of proximo-distal axis patterning?

Answer 19

• early limb bud stage, cells are broadly specified to for the three compartments of the limb, the cells undergo expansion before becoming determined to form different skeletal elements
• all layers (S, Z, A) are present from the beginning, but grow and differentiate

Question 20

Cartilage condensation process?

Answer 20

1. cells exit progress zone
2. condense or aggregate
3. differentiate into chondrocytes
4. becomes Cartilaginous Anlage
5. Chondrocytes respell
6. Interior: differentiated Cartilage
7. Exterior: flatted cells Perichondrium

Question 21

Which limbs develops first?

Answer 21

• forelimb first (arm)

- develops from top down

Question 22

What process defines the digits?

Answer 22

Interdigital apoptosis

Question 23

Syndacyly?

Answer 23

• caused by failure of interdigital apoptosis

- two or more digits are fused together

Question 24

Joint formation process?

Answer 24

1. cartilage condensation divided by interzones
2. Articular cartilage differentiates
3. Joint cavity forms (anlage)

Question 25

Bone and cartilage development process?

Answer 25

1. Bone collar stage: cartilage cells undergo hypertrophy 2. Periosteal invasion: periosteum penetrates interior part of bone and replaces cartilage, osteoblast put down mineralized matrix 3. Primary Ossification Center 4. Creation of Epiphyseal Growth Plate 5. Secondary Ossification Center - Epiphyses - Apophyses (trochanter)

Question 26

Layers of Growth plate?

Answer 26

1. Resting Cartilage (stem cells) 2. Proliferative zone (stacks) 3. Hypertrophic zone (bigger chondrocytes) 4. Calcification zone (replace cartilage with bone) 5. Ossification zone - grow at fast rate

Question 27

How many growth plates start to fuse between ages 14-20?

Answer 27

12-15 | -stop growing taller once long bones fuse

Question 28

Brachydactyly?

Answer 28

- abnormal shortening of fingers and toes | - caused by dysfunction or premature closure of growth plate of fingers and toes

Question 29

What forms muscle development?

Answer 29

- Myoblasts (PAX3 positive cells) migrate from dermomyotome into limb bud and enlarge - several myoblasts fuse to form a Myotube, which matures into skeletal muscle fiber - form Dorsal (dmm) and ventral (vmm) muscle masses

Question 30

Development of Ligaments and Tendons? Gene precursor?

Answer 30

- Scx expressing cells (tendon precursors) arise from sclerotome - Scx expression (tendon induction) depends on ectoderm, also skeletal condensations

Question 31

Ligaments and tendons development process?

Answer 31

1. Scx expressing tendon progenitors differentiate within limb by FGF signaling - TGF beta plays a role 2. tendon progenitors organize between skeletal and muscular tissues - most tendons depend on presence of muscles to form (extensor and flexor tendons on hand form independent of muscle) 3. tendon cells signal ridge formation in skeletal tissues - muscle contraction dictates ridge robusticity

Question 32

Vascular invasion?

Answer 32

1. endothelial cells recruited (from local mesenchyme and somites) 2. vasculogenesis: vessels form de novo in embryonic mesenchyme 3. angiogenesis: primitive vessels remodeled to form adult networks 4. regression (loss of signals) occurs at site of cartilage condensation

Question 33

Nerve invasion?

Answer 33

1. innervation is targeted 2. innervation pattern dictated by local pattern signals 3. correlation between craniocaudal location of neurons and anteroposterior pattern of muscle innervation

Question 34

Limb rotation and dermatome formation?

Answer 34

1. initially point caudally 2. grow at right angle to body 3. elbow and knee point outward 4. palm and sole toward trunk 5. lower limb: rotates cranially significantly 6. upper limb: rotates caudally Bone homologies: - radius: tibia - fibula: ulna