Control of Cell Pattern in the Developing Nervous System 
Floor Plate-Specific Genes 
in Axon Guidance 
After the identity of motor neurons and other 
spinal cord neurons has been established, the 
floor plate appears to provide both long-range 
and local guidance cues that promote the growth 
of axons to and across the ventral midline of the 
jspinal cord. First, the floor plate secretes a diffu- 
sible chemoattractant that can orient the growth 
of axons of commissural neurons in vitro and 
may account for the homing of these axons to the 
floor plate in vivo. Second, the floor plate may 
contribute to the change in trajectory of commis- 
sural axons from the transverse to the longitu- 
dinal plane that occurs immediately after cross- 
ing the ventral midline. In support of this 
proposal, genetic mutations in mice and zebra 
fish that result in the absence of the floor plate 
during embryonic development lead to errors in 
the pathfinding of commissural axons at the mid- 
line of the spinal cord. Third, the floor plate may 
promote the fasciculation of commissural axons 
that occurs after they cross the midline of the spi- 
nal cord by regulating the expression of glyco- 
proteins of the immunoglobulin superfamily. 
The specialized role of the floor plate in verte- 
brate neural development has parallels in inver- 
tebrate organisms, in that cells at the midline of 
the embryonic Drosophila and C. elegans central 
nervous systems have been implicated in neural 
patterning and axon guidance. 
To identify molecules that may mediate the di- 
verse functions of the floor plate during early 
neural development, we have used subtractive hy- 
bridization techniques to isolate cDNA clones ex- 
pressed selectively by the floor plate. One of 
these cDNA clones encodes a novel secreted pro- 
tein, F-spondin, which is expressed at high levels 
by the rat floor plate during embryonic develop- 
ment. F-spondin contains domains similar to 
those present in thrombospondin and other pro- 
teins implicated in cell adhesion and neurite out- 
growth. In vitro assays show that F-spondin 
promotes neural cell adhesion and neurite 
outgrowth, suggesting that its secretion by the 
floor plate contributes to the growth and guid- 
ance of axons in the developing central nervous 
system. 
The F-spondin protein may be associated with 
the extracellular matrix, since it has several clus- 
ters of basic residues that function as glycosami- 
noglycan-binding domains in other secreted pro- 
teins. The restricted distribution of F-spondin 
mRNA in the embryonic nervous system contrasts 
with the distribution of other secreted glycopro- 
teins that promote neural cell adhesion and neu- 
rite outgrov^h. For example, the expression of 
F-spondin mRNA is more restricted than that of 
thrombospondin and of tenascin, which appear 
to be expressed widely in the embryonic central 
nervous system. 
The prominent expression of F-spondin in the 
floor plate suggests that the protein may be in- 
volved in the plate's development or functions. 
Midline neural plate cells that give rise to the 
floor plate undergo marked changes in cell shape 
during the closure of the neural tube. Thus one 
possible function of F-spondin could be to medi- 
ate adhesive plate cell interactions that maintain 
the integrity of the floor plate during formation 
of the embryonic spinal cord. 
The expression of F-spondin mRNA in floor 
plate cells is highest at the time of the plate's 
suggested role in the chemotropic guidance of 
commissural axons. However, recombinant F- 
spondin does not mimic the ability of the floor 
plate-derived chemoattractant to promote the 
outgrowth of commissural axons from dorsal spi- 
nal cord explants. This suggests that F-spondin 
may not be involved in the long-range guidance 
of commissural axons to the floor plate, at least 
through chemotropism. 
F-spondin is more likely to be involved in the 
contact-dependent guidance of commissural 
axons once they reach the ventral midline of the 
spinal cord under the influence of distinct che- 
motropic guidance cues. The growth cones of 
commissural neurons cross the midline by grow- 
ing between the basal surface of floor plate cells 
and the underlying basal lamina. Floor plate- 
secreted F-spondin may accumulate at high levels 
in association with the basal surface of floor plate 
cells or with the underlying basal lamina, thus 
generating a difference in adhesive properties of 
the floor plate and the lateral neuroepithelium. 
The growth cones of commissural neurons may 
adhere preferentially to F-spondin, prompting 
them to change trajectory at the boundary of the 
floor plate and lateral neuroepithelium. It is also 
possible that F-spondin has a more active signal- 
ing role that induces changes in the properties of 
commissural growth cones, permitting them to 
respond to other midline guidance cues. 
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