A role for midbrain arcs in nucleogenesis
Seema Agarwala
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Clifton W. Ragsdale
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Department of Neurobiology, Pharmacology and Physiology, The University of Chicago
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Chicago, IL 60637, USA
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Nuclei are fundamental units of vertebrate brain
organization, but the mechanisms by which they are
generated in development remain unclear. One possibility
is that the early patterning of brain tissue into reiterated
territories such as neuromeres and columns serves to
allocate neurons to distinct nuclear fates. We tested this
possibility in chick embryonic ventral midbrain, where a
periodic pattern of molecularly distinct stripes (midbrain
arcs) precedes the appearance of midbrain nuclei. We
found that midbrain arc patterning has a direct
relationship to the formation of nuclei. Both differential
homeobox gene expression and diagnostic axon tracing
studies established that the most medial arc contains
primordia for two major midbrain nuclei: the oculomotor
complex and the red nucleus. We tested the relationship of
the medial arc to oculomotor complex and red nucleus
In the adult brain, many neurons are organized into
welldelineated clusters of characteristic shape and size. The role of
these clusters, known as nuclei, is to organize neuronal
circuitry. Thus, for a brain neuron, the projections of its axons,
the neurotransmitters it employs and the local and
longdistance inputs it receives, are governed by its nuclear
assignment. Despite major recent insights into the early
patterning events of the neural tube (Jessell and Lumsden,
1997), very little is known about the cellular and molecular
patterning mechanisms that generate discrete brain nuclei of
appropriate size, location and cell-type composition.
The ventral midbrain provides an elegant model system for
addressing the mechanisms of brain nucleogenesis. The adult
midbrain tegmentum exhibits a complex structural architecture
featuring spherical, ovate and plate-shaped nuclei. During
embryogenesis, this nuclear organization is preceded by a more
regular organization into a reiterated series of arcuate
territories called midbrain arcs (Agarwala et al., 2001; Sanders
et al., 2002). Arcs are known to be molecularly distinct,
differing, for example, in their expression of homeobox genes
(Agarwala et al., 2001). Whether the arcs simply reflect a
mechanism for regulating the numbers of distinct midbrain
cell-types or serve a more specific role in allocating neurons to
particular nuclear fates is unknown.
The possibility that developmental periodicities might
development by perturbing arc pattern formation in Sonic
Hedgehog and FGF8 misexpression experiments. We found
that Sonic Hedgehog manipulations that induce ectopic
arcs or expand the normal arc pattern elicit precisely
parallel inductions or expansions of the red nucleus and
oculomotor complex primordia. We further found that
FGF8 manipulations that push the medial arc rostrally
coordinately move both the red nucleus and oculomotor
complex anlagen. Taken together, these findings suggest
that arcs represent a patterning mechanism by which
midbrain progenitor cells are allocated to specific nuclear
fates.
provide a patterning substrate for nucleogenesis has been
addressed in connectional and fate-mapping studies of
hindbrain rhombomeres. The striking finding is that most
brainstem nuclei are generated from serially adjoining sets
of rhombomeres with few nuclei developing from single
rhombomeres (Cramer et al., 2000; Diaz et al., 1998; Lumsden
and Keynes, 1989; Marin and Puelles, 1995). Thus, the extent
to which anteroposterior periodicities are crucial to hindbrain
nuclear specification remains uncertain (Wingate and
Lumsden, 1996). Indeed, the generation of hindbrain nuclei,
many of which form longitudinal columns, may be more tightly
regulated in the mediolateral dimension (Clarke et al., 1998;
Marin and Puelles, 1995). This observation is of particular
interest because midbrain arcs, unlike hindbrain rhombomeres,
are arrayed along the mediolateral axis parallel to the ventral
midline.
In these studies, we explored the relationship between arc
pattern formation and the generation of midbrain nuclei by
focusing on the most medial arc as a prototypical arc. Using
diagnostic connectional and molecular criteria we identified
the anlagen for at least two midbrain nuclei within the medial
arc: the oculomotor complex (OMC) and the red nucleus
(RN). Both of these nuclei are part of the motor system,
but their functions and connections are so dissimilar that a
shared origin in the same arc was unexpected. The OMC
contains motoneurons that control eye movements and the
parasympathetic regulation of accommodation and pupil
contraction (Evinger, 1988). The RN, by contrast, contains no
motoneurons but is a cerebellar-related nucleus mediating
motor cortex and cerebellar outflow to spinal cord in the
control of limb movements (Holstege and Tan, 1988; Keifer
and Houk, 1994; ten Donkelaar, 1988). The two medial arc
pronuclei are also molecularly distinct, differing (...truncated)