Notochordal development as influenced by the insecticide dicrotophos (Bidrin)

White Leghorn chicken embryos were treated at different ages with the insecticide dicrotophos to determine the time period of maximum effect upon notochordal development. Doses of insecticide ranging from 250 micrograms to 2.0 mg were injected into eggs at 8, 16, 24, 32, 40, 48, 72, or 96 hr of incubation and the eggs allowed to incubate for an additional 48 hr. Dicrotophos treatment caused dorsoventral and lateral folding of the notochord, with the cervical region being most severely affected. Although there was no apparent difference in dose responsiveness at any one age, there was an obvious age relationship. Notochordal responsiveness, expressed as both the number and severity of folds, was low among the 8- and 16-hr treated embryos, increased to a maximum in the 48-hr treatment group, and then declined among the older embryos. The time of maximum effect correlates closely with the time of sheath deposition and vacuolization of the notochord, but not to initial formation of the notochord from the mesoblast or later extracellular matrix production by sclerotome cells. It is proposed that dicrotophos interferes with some aspect of sheath formation. The pressure exerted by the vacuolization upon a structurally weakened sheath is thought to cause the observed folding.     

Effects of the organophosphate insecticides diazinon and parathion on bobwhite quail embryos: skeletal defects and acetylcholinesterase activity

Bobwhite quail eggs were injected at 48 or 72 hr of incubation with various doses of the organophosphate (OP) insecticides diazinon or parathion and the embryos were examined after an additional 48 hr of incubation by both histological and cartilage-staining methods. Bobwhite embryos did not display the notochordal folding or vascular enlargement reported for OP-injected chicken embryos. Cartilage staining of embryos injected with insecticide at 72 hr of incubation and recovered at day 12 of incubation revealed severe shortening and contortion of the vertebral axis, as well as tibiotarsal, rib, and sternum defects. Parathion was more potent in causing skeletal defects than diazinon. No type I defects (micromelia, parrot beak) were detected. Radiometric acetylcholinesterase (AChE) assays of whole embryo homogenates were performed for day 6, 9, and 12 diazinon-injected and control embryos. Diazinon effected drastic reductions in AChE activity. Although the AChE and axial skeletal responses of bobwhite embryos to OP injection are similar to those reported in the literature for other species, some major differences in the bobwhite response were noted: namely, the absence of notochordal folding in the young bobwhite embryo and the absence of type I defects at day 12. These differences suggest that further studies with the bobwhite quail would be useful in clarifying the mechanisms involved in OP-induced teratogenesis.     

The effects of the organophosphate insecticide malathion on very young chick embryos: malformations detected by histological examination

This histological study sought to determine the nature and incidence of developmental abnormalities induced by one of the reportedly least teratogenic of insecticides injected into very young chick embryos. Using techniques to assure rapid contact between injectant and embryo, eggs incubated for 24, 48, or 72 hr were injected with corn oil or 125 micrograms-4.0 mg malathion. The embryos were recovered 48 hr later, paraffin-embedded, serially cross-sectioned, and examined in detail. Structures affected (and the nature of the defects) were as follows: wing level notochord and spinal cord (folded or undulated); trunk/leg level spinal cord (variously, neural folds unfused, roof infolded, canal partitioned, etc.); eye (lens misshapen or severely thinned, optic cup incompletely invaginated); diencephalon (epiphysis bifurcated or off-center, supernumerary outgrowths); cardiovascular structures (atrium and major blood vessels enlarged); and tailbud (curled into hindgut: ourentery). Overall incidence was both dose- and age-related, doubling for each doubling of dose and tripling for each 24 hr less age at exposure. For most (not all) individual structures, incidence was greatest when exposed at 24 hr and nil at 72 hr. Severity of effect was not consistently dose- or age-dependent. We conclude that contrary to previous reports, 24- to 72-hr embryos are highly vulnerable to insecticide exposure, with the youngest the most vulnerable, and many of the defects detected may be attributed to either of two mechanisms: failure in formation of the supportive sheath, or factors that cause epithelial morphogenesis (e.g., microtubules, microfilaments, extracellular material, cell-to-cell adhesion mechanisms). Previous observations that 1- to 3-day embryos are relatively unresponsive to insecticides are probably artifactual owing to imprecise techniques.     

Neural tube defects caused by local anesthetics in early chick embryos

The effects of local anesthetics (ketamine HCl, lidocaine HCl, procaine HCl, and tetracaine HCl) on stage 8 (four-somite) chick embryos were investigated. In general, embryos responded to drug treatment in a dose-related manner during the first 6 hr of incubation. Concentrations of 500 micrograms/ml (ca. 2 mM) or higher were embryolethal, whereas 100-200 micrograms/ml (0.1-0.8 mM) preferentially inhibited elevation of neural folds. The latter effect was detectable within 3 hr of treatment and was readily reversible. Tetracaine was the most potent among the four local anesthetics tested at any given dose. Compared to controls, cells in the defective neuroepithelium were less elongated and exhibited smoother apical (luminal) surfaces, thinner microfilament bundles, and less intense actin-specific fluorescence. Furthermore, the effects of local anesthetics (100-200 micrograms/ml) on stage 8 chick embryos were not identical to those of cytochalasin D (0.05 micrograms/ml), colchicine (1 microgram/ml), or ionophore A23187 (25 micrograms/ml), although all treatments produced neural tube defects. Overall results suggest that local anesthetics inhibit closure of the neural tube through their disruptive action on the organization and function of microfilaments in developing neuroepithelial cells.     

Bovine oocytes treated prior to in vitro maturation with a combination of butyrolactone I and roscovitine at low doses maintain a normal developmental capacity.

Butyrolactone I (BL-I) and Roscovitine (ROS), two specific and potent inhibitors of M-phase promoting factor (MPF) kinase activity, were used to block germinal vesicle breakdown (GVBD) of cattle oocytes. A concentration 6.25 microM BL-I and 12.5 microM ROS blocked over 93.3 +/- 2.5% of oocytes in germinal vesicle (GV) stage during a 24-hr culture period. Following a second 24-hr culture step in maturation medium (IVM) almost all (91.5 +/- 3.0%) inhibited oocytes resumed meiosis and reached the metaphase II (MII) stage. The MII kinetics was different for inhibited and control oocytes. Fifty percent MII was reached at 13-14 hr in BL-I + ROS treated oocytes, compared to 18 hr in control oocytes. Therefore, control oocytes were fertilised (IVF) after 22 hr IVM and inhibited oocytes after 16 or 22 hr IVM. After IVF, percentage of grade 1 freezable embryos on day 7 (D + 7) as well as percentage of blastocyst formation on D + 8 in the group of BL-I + ROS treated oocytes fertilised after 16 hr IVM were higher (P < 0.05) compared with the other experimental group fertilised after 22 hr IVM but not different in comparison with the control. Survival to freezing and thawing of grade 1 embryos frozen on D + 7 was employed as viability criteria and was similar in all groups. Thus, the presence of BL-I + ROS in the prematuration medium of bovine oocytes determines a reversible meiotic block, without compromising their subsequent developmental competence.     

Time-lapse photographic study of neural tube closure defects caused by xylocaine in the chick

Sequential changes in the morphology of early chick embryos were, for the first time, photographically recorded. Embryos were explanted at stage 8 (four-somite) or 9- (six-somite) of development using New's technique and grown in nutrient medium (thin albumen) with or without a teratologic dose (200 micrograms/ml) of xylocaine. They were photographed using a Nikon Diaphot inverted microscope equipped with both phase-contrast optics and photomicrographic accessories maintained in an incubator. It was found, among other things, that a characteristic neural tube closure defect often seen in the midbrain and anterior portion of the hindbrain of xylocaine (200 micrograms/ml)-treated chick embryos was a consequence of failure of the neural tube to withstand the tension generated by the rapidly expanding cephalic region, which occurred, regardless of the stage at explanation, when corresponding control embryos had advanced to stage 10+ (11-somite) of development.     

Do organophosphate insecticides inhibit the conversion of tryptophan to NAD+ in ovo?

The hypothesis that organophosphate (OP) insecticides reduce the NAD+ levels of chick embryos by inhibiting kynurenine formamidase was tested. Fertile chicken eggs at 3 days of incubation were treated with a teratogenic dose of the organophosphate insecticide diazinon (DZN) in the presence or absence of exogenous L-tryptophan or nicotinamide, or one of the metabolic intermediates (L-kynurenine, 3-hydroxyanthranilic acid, quinolinic acid) between tryptophan and NAD+. By day 10 of development, DZN reduced the NAD+ content of the hind limbs of the embryos to less than 20% of normal and by day 15 it caused severe type I and type II teratogenic responses. The co-presence of tryptophan or one of its metabolites served to maintain the NAD+ levels of DZN-treated embryos close to or above normal and significantly alleviated the symptoms of type I teratisms. Tryptophan is virtually as effective as most of its metabolites in suppressing the effects of DZN on the NAD+ content and physical development of the embryos. This equivalence does not support the proposition that the inhibition of kynurenine formamidase causes the lowered NAD+ levels involved in OP-induced type I teratogenesis. It is consistent with the concept that the insecticide acts to decrease the availability of tryptophan to the embryo.     

Expression of the sonic hedgehog gene in human embryos with neural tube defects

BACKGROUND: To estimate the rate of malformations observed during early human development, a series of 38,913 first-trimester abortions were studied. Neural tube defects (NTD) were found in 57 cases. METHODS: A histological study of serial sections performed in 25 embryos revealed a spectrum of axial structure abnormalities. Expression of the SHH gene was studied by in situ hybridization in one case of CRS and in two cases of SB. RESULTS: A cervical notochord duplication was always found in craniorachischisis (CRS, n = 8), but not in spina bifida (SB, n = 10) or diplomyelia (split cord malformation, n = 3). In the embryo with CRS, expression of SHH was found in both domains, corresponding to the duplicated part of the notochord, whereas a single signal was observed in the nonduplicated part. This expression was associated at the cervical level of the open neural tube with a broad SHH expression domain and with two or even three domains in its lumbar region, suggesting multiple functional floor plates. Similarly, in two embryos with SB, two domains of SHH expression were found in the ventral neural tube. CONCLUSIONS: Our findings suggest that notochord splitting in the cervical region might be involved in the pathogenesis of CRS. Interestingly, similar notochord abnormality and altered expression of the shh gene are observed in Lp mice with NTD. This suggests that the Lp gene could be a candidate gene for human CRS. Further studies are needed to establish the primary event responsible for the notochord splitting and for the abnormal expression of the SHH gene in the floor plate in embryos with CRS and SB.     

Parthenogenetic development of bovine oocytes treated with ethanol and cytochalasin B after in vitro maturation.

The present study was conducted to investigate the effects of different culture durations (24-36 hr) on bovine oocyte maturation in vitro and the effect of the presence or absence of cumulus cells at the time of treatment to induce parthenogenetic activation (exposure to ethanol and cytochalasin B; CB) (experiment I). The effects of dosage (2.5 or 5.0 micrograms/ml) and incubation time (2.5, 5, or 10 hr) in CB (experiment II) on the subsequent development to the blastocyst stage in vitro was also investigated. In experiment I, cleavage and development to the blastocyst stage were not affected by the presence or absence of cumulus cells at the time of parthenogenetic activation. However, the 24-hr culture duration for in vitro maturation had a significantly lower rate of development to the blastocyst stage than the longer culture durations (27-36 hr). In experiment II, treatment with 5 micrograms/ml CB for 5 hr showed the highest percentage of development to blastocyst in the oocytes matured for both 27 and 30 hr. To determine the viability of the parthenogenetic embryos (morulae and blastocysts), four recipient heifers received two embryos each, and one heifer was found to be pregnant on day 35 following transfer. Although fetal heartbeat was not observed, the subsequent estrus was prolonged in all heifers. The present results demonstrate development of in vitro-matured, parthenogenetically activated bovine embryos up to the preimplantation stage.     

The influence of axial structures on chick somite formation

The influence of the axial structures on somite formation was investigated by culturing, on a nutritive agar substrate, segmental plates from chick embryos having 8 to 20 pairs of somites. In the first set of experiments, segmental plate was explanted together with adjacent notochord and approximately the lateral halves of the neural tube and node region. These explants formed 18 to 20 somites within 30 hr. In a second series of experiments, the notochord and neural tube were included as before, but further regression movements in the explants were prevented by removing the node region. These explants formed only 11.9 ± 1.1 somites. Finally, explants of segmental plate that included no neural tube, notochord, or node region were made. These explants had formed 10.7 ± 1.1 somites 14 to 17 hr later. When such explants were cultured for periods longer than 17 hr, there was a marked tendency for the more posterior somites to disperse and for all of the somites to develop a peculiar “hollow” morphology. It was concluded from these results that during the period of development when chick embryos possess 8 to 20 pairs of somites, the segmental plate mesoderm (1) represents about 12 prospective somites, (2) may segment into its full complement of somites without further contact with the axial structures, but (3) requires continued intimate contact with the axial structures for normal somite morphologic differentiation and stability.     

Experimental analysis of the mechanisms of somite morphogenesis

Earlier studies have suggested influences on somite morphogenesis by “somite-forming centers,” primitive streak regression, Hensen's node and notochord, and neural plate. Contradictions among these studies were unresolved.Our experiments resolve these conflicts and reveal roles of the primitive streak and notochord in shearing the prospective somite mesoderm into right and left halves and releasing somite-forming capabilities already present. The neural plate appears to be the principal inductor of somites.Embryo fragments containing no somite-forming centers, node, notochord, or streak nevertheless formed somites within 10 hr. Such somites disperse within the next 14–24 hr, which may explain why others failed to see them. In these fragments, an incision alongside the streak substitutes for streak regression in releasing somite formation. All such somites form simultaneously rather than in the normal anteroposterior progression. These fragments contain neural plate, but not notochord. We believe that physical attachment of somites to notochord in normal embryos stabilizes them and prevents dispersal.Pieces of epiblast were rotated 180° putting neural plate over lateral plate mesoderm regions. Somites were induced from the lateral plate by the displaced neural plate region. This is additional evidence of the powerful ability of neuroepithelium to induce somites.     

Evidence that sclerotomal cells do not migrate medially during normal embryonic development of the rat.

During embryonic development the medial part of the somite disorganizes or breaks up into sclerotomal cells which, according to many published reports, migrate medially to surround the notochord. The purpose of the study was to determine whether these cells actually migrate medially toward the notochord. Distances were measured between the notochord and the adjacent neural tube and the somite or its remnant during the period of somite disorganization. Serially sectioned, normal 10.5- to 13.5-day (d) rat embryos were used. Only transverse sections through the middle of the fourth cervical (C-4) body segment were measured, corresponding to the level of somite No. 8 (10.5 d) or its dermatomyotome remnant (10.5-11.5d) or spinal nerve C-4 (12.5-13.5d). Measurements were taken at six stages from photographic montages, all of which were made at precisely the same magnification. The notochord was the central axial structure from which the measurements were determined. The changes in distance show that during the period of somite breakup the neural tube grows dorsally, away from the notochord which lies adjacent to its ventral surface. Simultaneously the somite remnant moves laterally and dorsally, all the while maintaining its position relative to the overlying ectoderm and leaving behind a trail of sclerotomal cells. Also at each stage cell counts were made on the medial sclerotomal region of the C-4 segment. The average counts reveal that not only does the total number of cells increase substantially over the three-day period (42-7,546), but also the total number of mitoses (3.5-200), while the mitotic index decreases (9.0-2.7). High proliferative activity is apparent in the medial sclerotomal cells throughout the 3-day period. The evidence supports the conclusion that local proliferation of the trailing cells, which were left by the somite remnant as it moved dorsolaterally, causes the subsequent increase in density of the perichordal tissue, rather than an influx of migrating cells. Instead of sclerotomal cells migrating medially toward the notochord, the present study suggests that these cells retain their position relative to the notochord or central axis and that the medial sclerotomal region forms as a result of the growth movements of the surrounding structures.     

Teratogenic effects of bis-diamine on early embryonic rat heart: an in vitro study

BACKGROUND: Bis-diamine induces cardiac defects, including conotruncal anomalies in rat embryos when the agent is administered to the mother. To evaluate the teratogenic effects and mechanism of bis-diamine, we performed morphological and immunohistochemical analyses of early rat embryos cultured in medium containing bis-diamine. METHODS: The embryos were removed from mother rats on gestational day 10.5 and cultured in medium containing 1 mg of bis-diamine for 6 hr. The embryos were then cultured in medium only for another 6, 12, 18, and 42 hr, corresponding to embryonic day (ED) 11.0, 11.25, 11.5, and 12.5, respectively. Some embryos from the same mothers were used as controls and were cultured in medium only for the corresponding periods to the embryos exposed to bis-diamine. Some mother rats were given a single oral dose of 200 mg of bis-diamine on gestational day 10.5. Embryos from these pregnant rats were removed 6 hr after the oral administration of bis-diamine, and were also cultured in medium only for 6, 12, 18, and 42 hr. RESULTS: No cardiac abnormalities were detected in the controls at any stage of development. Thirty-three of 51 (65%) embryos exposed to bis-diamine and 15 of 20 (75%) embryos removed from bis-diamine-administered mothers showed abnormal cardiac development, including dilated ventricle, elongation of outflow tract, and pericardial defect on ED 11.5. Four of six (67%) embryos exposed to bis-diamine, and five of seven (71%) removed from bis-diamine-administered mothers also presented almost the same cardiac abnormalities on ED 12.5. No cardiac abnormalities were detected in bis-diamine-treated embryos before ED 11.5. In addition, the expression of neural cell adhesion molecule (N-CAM) was examined using immunohistochemical methods. Fewer N-CAM immunoreactive cells were detected in the third and fourth aortic arches in the bis-diamine-treated embryos than in controls on ED 11.5. However, more N-CAM immunoreactive cells were detected in the bis-diamine-treated embryos than in controls on ED 12.5. CONCLUSIONS: These results suggest that bis-diamine induces cardiac anomalies by delaying the migration of neural crest cells into the heart and by disturbing the proliferation of pericardial precursor during early cardiac development.     

Organophosphorus diazinon induced toxicity in the fish anguilla angvilla L.

1. Acute toxicity effects of diazinon on European eel (Anguilla anguilla) were examined using short-term exposures in static conditions.2. The lc₅₀ values found were: 0.16, 0.11, 0.09 and 0.08 mg/1 at 24, 48, 72 and 96 hr exposure, respectively.3. Eels were exposed to 0.056 mg/l of diazinon and the bioaccumulation and elimination of this insecticide in liver, muscle, gill and blood tissues were studied.4. BCF were 800 in liver, 1600 in muscle tissue and 2300 and 2730 in gill and blood tissue, respectively.5. The BCF₁ were 0.30 for liver, 0.60 for muscle and 0.84 for gill. Higher accumulation capacity of the gill was observed for the first hour of exposure.6. Diazinon elimination from the selected tissues was rapid, diazinon levels were not detected in any tissue after 24 hr in clean water.7. The excretion rate constants (K₂) of this insecticide were 0.023 hr⁻¹ for liver, 0.005 hr⁻¹ for gill and 0.019 ⁻¹ for muscle.8. Diazinon half-lives were calculated as 30.6, 32.2 and 38.3 hr for liver, muscle and gill, respectively.     

EFFECT OF AGING ON NEURAL COMPETENCE OF PRESUMPTIVE ECTODERM, AND EFFECT OF AGED ECTODERM ON DIFFERENTIATION OF THE ORGANIZER IN CYNOPS PYRRHOGASTER II. ROLE OF EXTREME POSTERIOR OF THE ARCHENTERIC ROOF IN THE SLIT-BLASTOPORE STAGE IN NORMAL DEVELOPMENT

The effect of aging on the neural competence of the presumptive ectoderm in gastrulae of Cynops pyrrhogaster and the effect of aged ectoderm on differentiation of the extreme posterior of the archenteric roof in the slit-blastopore stage were examined by a sandwich method in which this organizer was wrapped in the presumptive ectoderm taken from the 0- to 42-hr aged exogastrulae. Vital staining showed that this organizer becomes mainly tail notochord. Therefore it should be called tail or trunk-tail organizer. In 0- to 18-hr explants, typical trunk-tail structures were formed. With further aging of the presumptive ectoderm, a decrease of spinal cord and muscle with a concomitant increase of mesenchyme and mesothelium was observed. In 36- (corresponding to the slit-blastopore-initial neural stage) and 42-hr explants, neural competence had disappeared markedly. The notochord appeared in all explants, indicating this organizer is more firmly determined than the uninvaginated dorsal lip in small yolk-plug stage. Conclusively, this organizer does not play an important role in the induction of the neural plate, but induces the tail in normal development.     

Neural plate morphogenesis and axial stretching in “notochord-defective” Xenopus laevis embryos

The morphogenetic movements of neural ectoderm cells associated with neural plate development and neural fold fusion were examined in notochord-defective embryos. Those movements were apparently normal in embryos which displayed a notochord reduced in size or which completely lacked a notochord. Likewise, axial stretching in the anterior-posterior direction was also normal in “notochord-defective” embryos. A role for the anuran notochord in directing neural fold fusion and axial stretching can, therefore, be ruled out.     

Nerve growth factor induces neural differentiation in undifferentiated cell of early chick embryos

Nerve growth factor (NGF) induced differentiation in postnodal pieces (PNPs) of stage 4 chick embryos. This induction was highly selective for neural tissue; no other structures developed in the NGF-treated PNPs. Furthermore, the number of PNPs showing neural differentiation was dependent on the concentration of NGF, but there was no correlation between the concentration of NGF (5-100 ng/ml) and extent of neuralization. The neural inducing capacity of NGF could be abolished by anti-NGF antibody. NGF-induced neural differentiation was accompanied by elevated intracellular levels of cyclic AMP. Exogenous cyclic AMP (175 micrograms/ml) was able to stimulate neural differentiation but, unlike NGF, induced other structures (e.g., notochord and pulsatile tissue). Overall results suggest that cells from chick embryos at developmental stages much earlier than previously thought are responsive to NGF and NGF or a a closely related substance may serve as a neural inducer in the chick embryo.     

Notochord grafts do not suppress formation of neural crest cells or commissural neurons.

Grafting experiments previously have established that the notochord affects dorsoventral polarity of the neural tube by inducing the formation of ventral structures such as motor neurons and the floor plate. Here, we examine if the notochord inhibits formation of dorsal structures by grafting a notochord within or adjacent to the dorsal neural tube prior to or shortly after tube closure. In all cases, neural crest cells emigrated from the neural tube adjacent to the ectopic notochord. When analyzed at stages after ganglion formation, the dorsal root ganglia appeared reduced in size and shifted in position in embryos receiving grafts. Another dorsal cell type, commissural neurons, identified by CRABP and neurofilament immunoreactivity, differentiated in the vicinity of the ectopic notochord. Numerous neuronal cell bodies and axonal processes were observed within the induced, but not endogenous, floor plate 1 to 2 days after implantation but appeared to be cleared with time. These results suggest that dorsally implanted notochords cannot prevent the formation of neural crest cells or commissural neurons, but can alter the size and position of neural crest-derived dorsal root ganglia.     

HrzicN,a new Zic family gene of ascidians,plays essential roles in the neural tube and notochord development

Two axial structures, a neural tube and a notochord, are key structures in the chordate body plan and in understanding the origin of chordates. To expand our knowledge on mechanisms of development of the neural tube in lower chordates, we have undertaken isolation and characterization of HrzicN, a new member of the Zic family gene of the ascidian, Halocynthia roretzi. HrzicN expression was detected by whole-mount in situ hybridization in all neural tube precursors, all notochord precursors, anterior mesenchyme precursors and a part of the primary muscle precursors. Expression of HrzicN in a- and b-line neural tube precursors was detected from early gastrula stage to the neural plate stage, while expression in other lineages was observed between the 32-cell and the 110-cell stages. HrzicN function was investigated by disturbing translation using a morpholino antisense oligonucleotide. Embryos injected with HrzicN morpholino ('HrzicN knockdown embryos') exhibited failure of neurulation and tail elongation, and developed into larvae without a neural tube and notochord. Analysis of neural marker gene expression in HrzicN knockdown embryos revealed that HrzicN plays critical roles in distinct steps of neural tube formation in the a-line- and A-line precursors. In particular HrzicN is required for early specification of the neural tube fate in A-line precursors. Involvement of HrzicN in the neural tube development was also suggested by an overexpression experiment. However, analysis of mesodermal marker gene expression in HrzicN knockdown embryos revealed unexpected roles of this gene in the development of mesodermal tissues. HrzicN knockdown led to loss of HrBra (Halocynthia roretzi Brachyury) expression in all of the notochord precursors, which may be the cause for notochord deficiency. Hrsna (Halocynthia roretzi snail) expression was also lost from all the notochord and anterior mesenchyme precurosrs. By contrast, expression of Hrsna and the actin gene was unchanged in the primary muscle precursors. These results suggest that HrzicN is responsible for specification of the notochord and anterior mesenchyme. Finally, regulation of HrzicN expression by FGF-like signaling was investigated, which has been shown to be involved in induction of the a- and b-line neural tube, the notochord and the mesenchyme cells in Halocynthia embryos. Using an inhibitor of FGF-like signaling, we showed that HrzicN expression in the a- and b-line neural tube, but not in the A-line lineage and mesodermal lineage, depends on FGF-like signaling. Based on these data, we discussed roles of HrzicN as a key gene in the development of the neural tube and the notochord.     

Differential expression of R- and N-cadherin in neural and mesodermal tissues during early chicken development

R-cadherin is a newly identified member of the cadherin family of cell adhesion receptors. The expression of R-cadherin in early chicken embryos was studied using affinity-purified antibodies to this molecule, comparing it with that of N-cadherin. Immunoblot analysis of various organs of 10.5-day embryos showed that R-cadherin is most abundantly expressed in the retina and brain. Immunostaining of the cervical and thoracic regions of embryos revealed that R- and N-cadherin are expressed in all neural tissues. In the neural tube, R-cadherin appears at around stage 21, although N-cadherin expression begins at a much earlier stage. The distribution of R-cadherin in the neural tube differs from that of N-cadherin; for example, some regions of the tube express only R-cadherin, and other regions only N-cadherin. In the peripheral ganglia, these two cadherins are also expressed in different patterns which change during development. Some mesenchymal tissues including the notochord, the myotome, myotubes and perichondria also express these cadherins, again in different patterns. Thus, R- and N-cadherin are differentially expressed in all the tissues examined, and they may contribute to the spatial segregation of heterogeneous cells in a tissue.