neural crest cells migrate along anterior portion of somite to periphery
fibronectin and laminin not localized and cannot account for migration
posterior of somite also contains inhibitory elements – more likely, not well known
final differentiation influenced by environment during and after migration
evidence suggests combo of multipotent and committed precursors in neural crest
GGF promotes glial differentiation
from one precursor (multipotent):
FGF/NGF promotes neural cells
glucocorticoids promote adrenal chromaffin cells
Axon Outgrowth
axons establish circuits with specific targets via directed navigation
regulated by mech constraints, molecular and cellular composition of substrate, diffusible factors
growth cones at tips of axon send out contacts to environment
selective adhesion is important factor in direction
mylenation of axons – performed by Schwann cells
signals from neuron regulate proliferation and differentiation of Schwann cells
signals from Schwann cells influence electrical properties of axon
one axon per Schwann cell, many Schwann per axon
defects can result in Charcot-Marie-Tooth disease
satellite cells – surround cell bodies in ganglia
Molecular Mediators of Neuron Survival
many neurons die during normal development
up to 75% in some areas
often due to competition for same target
many neurons produced for a few targets, losers die
appears that it is a competition for trophic (nourishing) molecules produced by the target cells (such as nerve growth factor (NGF)), so target cells control life/death
Acquisition of Transmitter Properties
must have correct matching of neurotransmitter and receptors
environmental factors very important here
adrenergic can be stimulated to become cholinergic and vv in vitro
target tissues important part of environmental influence