Lecture - Embryology Flashcards
What does the limb skeleton develop from?
Somatic mesoderm
What does the limb musculature develop from?
The somites
When do limb buds appear?
Towards the end of week 4
Where do limb buds appear?
On the ventro-lateral body wall
What do limb buds consist of?
A core of mesenchyme, surrounded by ectoderm
What is the AER?
The apical ectodermal ridge - a thickening of ectoderm at the apex of the limb bud
What does the AER do?
Ensures the limb bud grows to the right length and ensures it develops proximally to distally
How does the AER ensure that the limb bud elongates?
Sends out signals to the immediately underlying mesenchyme which causes it to remain undifferentiated, and hence it continues to proliferate
How does the AER ensure the limb develops proximal-distal asymmetry?
The proximal mesenchyme is too far away from the AER to receive signals, and hence begins to differentiate.
What is the ZPA, and where is it found?
The zone of polarising activity
Found at the posterior base of the limb bud
What is the function of the ZPA?
Maintains the AER and controls patterning - ensures the limb develops anterior-posterior asymmetry
What marks the boundary between the dorsal and ventral limb ectoderm?
The AER
How does dorsoventral patterning occur?
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.
What is the dorsal-ventral foetal axis in an adult?
Anterior-posterior axis
What does the anterior-posterior foetal axis become after birth?
The superior-inferior axis
What controls anterior-posterior development?
The ZPA
What controls dorsal-ventral development?
The ectoderm
What controls proximal-distal development?
The AER
What are digital rays?
Condensations of mesenchyme within the hand and foot plates that eventually become cartilaginous models of the digital bones
Explain how the digits develop
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.
How do the digital bones form?
Signals from the AER stop, allowing the plate mesoderm to condense and differentiate into a cartilaginous model which then ossifies
How do the 2 common muscle masses form in the limbs?
Myogenic precursors migrate from the somites into the limb. These then merge into 2 common muscle masses.
Which muscle compartment is formed by the ventral muscle mass?
The flexor compartment
The extensor compartment is formed from which muscle mass?
The dorsal muscle mass
How is the limb innervated early in development?
Limb buds opposite spinal segments - spinal nerves grow into common muscle masses.
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.
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.
Which direction does the upper limb rotate in as it extends?
Laterally
Which way does the lower limb extend in as it grows?
Medially
Why is the flexor compartment of the upper limb anterior, but the flexor compartment of the lower limb posterior?
The upper limb rotates laterally as it extends, whereas the lower limb rotates medially as it extends.
What is “amelia”?
The complete absence of a limb
What is meromelia?
The partial absence of 1 or more limb structures
What is the type of meromelia called which occurred as a result of thalidomide exposure?
Phocomelia
What is it called when there is lack of apoptosis between the digits?
Syndactole
What is polydactyly?
Extra digits
How does limb development begin?
With the activation of mesenchyme within the somatic layer of lateral mesoderm
