Revision: Embryology

Question 1

where do limbs develop from?

Answer 1

somatic mesoderm -> activation of mesenchyme -> limb skeleton

somites form limb musculature

Question 2

limb buds

Answer 2

appear towards the end of week 4, l/limb about 2 days before u/limb

appear on ventro-lateral body wall and extend vertically first

consist of mesenchyme (flexible mesoderm) core w/ a thickened ectoderm at apex, the AER

elongation is through proliferation of mesenchyme core

Question 3

AER

Answer 3

apical ectodermal ridge

proximal-distal development

critical for limb bud growth

orchestrates limb development proximal -> distal

keeps the mesenchyme beneath it undifferentiated -> proliferates -> elongates

proximal mesenchyme begins to differentiate

AER then stimulates development of the digits w/in hand/foot plates and regresses

Question 4

ZPA

Answer 4

anterior-posterior development

zone of polarising activity

located in posterior base of limb bud

responsible for generation of asymmetry in limbs

controls the patterning and maintains the AER

Question 5

dorsoventral patterning

Answer 5

AER marks boundary between the dorsal and ventral limb ectoderm

exerts both dorsal and ventral patterning over the mesenchyme core

Question 6

axes

Answer 6

Question 7

hand and foot development

Answer 7

digital rays: mesenchyme condensations w/in plates forming cartilaginous models of the digital bones, AER is broken up and only maintained over the tips of the digital rays, interdigital spaces are progressively sculpted through apoptosis

Formation of bones: signals from AER to remain undifferentiated stop, lateral plate mesoderm condenses and differentiates, cartilage model forms, endochondral ossification, primary and secondary ossification centres form

Question 8

compartmentalisation of musculature

Answer 8

myogenic precursors migrate into limbs from somites and coalesce into two common muscle masses around skeleton - ventral (flexor) and dorsal (extensor), individual muscles then split from these common masses

Question 9

rotation of limbs

Answer 9

both u/ and l/ limbs have flexor and extensor compartments, however they are on the opposite sides

therefore, as the limb extends ventrally, they rotate

u/limb rotates laterally -> thumb is lateral

l/limb rotates medially -> big toe is medial

Before rotation: thumbs up, elbows out/soles facing in, knees out

After rotation: thumbs out, elbows down/soles down knees up

Question 10

development of limb innervation

Answer 10

u/limb bud appears opposite caudical cervical spinal segments, l/limb bud appears opposite lumbar and sacral spinal segments

these spinal nerves enter the limb bud early during development, w/o this development will stall

Question 11

development of brachial plexus

Answer 11

muscles are compartmentalised and nerves grow into the common muscle masses

anterior divisions for the ventral compartment regroup to form medial and lateral cords -> therefore they supply the flexors

posterior divisions for the dorsal compartment regroup to form the posterior cord -> therefore they supply the extensors

Question 12

nail development

Answer 12

develop from nail fields on the tips of the fingers, nail fields migrate onto the dorsal surface, bringing the nerve supply w/ them

Question 13

common developmental abnormalities

Answer 13

limb defects - u/ affected more than l/limb, the lag time of 2 days between them means that they have different critical periods, 6/10,000 births

limb: amelia - complete absence of a limb, meromelia - partial absence of 1/more limb structures, eg phocomelia from thalidomide
digits: syndactole - lack of apoptosis between digits leading to fused digits, poldactyly - too many digits