Effects of retinoic acid on chick tail bud development

Dynamic regulation of gene expression and morphogenesis in the zebrafish embryo test after exposure to all-trans retinoic acid

The zebrafish embryotoxicity test (ZET) is widely used in developmental toxicology. The analysis of gene expression regulation in ZET after chemical exposure provides mechanistic information about the effects of chemicals on morphogenesis in the test. The gene expression response magnitude has been shown to change with exposure duration. The objective of this work is to study the effect of the exposure duration on the magnitude of gene expression changes in the all-trans retinoic acid (ATRA) signaling pathway in the ZET. Retinoic acid regulation is a key driver of morphogenesis and is therefore employed here as an indicator for the regulation of developmental genes. A teratogenic concentration of 7.5 nM of ATRA was given at 3 hrs post fertilization (hpf) for a range of exposure durations until 120 hrs of development. The expression of a selection of genes related to ATRA signaling and downstream developmental genes was determined. The highest magnitudes of gene expression regulation were observed after 2-24 hrs exposure with an optimal response after 4 hrs. Longer exposures showed a decrease in the gene expression response, although continued exposure to 120 hpf caused malformations and lethality. This study shows that assessment of gene expression regulation at early time points after the onset of exposure in the ZET may be optimal for the prediction of developmental toxicity. We believe these results could help optimize sensitivity in future studies with ZET.

Inverse agonism of retinoic acid receptors directs epiblast cells into the paraxial mesoderm lineage

We have investigated the differentiation of paraxial mesoderm from mouse embryonic stem cells utilizing a Tbx6-EYFP/Brachyury (T)-Cherry dual reporter system. Differentiation from the mouse ESC state directly into mesoderm via Wnt pathway activation was low, but augmented by treatment with AGN193109, a pan-retinoic acid receptor inverse agonist. After five days of differentiation, T⁺ cells increased from 12.2% to 18.8%, Tbx6⁺ cells increased from 5.8% to 12.7%, and T⁺/Tbx6⁺ cells increased from 2.4% to 14.1%. The synergism of AGN193109 with Wnt3a/CHIR99021 was further substantiated by the increased expression of paraxial mesoderm gene markers Tbx6, Msgn1, Meox1, and Hoxb1. Separate to inverse agonist treatment, when mouse ESCs were indirectly differentiated into mesoderm via a transient epiblast step the efficiency of paraxial mesoderm formation markedly increased. Tbx6⁺ cells represented 65-75% of the total cell population after just 3 days of differentiation and the expression of paraxial mesoderm marker genes Tbx6 and Msgn increased over 100-fold and 300-fold, respectively. Further evaluation of AGN193109 treatment on the indirect differentiation protocol suggested that RARs have two distinct roles. First, AGN193109 treatment at the epiblast step and mesoderm step promoted paraxial mesoderm formation over other mesoderm and endoderm lineage types. Second, continued treatment during mesoderm formation revealed its ability to repress the maturation of presomitic mesoderm into somitic paraxial mesoderm. Thus, the continuous treatment of AGN193109 during epiblast and mesoderm differentiation steps yielded a culture where ~90% of the cells were Tbx6⁺. The surprisingly early effect of inverse agonist treatment at the epiblast step of differentiation led us to further examine the effect of AGN193109 treatment during an extended epiblast differentiation protocol. Interestingly, while inverse agonist treatment had no impact on the conversion of ESCs into epiblast cells based on the expression of Rex1, Fgf5, and pluripotency marker genes Oct4, Nanog, and Sox2, after three days of differentiation in the presence of AGN193109 caudal epiblast and early paraxial mesoderm marker genes, T, Cyp26a1, Fgf8, Tbx6 and Msgn were all highly up-regulated. Collectively, our studies reveal an earlier than appreciated role for RARs in epiblast cells and the modulation of their function via inverse agonist treatment can promote their differentiation into the paraxial mesoderm lineage.

Rare Deleterious PARD3 Variants in the aPKC-Binding Region are Implicated in the Pathogenesis of Human Cranial Neural Tube Defects Via Disrupting Apical Tight Junction Formation

Increasing evidence that mutation of planar cell polarity (PCP) genes contributes to human cranial neural tube defect (NTD) susceptibility prompted us to hypothesize that rare variants of genes in the core apical-basal polarity (ABP) pathway are risk factors for cranial NTDs. In this study, we screened for rare genomic variation of PARD3 in 138 cranial NTD cases and 274 controls. Overall, the rare deleterious variants of PARD3 were significantly associated with increased risk for cranial NTDs (11/138 vs.7/274, P < 0.05, OR = 3.3). These NTD-specific variants were significantly enriched in the aPKC-binding region (6/138 vs. 0/274, P < 0.01). The East Asian cohort in the ExAC database and another Chinese normal cohort further supported this association. Over-expression analysis in HEK293T and MDCK cells confirmed abnormal aPKC binding or interaction for two PARD3 variants (p.P913Q and p.D783G), resulting in defective tight junction formation via disrupted aPKC binding. Functional analysis in human neural progenitor cells and chick embryos revealed that PARD3 knockdown gave rise to abnormal cell polarity and compromised the polarization process of neuroepithelial tissue. Our studies suggest that rare deleterious variants of PARD3 in the aPKC-binding region contribute to human cranial NTDs, possibly by disrupting apical tight junction formation and subsequent polarization process of the neuroepithelium.

In vitro and in vivo toxicity evaluation of plant virus nanocarriers

The use of biological self-assembling materials, plant virus nanoparticles in particular, appears very intriguing as it allows a great choice of symmetries and dimensions, easy chemical and biological engineering of both surface and/or internal cavity as well as safe and rapid production in plants. In this perspective, we present an initial evaluation of the safety profile of two structurally different plant viruses produced in Nicotiana benthamiana L. plants: the filamentous Potato virus X and the icosahedral Tomato bushy stunt virus. In vitro haemolysis assay was used to test the cytotoxic effects, which could arise by pVNPs interaction with cellular membranes, while early embryo assay was used to evaluate toxicity and teratogenicity in vivo. Data indicates that these structurally robust particles, still able to infect plants after incubation in serum up to 24h, have neither toxic nor teratogenic effects in vitro and in vivo. This work represents the first safety-focused characterization of pVNPs in view of their possible use as drug delivery carriers.

From dinosaurs to birds: a tail of evolution

A particularly critical event in avian evolution was the transition from long- to short-tailed birds. Primitive bird tails underwent significant alteration, most notably reduction of the number of caudal vertebrae and fusion of the distal caudal vertebrae into an ossified pygostyle. These changes, among others, occurred over a very short evolutionary interval, which brings into focus the underlying mechanisms behind those changes. Despite the wealth of studies delving into avian evolution, virtually nothing is understood about the genetic and developmental events responsible for the emergence of short, fused tails. In this review, we summarize the current understanding of the signaling pathways and morphological events that contribute to tail extension and termination and examine how mutations affecting the genes that control these pathways might influence the evolution of the avian tail. To generate a list of candidate genes that may have been modulated in the transition to short-tailed birds, we analyzed a comprehensive set of mouse mutants. Interestingly, a prevalent pleiotropic effect of mutations that cause fused caudal vertebral bodies (as in the pygostyles of birds) is tail truncation. We identified 23 mutations in this class, and these were primarily restricted to genes involved in axial extension. At least half of the mutations that cause short, fused tails lie in the Notch/Wnt pathway of somite boundary formation or differentiation, leading to changes in somite number or size. Several of the mutations also cause additional bone fusions in the trunk skeleton, reminiscent of those observed in primitive and modern birds. All of our findings were correlated to the fossil record. An open question is whether the relatively sudden appearance of short-tailed birds in the fossil record could be accounted for, at least in part, by the pleiotropic effects generated by a relatively small number of mutational events.

Consequences of metal exposure on retinoid metabolism in vertebrates: a review

What we generally refer to as 'vitamin A' is a group of naturally-occurring molecules structurally similar to retinol that are capable of exerting biological activity. These retinoids are essential to diverse physiological functions including vision, immune response, bone mineralization, reproduction, cell differentiation, and growth. As well, some retinoids have antioxidant properties. Independent studies published over the last few decades have revealed that many fish and wildlife populations living in highly polluted environments have altered retinoid status possibly associated with retinoid metabolic or homeostatic mechanisms. Substantial evidence links organic contaminant exposure with changes in retinoid status in animal populations, but only a few detailed studies have been published implicating inorganic contaminants such as metals. This mini-review selectively deals with field and laboratory studies reporting associations between environmental contaminants, especially trace metals, and alterations in retinoid status. Both essential and non-essential trace metals have been reported to affect retinoid status. This review focuses on metabolic imbalances of retinoids in relation to metal contamination and illustrates possible modes of action. The role of retinoids as antioxidants and their potential as biomarkers of metal contamination are discussed.

Stem cells, signals and vertebrate body axis extension

The progressive generation of chick and mouse axial tissues - the spinal cord, skeleton and musculature of the body - has long been proposed to depend on the activity of multipotent stem cells. Here, we evaluate evidence for the existence and multipotency of axial stem cells. We show that although the data strongly support their existence, there is little definitive information about their multipotency or extent of contribution to the axis. We also review the location and molecular characteristics of these putative stem cells, along with their evolutionary conservation in vertebrates and the signalling mechanisms that regulate and arrest axis extension.

Evidence that the notochord may be pivotal in the development of sacral and anorectal malformations

BACKGROUND/PURPOSE

The notochord is known to organize normal development of central axial structures, such as the spinal cord, vertebral column, and anorectum, but its role in abnormal development of these organs has not been well documented. The current study has used Ethylenethiourea to induce anorectal malformations in fetal rats, allowing investigation of abnormalities of the notochord and their relationship to the axial structural abnormalities that occur.

METHODS

Timed-mated pregnant rats were fed Ethylenethiourea by gavage on gestational day 10. Their embryos were harvested on gestational days 13 to 16 and sectioned in either the transverse or sagittal plane. Sections were stained with H and E and examined serially.

RESULTS

Anorectal malformations were identified in 29 of 34 embryos and neural tube defects in 24, ranging from an accessory neural tube to lumbo-sacral rachischisis. There was no tail or only a rudimentary tail in the majority of embryos. Abnormalities of the notochord in the lumbo-sacral area included ventro-dorsal branching, ventral deviation, and ectopic notochordal tissue. Most abnormal notochord branches and ectopic notochordal tissue were abnormally close to or in contact with the wall of the cloaca or neural tube.

CONCLUSIONS

Given the known role of the notochord in controlling normal development, this study would suggest that abnormal notochord development may be pivotal in producing neural tube defects and anorectal malformations, possibly by altering sonic hedgehog signalling.

In vitro fertilization and the cloacal-bladder exstrophy-epispadias complex: is there an association?

PURPOSE

The cloacal-bladder exstrophy-epispadias complex represents a collection of congenital malformations caused by failed mesenchymal development during the first trimester. We sought to determine whether the exstrophy-epispadias complex is overrepresented among children conceived by in vitro fertilization (IVF).

MATERIALS AND METHODS

The clinic charts of all patients born between 1998 and 2001 who presented to our institution with the exstrophy-epispadias complex were reviewed. Four patients conceived by IVF and born with exstrophy-epispadias were identified and telephone interviews were conducted with the parents. Statistical analysis using clinical data and available United States population data was performed.

RESULTS

Of 78 clinic patients with exstrophy-epispadias born during a 4-year period 4 were conceived using IVF. An estimated 12% to 14% of children born in the United States with the exstrophy-epispadias complex are evaluated annually at our institution. During the years 1997 to 2000, 112,137 children in the United States were conceived using IVF. According to published incidence data, approximately 5 children with exstrophy-epispadias would be expected among this entire population. Expected numbers of children in the United States conceived by IVF and born with exstrophy-epispadias during a 4-year period were calculated based on the observed number in our clinic population. A resulting 7.3-fold relative increase in incidence was determined (p = 0.0021).

CONCLUSIONS

The exstrophy-epispadias complex appears to occur more frequently in children conceived by in vitro fertilization.

Developing with lethal RA levels: genetic ablation of Rarg can restore the viability of mice lacking Cyp26a1

We have previously reported that the retinoic acid (RA) catabolizing enzyme CYP26A1 plays an important role in protecting tail bud tissues from inappropriate exposure to RA generated in the adjacent trunk tissues by RALDH2, and that Cyp26a1-null animals exhibit spina bifida and caudal agenesis. We now show that, in the absence of Cyp26a1, retinoic acid receptor gamma (RARgamma) mediates ectopic RA-signaling in the tail bud. We also show that activated RARgamma results in downregulation of Wnt3a and Fgf8, which integrate highly conserved signaling pathways known for their role in specifying caudal morphogenesis. Ablation of the gene for RARgamma (Rarg) rescues Cyp26a1-null mutant animals from caudal regression and embryonic lethality, thus demonstrating that CYP26A1 suppresses the RA-mediated downregulation of WNT3A and FGF8 signaling pathways by eliminating ectopic RA in gastrulating tail bud mesoderm.

Homocysteine modifies development of neurulation and dorsal root ganglia in chick embryos

BACKGROUND

The formation of the neural tube (neurulation) involves two mechanisms: primary and secondary neurulation. In chicks, there is also an overlap zone, where both mechanisms work together. Homocysteine (Hcy) may have an important teratogenic role in neural tube defects (NTD) when folic acid levels are considered normal. Recently, Hcy capability to generate NTD and modify neural crest cell migration has been demonstrated in chick embryos. This study was aimed to evaluate the effects of Hcy on neurulation and the development of the dorsal root ganglia (DRG).

METHODS

Chick embryos were treated with L-Hcy thiolactone 20 micromol to produce the highest rate of survival with embryos carrying neural tube defect (NTD) in the spine. Embryos at stages (st) 3-10 were treated and harvested at st 18-23. Only externally normal embryos or those carrying spinal NTD embryos were considered.

RESULTS

Histological sections of Hcy-treated embryos showed: open spina bifida (39% of embryos), more than one tube forming the spinal cord (26%), disorganized spinal cord (26%), always affecting lumbosacral regions, probably in the overlap zone. Additionally, 32% of embryos had small and continuous DRG, associated with a slimmed roof plate. Three-dimensional reconstruction showed unsegmented DRG until the C8 ganglion level. There was a 75% reduction of C3 DRG cells in treated embryos in comparison to untreated ganglia.

CONCLUSION

Hcy teratogenicity in avian embryos affected the neural tube in the overlap zone, secondary neurulation and the cervical DRG.

Role and distribution of retinoic acid during CNS development

Retinoic acid (RA), the biologically active derivative of vitamin A, induces a variety of embryonal carcinoma and neuroblastoma cell lines to differentiate into neurons. The molecular events underlying this process are reviewed with a view to determining whether these data can lead to a better understanding of the normal process of neuronal differentiation during development. Several transcription factors, intracellular signaling molecules, cytoplasmic proteins, and extracellular molecules are shown to be necessary and sufficient for RA-induced differentiation. The evidence that RA is an endogenous component of the developing central nervous system (CNS) is then reviewed, data which include high-pressure liquid chromotography (HPLC) measurements, reporter systems and the distribution of the enzymes that synthesize RA. The latter is particularly relevant to whether RA signals in a paracrine fashion on adjacent tissues or whether it acts in an autocrine manner on cells that synthesize it. It seems that a paracrine system may operate to begin early patterning events within the developing CNS from adjacent somites and later within the CNS itself to induce subsets of neurons. The distribution of retinoid-binding proteins, retinoid receptors, and RA-synthesizing enzymes is described as well as the effects of knockouts of these genes. Finally, the effects of a deficiency and an excess of RA on the developing CNS are described from the point of view of patterning the CNS, where it seems that the hindbrain is the most susceptible part of the CNS to altered levels of RA or RA receptors and also from the point of view of neuronal differentiation where, as in the case of embryonal carcinoma (EC) cells, RA promotes neuronal differentiation. The crucial roles played by certain genes, particularly the Hox genes in RA-induced patterning processes, are also emphasized.

Retinoic acid, midkine, and defects of secondary neurulation

BACKGROUND

Retinoic acid (RA) is necessary for normal differentiation of the tail bud into the secondary neural tube. Excess RA, however, is teratogenic and causes neural tube defects (NTDs). The way in which RA modulates secondary neurulation is unclear but probably involves RA-regulated downstream genes such midkine (MK), which encodes a growth factor implicated in tail bud mesenchymal-neuroepithelial conversion. Our objective was to determine whether RA-deficiency would produce similar defects and if MK is involved.

METHODS

Citral, a drug that blocks endogenous RA formation, as well as a neutralizing antibody, were used to block RA activity in chick embryos. Immunohistochemistry and in situ hybridization were used to localize RA and MK in the tail bud. Competitive RT-PCR was used to examine the effects of excess RA and RA deficiency due to citral on the expression of MK mRNA.

RESULTS

Citral-induced NTDs displayed a morphological resemblance to those caused by excess RA. However, citral treatment did not significantly increase embryonic mortality, and RA rescue of citral-treated embryos proved unsuccessful. MK mRNA was detected in the differentiating tail bud by in situ hybridization. Competitive RT-PCR showed that excess RA decreased MK expression by 60%. Doses of citral that caused a comparable incidence of defects, however, caused only a 25% decrease.

CONCLUSIONS

The results show that excess RA and RA deficiency both cause defects of secondary neurulation. While excess RA decreased MK expression, RA deficiency had minimal effects. However, whether or not MK is an intermediary in the developmental phenomena regulated physiologically or pathologically by RA remains to be elucidated.

Effects of concanavalin A on limb regeneration in the axolotlAmbystoma mexicanum

The purpose of this investigation was to determine the effects, if any, of concanavalin A (Con A) on the process of pattern formation during limb regeneration in the axolotl Ambystoma mexicanum. Con A was administered to regenerating axolotl forelimbs in a Hydron disk that released the Con A slowly over a period of time. The results indicate that Con A is teratogenic to the regenerating axolotl limb, causing reductions in the number, size, and quality of skeletal elements present, with increasing reductions and malformations as the dose is increased. Low doses resulted in a reduction in the number of carpals or phalanges. Higher doses resulted in more drastic malformations, including the absence of digits. The implications of this teratogenic effect of Con A on limb regeneration are discussed in the context of the possible mechanisms of pattern formation.

Midkine and secondary neurulation

Midkine (MK) is a growth factor with documented neurotrophic activity for central nervous system neurons. It has also been shown to induce conversion of pluripotent mesenchymal P19 embryonic carcinoma cells into neuroepithelial derivatives. During the development of the chick embryonic neuraxis, two separate events occur. The anterior, greater portion of the neural tube, the primary neural tube, develops first from the neuroectoderm. The posterior section of the neural tube or the secondary neural tube forms next, from the cells of the tail bud, as tail bud cells undergo mesenchymal-neuroepithelial conversion. Disruptions in tail bud differentiation lead to defects of the secondary neural tube. In this study, antisense oligodeoxynucleotides (ODNs) were used to inhibit MK expression in order to determine if MK has a role in the mesenchymal-neuroepithelial conversion process. Chick embryos at the tail bud anlagen stage were treated with antisense ODNs to MK mRNA by sub-blastodermal injection and reincubated for 24 or 48 hr. The antisense ODN treatment resulted in a significant increase in the incidence of secondary neural tube defects, compared to control treatments with the saline vehicle only, sense ODNs, scrambled antisense ODNs, or non-sense ODNs. The loss of MK mRNA in the antisense ODN-treated embryos was confirmed by in situ hybridization. The results therefore suggest a role for MK in the mesenchymal-neuroepithelial conversion of tail bud mesenchyme into the secondary neural tube.

Pathogenesis of caudal dysgenesis/sirenomelia induced by ochratoxin A in chick embryos

Caudal dysgenesis/sirenomelia is a malformation complex for which the pathogenesis is controversial. This report describes the particular vulnerability of specific caudal structures to Ochratoxin A (OA), a fungal toxin, as the basis for caudal dysgenesis in an avian model. The experimental procedure involved injection of 1 microgram of OA into the air sac of eggs that had been incubated for 48 hours prior to treatment (i.e., embryos that had reached Hamburger and Hamilton stage 9-10 (6-10 somite pairs) [Hamburger and Hamilton (1951) Dev. Dyn. 195:231-272] by the time of treatment). Six to twelve hours following OA injection, excessive cell death, as shown by vital staining and routine histology, was evident in selected cell populations, including cells of the caudal-most mesoderm (the mesoderm that apparently forms the external genitalia and median infraumbilical region), the tail bud, and the neural tube caudal to the wing buds (corresponding to the level of the presomitic mesoderm). The notochord was not severely affected, although there were degenerative changes in the presomitic mesoderm. Except for positional abnormalities, development of the lateral plate mesoderm from which the leg buds are derived appeared relatively normal in most of the treated embryos. Six days post-treatment, varying degrees of caudal dysgenesis, presenting in severely affected specimens as sirenomelia, were observed in approximately 30% of the surviving treated embryos. The potential basis for the differential vulnerability of the affected cell populations and, therefore, the cellular basis for the genesis of caudal dysgenesis/sirenomelia in this model are discussed.

Glycoconjugates in normal and abnormal secondary neurulation

In chick embryos, the anterior greater portion of the neural tube develops by the folding, apposition, and fusion of the neuroectoderm. The smaller caudal portion that forms the secondary neural tube (lumbosacral and coccygeal regions) is derived from the tail bud, an aggregate of mesenchymal cells located at the caudal limit of the body. Tail bud mesenchyme, arranged in a solid cord, undergoes mesenchymal-epithelial transformation to form the secondary neural tube. Previous evidence suggests that this transformation is accompanied by modulation of cell surface glycoconjugates in the differentiating tissues. In this study, we show by lectin histochemistry and lectin blotting of proteins isolated by SDS-PAGE, that Datura stramonium agglutinin (DSA) binds preferentially to differentiating tail bud cells. This lectin is specific for beta 1-4-linked N-acetylglucosamine oligomers, such as the oligosaccharides of the poly-N-acetyllactosamine series that have been previously implicated in cell differentiation. Ultrastructural lectin cytochemistry indicates that at least some of the proteins binding DSA are localized extracellularly. The use of DSA as a teratogen resulted in embryos showing a variety of neural tube and notochord defects. We have also examined the binding of DSA to embryos that were treated with teratogenic doses of retinoic acid by sub-blastodermal injection, and find that the DSA-binding patterns are perturbed. Analysis of DSA-treated embryos using the TUNEL technique indicated that cell death was not a factor in DSA teratogenesis. This strongly suggests that the glycoconjugates of the cell surface have a role in the normal differentiation of tail bud mesenchyme into the neuroepithelium of the secondary neural tube. Perturbations of glycoconjugate activity results in defects of the secondary neural tube and associated tail bud derivatives.

Distribution of cellular retinoic acid-binding proteins I and II in the chick embryo and their relationship to teratogenesis

The distribution of cellular retinoic acid-binding proteins I and II (CRABP I and II) during the first 6 days of chick development has been investigated using immunoblotting. Since retinoic acid (RA) is teratogenic to some parts of the embryo, stimulatory to other parts, and has no effect on others it may be that the distribution of cytoplasmic proteins such as CRABP I and II plays some role in this differential activity. Neither protein is expressed in the day 2 embryo, but from day 3 onwards both proteins are expressed and CRABP I is in considerable excess over CRABP II. Within the day 4 embryo there is some significant variation in the distribution according to tissue type. Neural tissues, neural crest derivatives, and limb buds most strongly express CRABP I whilst other tissues contain only moderate levels, and heart and epidermis do not express CRABP I at all. CRABP II has a widespread distribution, although at a lower level than CRABP I, with the exception of somites and ectoderm which do not express it at all. In the limb buds, there is a significant variation in CRABP I levels across the anteroposterior axis which suggests that these two CRABPs may have different functions during development. The relationship of these distributions in the embryo to the role of endogenous RA and the teratogenic effects of RA is discussed.

Differentiation of the chick embryo floor plate

In a number of species, the floor plate of the developing neural tube and spinal cord has been ascribed specialized functions associated with the patterning of neuronal differentiation. The differentiation of the floor plate itself is believed to be closely related to the presence of the underlying notochord. Grafting experiments have previously shown that in the chick embryo an implanted segment of notochord is capable of inducing the adjacent host neural plate or neural tube to produce an additional floor plate, although the inductive effect diminishes with increasing age of the host. We have examined the potential of notochord to promote the appearance of floor plate-like structures from neural tube tissue in culture. To facilitate this, it was necessary initially to examine the immunoreactivity of the early neural tube and floor plate in situ and in vitro with a panel of antibodies to identify a suitable marker for floor plate differentiation in vitro. In situ, the differentiation of the floor plate was characterized by a lack of immunoperoxidase staining with antibody to neurofilaments and the monoclonal antibody HNK-1 throughout the period examined. This distinguished the floor plate from other regions of the neural tube, and was in contrast to its conspicuous affinity for antibodies to N-CAM and highly sialylated N-CAM, which also stained several closely adjacent regions of the neural tube over the period examined. We also found that oligodendrocytes occurred both in the floor plate and in the flanking ventral neural tube, and that astrocytes were too poorly represented throughout the neural tube at the stages examined to be useful markers of floor plate differentiation. We therefore concluded that only the anti-neurofilament and the HNK-1 antibodies were potentially useful markers for floor plate differentiation. When these antibodies were tested on cells in culture, neural tube tissue showed the presence of neurofilament- and HNK-1-positive neurites, while floor plate cultures showed few of these. These distributions were consistent with those demonstrated in situ. However, cells staining positively for N-CAM, sialylated N-CAM and the glial cell markers were relatively sparse in floor plate cultures, suggesting that these epitopes were not retained or were masked in cultured cells. As a result of these experiments, we selected the absence of neurofilament-positive cells as a marker for floor plate differentiation in culture.(ABSTRACT TRUNCATED AT 400 WORDS)