Embryology Flashcards
What cells is the limb bud formed of?
It has a core of proliferating mesenchymal cells with an ectoderm covering.
When do the limb buds first appear?
Towards the end of the 4th week
What is the Apical Ectodermal Ridge and why is it critical for limb bud outgrowth?
A thickened layer of ectoderm at the apex of the limb bud.
It secretes inductive signals which act in the immediately underlying mesenchymal so that it remains undifferentiated and continues to divide.
AER also induces development of the digits.
What is the role of the AER in dorsoventral patterning of the limb bud?
The AER marks the boundary between the dorsal and ventral limb ectoderm, and exerts dorsalising and ventralising influences over the underlying mesenchyme.
What is the Zone of Polarising Activity (ZPA)?
A signalling centre at the posterior of the limb bud that controls proximal-distal axis generation, dorsoventral patterning and maintains the AER.
What is syndactyly?
Fusion of the digits. May just involve connective tissue (cutaneous) or bones may also be fused (osseous).
What is polydactyly?
Extra digits (genetic recessive trait)
What is Amelia?
Complete absence of a limb
What is meromelia?
Partial absence of one or more limb structures
What is phocomelia?
Partial absence of one or more limb structures produced by exposure to thalidomide, leading to reduced AER function.
Which way do the limbs rotate?
Upper limb rotates laterally
Lower limb rotates medially
What does rotation of the limbs result in?
In the upper limbs the flexor compartment is anterior and the extensor compartment is posterior
In the lower limbs the flexor compartment is posterior and the extensor is anterior.