Maintenance of neuroepithelial progenitor cells by Delta-Notch signalling in the embryonic chick retina

BACKGROUND

Neurons of the vertebrate central nervous system (CNS) are generated sequentially over a prolonged period from dividing neuroepithelial progenitor cells. Some cells in the progenitor cell population continue to proliferate while others stop dividing and differentiate as neurons. The mechanism that maintains the balance between these two behaviours is not known, although previous work has implicated Delta-Notch signalling in the process.

RESULTS

In normal development, the proliferative layer of the neuroepithelium includes both nascent neurons that transiently express Delta-1 (Dl1), and progenitor cells that do not. Using retrovirus-mediated gene misexpression in the embryonic chick retina, we show that where progenitor cells are exposed to Dl1 signalling, they are prevented from embarking on neuronal differentiation. A converse effect is seen in cells expressing a dominant-negative form of Dl1, Dl1(dn), which we show renders expressing cells deaf to inhibitory signals from their neighbours. In a multicellular patch of neuroepithelium expressing Dl1(dn), essentially all progenitors stop dividing and differentiate prematurely as neurons, which can be of diverse types. Thus, Delta-Notch signalling controls a cell's choice between remaining as a progenitor and differentiating as a neuron.

CONCLUSIONS

Nascent retinal neurons, by expressing Dl1, deliver lateral inhibition to neighbouring progenitors; this signal is essential to prevent progenitors from entering the neuronal differentiation pathway. Lateral inhibition serves the key function of maintaining a balanced mixture of dividing progenitors and differentiating progeny. We propose that the same mechanism operates throughout the vertebrate CNS, enabling large numbers of neurons to be produced sequentially and adopt different characters in response to a variety of signals. A similar mechanism of lateral inhibition, mediated by Delta and Notch proteins, may regulate stem-cell function in other tissues.

Alterations of the endogenous surfactant system in septic adult rats

Sepsis is the most common factor leading to the acute respiratory distress syndrome (ARDS) and is associated with the highest mortality rate. It has been suggested that the pulmonary surfactant system is altered and contributes to the lung dysfunction associated with ARDS. The objective of this study was to characterize the lung injury, specifically the endogenous surfactant system in septic adult rats. Sepsis was induced in male Sprague-Dawley rats by cecal ligation and perforation and resulted in significant increases in heart rates, respiratory rates, and lactate levels along with positive blood cultures in septic animals compared with a sham control group. Two distinct septic groups were developed, a septic group and a sepsis with lung injury (septic+LI) group. The septic group had no significant differences in oxygenation compared with the sham group, whereas the septic+LI group had significantly lower PaO2 and higher A-a gradient values compared to both the sham and septic groups. The total surfactant pool size was significantly lower in the septic+LI group compared with the sham group. The small surfactant aggregate to large surfactant aggregate ratio was significantly lower in the septic group and was further reduced in the septic+LI group. There were also significantly higher levels of surfactant protein A (SP-A) in both septic and septic+LI groups compared to the sham group. These results demonstrated that the endogenous surfactant system was altered in systemic sepsis without lung dysfunction and is further altered when a lung injury is present.

UKCCSG study of accelerated radiotherapy for pediatric brain stem gliomas. United Kingdom Childhood Cancer Study Group

PURPOSE

Between 1991 and 1994, the United Kingdom Childhood Cancer Study Group (UKCCSG) conducted a multicenter study to assess the efficiency and tolerability of accelerated radiotherapy in children with a diagnosis of poor-prognosis brain stem glioma.

METHODS AND MATERIALS

Patients eligible for study were those aged 3-16 years with tumors arising in the pons, medulla, or midbrain, not previously treated with radiotherapy or chemotherapy. Histologic confirmation was not mandatory, but computed tomography or magnetic resonance imaging and clinical findings had to be typical, and patients were selected with short prediagnosis symptom history (<3 months), cranial nerve palsies or long tract signs, and intrinsic diffuse lesions on scanning. The treatment dose was 48.6 Gy in 27 fractions, increased to 50.4 Gy in 28 fractions in January 1992, delivered twice daily (except weekends) with an interfraction interval of at least 8 h. Between January 1991 and July 1994, 28 available patients were recruited: 15 boys and 13 girls with ages ranging between 3 and 13 years (median 6).

RESULTS

After treatment, neurologic improvement sustained for a period of at least 6 weeks without steroids was reported in 13 children (46%). On central review of postradiotherapy imaging, 50% of children showed evidence of partial response, but none exhibited a complete response. A further six patients (22%) had stable disease. The median survival time was 37 weeks (8.5 months); 1-year survival was 32%, and 2-year survival 11%. The pattern of relapse was local in all 26 patients who died of their disease; 1 patient had evidence of leptomeningeal seeding. Acute radiation morbidity was minimal, with only three patients (11%) exhibiting mild toxicity. No evidence of radiation-induced necrosis was found radiologically or histologically at postmortem. Ability to withdraw steroids following radiotherapy was the single most important prognostic variable in our study.

CONCLUSION

The results of this study are comparable to previous outcomes of studies with conventional and hyperfractionated radiotherapy in poor-prognosis brain stem glioma. The fractionation regimen was shown to be tolerable with an acceptable morbidity profile. However, further research is required to improve the poor prognosis of these unfortunate children.

Organization and development of facial motor neurons in the kreisler mutant mouse

The adult facial nerve contains the axons from two populations of efferent neurons. First, the branchiomotor efferent neurons that innervate the muscles of the second arch. These neurons project out of the hindbrain in the motor root and form the facial motor nuclei. Second, the preganglionic efferent neurons that innervate the submandibular and pterygopalatine ganglia. These neurons project from the hindbrain via the intermediate nerve and form the superior salivatory nucleus. The motor neurons of the facial nerve are known to originate within rhombomeres 4 and 5. In the kreisler mouse mutant there is a specific disruption of the hindbrain rhombomeres 5 and 6 appear to be absent. To investigate changes in the organization of the facial motor neurons in this mutant, we have used lipophilic dyes to trace the facial motor components both retrogradely and anterogradely. As expected, facial motor neurons are missing from rhombomere 5 in this mutant. In addition, the loss of these neurons correlates with the specific loss of the superior salivatory nucleus. In contrast, the branchiomeric neurons, that originate in rhombomere 4, appear to develop normally. This includes the caudal migration of their cell bodies forming the genu of the facial nerve. Our studies confirm that rhombomeres are critical to hindbrain development and that they are the fundamental unit at which motor neurons are specified.

Intercellular signals and cell-fate choices in the developing inner ear: origins of global and of fine-grained pattern

The major regions of the inner ear begin to be distinguishable by their patterns of gene expression very early, before the otocyst has closed. Later, individual cells within a neurogenic or sensory patch become committed to specific pathways of differentiation. Insights gained from homologies with invertebrates and from studies of tissues other than the ear, combined with discoveries from screens for mutations affecting development in the zebrafish, are beginning to reveal the genes and signalling mechanisms that control these cell-fate choices in the developing inner ear.

Device choice and experience level in endoscopic foreign object retrieval: an in vivo study

BACKGROUND

Successful foreign object retrieval may depend on device choice and the experience level of the endoscopist, although these factors have not been systematically evaluated.

METHODS

In anesthetized pigs, the ability to retrieve foreign objects (metal tack, button disc battery, wooden toothpick) placed endoscopically into the stomach was assessed. Seven university medical center gastroenterology attending physicians (5 clinical and 2 basic science research [BSR]), and 4 fellows-in-training participated. The devices used were the Roth retrieval net, rat tooth forceps, Dormia basket, polypectomy snare, and radial jaw forceps. The time to retrieve each object into an esophageal overtube within a 5 minute maximum was measured.

RESULTS

Only the Roth net and Dormia basket were successful in retrieving the button disc battery, although the Roth net was superior (100% vs 27%, Fisher p < 0.025). All devices were equally successful at retrieving the tack (82% to 100%, p = NS). The snare was significantly faster than the Roth net (p < 0.05). For the tack, there was significantly fewer difficulties encountered with the snare than the Roth net (Fisher p < 0.03). The Roth net was incapable of retrieving the toothpick; the other devices were equally successful (91% to 100%). The clinical attendings had a significantly higher success rate (95%) than the fellows (82%, chi squared p < 0.05) or combined fellows/BSR attendings (80%, p < 0.02), and were significantly faster than the fellows (p < 0.0002) or the fellows/BSR attendings (p < 0.0003).

CONCLUSIONS

The Roth net is the best device for retrieving smooth objects such as the button disc battery. For sharp objects, such as the tack and toothpick, best results were achieved with the snare, although the forceps were also effective. More experienced endoscopists had higher success rates and faster retrieval times. Both device choice and the experience level of the endoscopists have an impact on successful foreign object retrieval.

Inter-therapist reliability in locating latent myofascial trigger points using palpation

SUMMARY. Myofascial trigger points (TPs) are sites in muscle or fascia which are tender to palpate and are located in a taut band of tissue. The significance of TPs is underlined by claims that treatment applied to TPs can be effective in treating pain. Before such claims can be validated, the ability of clinicians to identify these points reliably using palpation needs to be established. The location of such sites is also important for studies which try to determine the relationship of these points to other measures of pathology, e.g. EMG and thermographic abnormalities. Intertester reliability in the localization of these points, therefore, appears to be of primary importance. This study examined the concordance between two experienced clinicians in being able to identify TPs in the upper trapezius muscle. The results indicated that the agreement between these clinicians in identifying TPs in asymptomatic subjects was poor. There was a lack of concordance not only in the location of sites of TPs but also in the numbers of TPs identified. This outcome challenges claims that TPs can be reliably identified using palpation. Copyright 1997 Harcourt Publishers Ltd.

Primary care--opportunities and threats. The changing meaning of the GP contract

The meaning of the GP contract has changed since the last major upheaval in the mid-1960s. The government has always dealt with general practitioners as independent contractors, but the way in which it treated them in 1990 was entirely different from the way in which they were treated in 1966. In 1966, the profession's independent contractor status effectively served to protect professional autonomy. In 1990, with the change in the form of government towards a "contract state," general practitioners were treated as independent contractors more in the sense of business entrepreneurs. The article finishes by raising the issue of how general practitioners can gain control over the medicopolitical agenda in the future.

Evaluation of color kinesis, a new echocardiographic method for analyzing regional wall motion in patients with dilated left ventricles

The recently developed echocardiographic technology of color kinesis (CK) displays endocardial motion in color layers on a single end-systolic 2-dimensional echocardiographic frame. Previous work using this method is promising for quantitation of regional function, but there is limited experience in patients with severely reduced left ventricular function. Twenty patients (age 59 +/- 10 years) with dilated cardiomyopathy (left ventricular ejection fraction 22 +/- 8%) underwent CK imaging. Endocardial motion was quantitated by measuring the distance of endocardial motion during the systolic interval and also by calculating the endocardial velocity. CK measurements were compared among 4 wall motion grades (i.e., normal, hypokinetic, akinetic, and dyskinetic) assessed by qualitative wall motion scoring. There was a significant overall difference (p < 0.0001) in the mean systolic endocardial inward motion (i.e., contraction) and outward motion (i.e., expansion) among wall motion grades. The mean endocardial outward distance was significantly greater for the dyskinetic segments than for the other grades (p < 0.001). There were also differences in the mean velocity of endocardial motion among the wall motion grades. In the presence of left bundle branch block, there was no difference in the mean endocardial inward distance of the hypokinetic, akinetic, and dyskinetic septal segments. We conclude that in the absence of left bundle branch block, normal, hypokinetic, akinetic, and dyskinetic ventricular wall segments may be distinguished in patients with dilated cardiomyopathy on the basis of endocardial motion measured with CK.

Surfactant: current and potential therapeutic application in infants and adults

Exogenous surfactant administration is currently being evaluated for the Acute Respiratory Distress Syndrome (ARDS). Although surfactant supplementation is now a routine therapy for babies born with neonatal RDS, this treatment modality for adults does not appear to result in a predictable improvement in lung function as is noted in neonates. This article will review the basic abnormalities of the surfactant system in patients with ARDS and contrast them with the primary surfactant deficient state of nRDS. Various factors that have been shown to influence an individual's response to exogenous surfactant will the be outlined. Finally, potential treatment approaches for patients with ARDS utilizing exogenous surfactant will be proposed.

Subclavian artery stenosis presenting as posterior cerebrovascular events after adjuvant radiotherapy for breast cancer

Late radiotherapy-induced arteriopathy is recognized as an important cause of cerebrovascular disease in patients treated with radiotherapy, usually for head and neck cancers. This case report is an example of radiotherapy-induced left subclavian artery stenosis, which resulted in serious posterior cerebral circulation events after adjuvant radiotherapy for carcinoma of the left breast. In view of the relatively good long term outlook for this disease, it is important to recognize and consider early medical or surgical intervention of such arterial lesions. In this patient, earlier intensive investigations probably would have prevented disabling iatrogenic morbidity.

Healing of incisional wounds in the embryonic chick wing bud: characterization of the actin purse-string and demonstration of a requirement for Rho activation

Small skin wounds in the chick embryo do not heal by lamellipodial crawling of cells at the wound edge as a skin wound does in the adult, but rather by contraction of an actin purse-string that rapidly assembles in the front row of epidermal cells (Martin, P., and J. Lewis. 1992. Nature (Lond.). 360:179-183). To observe the early time course of actin purse-string assembly and to characterize other cytoskeletal components of the contractile machinery, we have followed the healing of incisional or slash wounds on the dorsum of the chick wing; these wounds take only seconds to create and heal within approximately 6 h. Healing of the epithelium depends on a combination of purse-string contraction and zipper-like closure of the gap between the cut edges of the epithelium. Confocal laser scanning microscope studies show that actin initially aligns into a cable at the wound margin in the basal layer of the epidermis within approximately 2 min of wounding. Coincident with actin cable assembly, we see localization of cadherins into clusters at the wound margin, presumably marking the sites where segments of the cable in adjacent cells are linked via adherens junctions. A few minutes later we also see localization of myosin II at the wound margin, as expected if myosin is being recruited into the cable to generate a contractile force for wound healing. At the time of wounding, cells at the wound edge become transiently leaky, allowing us to load them with reagents that block the function of two small GTPases, Rho and Rac, which recently have been shown to play key roles in reorganiztion of the actin cytoskeleton in tissue-culture cells (Hall, A. 1994. Annu. Rev. Cell Biol. 10:31-54). Loading wound edge epidermal cells with C3 transferase, a bacterial exoenzyme that inactivates endogenous Rho, prevents assembly of an actin cable and causes a failure of healing. No such effects are seen with N17rac, a dominant inhibitory mutant Rac protein. These findings support the view that in this system the actin cable is required for healing-both the purse-string contraction and the zipping up-and that Rho is required for formation of the actin cable.

Primary health care for homeless people in A&E

Homeless people often present in A&E departments for primary health-care. A primary health-care nurse practitioner can provide for health needs and refer people to other sources of support and help. A nurse practitioner can liaise with other relevant services and educate colleagues about the causes and effects of homelessness.

Different forms of bone grafts

Successful incorporation of a bone graft depends on a number of factors. The source, size, function, and form of the bone graft necessary will influence decision making. This article presents the various forms of bone grafts, along with their applications in foot and ankle surgery.

A comparison of the effects of oral conjugated equine estrogen and transdermal estradiol-17 beta combined with an oral progestin on quality of life in postmenopausal women

OBJECTIVE

To compare the effect of transdermal estradiol-17 beta and oral conjugated equine estrogen when combined with an oral progestin on quality of life in post-menopausal women.

DESIGN

Randomized controlled double-blind trial. A randomization error lead to the exclusion of six subjects but the soundness of the remaining randomization was confirmed.

SETTING

Large urban community.

PATIENTS

Women 2-7 years after menopause with a uterus and ovaries, and not currently using hormone replacement therapy. Seventy-four women completed the trial.

INTERVENTIONS

After baseline measures of quality of life, subjects were randomly assigned to either continuous oral conjugated equine estrogen 0.625 mg daily or continuous transdermal estradiol-17 beta 50 mcg twice weekly, for four 4-week cycles. Medroxyprogesterone acetate 10 mg oral tablets was administered to both groups for the last 12 days of each cycle.

OUTCOMES MEASURED

Quality of life was determined using the Menopause-Specific Quality of Life Questionnaire. Tolerability was determined by a specifically designed list of adverse effects. Both measures were recorded at base-line and in mid-cycle during the second, third and fourth cycles of treatment.

RESULTS

There were no statistically significant differences in any of the domains at baseline between the oral and transdermal treatment groups. In the vasomotor domain-scores for the oral and transdermal groups improved from baseline levels of 3.14 and 3.09, respectively, to 1.32 and 1.23; physical domain scores improved from 2.45 and 2.73 to 2.04 and 1.78; psychosocial domain scores improved from 2.72 and 3.04 to 2.21 and 1.94; sexual domain scores improved from 2.32 and 2.16 to 1.64 and 1.30. There were no statistically significant group differences or time/group interactions. Both forms of therapy were equally well tolerated.

CONCLUSIONS

Improvement in all domains, measured by the Menopause-Specific Quality of Life Questionnaire, was observed in both the oral and transdermal groups. In the absence of a placebo control group, the improvements observed cannot be attributed solely to the therapy. Neither form of therapy offered an advantage over the other in respect to improvement in quality of life.

A menopause-specific quality of life questionnaire: development and psychometric properties

OBJECTIVE

To develop a condition-specific quality of life questionnaire for the menopause with documented psychometric properties, based on women's experience.

METHODS

SUBJECTS

Women 2-7 years post-menopause with a uterus and not currently on hormone replacement therapy. Questionnaire development: A list of 106 menopause symptoms was reduced using the importance score method. Replies to the item-reduction questionnaire from 88 women resulted in a 30-item questionnaire with four domains, vasomotor, physical, psychosocial and sexual, and a global quality of life question. Psychometric properties: A separate sample of 20 women was used to determine face validity, and a panel of experts was used to confirm content validity. Reliability, responsiveness and construct validity were determined within the context of a randomized controlled trial. Construct validation involved comparison with the Neugarten and Kraines'Somatic, Psychosomatic and Psychologic subscales, the reported intensity of hot flushes, the General Well-Being Schedule, Channon and Ballinger's Vaginal Symptoms Score and Libido Index, and the Life Satisfaction Index.

RESULTS

The face validity score was 4.7 out of a possible 5. Content validity was confirmed. Test-retest reliability measures, using intraclass correlation coefficients were 0.81, 0.79, 0.70 and 0.55 for the physical, psychosocial, sexual domains and the quality of life question. The intraclass correlation coefficient for the vasomotor domain was 0.37 but there is evidence of systematic change. Discriminative construct validity showed correlation coefficients of 0.69 for the physical domain, 0.66 and 0.40 for the vasomotor domain, 0.65 and -0.71 for the psychosocial domain, 0.48 and 0.38 for the sexual domain, and 0.57 for the quality of life question. Evaluative construct validity showed correlation coefficients of 0.60 for the physical domain, 0.28 for the vasomotor domain, 0.55 and -0.54 for the psychosocial domain, 0.54 and 0.32 for the sexual domain, and 0.12 for the quality of life question. Responsiveness scores ranged from 0.78 to 1.34.

CONCLUSIONS

The MENQOL (Menopause-Specific Quality of Life) questionnaire is a self-administered instrument which functions well in differentiating between women according to their quality of life and in measuring changes in their quality of life.

Crystal structures of a complexed and peptide-free membrane protein-binding domain: molecular basis of peptide recognition by PDZ

Modular PDZ domains, found in many cell junction-associated proteins, mediate the clustering of membrane ion channels by binding to their C-terminus. The X-ray crystallographic structures of the third PDZ domain from the synaptic protein PSD-95 in complex with and in the absence of its peptide ligand have been determined at 1.8 angstroms and 2.3 angstroms resolution, respectively. The structures reveal that a four-residue C-terminal stretch (X-Thr/Ser-X-Val-COO(-)) engages the PDZ domain through antiparallel main chain interactions with a beta sheet of the domain. Recognition of the terminal carboxylate group of the peptide is conferred by a cradle of main chain amides provided by a Gly-Leu-Gly-Phe loop as well as by an arginine side chain. Specific side chain interactions and a prominent hydrophobic pocket explain the selective recognition of the C-terminal consensus sequence.

Immunogenicity of full length and truncated forms of the human immunodeficiency virus type I envelope glycoprotein

We have monitored the immunogenicity of a V1V2 sub-fragment of gp 120 in contrast to the full length protein and to a truncated form (PR12) where the V1, V2 and V3 regions were removed. In contrast to previously published work [1] these studies show that monomeric forms of envelope are capable of inducing antibodies specific for both linear and discontinuous epitopes. These antibodies are capable of neutralising HIV infectivity. The majority of neutralising antibodies were specific for epitopes within the V2 and V3 regions demonstrating the immunodominance of these regions in monomeric gp 120. Relatively few of the antibodies were specific for the CD4 binding site, suggesting that this region is poorly immunogenic. Immunisation of rats with the PR12 truncated protein did not significantly enhance the immunogenicity of the CD4 binding site. However, the immune response generated included antibodies capable of binding to diverse primary HIV-1 and HIV-2 envelope glycoproteins. We have shown that up to 30% of sera from HIV-1 infected individuals have antibodies that are capable of recognising conformation-dependent epitopes within the V1V2 region of the clone HXB10, suggesting the presence of conserved cross-reactive epitopes. Furthermore we have shown an association between the presence of V1V2 reactive antibodies and the neutralisation titre of the sera tested suggesting that antibodies to this region contribute to the cross-reactive neutralising response.

Response of oral mucosal cells to glass ionomer cements

Although glass ionomer cements are generally considered to be tissue-compatible, it has been suggested that unreacted components or setting reaction by-products can affect cell metabolism. The current study examined the effects of constituents leached out of three glass ionomer cements on growth and metabolism of oral epithelial cells. Aseptically prepared discs of Ketac-Cem Radiopaque (KCR), Ketac-Cem Maxicap (KCM) and Fuji I were incubated in Dulbecco's medium for 10 d, with daily medium changes. Cultures of hamster cheek pouch (HCP) cells, a line of hamster buccal pouch epithelial cells, were incubated in control or eluate-containing media for 24 h. Viable cell numbers were determined by the colorimetric MTS assay, and DNA and RNA syntheses were assessed using [3H]thymidine and [3H]uridine incorporation, respectively. Responses to materials were determined by comparison of cell numbers and radioisotope incorporation (counts per minute (cpm) per 1000 cells). Results were analysed by ANOVA and Duncan's multiple range test, then converted to percent control for comparison. The eluates of all three materials from the first 24 h of soaking inhibited HCP cell growth. The number of cells in cultures exposed to Fuji were 88% of control cultures, while those exposed to KCR and KCM were 58% and 59% of control, respectively. The difference between Fuji-exposed and control cultures was significant (P < 0.05). The two Ketac cements were different from Fuji-exposed and control cultures (P < 0.05) but not from each other. All of the materials caused significant increases in labelling of DNA compared to control cultures (P < 0.05) when calculated on a per cell basis, but the materials did not differ from each other. Both Ketac cements also significantly stimulated labelling of RNA per cell compared to control cultures (P < 0.05). All effects of the material decreased over time. Results suggest that leachable components of the materials may affect the rate of progression of HCP cells through the cell cycle, rather than overt toxicity that results in cell death.

Timing of exogenous surfactant administration in a rabbit model of acute lung injury

The purpose of this study was to evaluate early vs. late administration of exogenous surfactant in an adult rabbit model of acute lung injury. Lung injury was induced by repetitive whole lung saline lavage and subsequent mechanical ventilation. Bovine lipid extract surfactant was instilled either 1 (Early) or 4 h (Late) after the last lavage. Animals were monitored for 7 h after the last lavage. Although arterial PO2 values increased significantly immediately after treatment in both the Early and Late groups, this improvement was not sustained in the Late group. There was also a higher incidence of pneumothoraxes in the Late group vs. both the Early group and a nontreated control group. The ratio of poorly functioning small surfactant aggregates to superior functioning large aggregates was higher in the Late group compared with the Early group. Morphological analysis revealed that early surfactant treatment prevented the progression of lung injury over time. We conclude that administration of exogenous surfactant at an early time point in lung injury resulted in superior responses compared with later treatments.

A chick homologue of Serrate and its relationship with Notch and Delta homologues during central neurogenesis

In the Drosophila nervous system, lateral inhibition regulates commitment to a neural fate by preventing neighbouring cells from developing alike. This signalling process is mediated by two transmembrane proteins-Notch as receptor and Delta as its ligand. The Delta-related protein Serrate also acts as a Notch ligand in Drosophila, but in a different developmental process that organizes patterning of the wing. We have previously shown that lateral inhibition operates at early stages of neurogenesis in vertebrates, via genes homologous to Drosophila Delta and Notch. We report here the cloning of a chick Serrate homologue, C-Serrate-1. This gene is expressed in the central nervous system, as well as in the cranial placodes, nephric epithelium, vascular system, and distal limb-bud mesenchyme. In most of these sites, its expression is associated with expression of C-Notch-1 and C- Delta-1. All three genes are expressed in the ventricular zone of the hindbrain and spinal cord, throughout the period when neurons are being born. Within this zone, C-Delta-1 and C-Serrate-1 are expressed in complementary subsets of nondividing cells that appear to be nascent neurons: C- Serrate-1 expression is restricted to specific locations along the dorsoventral axis, forming narrow bands extending from the anterior hindbrain to the tail. Our observations strongly suggest that Delta-Notch signalling delivers lateral inhibition not only early but throughout vertebrate neurogenesis to regulate neuronal commitment, and that Serrate-Notch signalling may act similarly in this process. By analogy with its role in Drosophila wing patterning, C-Serrate-1 may also have a role in organising the dorso-ventral pattern of the neural tube. We argue that signalling via Notch maintains neurogenesis, both in vertebrates and in flies, by keeping a proportion of the neuroepithelial cells in an uncommitted stem-cell-like state.

The role of FGF-3 in early inner ear development: an analysis in normal and kreisler mutant mice

The development of the otic placode is believed to depend on an inductive signal from the adjacent hindbrain. A candidate for this signal is FGF-3 (Int-2), which is expressed in the hindbrain adjacent to the future ear in rhombomeres 5 and 6 (r5 and r6). However, in vitro tests (Represa et al. (1991), Nature 353, 561-563) conflict with findings from FGF-3 knockout mice (Mansour et al. (1993), Development 117, 13-28). The former suggest that FGF-3 from the hindbrain is required to induce formation of the otocyst, while the latter imply that FGF-3 is required only in the later process of otocyst differentiation. We find that in normal embryos at early stages the gene is expressed not only in r5 and r6, but also in most of the hindbrain anterior to this and in the head ectoderm in the prospective otic placode region. In kreisler mutant embryos, however, there is no heightened expression in r5 and r6, but the early patch of expression in the prospective otic placode ectoderm is still seen and the otic vesicle still forms at nearly the normal place. Subsequent malformations of the inner ear in kreisler and in FGF-3 knockout mice are similar, involving failure of the development of the endolymphatic appendage. These findings argue that FGF-3 is not required as an inductive signal for invagination of the otic placode to form a vesicle, whose future site is already marked out independently of any localized FGF-3 signal from r5 and r6. FGF-3 does, however, appear to be required for a correct pattern of differentiation within the vesicle.