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Flashcards in MSK - Embryology Deck (23):
1

Give a very brief overview of what structures of limbs develop from

Begins with activation of mesenchyme within the somatic layer of the lateral mesoderm

This somatic mesoderm forms the limb skeleton

Somites form the limb structure

2

When, where and what are limb buds?

Limb buds appear in the 4th week of development, lower limb buds lag two weeks behind the upper limbs

Limb buds appear on the ventro-lateral body wall and extend ventrally first

They consist of a mesenchyme (flexible mesoderm) core with a thickened ectoderm at the apex at the apical ectodermal ridge

3

How does elongation of a limb bud occur?

Elongation is through proliferation of the mesenchymal core

4

Describe the apical ectodermal ridge and its role in limb bud development

Critical for limb bud outgrowth

Orchestrates limb development proximally to distally AER keeps the mesenchyme underlying it undifferentiated (this proliferates and elongates the limb)

Proximal mesenchyme begins to differentiate into constituent tissues as it is too far away from the AER to receive signals to stay undifferentiated

Finally, AER induces development of the digits of the hand and foot plates then regresses

5

What is the zone of polarising activity and how does it influence development?

The ZPA is the signalling centre located at the posterior base of the limb bud It is responsible for the generation of asymmetry in the limbs

Controls both patterning and the maintains the apical ectodermal ridge

6

What is dorsoventral patterning?

The AER marks the boundary between dorsal and ventral ectoderm

Ectoderm exerts 'dorsalising' or 'ventralising' influences over the mesenchymal core

7

What are the important axes in limb development?

Give structures that control developmental changes along each axis

Antero-posterior:

- ZPA

Dorso-ventral:

- Dorsal/Ventral ectoderm

Proximal/Distal:

- AER

8

Describe the initial development of the hand and foot plates

AER controls development of widened plates at the distal region of the limbs

Mesenchyme condensations within the plates form cartilaginous models of the digital bones

AER then breaks up and is maintained only over the tips of the digital rays

Interdigital spaces sculpted by apoptosis

9

Describe the formation of bones in the limb buds

Signals from the AER to remain undifferentiated stop as AER moves away from region about to differentiate

Lateral plate mesoderm condenses and differentiates

Cartilage models form

Endochondral ossification occurs (primary and secondary ossification centres)

10

Describe the differentiation of muscle tissue

Myogneic precursors from the somites coalesce into two common muscle masses around newly formed skeletal elements

Ventral - Flexors 

Dorsal - Extensors

Individual muscles then split from the common masses

11

Describe rotation of the limbs during development

Both upper and lower limbs have flexor and extensor compartments but they are on opposite sides

As the limbs extend ventrally they rotate:

- Upper limb rotates laterally (thumb is lateral)

- Lower limb rotates medially (Big toe is medial)

12

Compare the positions of the limbs before and after rotation

Before:

- Thumbs up, elbows out

- Soles facing in, knees out

After:

- Thumbs out, elbows down

- Soles down, knees up

13

Describe limb innervation during development

Upper limb appear opposite the caudal cervical spine segments

Lower limb appears opposite the lumbar and sacral spinal segments

Spinal nerves from these areas enter the limb bud early in development, without this, development stalls

14

Describe how the brachial plexus relates to limb development

Anterior (ventral) compartment supplied by medial and lateral cords, these innervate flexors

Posterior (dorsal) compartment supplied by the posterior cord, this innervates extensors

15

Describe how the development of nails relates to their innervation

Nails develop from the nail fields at the tips of fingers then move dorsally, they bring their nerve supply with them (Median and ulnar)

16

What is meant by:

- Post-axial

- Pre-axial

Post-axial:

- Little finger and toe

Pre-axial:

- Thumb and big toe

17

Give a brief overview of limb defects

Upper limb affected more

6/10,000 live births

Rare, usually hereditary but teratogen induced has been seen

18

Give two common limb defects

Amelia:

- Complete absence of limb

Meromelia:

- Partial absence of one or more limbs E.g. Phocomelia (Thalidomide)

19

Give two digit defects

Syndactole:

- Lack of apoptosis between digits

- Digits fused

- May involve just connective tissues or bones

Polydactyly:

- Too many digits

- Genetic recessive trait

20

Label this diagram

What is this a diagram of?

Q image thumb

Labels (top to bottom):

- Undifferentiated mesenchyme

- AER

- Differentiating mesenchyme

This diagram shows a growing limb bud and the process of mesenchymal differentiation occuring as AER moves away from the mesenchyme in question

21

What does this diagram show?

Label the boxes

Q image thumb

This diagram shows the process of ectoderm overlying the limb bud directing dorsal or ventral differentiation in the underlying mesoderm (Dorsoventral patterning)

Blue:

- Dorsal ectoderm

- Dorsalising

Purple:

- Ventral ectoderm

- Ventralising

Green:

- AER (Dorsoventral boundary)

22

Label the boxes

What does the second picture (right) show?

Q image thumb

Top to bottom, left to right:

- AER

- Loose mesenchyme core ( of the Limb bud)

- ZPA

- Dorsal ectoderm

- Ventral ectoderm

 

Right image shows a limb bud in cross section

23

What does this image show?

Label the boxes

This image shows the regression of the AER around the limb plates and the process of apoptosis sculpting the digits

Labels left to right:

- Digital rays

- AER maintained

- AER regresses