proximal to distal growth of the limb skeleton
o The mesoderm closest to the AER is referred to as
the
progress zone (PZ)
A zone which focuses on proliferation instead of
differentiation
o Meanwhile, the mesoderm far from the AER focuses
on differentiation instead of proliferation
Growth factors are not saturated in this area
The mesoderm here differentiates into cartilage
and bone
The most proximal area forms the
stylopod,
which differentiates into the
humerus
As the progress zone keeps extending/moving distally,
the relatively proximal areas left behind differentiate into
cartilage and bone
o This forms the zeugopod, which differentiates into
the
forearm
o The autopod differentiates into:
Carpal bones
Metacarpals
Phalanges
Some tissue in the AER will undergo apoptosis due to
bone morphogenetic protein (BMP) and retinoic acid
o The apoptosis of these tissues leads to the formation
of
fingers
o AER still releases FGF, which stimulates the progress
zone to proliferate/extend the fingers/digits
Eventually, the AER will start dying and the FGF levels
will drop
o As a result, the progress zone will stop proliferating
and just differentiate
In summary, the AER drives the activity of the progress
zone
Figure 7. Development of the Limb Buds
(C) DORSAL-VENTRAL DIFFERENTIATION
Within the hand, the dorsal portion has condensing tissue
called the
zone of polarizing activity (ZPA), which
drives the anterior and posterior development of the
thumb
The ZPA releases molecules which create a gradient that
stimulates the differences in our fingers
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