Mechanical reduction in intracanal Enterococcus faecalis when using three different single-file systems: an ex vivo comparative study.

AIM: To evaluate the bacterial reduction achieved by WaveOne Gold (Dentsply Sirona Endodontics, Ballaigues, Switzerland), Hyflex EDM (Coltene-Whaledent, Altstätten, Switzerland), and XP-endo Shaper (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland) in canals infected with Enterococcus faecalis. The null hypothesis (H0 ) was that there would be no difference amongst the NiTi systems in reducing the root canal bacterial load. METHODOLOGY: Seventy-four mandibular premolar teeth with straight and round root canals were selected. Sixty-two root canals were contaminated with E. faecalis for 4 weeks and subjected to mechanical preparation with one of three single-file NiTi rotary systems (WaveOne Gold, Hyflex EDM, and XP Endo Shaper). Twelve non-contaminated root canals were used as negative controls. Bacterial samples were collected with sterile paper points (Dentsply Sirona Endodontics) before and after rotary instrumentation to quantify the bacterial load. Five roots from each instrumentation group were evaluated topographically from the coronal, middle and apical thirds by SEM. The bacterial reduction was calculated and analysed statistically by the Kruskal-Wallis and Dunn post hoc tests, all at 5% significance. RESULTS: The samples taken before instrumentation showed approximately 9.64 log CFU bacterial load. Instrumentation with the Hyflex EDM and XP-endo Shaper resulted in significantly greater bacterial reduction than those with WaveOne Gold (P = 0.00). All specimens had bacterial growth. SEM images supported the CFU/mL data. Thick and dense debris was observed in WaveOne Gold samples. Instrumentation resulted in the disruption of the biofilm-like structure, and both areas of clean and open dentinal tubules and areas with thick debris were observed in Hyflex EDM and XP-endo Shaper samples. CONCLUSIONS: Instrumentation in straight and round canals of premolar teeth with the Hyflex EDM and XP-endo Shaper resulted in significantly greater bacterial reduction than WaveOne Gold. No instrumentation system rendered root canals completely free from bacteria.

The evaluation of hemostatic dysfunction and disseminated intravascular coagulation in dairy cows with abomasal displacement.

Disseminated intravascular coagulation (DIC) is a complex, dynamic and hemostatic disorder which develops secondarily to a disease characterized with an imbalance in the pro-coagulant and anti-coagulant components of hemostasis. The aim of the study is to evaluate hemostatic dysfunc- tion and the DIC syndrome in cattle with displaced abomasum (DA), with using the hematologic analyses and an extensive coagulation profile in the 96 hour-period including before and after surgery. The animal material of the study consisted of 12 dairy cows diagnosed with displaced abomasum (9 LDA and 3 RDA without volvulus) in the 2-4 week period after parturation and with no other post-partum disease. In dairy cows diagnosed with DA, hematological, coagulomet- ric (PT, APTT, Fibrinogen) and coagulation factor analyses [D-Dimer, TAT (thrombin-anti- thrombin complex), ATIII (antithrombin III), PAI-1 (plazminogen activator inhibitor-1] were performed in blood samples obtained before the operation as well as 30 minutes, 60 minutes and 2, 5, 10, 24, 48, 72 and 96 hours after the operation. In the DA cases, abnormalities were found in 6 of the 8 coagulation parameters. In the LDA and RDA groups, prolonged PT (sec), PT (INR) and APTT, hypofibrinogenemia, an increase in serum D-Dimer concentration at 72 and 96 hours after the operation and an increase in serum ATIII concentrations before and 30, 60 minutes and 2, 5, 72 and 96 hours after the operation was found (p⟨0.05). Hemostatic dysfunction and the risk of DIC developing in DA cases and continuing in the post-operative period was determined.

Reporting and Methodological Quality of Randomised Controlled Trials in Vascular and Endovascular Surgery.

BACKGROUND: Randomised controlled trials (RCTs) are subject to bias if they lack methodological quality. Moreover, optimal and transparent reporting of RCT findings aids their critical appraisal and interpretation. OBJECTIVES: The aim of this study was to ascertain whether the methodological and reporting quality of RCTs in vascular and endovascular surgery is improving. METHODS: The most recent 75 and oldest 75 RCTs published in leading journals over a 10-year period (2003-2012) were identified. The reporting quality and methodological quality data of the old and new RCTs were extracted and compared. The former was analysed using the Consolidated Standards of Reporting Trials (CONSORT) statement, the latter with the Scottish Intercollegiate Guidelines Network (SIGN) checklist. RESULTS: Reporting quality measured by CONSORT was better in the new studies than in the old studies (0.68 [95% CI, 0.66-0.7] vs. 0.60 [95% CI, 0.58-0.62], p < .001); however, both new and old studies had similar methodological quality measured by SIGN (0.9 [IQR 0.1] vs. .09 [IQR: 0.2], p = .787). Unlike clinical items, the methodological items of the CONSORT statement were not well reported in old and new RCTs. More trials in the new group were endovascular related (33.33% vs. 17.33%, p = .038) and industry sponsored (28% vs. 6.67%, p = .001). CONCLUSIONS: Despite some progress, there remains room for improvement in the reporting quality of RCTs in vascular and endovascular surgery. The methodological quality of recent RCTs is similar to that of trials performed >10 years ago.

Odontoblast-like cell numbers and reparative dentine thickness after direct pulp capping with platelet-rich plasma and enamel matrix derivative: a histomorphometric evaluation.

AIM: To collect quantitative information about the numbers of odontoblast-like cells and reparative dentine thickness after direct pulp capping with platelet-rich plasma (PRP) and enamel matrix derivative (EMD). METHODOLOGY: The experiment was conducted on 36 Wistar albino rats and a total of 144 incisor teeth. Calcium hydroxide, mineral trioxide aggregate, PRP and EMD were applied as direct capping agents on the pulps of 96 incisors (n = 24). Positive and negative control groups were created on the remaining 48 incisors. The teeth were extracted on the 7th and 28th days. After routine histological preparation, cross-sections were stained with haematoxylin and eosin. The numbers of the odontoblast-like cells were measured histomorphometrically on day 7 and day 28. The thickness of the reparative dentine was also measured. The number of odontoblast-like cells was also measured beneath the dentine bridge. The normal distribution of all data was tested with the Mann-Whitney U test. The statistical differences between groups were analysed using the Kruskal-Wallis test. RESULTS: The mean number of odontoblast-like cells increased between day 7 and 28 following pulp exposure (P > 0.01) in all groups except for the EMD group (P < 0.01), when compared with both the experimental and negative control groups (P < 0.01). Reparative dentine formation was observed in all groups of teeth (P > 0.01). CONCLUSIONS: Reparative dentine formation was observed, but with no significant difference between the groups. Odontoblast-like cells were observed in association with the outcome of pulps capped with PRP and EMD. PRP and EMD are possible capping agents that influence the thickness of reparative dentine formation.

Minimal intervention dentistry for managing carious lesions into dentine in primary teeth: an umbrella review.

PURPOSE: This umbrella review systematically appraised published systematic reviews on Minimal Intervention Dentistry interventions carried out to manage dentine carious primary teeth to determine how best to translate the available evidence into practice, and to provide recommendations for what requires further research. METHOD: An experienced information specialist searched MEDLINE, Embase, Cochrane Database of Systematic Reviews, Epistemonikos, Joanna Briggs Institute Database of Systematic Reviews and Implementation Reports, and the NIHR Journals Library. In addition, the PROSPERO database was searched to identify forthcoming systematic reviews. Searches were built around the following four concepts: primary teeth AND caries/carious lesion AND Minimal Intervention Dentistry AND systematic review/meta-analysis. Searches were restricted to English language, systematic reviews with/without meta-analyses published between January 2000 and August 2020. Two reviewers independently screened all titles and abstracts. Interventions included involved no dentine carious tissue removal (fissure sealants, resin infiltration, topical application of 38% Silver Diamine Fluoride, and Hall Technique), non-restorative caries control, and selective removal of carious tissue involving both stepwise excavation and atraumatic restorative treatment. Systematic reviews were selected, data extracted, and risk of bias assessed using ROBIS by two independent reviewers. Studies overlap was calculated using corrected covered area. RESULTS: Eighteen systematic reviews were included in total; 8 assessed the caries arresting effects of 38% Silver Demine Fluoride (SDF), 1 on the Hall Technique (HT), 1 on selective removal of carious tissue, and eight investigated interventions using atraumatic restorative treatment (ART). Included systematic reviews were published between 2006 and 2020, covering a defined time frame of included randomised controlled trials ranging from 1969 to 2018. Systematic reviews assessed the sealing efficacy of fissure sealants and resin infiltration in carious primary teeth were excluded due to pooled data reporting on caries arrest in both enamel and outer third of dentine with the majority of these carious lesions being limited to enamel. Therefore, fissure sealants and resin infiltration are not recommended for the management of dentinal caries lesions in primary teeth. Topical application of 38% SDF showed a significant caries arrest effect in primary teeth (p < 0.05), and its success rate in arresting dental caries increased when it was applied twice (range between 53 and 91%) rather than once a year (range between 31 and 79%). Data on HT were limited and revealed that preformed metal crowns placed using the HT were likely to reduce discomfort at time of treatment, the risk of major failure (pulp treatment or extraction needed) and pain compared to conventional restorations. Selective removal of carious tissue particularly in deep carious lesions has significantly reduced the risk of pulp exposure (77% and 69% risk reduction with one-step selective caries removal and stepwise excavation, respectively). ART showed higher success rate when placed in single surface compared to multi-surface cavities (86% and 48.7-88%, respectively, over 3 years follow-up). CONCLUSION: Minimal Intervention Dentistry techniques, namely 38% SDF, HT, selective removal of carious tissue, and ART for single surface cavity, appear to be effective in arresting the progress of dentinal caries in primary teeth when compared to no treatment, or conventional restorations. There is clear need to increase the emphasis on considering these techniques for managing carious primary teeth as a mainstream option rather than a compromise option in circumstances where the conventional approach is not possible due to cooperation or cost.

Best clinical practice guidance for treating deep carious lesions in primary teeth: an EAPD policy document.

PURPOSE: The European Academy of Paediatric Dentistry (EAPD) has developed this best clinical practice guidance to help clinicians manage deep carious lesions in primary teeth. METHODS: Three expert groups conducted systematic reviews of the relevant literature. The topics were: (1) conventional techniques (2) Minimal Intervention Dentistry (MID) and (3) materials. Workshops were held during the corresponding EAPD interim seminar in Oslo in April 2021. Several clinical based recommendations and statements were agreed upon, and gaps in our knowledge were identified. RESULTS: There is strong evidence that indirect pulp capping and pulpotomy techniques, and 38% Silver Diamine Fluoride are shown to be effective for the management of caries in the primary dentition. Due to the strict criteria, it is not possible to give clear recommendations on which materials are most appropriate for restoring primary teeth with deep carious lesions. Atraumatic Restorative Technique (ART) is not suitable for multi-surface caries, and Pre-formed Metal Crowns (PMCs) using the Hall technique reduce patient discomfort. GIC and RMGIC seem to be more favourable given the lower annual failure rate compared to HVGIC and MRGIC. Glass carbomer cannot be recommended due to inferior marginal adaptation and fractures. Compomers, hybrid composite resins and bulk-fill composite resins demonstrated similar values for annual failure rates. CONCLUSION: The management of deep carious lesions in primary teeth can be challenging and must consider the patient's compliance, operator skills, materials and costs. There is a clear need to increase the use of MID techniques in managing carious primary teeth as a mainstream rather than a compromise option.

The clinical efficacy of pentoxifylline and l-glutamine on ischemia and reperfusion injury in cattle with displaced abomasum: a longitudinal study.

This study aimed to assess the clinical efficacy of pentoxifylline (PTX) and L-glutamine (L-Gln) treatment on ischemia and reperfusion (I/R) injury in the abomasal tissue, acute phase response (APR), oxidative stress (OS), cytokine response, hemostatic, and coagulation disorders in the 96-h period before and after surgery in displaced abomasum (DA) cases. The study sample consisted of 48 dairy cows with DA that were categorized into four groups as group S (Sham group) (9 Left displaced abomasum (LDA)+3 Right displaced abomasum (RDA), group P (PTX) (10 LDA+2 RDA), group G (L-Gln) (10 LDA+2 RDA), and group P+G (PTX+L-Gln) (10 LDA+2 RDA). Acute-phase protein (Haptoglobin), oxidative stress indicators (malondialdehyde, nitric oxide, and glutathione), cytokines (tumor necrosis factor (TNF)-α and interleukin-1ß (IL-1ß), coagulation factors (D-Dimer, Antithrombin (ATIII), Thrombin-antithrombin complex, Plasminogen activator inhibitor-1), and enzyme activities (lactate dehydrogenase, gamma- -glutamyl transferase, sorbitol dehydrogenase, glutamate dehydrogenase, adenosine deaminase, myeloperoxidase, and creatine phosphokinase) in blood serum samples and coagulometric analyses of blood plasma were performed in samples taken before the operation and at 30 and 60 min and 2, 5, 10, 24, 48, 72, and 96 h after the operation. In DA cases, while post-operative treatment procedures with PTX and L-Gln were effective in decreasing APR and OS, these were ineffective in prohibiting the inflammatory response coordinated by cytokines. For the treatment and prevention of I/R injury in the DA cases, PTX and L-Gln procedures hold promise with their effects on APR, OS, and hemostatic dysfunction. Additional treatment procedures are required for the suppression of inflammatory response, and the effectiveness of preconditioning treatment may be evaluated.

Retinoic acid receptor-α signalling antagonizes both intracellular and extracellular amyloid-ß production and prevents neuronal cell death caused by amyloid-ß.

Alzheimer's disease (AD) is characterized by amyloid-ß (Aß) deposition in the brain, neuronal cell loss and cognitive decline. We show here that retinoic acid receptor (RAR)α signalling in vitro can prevent both intracellular and extracellular Aß accumulation. RARα signalling increases the expression of a disintegrin and metalloprotease 10, an α-secretase that processes the amyloid precursor protein into the non-amyloidic pathway, thus reducing Aß production. We also show that RARα agonists are neuroprotective, as they prevent Aß-induced neuronal cell death in cortical cultures. If RARα agonists are given to the Tg2576 mouse, the normal Aß production in their brains is suppressed. In contrast, neither RARß nor γ-agonists affect Aß production or Aß-mediated neuronal cell death. Therefore, RARα agonists have therapeutic potential for the treatment of AD.

Radiculomegaly of permanent canines: report of endodontic treatment in OFCD syndrome.

AIM: To present a case describing the endodontic treatment of a permanent canine with radiculomegaly and to draw the attention of the endodontic community to this rare anomaly. SUMMARY: Oculo-facio-cardio-dental (OFCD) syndrome is a rare X-linked dominant syndrome characterized by canine teeth with extremely large roots (radiculomegaly), dental abnormalities, congenital cataracts, dysmorphic facial features and congenital heart disease. The diagnosis of this syndrome is difficult. Unique and specific symptoms, which can be diagnosed by an endodontist or a general dentist, are present in the dental, skeletal, ocular and cardiac structures. Worldwide, only 20 cases of this syndrome have been reported to date. The patient presented in this study was a 17-years-old girl with a mandibular right canine (47.5mm) tooth with two canals. KEY LEARNING POINTS: • Clinicians should be aware of anatomical variations in cases with OFCD syndrome. • Root canal treatment in such cases is challenging. • In general, the majority of mandibular canines have one root and root canal, but 15% may have two canals, and a smaller number may have two distinct roots.

Retinoid induction of alveolar regeneration: from mice to man?

The use of retinoids to induce human lung regeneration is under investigation in a number of studies in patients with chronic obstructive pulmonary disease (COPD). Retinoic acid (RA) has complex pleiotropic functions during vertebrate patterning and development and can induce regeneration in a number of different organ systems. Studies of retinoid signalling during lung development might provide a molecular basis to explain pharmacological induction of alveolar regeneration in adult models of lung disease. In this review the role of endogenous RA signalling during alveologenesis is explored and data suggesting that a number of exogenous retinoids can induce regeneration in the adult lung are discussed. Current controversies in this area are highlighted and a hypothesis of lung regeneration is put forward. Understanding the cellular and molecular mechanisms of induction of regeneration will be central for effective translation into patients with lung disease and may reveal novel insights into the pathogenesis of alveolar disease and senescence.

Biolistics. The retinoic acid supergun affair.

By using biolistics to transfect the regenerating amphibian limb with cDNAs encoding chimeric receptors, the functions of individual retinoic acid receptors have been pinpointed.

Retinoids in patterning: chimeras win by a knockout.

Recent studies on the regenerating newt limb, using cells transfected with chimeric retinoic acid receptors that can be activated by thyroid hormone, have provided unique insights into the function of specific retinoic acid receptor isoforms.

Vertebrate development: a nervous vitamin.

The vitamin A-related retinoids have recently been shown to be involved in the development of the central nervous system, both in specifying position along the head-to-tail axis and in the development of a specific subset of motor neurons.

Retinoic acid is required for the initiation of outgrowth in the chick limb bud.

BACKGROUND: Retinoic acid (RA) is present in the chick limb bud, and excess RA induces limb duplications. Here, we have investigated the role of endogenous RA during chick limb development by preventing the synthesis of RA and testing the effect on various genes expressed during limb initiation and outgrowth. RESULTS: We demonstrate that the stage 20/21 limb bud synthesizes didehydroretinoic acid (ddRA), and that the posterior half of the limb bud synthesizes ddRA at a higher rate than the anterior half. Disulphiram inhibits this synthesis at micromolar concentrations. Administering disulphiram to embryos prior to limb bud outgrowth (stages 12-18) abolishes outgrowth, and no limb develops in the majority of cases. Disulphiram treatment also prevents the expression of Sonic hedgehog (Shh), but the expression of the fibroblast growth factor-8 gene (Fgf-8) appears as normal in the ectoderm over the prospective limb bud. The application of a bead soaked in RA can rescue Shh expression. Disulphiram treatment of later limb buds (stages 20-23) similarly down-regulates Shh, and also Fgf-4, expression, whereas the expression of Fgf-8, as at earlier stages, is initially unaffected. Again, RA can rescue the expression of Shh in these limb buds. CONCLUSIONS: RA, in conjunction with Fgf-8, may be needed for the induction of the chick limb bud and the induction of Shh and Fgf-4 expression. The expression of Shh and Fgf-4 remains dependent upon the continued synthesis of RA within the limb bud. Didehydroretinoic acid is the major active retinoid in the stage 20 chick limb bud.

Vitamin A-deficient quail embryos have half a hindbrain and other neural defects.

BACKGROUND: Retinoic acid (RA) is a morphogenetically active signalling molecule thought to be involved in the development of severely embryonic systems (based on its effect when applied in excess and the fact that it can be detected endogenously in embryos). Here, we adopt a novel approach and use the vitamin A-deficient (A-) quail embryo to ask what defects these embryos show when they develop in the absence of RA, with particular reference to the nervous system. RESULTS: We have examined the anatomy, the expression domains of a variety of genes and the immunoreactivity to several antibodies in these A- embryos. In addition to the previously documented cardiovascular abnormalities, we find that the somites are smaller in A- embryos, otic vesicle development is abnormal and the somites continue up to and underneath the otic vesicle. In the central nervous system, we find that neural crest cells need RA for normal development and survival, and the neural tube fails to extend any neurites into the periphery. Using general hindbrain morphology and the expression patterns of Hoxa-2, Hoxb-1, Hoxb-4, Krox-20 and FGF-3 as markers, we conclude that segmentation in the myelencephalon (rhombomeres 4-8) is disrupted. In contrast, the dorsoventral axis of the neural tube using Shh, islet-1 and Pax-3 as markers is normal. CONCLUSIONS: These results demonstrate at least three roles for RA in central nervous system development: neural crest survival, neurite outgrowth and hindbrain patterning.

Retinoic acid, a developmental signalling molecule.

Retinoic acid has been used as a tool both by embryologists studying the spatial organization of cells in the embryo and by molecular biologists studying the control of gene expression in the nucleus. Embryologists have shown that retinoic acid can modify the pattern of cell differentiation so as to duplicate complete parts of the embryo in a well-organized way; molecular biologists have shown that retinoic acid can act as the switch starting the sequence of differential gene expression that results in cell differentiation. In the past year these two approaches have converged so that there now seems a real possibility that we may soon for the first time understand how a particular vertebrate development system works.

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.

Hindbrain respecification in the retinoid-deficient quail.

We report here the development and rescue of the truncated hindbrain of retinoid-deprived quail embryos. The embryo is completely rescued by an injection of retinol into the egg; this confirms retinol, or a related retinoid, as a required molecule in hindbrain development. Staging the retinoid replacement enabled us to determine that the 3-4 somite stage is the period when retinoids are required for normal development. Analysis of the development of the retinoid-deprived hindbrain phenotype through somitogenesis has revealed a pathway of retinoid action in early hindbrain regionalization. The hindbrain of the retinoid-deprived embryo is normal in size, during early somitogenesis, but has a respecified pattern of Krox-20 expression. From the earliest expression of Krox-20, at the 5 somite stage, the rhombomere 3 stripe fills the caudal third of the developing hindbrain to the level of the first somite. Morphologically only 2, instead of the normal 5, rhombomere bulges form. These 2 bulges express genes and, later, develop morphology characteristic of rhombomeres 1 and 2 and rhombomere 3. Posterior hindbrain specific genes, Hoxb-1, Fgf3, MafB, and the rhombomere 5 stripe of Krox-20 are never expressed in the head neuroepithelium of these embryos. From the initial formation of the neural plate, there is no evidence of rhombomere 4-7 specific characteristics. These results indicate the specification of the posterior hindbrain is lost and its cells participate in the formation of an enlarged anterior hindbrain. In our previous study, we reported the absence of the posterior hindbrain in retinoid-deprived quails (Maden, M., Gale, E., Kostetskii, I., Zile, M., 1996. Vitamin A-deficient quail embryos have half a hindbrain and other neural defects. Curr. Biol. 6, 417-426). Here, we show this phenotype to be the result of respecification of the hindbrain cells. This provides evidence for a region specific response to a single stimulus, retinol, which suggests a pre-rhombomeric regionalization of the hindbrain.

Abnormal anteroposterior and dorsoventral patterning of the limb bud in the absence of retinoids.

We describe here how the early limb bud of the quail embryo develops in the absence of retinoids, including retinoic acid. Retinoid-deficient embryos develop to about stage 20/21, thus allowing patterns of early gene activity in the limb bud to be readily examined. Genes representing different aspects of limb polarity were analysed. Concerning the anteroposterior axis, Hoxb-8 was up-regulated and its border was shifted anteriorly whereas shh and the mesodermal expression of bmp-2 were down-regulated in the absence of retinoids. Concerning the apical ectodermal genes, fgf-4 was down-regulated whereas fgf-8 and the ectodermal domain of bmp-2 were unaffected. Genes involved in dorsoventral polarity were all disrupted. Wnt-7a, normally confined to the dorsal ectoderm, was ectopically expressed in the ventral ectoderm and the corresponding dorsal mesodermal gene Lmx-1 spread into the ventral mesoderm. En-1 was partially or completely absent from the ventral ectoderm. These dorsoventral patterns of expression resemble those seen in En-1 knockout mouse limb buds. Overall, the patterns of gene expression are also similar to the Japanese limbless mutant. These experiments demonstrate that the retinoid-deficient embryo is a valuable tool for dissecting pathways of gene activity in the limb bud and reveal for the first time a role for retinoic acid in the organisation of the dorsoventral axis.

Domains of cellular retinoic acid-binding protein I (CRABP I) expression in the hindbrain and neural crest of the mouse embryo.

We describe here the distribution of cellular retinoic acid-binding protein I (CRABP I) in the head of the early mouse embryo from day 8 to day 13 of gestation, using both in situ hybridisation to localise mRNA and immunocytochemistry to localise protein. The distribution of mRNA and protein was found to be identical. CRABP I first appeared in part of the presumptive hindbrain of the presomite embryo and then became localised to rhombomeres 2, 4, 5 and 6. The only other area of expression in the cephalic neuroepithelium was in a part of the midbrain roof. The neural crest and its mesenchymal derivatives, the branchial arches, expressed CRABP I and crest could be seen streaming from the neuroepithelium of individual rhombomeres into particular branchial arches. This suggested a fate map could be constructed describing the rhombomeric origin of branchial arch mesenchyme. Later in development, axons throughout the hindbrain expressed CRABP I. The results are considered in terms of the role of retinoic acid in the specification of neuronal phenotype in the hindbrain and in axon outgrowth.