The role of retinoic acid in the morphogenesis of the neural tube

We have examined the role of the signalling molecule, retinoic acid, in the process of neurulation and the subsequent growth and differentiation of the central nervous system using quail embryos that have developed in the absence of retinoic acid. Such retinoic acid-free embryos undergo abnormal neural tube formation in terms of its shape and structure, but the embryos do not display spina bifida or exencephaly. The neural tubes have a wider floor plate, a thicker roof plate and a different dorsoventral shape. Phalloidin staining and electron microscopy revealed alterations in the actin filaments and the junctional complexes of the cell layer lining the lumen. Initially the neural tubes proliferated at the same rate as normal, but later the proliferation rate declined drastically and neuronal differentiation was highly deficient. There were very few motoneurons extending neurites into the periphery, and within the neural tube axon trajectories were chaotic. These results reveal several functions for retinoic acid in the morphogenesis and growth of the neural tube, many of which can be explained by defective notochord signalling, but they do not suggest that this molecule plays a role in neural tube closure.

Detecting radiographic knee osteoarthritis: what combination of views is optimal?

OBJECTIVE

The failure to image the patellofemoral joint or the posterior knee compartment when evaluating persons for knee osteoarthritis may result in missed cases. While the skyline view has been recommended due to more reproducible assessment of the patellofemoral joint space, the lateral view may be easier to acquire and provides different information. We evaluated the sensitivity of different combinations of X-ray views (anteroposterior and lateral; anteroposterior and skyline; all three views) in 377 persons with knee symptoms who had all three views available and had a definite osteophyte on at least one view.

RESULTS

Of the different views, skylines had to be excluded most often because the image of the patellofemoral joint was technically unsatisfactory. In the remaining knees, adding either a lateral or a skyline view to an anteroposterior view yielded roughly equal and high sensitivity (94-97%) when compared with the gold standard of a positive X-ray on any of the three views.

CONCLUSION

As long as at least an anteroposterior view and one image of the patellofemoral joint is obtained (either skyline or lateral), few cases with radiographic disease will be missed. For clinical or epidemiological studies the lateral view may be easier to acquire with high quality than the skyline view.

Abnormalities of somite development in the absence of retinoic acid

We describe the effects of an absence of retinoic acid (RA) on the development of somites in the quail embryo. RA was removed by generating vitamin A deficient quail embryos whereupon the resulting defects in the embryos can be analysed. The effect on the somites is threefold. Firstly, they are half the size of normal, but the total number of somites is the same as normal. There has therefore been some global regulation event. Secondly, by TUNEL staining and TEM we show that the lateral halves of all of the somites undergo apoptosis between stages 11 and 14. This effect is confined to the sclerotome of the somites. Thirdly, some of the genes involved in somite differentiation are down-regulated such as fgf-4, fgf-8, engrailed and myogenin whereas others we examined such as cek-8, Delta, follistatin and myf5 are not affected. These studies reveal remarkably specific effects of RA on developmental gene pathways in the embryo.

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.

The distribution of endogenous retinoic acid in the chick embryo: implications for developmental mechanisms

The aim of these experiments was to determine the endogenous distribution of retinoic acid (RA) across a wide range of embryonic stages in the chick embryo. By high pressure liquid chromatography, it was revealed that didehydroRA is the most prevalent retinoic acid in the chick embryo and that the tissues of the stage 24 embryo differed widely in their total RA content (didehydroRA + all-trans-RA). Some tissues such as the heart had very little RA and some such as the neural tube had very high levels, the total variation between these two being 29-fold. We showed that these tissues also synthesised RA and released it into the medium, thus validating the use of the F9 reporter cell system for further analyses of younger staged embryos. With these F9 cells, we showed that, at stage 4, the posterior end of the embryo had barely detectably higher levels of RA than the anterior end, but that a significant level of RA generation was detected as soon as somitogenesis began. Then a sharp on/off boundary of RA was present at the level of the first somite. We could find no evidence for a posterior-to-anterior gradient of RA. Throughout further development, various consistent observations were made: the developing brain did not generate RA, but the spinal part of the neural tube generated it at very high levels so there must be a sharp on/off boundary in the region of the hindbrain/spinal cord junction; the mesenchyme surrounding the hindbrain generated RA whereas the hindbrain itself did not; there was a variation in RA levels from the midline outwards with the highest levels of RA in the spinal neural tube followed by lower levels in the somites followed by lower levels in the lateral plate; the posterior half of the limb bud generated higher levels than the anterior half. With these observations, we were able to draw maps of endogenous RA throughout these early stages of chick embryogenesis and the developmental implications of these results are discussed.

The role of vitamin A in the development of the central nervous system

We describe here the defects that arise in the central nervous system (CNS) of quail embryos when they develop in the absence of vitamin A. It has been assumed that because of the effects of excess vitamin A and its metabolites, particularly retinoic acid (RA), on the CNS they are involved in various aspects of CNS development. We show that this is indeed the case, because these deficient quail embryos have three defects in their CNS. First, the posterior hindbrain fails to develop because the cells fated to form this part of the CNS in the very early embryo die by apoptosis. Second, the neural tube fails to extend neurites into the periphery both in vivo and in vitro. Third, the neural crest cells throughout the embryo die by apoptosis. These results demonstrate a crucial requirement for vitamin A in CNS development.

Positional apoptosis during vertebrate CNS development in the absence of endogenous retinoids

We have previously shown that quail embryos that develop in the absence of vitamin A have severe defects in their central nervous system. One defect is a completely missing posterior hindbrain. Here we have studied how this comes about by examining cell death using a wholemount technique. In these A- embryos we observe two narrow bands of ectopic apoptosis. One is in the mesenchyme in the region of the first somite and occurs at the 4–6 somite stage, before neural tube closure. The second band follows immediately afterwards and occurs in the neuroepithelium of the presumptive posterior hindbrain at the 6–8 somite stage. Electron microscopy shows that the dying neuroepithelial cells exhibit the characteristics of apoptosis. Rescuing the embryos by injecting retinol before gastrulation completely prevents these apoptotic events. In an effort to identify some of the genes that may be involved in the apoptotic pathway we show that Msx-2 is upregulated in the apoptotic neuroepithelium and thus may be involved, whereas Bmp-4 is not altered and thus presumably not involved. Since these apoptotic event take place at the time of specification of axial identity and segmentation in the mesenchyme and neuroepithelium we conclude that these cells die because they are wrongly specified in terms of their rostrocaudal position, a novel phenomenon which we refer to as positional apoptosis.

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.

Late effects of retinoic acid on neural crest and aspects of rhombomere

We exposed st.10 chicks to retinoic acid (RA), both globally, and locally to individual rhombomeres, to look at its role in specification of various aspects of hindbrain derived morphology. Previous studies have looked at RA exposure at earlier stages, during axial specification. Stage 10 is the time of morphological segmentation of the hindbrain and is just prior to neural crest migration. Rhombomere 4 localised RA injections result in specific alterations of pathways some crest cells that normally migrate to sites of differentiation of neurogenic derivatives. The r4 crest cells that give rise to mesenchymal derivatives are unaffected. In addition, r4 gene expression is also partially altered by RA; within 6 hours of r4 exposure to RA, ectopic expression of Krox-20 is seen in r4 and Hoxb-1 expression is lost while Hoxa-2 expression continues normally. When we examined these RA-treated animals later in development, they showed an anterior displacement of the facial ganglion in addition to a mis-direction of the extensions of its distal axons and a dramatic decrease in the number of contralateral vestibuloacoustic neurons normally seen in r4. Only this r4-specific neuronal type is affected in r4; the motor neuron projections seem normal in experimental animals. The specificity of this result, combined with the loss of Hoxb-1 expression in r4 and the work by Krumlauf and co-workers showing gain of contralateral neurons co-localised with ectopic Hoxb-1 expression, indicates a role for Hoxb-1 and RA in the specification of this cell type in normal development. These results suggest that RA, at st.10, is able to affect some aspects of segment identity while leaving others unchanged.

Hemolytic anemia and acute renal failure associated with temafloxacin-dependent antibodies

Quinine-ingestion has been associated with immune-mediated recurrent pancytopenia, hemolysis, and renal failure. The structure of fluoroquinolone antibiotics is similar to the structure of quinine. Over a 3 month period, three patients at our institution developed hemolysis and renal failure following ingestion of the fluoroquinolone antibiotic temafloxacin. Two of the three patients required hemodialysis. Following withdrawal from the drug, the hemolysis resolved and the renal function eventually returned to normal in all three patients. One patient also had a transient mild thrombocytopenia. Sera from all three patients were tested for drug-dependent antibodies to red blood cells, platelets, and neutrophils. Temafloxacin-dependent red cell antibodies were detected in one patient, and temafloxacin-dependent red cell and neutrophil antibodies were detected in a second patient. No temafloxacin-dependent antibodies were detected in the third patient. Sera from all three patients were also tested for quinine and quinidine-dependent antibodies to red cells, platelets, and neutrophils. Sera from the patient without temafloxacin-dependent red cell antibodies reacted with red cells in the presence of quinine. These results suggest that, at least in some patients, the toxicities associated with temafloxacin are immune mediated.

Causes of hypoglycaemia

Hypoglycaemia represents a disturbance in carbohydrate metabolism that threatens to deprive the brain of its principal fuel, and its physiological consequences are mediated almost exclusively by the CNS. Although some find the distinction pedantic, it is useful to reserve the term hypoglycaemia for this biochemical state, and neuroglycopenia for the clinical syndrome that results.

Patient self-monitoring of blood glucose and refinements of conventional insulin treatment

The compelling evidence that blood glucose control will slow or prevent microvascular complications has stimulated research to find better ways of managing insulin-dependent diabetes. The excellent results obtained with "open loop" insulin infusion systems suggest that the relative failure of conventional treatment is the result of (1) a lack of appropriate feedback to the patient and (2) the use of insulin regimens which do not mimic physiologic insulinemia, particularly in the basal state. Doctors regard blood glucose measurements as an essential part of diabetic management and extension of this technology to patients has added a new dimension, particularly in the assessment of control. Nevertheless, home blood-glucose monitoring will not necessarily improve diabetic control; the best results have been obtained when it has been offered as part of a package deal which includes more investment of time and interest by patients and doctor together with joint discussions of problems and changes in treatment. The biggest problem with conventional twice daily insulin regimens is to sustain constant basal insulin levels during the night. Attempts to obtain fasting normoglycemia with an injection before supper often result in nocturnal hyperinsulinemia and hypoglycemia. This can usually be resolved by changing to a three times daily regimen with an extra injection of NPH insulin at bedtime. Three times daily insulin injections with feedback from home blood-glucose monitoring give as good blood glucose control as infusion systems and are cheaper and more acceptable to patients.

A critical evaluation of methods of monitoring diabetic control

Patients with insulin-treated diabetes need to be actively involved in their own treatment. We have found that measurement of blood rather than urinary glucose by our diabetic patients leads to greater understanding and enthusiasm as well as to better control of blood glucose. Home blood glucose monitoring is complementary to rather than a substitute for measurement of hemoglobin A1. The latter provides an objective index of long-term control that is of more use to the doctor than to the patient. The former gives the patient information that enables him to master his own disease from day to day.