3.1 Development of the limbs Flashcards
Where do the limb buds appear?
On the ventro-lateral body wall
In what order do the limb buds develop?
Upper limb slightly before the lower limb
When do the limb buds appear?
End of the forth week after fertilisation
What are the limb buds made up from?
Made up of a core of proliferating mesenchymal cells with an ectoderm covering
How does limb development begin?
Activation of the mesenchyme within the lateral mesoderm
What does remains of the notochord become in adults?
The nucleus pulposus
How do the limb buds grow?
Proliferation of the mesenchyme core
What is the thickened ectoderm at the apex of the limb bud called?
Apical ectodermal ridge (AER)
What is the function of the AER?
Drives elongation in a proximal to distal manner.
How does the AER cause proliferation?
Makes the underlying mesenchyme remain undifferentiated.
What induces development of the hand/foot plates?
The AER
What boundary does the AER mark?
The boundary between the dorsal and ventral limb ectoderm.
What is the ZPA?
Zone of polarising activity
Signalling centre located at the posterior base of the limb bud. It generates asymmetry in the limb and maintains the AER.
Explain how the hand and foot plates are formed
Digital plates form in the mesenchyme. They are the cartilaginous models of the digital bones. Apoptosis then occurs between the digits. The AER is only maintained over the digital plates. The digits are then sculpted.
What is syndactyly?
Fusion of digits
What is polydactyly?
Extra digits
What is amelia?
Complete absence of a limb
What is meromelia?
Partial absence of one or more limb structures
How do bones form from the cartilage skeleton?
Endochondrial ossification
How do bones grow in children?
Epiphyseal plates
Explain how muscles form
Myogenic precursors migrate into the limbs from the somites. They then unite to form two common muscle masses around the newly formed skeleton. Individual muscles then split from the common masses.
In what direction does the upper limb rotate?
Laterally
In what direction does the lower limb rotate?
Medially
Why is innervation important early on in limb formation?
To continue development
Where does the MSK system originate from?
Somites
Lateral plate mesoderm
