Limb Development

Question 1

Limb primordia

Answer 1

1. Mesoderm
   A. Paraxial: somites -> muscles only
   B. Lateral plate: somatic mesoderm -> mesenchyme-> cartilage -> bone, CT, blood vessels

Question 2

Nerve primordia

Answer 2

1. Ectoderm
   A. Neural crest: afferent innervation and myelination
   B. Neural tube: efferent innervation
   *sympathetics and parasympathetics

Question 3

AER (apical ectodermal ridge)

Answer 3

Ridge of ectoderm along cranial/caudal axis where limb buds form

Question 4

ZPA (zone of polarizing activity)

Answer 4

Forms at caudal end of limb bud

1. Creates SHH gradient

Question 5

FGF (fibroblast growth factors)

Answer 5

FGF10 -> AER secretes FGF8 -> ZPA

Question 6

SHH (sonic hedgehog)

Answer 6

Primary growth factor

1. Gradient defines anterior/posterior limb pattern

Question 7

BMP (bone morphogenic protein)

Answer 7

1. Expressed by ventral ectoderm
2. BMP -> EN1 to set up dorsal/ventral gradient
3. Helps -> AER via FGF10
4. Vital for finger differentiation
   A. Apoptosis in webbing

Question 8

HOX (homeobox domain genes)

Answer 8

1. HOXa and HOXd = primary determinants of bone morphology

2. HOXD 9-13 expression based on A/P and P/D concentration gradients of SHH, FGFs, and WNT7a

Question 9

WNT7a

Answer 9

Dorsal limb ectoderm

1. Establishes dorsal/ventral gradient
2. Interacts w/ AER-FGF -> SHH
3. Maintain proximal/distal axis by maintaining mesenchyme along AER

Question 10

EN1 (entrained 1)

Answer 10

Stimulated be BMP in ventral limb

1. Suppresses WNT7a ventrally => WNT7a dorsal/ventral gradient

Question 11

Abnormal limb development

Answer 11

1. Change in regulation of primary players -> cascade effect
2. Environmental factors: teratogens during let dev. Periods
   A. Alcohol
   B. Thalidomide

Question 12

Axes of growth

Answer 12

1. Ant/post -> lat/medial
2. Prox/distal
3. Dorsal/ventral -> post/ant
   * starts ant/post like quadruped

Question 13

Anterior/posterior gradient formation

Answer 13

3 weeks
1. Growth initiates as bud forms
A. FGF10 from mesenchyme -> AER forms outgrowth in ectoderm
B. Cells of AER
1. Stimulate FGF8 -> distal growth
2. Stimulate ZPA form at post aspect-> ant/post gradient
C. ZPA releases SHH -> ant/post gradient
1. SHH gradient dictated bone formation
A. Thumb and radius- low/absent [SHH]
B. 2nd and 3rd digits - med [SHH]
C. 4th and 5th digits and ulna - high [SHH]

2. Cross-sections
   A. Zeugopod- arm (radius/ulna)
   B. Autopod - hand
   C. Stylopod- humerus

Question 14

Amelia

Answer 14

Absence of limbs
1. Suppression of limb bud dev. During 4th week
A. No ZPA

Question 15

Polydactyly

Answer 15

Extra fingers
1. Cause: error in SHH transcription w/in limb buds -> A/P gradient mutations

2. Pre-axial: 2 thumbs

Question 16

Bradydactyly

Answer 16

Short terminal phalanges

1. Cause: HOXD 9-13 mutation

Question 17

Digit formation

Answer 17

1. Phalanx forming region stim interdigital mesenchyme to differentiate according to pre/post axial gradient
2. Based on SHH signaling
3. Mediated by BMP
4. Weeks 4-8
   A. Limb buds
   B. Paddle-shaped hand and footplates
   C. Digital rays
   D. Notched between digital rays
   E. Webbed fingers/toes
   F. Separate digits

Question 18

Syndactyly

Answer 18

Webbing
1. Incomplete formation of interdigital clefts
2. Cause: lack apoptosis between digits
  A. Usually BMP mutations
  B. Can involve SHH mutation

Question 19

Proximal/distal gradient

Answer 19

1. Differentiation front model
   A. PZ (proliferating zone) - undifferentiated front along AER
   B. AER produces FGF8 -> mesenchymal cell division in PZ
   C. PZ differentiates according to distal distance traveled
   D. PZ position relative to AER determines HOXD gene expression => prox/distal tissue density
   E. HOXD9-13 control tissue ID beyond differentiation front

Question 20

Meromelia

Answer 20

Partial absence of limbs
1. Terminal: missing hands
2. Intercalary: missing parts
A. Phocomelia: hands present, prox parts missing
B. Hemimelia: missing at least one zeugopod bone (radius/ulna)
3. Cause: disruption of prox/distal dev. In 5th week
A. Variable mutations
B. Mostly HOXD mutations

Question 21

Dorsal/ventral gradient

Answer 21

As limb bud elongates

1. WNT7a in dorsal ectoderm
2. EN1 in ventral ectoderm
3. EN1 inhibits WNT7a ventrally => WNT7a dorsal/ventral gradient
4. Mesenchyme differentiates ant/post limb compartments

Question 22

Limb rotation

Answer 22

7-8 weeks
1. Upper limb rotates 90 degrees laterally ->
  A. Elbow point post
  B. All extensors post
  C. All flexors ant
2. Lower limb rotates 90 degrees medially->
  A. Knee point ant
  B. Leg and foot flexors post
  C. Extensors ant

Question 23

Bone development

Answer 23

1. Not scleratomes of somites bc appendicular, not axial
2. Endochondral ossification
   A. Week 5: mesenchymal models made from lateral plate mesoderm
   B. Week 6: mesenchymal tissue in hand plates condense -> cartilaginous digital rays
   C. Week 7: start ossification long bones
   
   1. Ossification centers in all long bones 12th week
   2. Carpals don’t ossify until after birth

Question 24

Fusion of epiphyses

Answer 24

1. Growth plates between epiphyses and diaphyses
2. Happens at different time depending on location
3. Most fuse by 18-20 y/o

Question 25

Dwarfism

Answer 25

1. Hypochondroplasia 2. Achondroplasia 3. Chondrodysplasia 4. Cause: FGF3 mutation -> problems w/ collagen production or turning collagen to bone 5. Epiphyses and diaphyses fuse early or fail to fuse

Question 26

Muscle development

Answer 26

1. Week 5: ventral rami invade limb buds carrying myoblasts from C5-T1 somites in upper limb A. L2-S3 somites in lower limb 2. Myocytes aggregate and proliferate -> ventral/dorsal muscle mass 3. Mesenchyme from lateral plate mesoderm -> ligaments and blood vessels 4. Mytotomes: set of muscles innervation by specific spinal n. A. Muscle mass from each myotome porliferated into limb prox -> distal B. Overlap from migration/fusion of myotomes

Question 27

Dermatome development

Answer 27

Sensory axons enter limb bud after motor axons from neural crest 1. Sensory nerves envelope in epidermis and pulled along ventral axis pre -> post axial