Session 1 Lecture - Development of the limbs

Question 1

When do the limb buds appear?

Answer 1

End of 4th week of embryonic development

Question 2

Which develop first - the upper or lower limbs?

How many days difference between their development?

Answer 2

The upper limbs - the embryo develops in a head to tail sequence
The lower limbs develop 2 days later than the upper limbs

Question 3

What are the limb buds composed of?

Answer 3

Core of proliferating (multiplying) mesenchymal cells with an ectoderm covering

Question 4

What are mesenchymal cells?

Answer 4

Undifferentiated cells

Question 5

How does the limb bud elongate?

Answer 5

Through proliferation of the mesenchymal core

Question 6

The thickened ectoderm at the apex of the limb bud is known as what?

Answer 6

The AER - the apical ectodermal ridge

Question 7

What are the 3 spatial axis in limb development?

Answer 7

1. shoulder to fingertips (proximal to distal)
2. palmar surface to dorsal surface (dorsal to ventral)
3. side to side (anterior to posterior)

Question 8

What controls the anterior to posterior axis?

Answer 8

The ZPA - zone of polarising activity

Question 9

What controls the proximal to distal axis?

Answer 9

The AER - Apical ectodermal ridge

Question 10

What controls the dorsal to ventral axis?

Answer 10

the ectoderm (covering the limb bud)

Question 11

What are the final stages of the AER development?

Answer 11

Appearance of paddles (the limb bud flares) and then the AER regresses (cell apoptosis)

Question 12

What effect does the AER have on underlying mesenchyme core?

Answer 12

It keeps the mesenchyme undifferentiated

As the mesenchyme proliferates the proximal mesenchyme begins to differentiate into tissues

Question 13

Where is the ZPA located?

Answer 13

The zone of polarising activity is a signalling centre located at the posterior base of the limb bud

Question 14

What does the ZPA maintain?

Answer 14

The zone of polarising activity maintains the AER - the apical ectodermal ridge

Question 15

What happens if you implant a second ZPA?

Answer 15

A second zone of polarising activity would result in duplication of the digits

Question 16

What is happening to the mesenchymal cells as AER regresses?

Answer 16

As the apical ectodermal ridge regresses the mesenchymal cells start to differentiate, condensing and developing into cartilaginous models of bones. They will create digital rays (continuous digit of hand or foot)

Question 17

In the hand and foot plates, where does AER remain and where does it go through apoptosis?

Answer 17

Apical ectodermal ridges remains at the tips of the digital rays (continuous digit of hand or foot) and goes through apoptosis (programmes cells death) in the interdigital spaces

Question 18

What is the term used to describe fusion of digits?

Answer 18

Syndactyly

Question 19

What is the term used to describe extra digits?

Answer 19

Polydactyly

This is a genetic recessive trait

Question 20

What is the term used to describe a complete absence of limb?

Answer 20

Amelia

Question 21

What is the term used to describe partial absence of one of more limb structures?

Answer 21

Meromelia

Question 22

What is the term used to describe missing arms, forearms, thighs and legs (where hands and feet are attached directly to trunk)?

Answer 22

Phocomelia

Question 23

Give three ways in which a problem may occur in morphogenesis (embryo developing its shape)?

Answer 23

1. Malformation - error in co-ordination of morphogenesis (usually a signalling problem)
2. Deformation - due to construction bands (cutting off circulation to part of the embryo and causing the limb to fall off)
3. Disruption - e.g. through infections such as rubella or through cancer treatment (such as thalidomide)

Question 24

Define caudal

Answer 24

Near the tail or posterior of the body

Question 25

The upper limb bud appears opposite which spinal segments?

Answer 25

Caudal cervical spinal segments

Question 26

The lower limb bud appears opposite which spinal segments?

Answer 26

The lumbar and sacral spinal segments

Question 27

Where do myogenic precursors migrate into the limbs from?

Answer 27

Somites (body segments)

Question 28

What do myogenic precursors coalesce into? (come together as)

Answer 28

2 common muscle masses - individual muscles split from common masses

Question 29

In muscle development, where are the flexors located and where are the extensors located?
(one is ventral and one is dorsal)

Answer 29

Ventral (directed towards front) - flexor

Dorsal (directly towards back) - extensor

Question 30

In which direction do limbs first extend?

Answer 30

Ventrally (towards the front). Palms of hands and soles of feet are facing one another

Question 31

As limbs elongate they rotate. Which direction do the upper and lower limbs rotate?

Answer 31

Upper limbs - laterally (away from midline

Lower limbs - medially (towards midline)