Embryology

Question 1

When does limb development occur?

Answer 1

4 to 8 weeks

Question 2

When are limb buds visible?

Answer 2

End of week 4

Question 3

Which germ layer contribute to the development of limbs?

Answer 3

ectoderm and mesoderm

Question 4

What do the buds initially consist of?

Answer 4

Lateral plate mesoderm covered by a layer of ectoderm

Question 5

What does the mesoderm core condense into?

Answer 5

The mesenchyme which will form the bone and connective tissue of the limbs

Question 6

What are the skeletal muscles of the limbs derived from?

Answer 6

Somite- blocks of paraxial mesoderm

Question 7

What is the ventral part of the somite?

Answer 7

sclerotome which forms the vertebral column

Question 8

What is dorsolateral part of the somite?

Answer 8

dermomyotome which divides into the dermatome and myotome.

Question 9

What two components foes the myotome differentiate into?

Answer 9

Dorsal epimere: muscles which are innervated bu the dorsal rami of spinal nerves
Ventral hypomere: muscles of thoracic and abdominal wall and muscles of limbs which are innervated by the central ram of spinal nerves

Question 10

What does limb skeletal muscle arise from?

Answer 10

hypomere

Question 11

What two parts does the hypomeres divide into?

Answer 11

Posterior condensation and anterior condensation

Question 12

What does the posterior condensation form?

Answer 12

Extensors and supinators of the upper limbs

Extensions and abductors of lower limb

Question 13

What does the anterior condensation form?

Answer 13

Flexor pronator of the upper limbs

Flexor and abductors of the lower limb

Question 14

Which branches of the central primary rami is the upper limb innervated by?

Answer 14

Brachial plexus C5-T1

Question 15

Which branches of the central primary ram is the lower limb innervated by?

Answer 15

lumbrosacral plexus L4-S3

Question 16

When does limb rotation occur?

Answer 16

6-8 weeks

Question 17

What is the rotation of the limbs?

Answer 17

upper limbs 90 laterally so flexors lie anteriorly

lower limbs 90 medially so flexors lie posteriorly

Question 18

Which 3 axes does the limb develop along?

Answer 18

proximodistal- growing out form the body
craniocaudal- in the embryonic position it determines the fingers ending up in the correct position
dorsoventral- ensures the ventral surface is soft and the dorsal (nails and knuckles) are hard

Question 19

What controls the proximodistal outgrowth?

Answer 19

The lateral plate mesoderm secretes fibroblast growth factor 10
This induces a thickening of the overlying ectoderm alone the top of the limb bud- apical ectodermal ridge
The AER express Fgf 4 and 8 which cause apid proliferation of the cells underlying AER

Question 20

How is the dorsocentral growth controlled?

Answer 20

The dorsal ectoderm expressed Wnt7 and then ventral ectoderm expresses engrained-1 which inhibits Wnt7

Question 21

How is craniocaudal patterning controlled?

Answer 21

It is determined by the zone of polarising activity (ZPA) in the causal part of the limb bud where sonic hedgehog (Shi) is expressed. The concentration of Shh decreases as it moves crani ally. Low concs. Forms the thumb etc.

Question 22

When does digit formation occur?

Answer 22

Week 6

Question 23

How are fingers and toes formed?

Answer 23

Programmed cell apoptosis in the AER

Question 24

What is Amelia?

Answer 24

complete absence of a limb- early loss of Fgf signalling

Question 25

What is Meromelia?

Answer 25

Partial absence of a limb- later or partial loss of fgf signalling

Question 26

What is phocomelia?

Answer 26

The digits develop prematurely and the proximal elements of the limb are absent- can be due to thalidomide

Question 27

What is syndactyly?

Answer 27

Fished digits- failure of apoptosis to break out AER

Question 28

What is Polydactyly?

Answer 28

Extra digits- inappropriate/ectopic Shh expression

Question 29

What is ectrodactyly?

Answer 29

Split hand/foot- failure of the middle of the AER to form porperly