Opposing morphogen gradients pattern the D-V axis of the
spinal cord
Dorsal neural
tube: BMP4
Ventral neural
tube & notochord:
Shh
Motor neuron differentiation depends on Shh and the notochord
Sonic
hedgehog
One notochord
Two notochords
Neuronal fate of the intermediate zone of the spinal cord is
“plastic” & dependent on levels of Shh
low Shh: dorsalized fate; high Shh: ventralized
Increasing BMP/dorsal roof plate ectoderm dorsalizes neurons
Neural plate folds into neural tube to generate brain & spinal
cord; neural crest will form sensory neurons, autonomic NS
BMP
vs.
Shh
Brain forms from neural tube with 3 divisions: largest= TE/DE (FB)
Frog mesoderm/endoderm involutes at blastopore, then extends
along A/P axis
during
gastrulation
Neural tissue
is induced by
mesoderm &
the notochord
(neurulation)
Head organizer forms, neurulation occurs where BMP is inactive
Head organizer forms from the first endoderm + mesoderm
Signaling molecules retinoic acid (RA), FGFs & Wnts further
divide the developing nervous system along the A-P axis
Retinoic acid (vitA derivative) is essential to neural plate posterior
patterning
RA acts via homeobox (Hox) gene transcription to control
specific positional coding along the A-P axis
High RA:
posteriorizing
increasing
RA 
loss of head
structures
RA secreted
by paraxial
mesoderm
at posterior
Position of homeobox genes on the chromosomes correlates with their
regional expression pattern
Hox genes: highly conserved in sequence and function
A
P
Each rhombomere in the
hindbrain contains specific
populations of cranial motor
neurons
Hox genes pattern cranial motor neurons (R1-R7) & the
vagus nerve (organs from heart to colon)
Homeobox gene mutations disrupt rhombomere development
and lead to cranial nerve deletions
Vertebrate brain has a signaling center at the MB/HB boundary
expressing homeobox gene Engr
Engr
Engr
Patterns of gene expression at the midbrain-hindbrain border
Irx: activates
Otx2 & Gbx2
Fgf8: regulates
growth of NPCs
Fgf8 is a critical component of the MB-HB organizer
Opposing gradients of Emx2 & Pax6 establish brain regions
-/-
Fgf8: functions at MB/HB boundary, & in anterior FB
Fgf8 and BMP expression position the somatosensory map
Fgf8
BMP
Increased anterior Fgf8 displaces SS map to posterior
Fgf8 at the posterior causes duplication of the SS map
Ectopic Fgf8 at posterior leads to mirror duplication of the
barrel cx
Cyt oxidase staining (layer IV)
The fruit fly nervous system arises from ectoderm adjacent to
ventral mesoderm
Neuroblast
delamination
In Drosophila
NB
GMCs (many)
1 GMC
pair neurons
or glia
GMC= ganglion
mother cell
Embryonic ectodermal cells can switch their fate to neural
Lateral inhibition via As-c signaling generates a neuroblast
more Asc
Proneural cluster Asc+
Delta ON
Lateral inhibition Notch
Notch ON/Delta OFF
(neighbors)
NB selected
Neuroblast Asc+
& epidermal cells
Proneural cell clusters initially express achaete-scute
… then the clusters become refined to 1 As-c+ NB