Lecture - Embryology

Question 1

What does the limb skeleton develop from?

Answer 1

Somatic mesoderm

Question 2

What does the limb musculature develop from?

Answer 2

The somites

Question 3

When do limb buds appear?

Answer 3

Towards the end of week 4

Question 4

Where do limb buds appear?

Answer 4

On the ventro-lateral body wall

Question 5

What do limb buds consist of?

Answer 5

A core of mesenchyme, surrounded by ectoderm

Question 6

What is the AER?

Answer 6

The apical ectodermal ridge - a thickening of ectoderm at the apex of the limb bud

Question 7

What does the AER do?

Answer 7

Ensures the limb bud grows to the right length and ensures it develops proximally to distally

Question 8

How does the AER ensure that the limb bud elongates?

Answer 8

Sends out signals to the immediately underlying mesenchyme which causes it to remain undifferentiated, and hence it continues to proliferate

Question 9

How does the AER ensure the limb develops proximal-distal asymmetry?

Answer 9

The proximal mesenchyme is too far away from the AER to receive signals, and hence begins to differentiate.

Question 10

What is the ZPA, and where is it found?

Answer 10

The zone of polarising activity

Found at the posterior base of the limb bud

Question 11

What is the function of the ZPA?

Answer 11

Maintains the AER and controls patterning - ensures the limb develops anterior-posterior asymmetry

Question 12

What marks the boundary between the dorsal and ventral limb ectoderm?

Answer 12

The AER

Question 13

How does dorsoventral patterning occur?

Answer 13

The AER marks the boundary between the dorsal and ventral ectoderm. The dorsal ectoderm sends out signals to the underlying mesoderm telling it it is dorsal, and the same occurs with the ventral ectoderm.

Question 14

What is the dorsal-ventral foetal axis in an adult?

Answer 14

Anterior-posterior axis

Question 15

What does the anterior-posterior foetal axis become after birth?

Answer 15

The superior-inferior axis

Question 16

What controls anterior-posterior development?

Answer 16

The ZPA

Question 17

What controls dorsal-ventral development?

Answer 17

The ectoderm

Question 18

What controls proximal-distal development?

Answer 18

The AER

Question 19

What are digital rays?

Answer 19

Condensations of mesenchyme within the hand and foot plates that eventually become cartilaginous models of the digital bones

Question 20

Explain how the digits develop

Answer 20

As the limb develops, the AER breaks up and is only maintained over the tips of the digital rays, allowing elongation of these areas. Apoptosis then occurs between them, forming the interdigital spaces.

Question 21

How do the digital bones form?

Answer 21

Signals from the AER stop, allowing the plate mesoderm to condense and differentiate into a cartilaginous model which then ossifies

Question 22

How do the 2 common muscle masses form in the limbs?

Answer 22

Myogenic precursors migrate from the somites into the limb. These then merge into 2 common muscle masses.

Question 23

Which muscle compartment is formed by the ventral muscle mass?

Answer 23

The flexor compartment

Question 24

The extensor compartment is formed from which muscle mass?

Answer 24

The dorsal muscle mass

Question 25

How is the limb innervated early in development?

Answer 25

Limb buds opposite spinal segments - spinal nerves grow into common muscle masses.

Question 26

With reference to the development of the limbs, explain why the medial and lateral cords of the brachial plexus innervate the flexor compartment of the arm.

Answer 26

The anterior divisions of the relevant spinal nerves innervate the ventral muscle mass, which eventually becomes the anterior, or flexor, compartment. The anterior divisions then remodel to form the medial and lateral cords.

Question 27

Which direction does the upper limb rotate in as it extends?

Answer 27

Laterally

Question 28

Which way does the lower limb extend in as it grows?

Answer 28

Medially

Question 29

Why is the flexor compartment of the upper limb anterior, but the flexor compartment of the lower limb posterior?

Answer 29

The upper limb rotates laterally as it extends, whereas the lower limb rotates medially as it extends.

Question 30

What is “amelia”?

Answer 30

The complete absence of a limb

Question 31

What is meromelia?

Answer 31

The partial absence of 1 or more limb structures

Question 32

What is the type of meromelia called which occurred as a result of thalidomide exposure?

Answer 32

Phocomelia

Question 33

What is it called when there is lack of apoptosis between the digits?

Answer 33

Syndactole

Question 34

What is polydactyly?

Answer 34

Extra digits

Question 35

How does limb development begin?

Answer 35

With the activation of mesenchyme within the somatic layer of lateral mesoderm