E - Limb Development Flashcards
When does upper and lower limb development begin?
4th week of gestation
When are the limbs fully differentiated in gestation?
Week 8
What are the precursors to limbs?
Limb buds
How is limb bud formation initiated?
Activation of mesenchymal cells in the somatic layer of lateral plate mesoderm
Where do the limb buds appear and how do they extend?
On the ventrolateral body wall and extend ventrally.
What do limb buds consist of?
Undifferentiated mesenchyme and tipped with a layer of ectoderm called the apical ectodermal ridge (AER)
Elongation of limb buds occurs through…
Proliferation of the underlying mesenchymal core
What is the crucial role of the AER?
To ensure that the mesenchyme underneath it remains undifferentiated.
As limb bud growth proceeds…
The proximal mesenchyme loses signals from the AER and differentiates into the constituent tissues of the limb
What is the AER maintained by?
The zone of polarising activity
What does the ZPA do?
To maintain the AER and ensure asymmetry in the limbs
Where is the ZPA located?
The posterior base of the limb bud
Why is the position of the AER important?
It marks the boundary between the dorsal and ventral ectoderm. This therefore allows the ectoderm to extert ventralising and dorsalising influences on the mesenchyme core
The formation of digits starts with…
The mesenchyme condensing into plates to form the cartilaginous models for future digital bones
When digits are formed, the AER…
Breaks up and maintained only on the tips of future digits
