Accurate staging of chick embryonic tissues via deep learning of salient features

Recent work shows that the developmental potential of progenitor cells in the HH10 chick brain changes rapidly, accompanied by subtle changes in morphology. This demands increased temporal resolution for studies of the brain at this stage, necessitating precise and unbiased staging. Here, we investigated whether we could train a deep convolutional neural network to sub-stage HH10 chick brains using a small dataset of 151 expertly labelled images. By augmenting our images with biologically informed transformations and data-driven preprocessing steps, we successfully trained a classifier to sub-stage HH10 brains to 87.1% test accuracy. To determine whether our classifier could be generally applied, we re-trained it using images (269) of randomised control and experimental chick wings, and obtained similarly high test accuracy (86.1%). Saliency analyses revealed that biologically relevant features are used for classification. Our strategy enables training of image classifiers for various applications in developmental biology with limited microscopy data.

Fgf signalling triggers an intrinsic mesodermal timer that determines the duration of limb patterning

Complex signalling between the apical ectodermal ridge (AER - a thickening of the distal epithelium) and the mesoderm controls limb patterning along the proximo-distal axis (humerus to digits). However, the essential in vivo requirement for AER-Fgf signalling makes it difficult to understand the exact roles that it fulfils. To overcome this barrier, we developed an amenable ex vivo chick wing tissue explant system that faithfully replicates in vivo parameters. Using inhibition experiments and RNA-sequencing, we identify a transient role for Fgfs in triggering the distal patterning phase. Fgfs are then dispensable for the maintenance of an intrinsic mesodermal transcriptome, which controls proliferation/differentiation timing and the duration of patterning. We also uncover additional roles for Fgf signalling in maintaining AER-related gene expression and in suppressing myogenesis. We describe a simple logic for limb patterning duration, which is potentially applicable to other systems, including the main body axis.

A neuroepithelial wave of BMP signalling drives anteroposterior specification of the tuberal hypothalamus

The tuberal hypothalamus controls life-supporting homeostatic processes, but despite its fundamental role, the cells and signalling pathways that specify this unique region of the central nervous system in embryogenesis are poorly characterised. Here, we combine experimental and bioinformatic approaches in the embryonic chick to show that the tuberal hypothalamus is progressively generated from hypothalamic floor plate-like cells. Fate-mapping studies show that a stream of tuberal progenitors develops in the anterior-ventral neural tube as a wave of neuroepithelial-derived BMP signalling sweeps from anterior to posterior through the hypothalamic floor plate. As later-specified posterior tuberal progenitors are generated, early specified anterior tuberal progenitors become progressively more distant from these BMP signals and differentiate into tuberal neurogenic cells. Gain- and loss-of-function experiments in vivo and ex vivo show that BMP signalling initiates tuberal progenitor specification, but must be eliminated for these to progress to anterior neurogenic progenitors. scRNA-Seq profiling shows that tuberal progenitors that are specified after the major period of anterior tuberal specification begin to upregulate genes that characterise radial glial cells. This study provides an integrated account of the development of the tuberal hypothalamus.

Retinoic acid influences the timing and scaling of avian wing development

A fundamental question in biology is how embryonic development is timed between different species. To address this problem, we compared wing development in the quail and the larger chick. We reveal that pattern formation is faster in the quail as determined by the earlier activation of 5′Hox genes, termination of developmental organizers (Shh and Fgf8), and the laying down of the skeleton (Sox9). Using interspecies tissue grafts, we show that developmental timing can be reset during a critical window of retinoic acid signaling. Accordingly, extending the duration of retinoic acid signaling switches developmental timing between the quail and the chick and the chick and the larger turkey. However, the incremental growth rate is comparable between all three species, suggesting that the pace of development primarily governs differences in the expansion of the skeletal pattern. The widespread distribution of retinoic acid could coordinate developmental timing throughout the embryo.

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Quail wings develop faster than chick and turkey wings

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Retinoic acid can set the species timing of wing development

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Developmental timing is independent of growth and scales the skeletal pattern

Establishing the pattern of the vertebrate limb

The vertebrate limb continues to serve as an influential model of growth, morphogenesis and pattern formation. With this Review, we aim to give an up-to-date picture of how a population of undifferentiated cells develops into the complex pattern of the limb. Focussing largely on mouse and chick studies, we concentrate on the positioning of the limbs, the formation of the limb bud, the establishment of the principal limb axes, the specification of pattern, the integration of pattern formation with growth and the determination of digit number. We also discuss the important, but little understood, topic of how gene expression is interpreted into morphology.

Sonic hedgehog specifies flight feather positional information in avian wings

Classical tissue recombination experiments performed in the chick embryo provide evidence that signals operating during early limb development specify the position and identity of feathers. Here, we show that Sonic hedgehog (Shh) signalling in the embryonic chick wing bud specifies positional information required for the formation of adult flight feathers in a defined spatial and temporal sequence that reflects their different identities. We also reveal that Shh signalling is interpreted into specific patterns of Sim1 and Zic transcription factor expression, providing evidence of a putative gene regulatory network operating in flight feather patterning. Our data suggest that flight feather specification involved the co-option of the pre-existing digit patterning mechanism and therefore uncovers an embryonic process that played a fundamental step in the evolution of avian flight.

An autoregulatory cell cycle timer integrates growth and specification in chick wing digit development

A fundamental question is how proliferation and growth are timed during embryogenesis. Although it has been suggested that the cell cycle could be a timer, the underlying mechanisms remain elusive. Here we describe a cell cycle timer that operates in Sonic hedgehog (Shh)-expressing polarising region cells of the chick wing bud. Our data are consistent with Shh signalling stimulating polarising region cell proliferation via Cyclin D2, and then inhibiting proliferation via a Bmp2-p27^kip1 pathway. When Shh signalling is blocked, polarising region cells over-proliferate and form an additional digit, which can be prevented by applying Bmp2 or by inhibiting D cyclin activity. In addition, Bmp2 also restores posterior digit identity in the absence of Shh signalling, thus indicating that it specifies antero-posterior (thumb to little finger) positional values. Our results reveal how an autoregulatory cell cycle timer integrates growth and specification and are widely applicable to many tissues.

Polarizing Region Tissue Grafting in the Chick Embryo Limb Bud

The polarizing region of the developing limb bud is an important organizing center that is involved in anteroposterior (thumb to little finger) patterning and has three main functions that are now considered to depend on the secreted protein Sonic hedgehog (Shh). These are (1) specifying anteroposterior positional values by autocrine and graded paracrine signaling; (2) promoting growth in adjacent mesenchyme; (3) maintaining the distal epithelium that is essential for limb outgrowth by induction of a factor in adjacent mesenchyme. The polarizing region was identified using classical tissue grafting techniques in chicken embryos. Here we describe this procedure using tissue from transgenic Green Fluorescent Protein-expressing chicken embryos that allows the long-term fate of the polarizing region to be determined. This technique provides a highly useful and effective method to understand how the polarizing region patterns the limb and has implications for other organizing centers.

Development of the basal hypothalamus through anisotropic growth

The adult hypothalamus is subdivided into distinct domains: pre-optic, anterior, tuberal and mammillary. Each domain harbours an array of neurones that act together to regulate homeostasis. The embryonic origins and the development of hypothalamic neurones, however, remain enigmatic. Here, we summarise recent studies in model organisms that challenge current views of hypothalamic development, which traditionally have attempted to map adult domains to correspondingly located embryonic domains. Instead, new studies indicate that hypothalamic neurones arise from progenitor cells that undergo anisotropic growth, expanding to a greater extent than other progenitors, and grow in different dimensions. We describe in particular how a multipotent Shh/ Fgf10-expressing progenitor population gives rise to progenitors throughout the basal hypothalamus that grow anisotropically and sequentially: first, a subset displaced rostrally give rise to anterior-ventral/tuberal neuronal progenitors; then a subset displaced caudally give rise to mammillary neuronal progenitors; and, finally, a subset(s) displaced ventrally give rise to tuberal infundibular glial progenitors. As this occurs, stable populations of Shh+ive and Fgf10+ive progenitors form. We describe current understanding of the mechanisms that induce Shh+ive /Fgf10+ive progenitors and begin to direct their differentiation to anterior-ventral/tuberal neuronal progenitors, mammillary neuronal progenitors and tuberal infundibular progenitors. Taken together, these studies suggest a new model for hypothalamic development that we term the "anisotropic growth model". We discuss the implications of the model for understanding the origins of adult hypothalamic neurones.

An intrinsic cell cycle timer terminates limb bud outgrowth

The longstanding view of how proliferative outgrowth terminates following the patterning phase of limb development involves the breakdown of reciprocal extrinsic signalling between the distal mesenchyme and the overlying epithelium (e-m signalling). However, by grafting distal mesenchyme cells from late stage chick wing buds to the epithelial environment of younger wing buds, we show that this mechanism is not required. RNA sequencing reveals that distal mesenchyme cells complete proliferative outgrowth by an intrinsic cell cycle timer in the presence of e-m signalling. In this process, e-m signalling is required permissively to allow the intrinsic cell cycle timer to run its course. We provide evidence that a temporal switch from BMP antagonism to BMP signalling controls the intrinsic cell cycle timer during limb outgrowth. Our findings have general implications for other patterning systems in which extrinsic signals and intrinsic timers are integrated.

The chick limb: embryology, genetics and teratology

The chick embryo has a long history in investigations of vertebrate limb development because of the ease with which its limbs can be experimentally manipulated. Early studies elucidated the fundamental embryology of the limb and identified the key signalling regions that govern its development. The chick limb became a leading model for exploring the concept of positional information and understanding how patterns of differentiated cells and tissues develop in vertebrate embryos. When developmentally important molecules began to be identified, experiments in chick limbs were crucial for bridging embryology and molecular biology. The embryological mechanisms and molecular basis of limb development are largely conserved in mammals, including humans, and uncovering these molecular networks provides links to clinical genetics. We emphasise the important contributions of naturally occurring chick mutants to elucidating limb embryology and identifying novel developmentally important genes. In addition, we consider how the chick limb has been used to study mechanisms involved in teratogenesis with a focus on thalidomide. These studies on chick embryos have given insights into how limb defects can be caused by both genetic changes and chemical insults and therefore are of great medical significance.

Evolution of antero-posterior patterning of the limb: Insights from the chick

The developing limbs of chicken embryos have served as pioneering models for understanding pattern formation for over a century. The ease with which chick wing and leg buds can be experimentally manipulated, while the embryo is still in the egg, has resulted in the discovery of important developmental organisers, and subsequently, the signals that they produce. Sonic hedgehog (Shh) is produced by mesenchyme cells of the polarizing region at the posterior margin of the limb bud and specifies positional values across the antero-posterior axis (the axis running from the thumb to the little finger). Detailed experimental embryology has revealed the fundamental parameters required to specify antero-posterior positional values in response to Shh signaling in chick wing and leg buds. In this review, the evolution of the avian wing and leg will be discussed in the broad context of tetrapod paleontology, and more specifically, ancestral theropod dinosaur paleontology. How the parameters that dictate antero-posterior patterning could have been modulated to produce the avian wing and leg digit patterns will be considered. Finally, broader speculations will be made regarding what the antero-posterior patterning of chick limbs can tell us about the evolution of other digit patterns, including those that were found in the limbs of the earliest tetrapods.

Fgf10+ progenitors give rise to the chick hypothalamus by rostral and caudal growth and differentiation

Classical descriptions of the hypothalamus divide it into three rostro-caudal domains but little is known about their embryonic origins. To investigate this, we performed targeted fate-mapping, molecular characterisation and cell cycle analyses in the embryonic chick. Presumptive hypothalamic cells derive from the rostral diencephalic ventral midline, lie above the prechordal mesendoderm and express Fgf10Fgf10⁺ progenitors undergo anisotropic growth: those displaced rostrally differentiate into anterior cells, then those displaced caudally differentiate into mammillary cells. A stable population of Fgf10⁺ progenitors is retained within the tuberal domain; a subset of these gives rise to the tuberal infundibulum - the precursor of the posterior pituitary. Pharmacological approaches reveal that Shh signalling promotes the growth and differentiation of anterior progenitors, and also orchestrates the development of the infundibulum and Rathke's pouch - the precursor of the anterior pituitary. Together, our studies identify a hypothalamic progenitor population defined by Fgf10 and highlight a role for Shh signalling in the integrated development of the hypothalamus and pituitary.

Inhibition of Shh signalling in the chick wing gives insights into digit patterning and evolution

In an influential model of pattern formation, a gradient of Sonic hedgehog (Shh) signalling in the chick wing bud specifies cells with three antero-posterior positional values, which give rise to three morphologically different digits by a self-organizing mechanism with Turing-like properties. However, as four of the five digits of the mouse limb are morphologically similar in terms of phalangeal pattern, it has been suggested that self-organization alone could be sufficient. Here, we show that inhibition of Shh signalling at a specific stage of chick wing development results in a pattern of four digits, three of which can have the same number of phalanges. These patterning changes are dependent on a posterior extension of the apical ectodermal ridge, and this also allows the additional digit to arise from the Shh-producing cells of the polarizing region – an ability lost in ancestral theropod dinosaurs. Our analyses reveal that, if the specification of antero-posterior positional values is curtailed, self-organization can then produce several digits with the same number of phalanges. We present a model that may give important insights into how the number of digits and phalanges has diverged during the evolution of avian and mammalian limbs.

Highlighted Article: In the chick wing, the relative timing of the specification of antero-posterior positional values and self-organising mechanisms determines digit patterning and identity.

Transcriptional changes in chick wing bud polarization induced by retinoic acid

BACKGROUND

Retinoic acid is implicated in the induction of the gene encoding Sonic hedgehog (Shh) that specifies anteroposterior positional values and promotes growth of the developing limb bud. However, because retinoic acid is involved in limb initiation, it has been difficult to determine if it could have additional roles in anteroposterior patterning. To investigate this, we implanted retinoic acid-soaked beads to the anterior margin of the chick wing bud and performed microarray analyses prior to onset of Shh expression.

RESULTS

Retinoic acid up-regulates expression of Hoxd11-13 that encode transcription factors implicated in inducing Shh transcription and that are involved in digit development. In our assay, retinoic acid induces Shh transcription and, consequently, a new pattern of digits at a much later stage than anticipated. Retinoic acid represses many anteriorly expressed genes, including Bmp4, Lhx9, Msx2, and Alx4. We provide evidence that retinoic acid influences transcription via induction of dHAND and inhibition of Gli3 to establish a new anteroposterior pre-pattern. We show that transient exposure to retinoic acid can suppress distal development and expedite cells to transcriptionally respond to Shh.

CONCLUSIONS

Our findings reveal how retinoic acid and Shh signaling could cooperate in anteroposterior patterning of the limb. Developmental Dynamics 246:682-690, 2017. © 2017 Wiley Periodicals, Inc.

Sonic Hedgehog Signaling in Limb Development

The gene encoding the secreted protein Sonic hedgehog (Shh) is expressed in the polarizing region (or zone of polarizing activity), a small group of mesenchyme cells at the posterior margin of the vertebrate limb bud. Detailed analyses have revealed that Shh has the properties of the long sought after polarizing region morphogen that specifies positional values across the antero-posterior axis (e.g., thumb to little finger axis) of the limb. Shh has also been shown to control the width of the limb bud by stimulating mesenchyme cell proliferation and by regulating the antero-posterior length of the apical ectodermal ridge, the signaling region required for limb bud outgrowth and the laying down of structures along the proximo-distal axis (e.g., shoulder to digits axis) of the limb. It has been shown that Shh signaling can specify antero-posterior positional values in limb buds in both a concentration- (paracrine) and time-dependent (autocrine) fashion. Currently there are several models for how Shh specifies positional values over time in the limb buds of chick and mouse embryos and how this is integrated with growth. Extensive work has elucidated downstream transcriptional targets of Shh signaling. Nevertheless, it remains unclear how antero-posterior positional values are encoded and then interpreted to give the particular structure appropriate to that position, for example, the type of digit. A distant cis-regulatory enhancer controls limb-bud-specific expression of Shh and the discovery of increasing numbers of interacting transcription factors indicate complex spatiotemporal regulation. Altered Shh signaling is implicated in clinical conditions with congenital limb defects and in the evolution of the morphological diversity of vertebrate limbs.

The origins, scaling and loss of tetrapod digits

Many of the great morphologists of the nineteenth century marvelled at similarities between the limbs of diverse species, and Charles Darwin noted these homologies as significant supporting evidence for descent with modification from a common ancestor. Sir Richard Owen also took great care to highlight each of the elements of the forelimb and hindlimb in a multitude of species with focused attention on the homology between the hoof of the horse and the middle digit of man. The ensuing decades brought about a convergence of palaeontology, experimental embryology and molecular biology to lend further support to the homologies of tetrapod limbs and their developmental origins. However, for all that we now understand about the conserved mechanisms of limb development and the development of gross morphological disturbances, little of what is presented in the experimental or medical literature reflects the remarkable diversity resulting from the 450 million year experiment of natural selection. An understanding of conserved and divergent limb morphologies in this new age of genomics and genome engineering promises to reveal more of the developmental potential residing in all limbs and to unravel the mechanisms of evolutionary variation in limb size and shape. In this review, we present the current state of our rapidly advancing understanding of the evolutionary origin of hands and feet and highlight what is known about the mechanisms that shape diverse limbs.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Intrinsic properties of limb bud cells can be differentially reset

An intrinsic timing mechanism specifies the positional values of the zeugopod (i.e. radius/ulna) and then autopod (i.e. wrist/digits) segments during limb development. Here, we have addressed whether this timing mechanism ensures that patterning events occur only once by grafting GFP-expressing autopod progenitor cells to the earlier host signalling environment of zeugopod progenitor cells. We show by detecting Hoxa13 expression that early and late autopod progenitors fated for the wrist and phalanges, respectively, both contribute to the entire host autopod, indicating that the autopod positional value is irreversibly determined. We provide evidence that Hoxa13 provides an autopod-specific positional value that correctly allocates cells into the autopod, most likely through the control of cell-surface properties as shown by cell-cell sorting analyses. However, we demonstrate that only the earlier autopod cells can adopt the host proliferation rate to permit normal morphogenesis. Therefore, our findings reveal that the ability of embryonic cells to differentially reset their intrinsic behaviours confers robustness to limb morphogenesis. We speculate that this plasticity could be maintained beyond embryogenesis in limbs with regenerative capacity.

An intrinsic timer specifies distal structures of the vertebrate limb

How the positional values along the proximo-distal axis (stylopod-zeugopod-autopod) of the limb are specified is intensely debated. Early work suggested that cells intrinsically change their proximo-distal positional values by measuring time. Recently, however, it is suggested that instructive extrinsic signals from the trunk and apical ectodermal ridge specify the stylopod and zeugopod/autopod, respectively. Here, we show that the zeugopod and autopod are specified by an intrinsic timing mechanism. By grafting green fluorescent protein-expressing cells from early to late chick wing buds, we demonstrate that distal mesenchyme cells intrinsically time Hoxa13 expression, cell cycle parameters and the duration of the overlying apical ectodermal ridge. In addition, we reveal that cell affinities intrinsically change in the distal mesenchyme, which we suggest results in a gradient of positional values along the proximo-distal axis. We propose a complete model in which a switch from extrinsic signalling to intrinsic timing patterns the vertebrate limb.

Sonic hedgehog-expressing cells in the developing limb measure time by an intrinsic cell cycle clock

How time is measured is an enduring issue in developmental biology. Classical models of somitogenesis and limb development implicated intrinsic cell cycle clocks, but their existence remains controversial. Here we show that an intrinsic cell cycle clock in polarizing region cells of the chick limb bud times the duration of Sonic hedgehog (Shh) expression, which encodes the morphogen specifying digit pattern across the antero-posterior axis (thumb to little finger). Timing by this clock starts when polarizing region cells fall out of range of retinoic acid signalling. We found that timing of Shh transcription by the cell cycle clock can be reset, thus revealing an embryonic form of self-renewal. In contrast, antero-posterior positional values cannot be reset, suggesting that this may be an important constraint on digit regeneration. Our findings provide the first evidence for an intrinsic cell cycle timer controlling duration and patterning activity of a major embryonic signalling centre.

Mathematical modelling of digit specification by a sonic hedgehog gradient

BACKGROUND

The three chick wing digits represent a classical example of a pattern specified by a morphogen gradient. Here we have investigated whether a mathematical model of a Shh gradient can describe the specification of the identities of the three chick wing digits and if it can be applied to limbs with more digits.

RESULTS

We have produced a mathematical model for specification of chick wing digit identities by a Shh gradient that can be extended to the four digits of the chick leg with Shh-producing cells forming a digit. This model cannot be extended to specify the five digits of the mouse limb.

CONCLUSIONS

Our data suggest that the parameters of a classical-type morphogen gradient are sufficient to specify the identities of three different digits. However, to specify more digit identities, this core mechanism has to be coupled to alternative processes, one being that in the chick leg and mouse limb, Shh-producing cells give rise to digits; another that in the mouse limb, the cellular response to the Shh gradient adapts over time so that digit specification does not depend simply on Shh concentration.

Gradients of signalling in the developing limb

The developing limb is one of the first systems where it was proposed that a signalling gradient is involved in pattern formation. This gradient for specifying positional information across the antero-posterior axis is based on Sonic hedgehog signalling from the polarizing region. Recent evidence suggests that Sonic hedgehog signalling also specifies positional information across the antero-posterior axis by a timing mechanism acting in parallel with graded signalling. The progress zone model for specifying proximo-distal pattern, involving timing to provide cells with positional information, continues to be challenged, and there is further evidence that graded signalling by retinoic acid specifies the proximal part of the limb. Other recent papers present the first evidence that gradients of signalling by Wnt5a and FGFs govern cell behaviour involved in outgrowth and morphogenesis of the developing limb.

Cleaved secretory leucocyte protease inhibitor as a biomarker of chymase activity in allergic airway disease

BACKGROUND

Secretory leucocyte protease inhibitor (SLPI), which is present in many physiological fluids including saliva, sputum and nasal discharge, is the most effective inhibitor of chymase. Previously, we demonstrated that chymase is able to cleave SLPI and that the cleaved portion, cSLPI, is a biomarker of chymase activity.

OBJECTIVE

We investigated the potential of cSLPI as a biomarker of chymase activity in subjects with allergic rhinitis (AR) and asthmatic airway disease.

METHODS

Baseline sputum samples were collected from atopic asthmatics and healthy controls (HC). Nasal lavages (NAL) were performed in subjects with AR both at baseline and following a nasal challenge with allergen or placebo. Levels of cSLPI and chymase were determined by Western analysis, and tryptase and alpha-2 macroglobulin were measured by immunoassay.

RESULTS

As compared with HC, asthmatics showed a significant increase in baseline cSLPI/total SLPI ratios and an increase in chymase levels. There was a high correlation of cSLPI/SLPI ratios to chymase levels in normal individuals and untreated asthmatics. In the NAL of patients with AR, as compared with placebo, allergen challenge increased inflammatory biomarkers, including cSLPI/SLPI ratios, chymase levels, tryptase levels and alpha2-macroglobulin levels. Correlations were observed between cSLPI/SLPI ratios and chymase levels and cSLPI/SLPI ratios and alpha2-macroglobulin levels; no correlation was seen between cSLPI/SLPI ratios and tryptase levels.

CONCLUSION

Our data indicate that cSLPI reflects chymase activity in AR and asthma. Hence, cSLPI may serve as a biomarker for disease activity and for monitoring the efficacy of novel anti-inflammatory treatments in chymase-mediated diseases.

Growing models of vertebrate limb development

The developing limb has been a very influential system for studying pattern formation in vertebrates. In the past, classical embryological models have explained how patterned structures are generated along the two principal axes of the limb: the proximodistal (shoulder to finger) and anteroposterior (thumb to little finger) axes. Over time, the genetic and molecular attributes of these patterning models have been discovered, while the role of growth in the patterning process has been only recently highlighted. In this review, we discuss these recent findings and propose how the various models of limb patterning can be reconciled.

Micro-magnetic resonance imaging of avian embryos

Chick embryos are useful models for probing developmental mechanisms including those involved in organogenesis. In addition to classic embryological manipulations, it is possible to test the function of molecules and genes while the embryo remains within the egg. Here we define conditions for imaging chick embryo anatomy and for visualising living quail embryos. We focus on the developing limb and describe how different tissues can be imaged using micro-magnetic resonance imaging and this information then synthesised, using a three-dimensional visualisation package, into detailed anatomy. We illustrate the potential for micro-magnetic resonance imaging to analyse phenotypic changes following chick limb manipulation. The work with the living quail embryos lays the foundations for using micro-magnetic resonance imaging as an experimental tool to follow the consequences of such manipulations over time.

Integration of growth and specification in chick wing digit-patterning

In the classical model of chick wing digit-patterning, the polarizing region--a group of cells at the posterior margin of the early bud--produces a morphogen gradient, now known to be based on Sonic hedgehog (Shh), that progressively specifies anteroposterior positional identities in the posterior digit-forming region. Here we add an integral growth component to this model by showing that Shh-dependent proliferation of prospective digit progenitor cells is essential for specifying the complete pattern of digits across the anteroposterior axis. Inhibiting Shh signalling in early wing buds reduced anteroposterior expansion, and posterior digits were lost because all prospective digit precursors formed anterior structures. Inhibiting proliferation also irreversibly reduced anteroposterior expansion, but instead anterior digits were lost because all prospective digit precursors formed posterior structures. When proliferation recovered in such wings, Shh transcription was maintained for longer than normal, suggesting that duration of Shh expression is controlled by a mechanism that measures proliferation. Rescue experiments confirmed that Shh-dependent proliferation controls digit number during a discrete time-window in which Shh-dependent specification normally occurs. Our findings that Shh signalling has dual functions that can be temporally uncoupled have implications for understanding congenital and evolutionary digit reductions.

An effective electronic surgical referral system

INTRODUCTION

Electronic booking of out-patient appointments is being rolled out in England under the 'Choose and Book' programme. We set up and ran a local electronic surgical referral service before this. This paper assesses the effect of the electronic surgical referral service on patient waiting times and attendance rates.

PATIENTS AND METHODS

The study included 54 patients referred electronically and 189 referred on paper to a single colorectal surgical service over the same period.

RESULTS

The appointment booking was achieved on the same day as the referral was made for the majority of electronic referrals whereas it took an average of 7 days for paper referrals. There was no significant difference in the time from referral to being seen in clinic between the two groups. Patients referred electronically were much more likely to attend for their appointment.

CONCLUSIONS

This study shows that an electronic surgical referral system can improve efficiency. This may be because this system allows enhanced patient choice of appointment date and time.

The expression of D-cyclin genes defines distinct developmental zones in snapdragon apical meristems and is locally regulated by the Cycloidea gene

Three D-cyclin genes are expressed in the apical meristems of snapdragon (Antirrhinum majus). The cyclin D1 and D3b genes are expressed throughout meristems, whereas cyclin D3a is restricted to the peripheral region of the meristem, especially the organ primordia. During floral development, cyclin D3b expression is: (a) locally modulated in the cells immediately surrounding the base of organ primordia, defining a zone between lateral organs that may act as a developmental boundary; (b) locally modulated in the ventral petals during petal folding; and (c) is specifically repressed in the dorsal stamen by the cycloidea gene. Expression of both cyclin D3 genes is reduced prior to the cessation of cell cycle activity, as judged by histone H4 expression. Expression of all three D-cyclin genes is modulated by factors that regulate plant growth, particularly sucrose and cytokinin. These observations may provide a molecular basis for understanding the local regulation of cell proliferation during plant growth and development.

Computed tomography appearance of idiopathic aneurysm of the azygos vein

True idiopathic aneurysm of the azygos vein is a rarely described abnormality. The authors present a case of this anomaly in a 70-year-old woman with a history of rectal carcinoma. The aneurysm was found incidentally on computed tomography, performed to characterize the appearance of a lung nodule. The aneurysm decompressed when the patient was upright. The effects of respiratory and postural manoeuvres on the lesion are discussed and the computed tomography appearance is described.

Traumatic aortic disruption: false positive diagnosis on transesophageal echocardiography

A case is described in which there was a high index of suspicion for traumatic aortic disruption. Because angiography was equivocal, transesophageal echocardiography was performed, and interpreted as showing a noncircumferential aortic tear. At thoracotomy, however, no tear was seen. The false positive interpretation was caused by a crescent-shaped atherosclerotic plaque.

Steroid-responsive interstitial lung disease in patients receiving 2'-deoxy-5-fluorouridine-infusion chemotherapy. A report of three cases

BACKGROUND

Continuous infusion of 2'-deoxy-5-fluorouridine (FUdR) has shown promise in its activity against metastatic renal cell carcinoma. Its side-effect profile is dominated by gastrointestinal toxicity; there are no known adverse pulmonary reactions. To the authors' knowledge, this is the first report on the development of lung toxicity in three patients receiving FUdR-infusion chemotherapy for metastatic renal cell carcinoma.

METHODS

The hospital charts of three patients presenting with pulmonary symptoms during FUdR chemotherapy were reviewed. A literature search was performed regarding FUdR-related pulmonary toxicity.

RESULTS

Nonproductive cough, dyspnea, and fever appeared within the 10th chemotherapy cycle. Chest radiographs showed interstitial disease in each case, accompanied by a restrictive pattern by pulmonary-function testing. Lung biopsies were performed on two patients showing a pattern of interstitial inflammation. Discontinuing FUdR and instituting steroidal therapy invariably improved symptoms, as was evident by chest radiographs and pulmonary function tests. In one patient, resuming FUdR treatment resulted in a recurrence of the respiratory symptoms, which was controlled with an increased steroidal dose. All three patients required low dose steroids to maintain their baseline respiratory functions.

CONCLUSIONS

2'-deoxy-5-fluorouridine-related lung toxicity is an uncommon event and occurs late in the treatment course. It is rapidly symptomatic and responds readily to steroidal therapy.

Role of intraoperative ultrasonography in hepatic surgery: a preliminary report

Twenty-four patients who underwent ultrasonography during hepatic surgery in the 22 months from October 1989 to July 1991 were studied prospectively to determine the value, if any, of intraoperative ultrasonography. For 6 (25%) of the 24 patients the operative plan was altered because of the ultrasonographic findings. In one of them resection was extended from a left lateral segmentectomy or sectorectomy to a left hemihepatectomy. In the other five patients the planned liver procedure was abandoned. Two of these five patients were cirrhotic--in one of them an additional metastasis was seen, and in the other there was tumour invasion of the portal vein. Of the three noncirrhotic patients, one had tumour invasion of the right hepatic vein, and the other two patients had additional metastases in the caudate lobe (Couinaud segment 1). Ultrasonography was found to be useful in 25% of patients who underwent this investigation during hepatic surgery--it avoided liver resections that would have failed because of advanced malignant disease.

Prospective randomized double-blind trial of nabilone versus domperidone in the treatment of cytotoxic-induced emesis

A prospective randomized double-blind trial comparing the butyrophenone analogue domperidone (D) and the synthetic cannabinoid nabilone (N) in the treatment of cytotoxic-induced emesis was conducted in 38 patients receiving highly emetogenic chemotherapy regimens (70% containing cisplatin). Patients received 20 mg D or 1 mg N the night before chemotherapy and 8-hourly on each chemotherapy day for two consecutive cycles of treatment. Three of 19 patients randomized to N completed only one cycle because of disease progression or subjectively adverse effects. Four of 19 patients completed only one cycle of D because of lack of efficacy or chemotherapy toxicity. In all, 32 cycles of N and 33 cycles of D were evaluable for efficacy. The mean number of vomiting episodes in cycle 1 was 4.76 for N and 12.95 for D (P less than 0.02). The corresponding values for cycle 2 were 4.27 and 7.69 (P greater than 0.10), and for cycles 1 and 2 combined, 4.53 for N and 10.81 for D (P less than 0.01). Nausea and food intake scores did not differ significantly, although there was a trend towards less nausea and an increased food intake with N. Subjectively adverse effects were more frequent with N and included drowsiness, dizziness, dry mouth, and postural hypotension. N is superior to D for the control of cytotoxic-induced emesis.

Visual versus computerised assessment of left ventricular function from cinéangiography

Visual assessment of left ventricular function from cinéangiography was compared with computerised assessment in 48 randomly selected cinéangiograms. The parameters compared included end-diastolic volume, end-systolic volume, stroke volume, ejection fraction and left ventricular output. There was poor agreement between visual and calculated values for end-diastolic volume, stroke volume and left ventricular output, but good agreement for ejection fraction and moderately good agreement for end-systolic volumes. Absolute values are particularly difficult to assess.

Surgical treatment of mitral regurgitation caused by floppy valves: repair versus replacement

Between October 1969 and April 1980, 132 patients underwent operation for floppy mitral valve. The 46 patients in group A underwent valve replacement using an inverted, fresh, unstented, antibiotic-sterilized aortic homograft and the 86 patients in group B underwent valve repair. The onset of symptoms was sudden. More than 80% of the patients in both groups were in New York Association functional class III or IV. The dominant lesion in both groups was ruptured chordae to the posterior cusp (72% in group A and 58% in group B). There were five early deaths (11%) and 15 late deaths (32%) in group A and four early deaths (5%) and five late deaths (7%) in group B. The actuarial survival rate at 5 years was 62% in group A and 90% in group B. In group A, five patients (11%) had valve failure, which led to late death in two. Four patients (8%) in group A required reoperation. Two patients in group B developed a loud mitral systolic murmur soon after operation and required reoperation. Two patients in group B had thromboembolism before routine anticoagulation. Sixteen patients (61%) in group A and 56 (76%) in group B were functional class I after operation.

The use of "fresh" unstented homograft valves for replacement of the aortic valve. Analysis of 8 years' experience

Between August, 1969 and May 1978, 679 patients underwent homograft replacement of the aortic valve. Isolated elective valve replacement was performed in 411 patients. Thirty-four patients had total aortic root replacement with reimplantation of the coronary arteries. There were 16 early deaths (3.9%) and 43 late deaths (10.5%) during a follow-up between 3 and 102 months (mean 47 months). Actuarial analysis showed 87% survival at 5 years and 81% at 8 years. Valve failure occurred in 24 patients (5.9%) owing to prolapse of one cusp in eight patients (2.0%), infective endocarditis in seven patients (1.7%), and degeneration of the valve in nine patients (2.2%). Degenerative valve failure was encountered after the fourth year with an incidence of 4.8% of patients at risk and occurred only in grafts from donors over the age of 65 years. Diastolic murmurs were present in 28% of patients followed beyond 1 month and increased very slightly with time. Systemic embolism was not recorded in any patient despite the fact that anticoagulants were not used. The clinical results were judged to be good or excellent in 89% of patients. It is concluded that homograft replacement of the aortic valve gives satisfactory results with a low incidence of late valve failure.

Influence of preoperative left ventricular function on results of homograft replacement of the aortic valve for aortic stenosis

The effect of preoperative left ventricular function on early and late prognosis was assessed in 103 patients with aortic stenosis who underwent left ventricular cineangiography before homograft replacement of the aortic valve. The patients were separated into two groups: Group A (58 patients) with an ejection fraction of 0.46 or more and Group B (45 patients) with an ejection fraction of 0.45 or less. The two groups were compared with respect to clinical and hemodynamic data as well as operative result. There was poor correlation between clinical data and left ventricular function. In Group A there were three early deaths (5.2 percent) and three late deaths (5.2 percent) compared with no early and six late deaths (13.3 percent) in Group B during the follow-up period of 12 to 102 months (mean 43 months). Most patients in Group B showed considerable symptomatic improvement but less than that observed in Group A. Forty-two patients (13 in Group A and 29 in Group B) underwent repeat cardiac catheterization and coronary angiography. Improvement in left ventricular function as assessed by radial analysis of segmental wall motion and ejection fraction was observed in 20 of the 29 patients in Group B. Failure of left ventricular function to improve was associated with additional coronary artery disease in the majority of patients. It is concluded that poor left ventricular function does not increase the risk of aortic valve replacement for aortic stenosis and that improvement in left ventricular function can be expected in the majority of patients.

Influence of preoperative left ventricular function on results of homograft replacement of the aortic valve for aortic regurgitation

The effect of preoperative left ventricular function on eraly and late prognosis was assessed in 69 patients with aortic regurgitation who underwent homograft replacement of the aortic valve. Patients were divided into two groups: Group A (38 patients) had an ejection fraction of 0.46 or more and Group B (31 patients) had an ejection fraction of 0.45 or less. Clinical data, hemodynamic data, and operative results were compared in the two groups. In Group A there was one early death (2.6%) and there were two late deaths (5.3%) compared to two early deaths (6.5%) and seven late deaths (22.6%) in Group B during a follow-up period of 13 to 98 months (mean, 49 months). Actuarial analysis showed a 94% survival at 6 years in Group A compared to 80% in Group B. Twenty-four patients were reinvestigated by repeat cardiac catheterization and coronary angiography at a mean time of 38 months following valve replacement. Left ventricular function was assessed by computerized quantitative radial analysis of segmental wall motion. Improvement in left ventricular function occurred in eight of the 14 patients reinvestigated in Group B, and appeared to be closely related to the etiology of the initial valve lesion. Despite the higher mortality rate in patients with poor left ventricular function, most derived considerable benefit from operation.