Limb Formation

Question 1

What does patterning require?

Answer 1

A local signalling centre

Limited repertoire of signalling molecules

Question 2

When does development of the human limb bud take place?

Answer 2

Week 4s-8

Question 3

What is patterning?

Answer 3

Creation of specific structures according to a pattern- in specific locations

Question 4

Structure of limb bud

Answer 4

somatopleure- from lateral plate mesoderm. Hyaline cartilage model for endochondral ossification of bone

With an ectodermal epithelial covering- will form epidermis

Question 5

How many axes of symmetry will the limb eventually contain? + what are they

Answer 5

3
proximodistal- near to far

dorsoventral- back to fron

anteroposterior- head to tail

Question 6

What limbs develop first?

Answer 6

Hind limbs develop 1-2 days behind the upper

Question 7

What enables limb outgrowth, and thus the proximodistal axes formation?

Answer 7

Apical Ectodermal Ridge

Question 8

How is the AER formed?

Answer 8

FGF-10 formed by lateral plate mesoderm, induces AER formation-

causes the overlying ectoderm to thicken, forming the AER

Apical Ectodermal Ridge is formed at the tip of the limb bud, therefore enables elongation at the progress zone

Question 9

What is the function of the progress zone?

Answer 9

The dividing cells release FGF growth factors to stimulate the AER to produce factors that encourage mitosis in the progress zone

Question 10

What does the progress zone suggest about the formation of proximal structures?

Answer 10

Cells measure the time spent in the zone, and those staying a short time become more proximal structures

Question 11

Evidence that Apical Ectodermal Ridge is important in limb formation

Answer 11

Knock out mouse formed- limb will not develop past the initial bud- truncation of distal structures

Question 12

Explain dorso-ventral patterning

Answer 12

Wnt7a activity in the overlying ectoderm is restricted on dorsal side of the limb bud and En1 on ventral side

Question 13

Experimental evidence for dorsoventral patterning

Answer 13

Knockout mouse without Wnt7a activity on both sides, formed a foot pad to develop from ventral side

Question 14

Explain anteroposterior patterning

Answer 14

1. determines the first digit side of the limb to the 5th digit side
2. Shh is expressed in the caudal region of the limb but in the Zone of Polarising Activity
3. expression of Wnt7a maintains expression of Shh

Question 15

Explain ZPA role

Answer 15

Secretes Shh in only the posterior region of the limb

Question 16

Experimental evidence for ZPA function

Answer 16

ZPA grafted onto anterior region of limb bud- forces a publication of limb structures

Question 17

Morphogen definition

Answer 17

Chemical that produces different effects on cell fate at different concentrations

Example = Shh

Question 18

How does polydactyly develop?

Answer 18

Mutations in Hoxa- or Hoxd clusters

Zone of polarising activity forms a new organiser region more anterior than the normal posterior one.

Secretes Shh in an abnormal region, which results in the development of extra digits

Question 19

Digit formation stages

Answer 19

1. Apical Ectodermal Ridge acts as a signalling centre
2. AER persists locally- causing extension
3. Other regions of the AER regress and degenerate due to the BMP signalling inhibiting FGF
4. Apoptosis of the interdigital mesenchyme

Question 20

What happens next in the development of the limb?

Answer 20

Long bone growth from lateral plate mesoderm-somatopleure

Question 21

Explain stages of long bone growth

Answer 21

Week 12

1. lateral plate mesoderm- somitic migrates into the limb bud and forms a condensation in the long axis
2. chondrocytes then invade and secreted ECM to form a cartilaginous model of the bones
3. ossification begins in the primary ossification centres
4. endochondral ossification takes place, forming woven bone
5. at birth the diaphysis is completely ossified but not the epiphysis

Question 22

What may potentially inhibit early chondrogenesis?

Answer 22

Wnt6 signalling, secreted by overlying surface ectoderm

Question 23

Origin of embryonic skeletal muscle

Answer 23

somite myotome- giving rise to myogenic progenitor cells

Question 24

Explain initial structure of muscle in limbs

Answer 24

Limb level somites- hypxial lips of dermomyotome delaminate and migrate to the limb bud to form musculature or limbs

between limb somites- cells delaminate from the edges of the dermamyotome and form subjacent postmitotic myotome which gives rise to trunk muscles

Question 25

How are the muscles formed?

Answer 25

somitic Hypaxial demomyotome delaminates and secretes myogenic determination factors, such as Pax3, Myf5 and MyoD

Form two condensations on either side of bone , which gives rise to the flexor and extensor muscles respectively

Secondary satellite cells then become organised under the muscle

Question 26

How are the limbs innervated?

Answer 26

Ventral rami of spinal nerves move between the split somites into the upper limb and then the lower limb

Question 27

How are dermatomes and myotomes innervated?

Answer 27

Dorsal muscle masses and skin innervated by the dorsal branches of the ventral rami, whereas the ventral muscles are innervated by the ventral rami

Question 28

Process of limb rotation

Answer 28

1. ectoderm from body wall is recruited as limbs grow
2. weeks 6-8, rotation of the limbs along the long axis
   - lateral rotation upper limbs
   - medial rotation lower limbs

Question 29

What does the limb rotation cause?

Answer 29

Originally segmental innervation forms spiralling dermatomes

Question 30

3 sections of limb

Answer 30

stylopod- arm
zeugopod- forearm
auropod- digits

Question 31

two abnormalities of limb development

Answer 31

gigantism and dwarfism

Question 32

explain gigantism

Answer 32

excess of growth hormone leads to long limbs

Question 33

dwarfism explained

Answer 33

pituitary dwarfism- growth hormone deficiency

acromegaly- mutation in FGFR3 gene, constantly on so inhibits bone growth