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A
B
Epidermis
Roof plate
Midbrain
Sensory
Neurons
Hindbrain
BMP
Somite
Forebrain
Shh
Spinal
Cord
Shh
Shh
Basal
Plate
Shh
Floor Plate
Shh
Alar Plate
Notochord
Floor Plate
Roof Plate
Cervical
Thoracic
C
+
RA
Class I
HD genes
-
+
+
-
Paraxial
Mesoderm
Notochord
-
Class II
HD genes
Shh
LMC
CT
+
HoxC5
HoxC8
HoxC6
HoxC9
-
Anterior HoxC genes
Posterior HoxC genes
+
+
-
FGF
RA
FIGURE 4.24 Specification of motor neurons in the vertebrate spinal cord. A. The neural tube, shown
here for a mouse, is subdivided into four longitudinal domains: the floorplate, basal plate, alar plate, and
roof plate. Motor neurons are derived from the basal plate. B. Schematic cross section of the neural tube. The
notochord, which is located underneath the floorplate, releases Sonic hedgehog (Shh) which induces the floor-
plate to release its own Shh . This forms a gradient in the neural tube with high concentrations ventrally and
low concentrations dorsally. BMP molecules released from the dorsal epidermis and roof plate form an oppos-
ing gradient. C. (Left) A gradient of Shh emanates from the notochord and floorplate; threshold levels of Shh
turn on Class II HD genes. Retinoic acid (RA) expressed by the paraxial mesoderm induces the expression
of Class I HD genes that are turned off more ventrally by threshold levels of Shh . Class I and Class II HD
transcription factors cross-repress each other, creating sharp definitive boundaries at different dorso-ventral
levels in the cord. Thus the boundary between Dbx and Nkx6 is more dorsal than the boundary between
Pax6 and Nkx2.2 . Between these boundaries, the OLIG2 bHLH transcription factor necessary for motor neuron
specification is turned on by the concerted action of RA, Nkx6 and Pax6. OLIG2 and RA are necessary for
the expression of motor neuron differentiation genes of the pMN family. (Below) A gradient of FGF8 emanates
from the mesoderm. High levels of FGF8 turn on more caudal HoxC genes, whereas RA and low levels of
FGF8 turn on rostral HoxC genes. Rostral and caudal HoxC transcription factors cross-repress each other, cre-
ating sharp definitive boundaries at different rostrocaudal levels in the cord. The boundary between Hoxc6
and Hoxc9 establishes the boundary between the LMC of the cervical cord and the CT of the thoracic cord.
expression and is sufficient to drive spinal progenitor
cells down a motor neuron pathway. This is shown by
experiments in which motor neurons arise dorsally when
Mnr2 is expressed ectopically in dorsal progenitors.
The motor neurons in the spinal cord are organized
into functional columns that project to different muscle
groups in the mature animal along the anterior to pos-
terior axis of the body. Thus, in the cervical region is
the Lateral Motor Column (LMC) that innervates fore-
limb muscles and in the mid-thoracic region is the
Column of Terni (CT) that innervates the sympathetic
chain. The anterior to posterior patterning of motor
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