Previous studies have suggested that interactions with other axons are important in sensory axon pathfinding in the developing chick hindlimb. Yet the nature of these interactions remains unknown, in part because information about the spatial relationships among the different kinds of axons is lacking. To obtain this information, we combined retrograde axonal tracing with an immunofluorescent labelling approach that distinguishes between sensory and motoneuron axons. This allowed us to follow the trajectories of sensory axons having a known destination, while also identifying their neighbors. We found that as sensory and motoneuron axons meet in the spinal nerves and travel into the limb, sensory axons remain bundled together. The large bundles that are present proximally gradually split into smaller bundles as the axons course distally in the spinal nerves; more distally, some bundles join to again form large bundles. Younger, later-growing sensory axons appear to grow primarily along bundles of older sensory axons that grew out earlier. Starting from very proximal levels, axons projecting along an individual cutaneous nerve are found together in bundles that are situated in characteristic regions of each spinal nerve. Some of these bundles are initially interspersed with bundles of axons projecting along other nerves, thereby indicating that the initial position of a cutaneous axon in the spinal nerves does not strictly determine its subsequent trajectory. As they travel distally, bundles of axons projecting along one cutaneous nerve gradually join one another, becoming increasingly separated from axons having different destinations. In contrast, muscle sensory axons are situated adjacent to motoneuron axons innervating the same muscle for much of their course. This suggests that muscle sensory axons may be guided to the appropriate muscles by fasciculating along motoneuron axons. Taken together, the results show that sensory axons projecting along different nerves are different from one another and respond to cues in their environment to navigate through the spinal nerves and plexus. Thus, sensory neurons must be intrinsically specified with respect to their peripheral targets. Sensory axons appear to respond differentially to the axons they encounter, segregating from axons that project along different nerves and often growing with axons destined for the same nerve, suggesting that fasciculation may aid pathfinding.

REFERENCES

Dahm
L.
,
Landmesser
L. T.
(
1988
)
The regulation of intramuscular nerve branching during normal development and following activity blockade.
Dev. Biol
130
,
621
644
Halfter
W.
,
Yip
Y. P. L.
,
Yip
J. L.
(
1994
)
Axonin-1 is expressed primarily in subclasses of avian sensory neurons during outgrowth.
Dev. Brain Res
78
,
87
101
Hall
A. K.
,
Ai
X.
,
Hickman
G. E.
,
MacPhedran
S. E.
,
Nguaguba
C. O.
,
Robertson
C. P.
(
1997
)
The generation of neuronal heterogeneity in a rat sensory ganglion.
J. Neurosci
17
,
2775
2784
Hamburger
V.
(
1975
)
Cell death in the development of the lateral motor column of the chick embryo.
J. Comp. Neurol
160
,
535
546
Hamburger
V.
,
Hamilton
H. L.
(
1951
)
A series of normal stages in the development of the chick embryo.
J. Morphol
88
,
49
92
Hamburger
V.
,
Yip
J. L.
(
1984
)
Reduction of experimentally induced neuronal death in the spinal ganglia of the chick embryo by nerve growth factor.
J. Neurosci
4
,
767
774
Honig
M. G.
(
1982
)
The development of sensory projection patterns in embryonic chick hindlimb.
J. Physiol
330
,
175
202
Honig
M. G.
,
Burden
S. M.
(
1993
)
Growth cones respond in diverse ways upon encountering neurites in cultures of chick dorsal root ganglia.
Dev. Biol
156
,
454
472
Honig
M. G.
,
Kueter
J.
(
1995
)
The expression of cell adhesion molecules on the growth cones of chick cutaneous and muscle sensory neurons.
Dev. Biol
167
,
563
583
Honig
M. G.
,
Lance-Jones
C.
,
Landmesser
L. T.
(
1986
)
The development of sensory projection patterns in embryonic chick hindlimb under experimental conditions.
Dev. Biol
118
,
532
548
Honig
M. G.
,
Rutishauser
U. S.
(
1996
)
Changes in the segmental pattern of sensory neuron projections in the chick hindlimb under conditions of altered cell adhesion molecule function.
Dev. Biol
175
,
325
337
Lance-Jones
C.
,
Dias
M.
(
1991
)
The influence of presumptive limb connective tissue on motoneuron axon guidance.
Dev. Biol
143
,
93
110
Lance-Jones
C.
,
Landmesser
L. T.
(
1980
)
Motoneurone projection patterns in the chick hind limb following early partial reversals of the spinal cord.
J. Physiol
302
,
581
602
Lance-Jones
C.
,
Landmesser
L. T.
(
1981
)
Pathway selection by chick lumbosacral motoneurons during normal development.
Proc. Roy. Soc. London
214
,
1
18
Lance-Jones
C.
,
Landmesser
L. T.
(
1981
)
Pathway selection by embryonic chick motoneurons in an experimentally altered environment.
Proc. Roy. Soc. London
214
,
19
52
Landmesser
L. T.
(
1978
)
The development of motor projection patterns in the chick hind limb.
J. Physiol
284
,
391
414
Landmesser
L. T.
,
Honig
M. G.
(
1986
)
Altered sensory projections in the chick hindlimb following the early removal of motoneurons.
Dev. Biol
118
,
511
531
McConnell
S.
,
Kaznowski
C.
(
1991
)
Cell cycle dependence of laminar determination in developing neocortex.
Science
254
,
282
285
Oakley
R. A.
,
Lefcort
F. B.
,
Clary
D. O.
,
Reichardt
L. F.
,
Prevette
D.
,
Oppenheim
R. W.
,
Frank
E.
(
1997
)
Neurotrophin-3 promotes the differentiation of muscle spindle afferents in the absence of peripheral targets.
J. Neurosci
17
,
4262
4274
Scott
S. A.
(
1986
)
Skin sensory innervation patterns in embryonic chick hindlimb following dorsal root ganglion reversals.
J. Neurobiol
17
,
649
668
Thiery
J.-P.
,
Delouvee
A.
,
Grumet
M.
,
Edelman
G. M.
(
1985
)
Initial appearance and regional distribution of the neuron-glia cell adhesion molecule in the chick embryo.
J. Cell Biol
100
,
442
456
Tosney
K. W.
,
Landmesser
L. T.
(
1984
)
Pattern and specificity of axonal outgrowth folllowing varying degrees of chick limb bud ablation.
J. Neurosci
4
,
2518
2527
Tosney
K. W.
,
Landmesser
L. T.
(
1985
)
Development of the major pathways for neurite outgrowth in the chick hindlimb.
Dev. Biol
109
,
193
214
Tosney
K. W.
,
Landmesser
L. T.
(
1985
)
Growth cone morphology and trajectory in the lumbosacral regions of the chick embryo.
J. Neurosci
5
,
2345
2358
Tyrrell
S.
,
Schroeter
S.
,
Coulter
L.
,
Tosney
K. W.
(
1990
)
Distribution and projection pattern of motoneurons that innervate hindlimb muscles in the quail.
J. Comp. Neurol
298
,
413
430
Wang
G.
,
Scott
S. A.
(
1997
)
Muscle sensory innervation patterns in embryonic chick hindlimbs following dorsal root ganglion reversal.
Dev. Biol
186
,
27
35
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