Control of Cell Pattern in the Developing Nervous System 
Although these studies implicate the floor 
plate as a source of retinoids, definite evidence 
that the plate, or in fact any vertebrate tissue, can 
release morphogenetically active retinoids is still 
lacking. We therefore devised a sensitive and spe- 
cific assay to detect the release of morphogenetic 
retinoids from neural tissues. This has provided 
further evidence that the floor plate does indeed 
release retinoids. The basis of this assay is to use a 
fragment of DNA, which acts as a retinoic acid- 
responsive element, to drive expression of a re- 
porter gene in tissue culture cells. Using F9 cells 
transfected with the retinoid response element 
placed upstream of the (8-galactosidase (lacZ) 
gene, it has been possible to coculture floor 
plate, dorsal neural tube, and other chick and rat 
tissues, and to monitor retinoid release by the ap- 
pearance of histologically detectable /3-galactosi- 
dase in cells adjacent to the test tissue. 
Rat or chick floor plate tissue was found to be a 
potent inducer of /«cZ activity in nearby F9 cells. 
Dorsal neural tube tissue also induced lacZ ex- 
pression, but the incidence of labeled cells, 
when compared with that observed with the floor 
plate, was markedly reduced. The intensity of 
lacZ expression was also reduced. These results 
show that the floor plate is considerably more 
efi'ective than dorsal neural tube in inducing lacZ 
expression, and support the idea that the floor 
plate may be a local source of retinoids in the 
developing neural tube. 
The evidence that the floor plate is a local 
source of morphogenetically active retinoids, to- 
gether with the finding that it has polarizing activ- 
ity within the neural tube and limb, raises the 
question of whether the polarizing activity of the 
floor plate within the neural tube is mediated by 
retinoids. To address this, we grafted retinoic 
acid-soaked beads adjacent to the neural tube of 
chick embryos. When bead implants were intro- 
duced into early embryos, defects in neural tube 
formation were frequently observed. However, 
the use of antibodies that reveal the pattern of 
cell diff^erentiation within the neural tube indi- 
cates that a local source of retinoic acid does not 
mimic the eff^ect of a floor plate graft. 
There are several possible reasons for the fail- 
ure of retinoic acid-impregnated beads to mimic 
the floor plate. First, retinoic acid may be synthe- 
sized and released by its floor plate but may not, 
in fact, be involved in neural tube patterning. Sec- 
ond, retinoic acid may act in conjunction with 
another floor plate-derived signaling molecule. 
And third, the concentration of retinoic acid dif- 
fusing from the bead may be too low to cause 
changes in cell pattern. We are currently investi- 
gating these possibilities. 
Taken together, our findings indicate that a cas- 
cade of inductive interactions occurring early in 
the development of the vertebrate nervous sys- 
tem has a central role in the patterning of neural 
cells. Signals from the notochord induce the floor 
plate at the midline of the neural plate and neural 
tube. The floor plate and notochord then appear 
to control the differentiation of other neural cells 
and to contribute to the D-V pattern of the ner- 
vous system. Thus the pattern of cell differentia- 
tion in the neural tube, as in many other develop- 
ing tissues, appears to depend critically on the 
organizing properties of specialized cell groups. 
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