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PubMed1998

A model neuron with activity-dependent conductances regulated by multiple calcium sensors.

Membrane channels are subject to a wide variety of regulatory mechanisms that can be affected by activity. We present a model of a stomatogastric ganglion (STG) neuron in which several Ca2+-dependent pathways are used to regulate the maximal conductances of membrane currents in an activity-dependent manner. Unlike previous models of this type, the regulation and modification of maximal conductances by electrical activity is unconstrained. The model has seven voltage-dependent membrane currents and uses three Ca2+ sensors acting on different time scales. Starting from random initial conditions over a given range, the model sets the maximal conductances for its active membrane currents to values that produce a predefined target pattern of activity approximately 90% of the time. In these models, the same pattern of electrical activity can be produced by a range of maximal conductances, and this range is compared with voltage-clamp data from the lateral pyloric neuron of the STG. If the electrical activity of the model neuron is perturbed, the maximal conductances adjust to restore the original pattern of activity. When the perturbation is removed, the activity pattern is again restored after a transient adjustment period, but the conductances may not return to their initial values. The model suggests that neurons may regulate their conductances to maintain fixed patterns of electrical activity, rather than fixed maximal conductances, and that the regulation process requires feedback systems capable of reacting to changes of electrical activity on a number of different time scales.

Action Potentials↗
PubMed1991

The intrasession repeatability of pattern electroretinograms and the effects of digital filtering.

Transient pattern electroretinograms were recorded in 12 normal young volunteers with the use of 12' and 30' checkerboard stimuli and 1 c/deg and 2.5 c/deg vertical gratings. The intrasession repeatability of pattern electroretinograms was determined by employing alternative statistics to the frequently used correlation coefficients. Comparisons were made with the repeatability of cone electroretinograms elicited by ganzfeld stimulation. The coefficient of repeatability was substantially lower (better) for pattern electroretinogram b-wave implicit times than for a-wave implicit times and b-wave amplitudes, the latter having high coefficients of repeatability. Cone electroretinograms had better repeatability with respect to all three parameters. Digital filtering using a bandpass of 1-40 Hz was applied to the original pattern electroretinograms recorded with a bandpass of 1-250 Hz. Although peak definition was generally enhanced, repeatability did not show marked improvement. Differences between broadband analogue-filtered pattern electroretinograms and the same responses with high-frequency noise removed by digital filtering need to be taken into account. The coefficient of repeatability statistic appears to facilitate the assessment of pattern electroretinograms and permits the comparison of the repeatability of both implicit time and amplitude parameters irrespective of absolute values. The implications of the relatively poor repeatability of pattern electroretinogram b-wave amplitudes for clinical applications need to be considered.

Adult↗
PubMed1997

Monitoring expenditure in relation to epidemiological and demographical characteristics of AIDS in South East England.

In the UK, over 70% of AIDS, including new cases, is located in a few Districts in central London where the distribution of previously occurring and new cases is essentially confined to the original risk groups of homosexual/bisexual men, drug addicts of both sexes, and some of their sexual partners and consorts. But control policy is still based on the assumption that HIV has already spread from persons in these risk groups into the general population, and that it will spread hereafter at an increased rate because of heterosexual transmission to cause a widespread epidemic of AIDS. The basis and implications of this policy were investigated in the South East region adjacent to London. Analysis of demographic and epidemiological data shows that, with one exception, there is very little extension of AIDS from affected Districts in London to the surrounding region or even to the suburban fringe. Where AIDS is prevalent, as in this exception, the distribution follows the same, original pattern which relates essentially to risks arising from life-styles. There is no significant association between the prevalence or spread of AIDS and conventional markers of deprivation, economic and social disadvantage. Allocations of personnel, services and expenditure, assessed from returns required under the AIDS Control Act of 1987 and official registration data, continue to follow the original policy assumptions. They are therefore unrelated to the numbers of existing and new cases, disproportionate and unrealistic. Claims that this widespread excess of effort is justified by the fact that AIDS has not spread to the general population are falsified by the continuation of cases almost exclusively in risk groups. There is nothing in the data required under the Act or in registrations in this main locus of AIDS in the UK to suggest any change in this or to justify continuation of current expenditure and redundant activities. There is no evidence in these data that ethnic variations in the resident populations of Districts are associated with variations in the prevalence of AIDS. However, the larger figures available in some of the national data do indicate a disproportionate increase in some minority ethnic groups. Further detail about risks factors in these groups are required and, meanwhile, effort and expenditure should be re-orientated toward treatment, contact tracing and other public health measures for more effective containment of the continuing spread of AIDS in all the high risk groups.

Acquired Immunodeficiency Syndrome↗
PubMed1999

A case study on expanding the concept of caring from individuals to communities.

Caring has been described as the essence of nursing and some theorists assert that caring occurs only on the interpersonal level. The purpose of this paper is to illustrate caring beyond the individual to communities. Patterns of caring explicated from a case study were matched with concepts of caring defined in the literature. The concepts of caring were modified through a reevaluation of caring based on the findings from a community case study. Findings suggest concepts of caring as (1) a moral imperative, (2) an affect, (3) a personal interaction, and (4) an action, can be found at the community level. Modification of the original patterns of caring included broadening the concepts to include community interaction and preparatory caring action.

Caregivers↗
PubMed2000

Common threads? Palliative care service developments in seven European countries.

Since the late 1960s hospice and palliative care services have been developing in many European countries. Although attention has been given to patterns of development in specific national contexts, so far we lack a comparative understanding of how these services are organized and delivered. Such a comparison poses certain practical and methodological difficulties. It does, however, allow a wider view of the current provision of palliative care in Europe, together with a consideration of implications for the future. We report on an analysis of palliative care developments in seven European countries which gave attention to early origins, patterns of provision, and structural and policy integration. We conclude that, despite different processes of development, the emergent discipline of palliative care now finds its most congenial home within the structures of the formal health care system. Accordingly, inequities between the seven countries can be more clearly identified, posing continuing challenges to policy makers and planners who operate with a European perspective.

Europe↗
PubMedDate not supplied

[Ureteral colic and emotional stress].

UNLABELLED: Clinical practice often shows evidences of organic patterns originated from conflicting psychosocial configurations and excessively stressing contexts of life. The ureteral colic, a frequent and severe morbid entity owing to the lancinating, incapacitating and recurrent pain, however has been kept far from studies which corroborate this perception. MATERIAL AND METHOD: A kind of Case and Control research in 40 patients with an ureteral colic history, has quantified, through the use of the Holmes and Rahe Inventory, the stressing factors in periods of time preceeding the ureteral colic patterns, in comparison to normal controls. RESULTS: The results show a statistically significant difference in favor of cases in a 14 day period, through the stratifying analysis, an individual score higher or equal to 150 points (p = 0.00227949). There was no statistically significant difference in a 12 month period (p = 0.29990307), which makes evident the decreasing of the sensibility concerning the method to this period. CONCLUSION: This result corroborates the evidence of the influence of the emotional stress preceding, in a 14 day studied interval, the unchaining of these crises in predisposed people.

Case-Control Studies↗
PubMed1982

Laser diffraction studies of sarcomere dynamics during 'isometric' relaxation in isolated muscle fibres of the frog.

1. A study has been made of changes in sarcomere length and tension which occur during relaxation from isometric (;fixed ends') tetani in isolated muscle fibres of the frog. Sarcomere lengths were calculated from measurements of the separation of the zero-to-first-order intensity maxima in diffraction patterns generated by illuminating small segments of fibre with a He-Ne laser. Diffraction spectra were recorded continuously on cine-film using the method of ;streak' photography.2. Many sarcomeres in a muscle fibre are found to undergo active shortening during relaxation, at the expense of others located elsewhere which become passively extended. The time of onset of changes in sarcomere length coincides with the well known ;shoulder' on the tension record, and their amplitude is maximal at the time when isometric force approaches zero. The original pattern of sarcomere lengths is only re-established after tension has disappeared.3. The variability in the pattern of sarcomere length changes during relaxation was studied in a sample of twenty fibres, by making ;streak' recordings from successive 1 mm segments along the entire length of each fibre. This survey showed that segments which elongate are located predominantly, though not exclusively, close to the fibre ends. The fractional length of the fibre which underwent shortening was found to vary considerably in different preparations. In many ;streak' recordings the first order line fragmented into several distinct intensity maxima.4. The effects of varying the initial fibre length on the time course of the tension decay and on the accompanying changes in sarcomere length were studied. The range of fibre lengths investigated corresponded with sarcomere spacings of 1.9-3.2 mum. The rate constant, R, for the exponential (later) component of the tension decay decreased with increasing fibre length and this was accompanied by a reduction in the mean amplitude of sarcomere shortening [Formula: see text] and lengthening [Formula: see text]. However, the time interval during which sarcomeres shortened or elongated increased, and so the mean velocities of sarcomere length changes ([Formula: see text] and [Formula: see text]) also displayed an inverse dependence upon fibre length.5. The effects of altering temperature (0-20 degrees C) on the time course of the changes in sarcomere length and tension were investigated. The time to onset of the length changes, t(1), the time to maximum amplitude, t(2), and the time to full recovery, t(3), all decreased exponentially with increasing temperature, but to differing extents. Temperature co-efficients for the velocity of sarcomere length changes during the interval t(2) - t(1) and for the fast tension decay were 2.47 ([Formula: see text]), 2.54 ([Formula: see text]) and 2.45 (R). The rate of the slow tension decay (before the tension ;shoulder') also increased with temperature, with a Q(10) of 2.43.6. The complex patterns of sarcomere length changes seen during relaxation appear to be due to variations in the duration of mechanical activity in different fibre segments. A model is presented which shows that the characteristic form of the tension decay can be accounted for by the progressive emergence of local inequalities in the relative strengths of adjacent sarcomeres, as relaxation proceeds.

Animals↗
PubMed1975

Properties of the high-molecular-weight protein (spectrin) from human-erythrocyte membranes.

The high-molecular-weight protein component from human erythrocytes has been isolated and its solubility properties studied. In physiological solvent conditions the spectrin is not aggregated and is unaffected, both in hydrodynamic properties and conformation, as judged by circular dichroism and intrinsic fluorescence, by the addition of calcium ions. When the pH is decreased an opalescence first sets in, which corresponds to an associated fibrous state of the protein, and when a critical pH is reached precipitation ensues. The precipitation profile is characterised by extreme sharpness, of the kind observed in the phase separation of polyacid-polybase mixtures or of polyampholytes. The addition of calcium ions displaces this precipitation edge towards higher pH. Sodium ions have a similar but smaller effect. The position of the profile is significantly displaced in aged spectrin preparations, or those from frozen erythrocyte ghosts. Fresh preparations of spectrin consist predominantly of a component sedimenting at 9.7 S, with a minor component at 4.4 S (and traces of higher aggregates). The pattern is independent of the ionic strength, or of the presence or absence of calcium ions. The proportion of the small component increases with time, and in spectrin preparations from frozen ghosts it invariably predominates. At low concentrations of guanidine hydrochloride the larger component gives place progressively to the smaller, and vanishes completely when the concentration of the denaturant reaches 1 M. The two components coexist at concentrations below this, and are not in rapid interconversion equilibrium. On recovery of the protein from the guanidine hydrochloride by dialysis, the original pattern of two components is regained. On the other hand the larger component is not found in the material recovered from guanidine hydrochloride solutions of preparations that contain only the small component at the outset. The recovered spectrin is similar to the starting material in its circular dichroism, in its pH-precipitation profiles, and the manner in which the latter is modified by calcium ions. Molecular weight determination by sedimentation equilibrium shows that the 4.4-S species has a molecular weight of some 230 000, which is also the value derived from the extrapolated sedimentation coeffiecient in 6 M guanidine hydrochloride, and thus corresponds to single chains (of which two or more species are resolved in acrylamide gel electrophoresis in the presence of sodium dodecylsulphate); the 9.7-S species, which characterises what is evidently the native state of the extracted spectrin, is found to be a dimer. The frictional coefficients of the monomer and dimer are appreciably different. That of the dimer is compatible with a somewhat asymmetric structure, but by no means to the extent expected for a myosin-like or paramyosin-like molecule.

Blood Proteins↗
PubMed1997

"Cavernous sinus EEG": a new method for the preoperative evaluation of temporal lobe epilepsy.

PURPOSE: In presurgical evaluation of temporal lobe epilepsy (TLE), invasive methods are necessary if results of various noninvasive methods are not sufficiently convergent enough to identify the epileptogenic area accurately. To detect the epileptiform discharges originating specifically from the mesial temporal lobe, we applied the cavernous sinus catheterization technique. METHODS: We placed Seeker Lite-10 guide wire electrodes into bilateral cavernous sinus through the internal jugular veins to record EEG (cavernous sinus EEG) in 6 patients with intractable TLE. Scalp EEG was simultaneously recorded in all 6 and electrocorticogram (ECoG) was also recorded in 4. RESULTS: The cavernous sinus EEG demonstrated clear epileptiform discharges, sometimes even when they were absent on the simultaneously recorded scalp EEG. The epileptiform discharges recorded from the cavernous sinus electrodes were specifically associated with those in the mesial temporal region on ECoG. Ictal EEG pattern originating from mesial temporal lobe was also clearly documented on the cavernous sinus EEG. CONCLUSIONS: This new, semi-invasive method of identifying epileptogenic areas can detect the epileptiform discharges specifically arising from the mesial temporal lobe; it is as useful as or complements the invasive techniques such as foramen ovale or depth recording.

Adolescent↗
PubMed1975

Topography and topology in functional recovery of regenreated sensory and motor systems.

Motor and sensory nerves can re-establish coordinated movement and accurate sensation when they regrow into denervated tissues of some lower vertebrates. Motor nerves achieve their end by a competitive process in which each motoneuron innervates many muscle fibres but, in the presence of many applicants, only those synapses from motoneurons most appropriate to a muscle cell, with respect to the original pattern of development, are retained in a functional state. The discharge pattern of a motoneuron, determined by its connections with the network of central interneurons, is not sensitive to the location of the muscles in which the axon terminates, but the efficacy of transmission from the terminals is. Sensory nerves re-establish their functional specificity as to receptor type by an inductive process occurring at the terminals along with the cessation of growth. However, in the case of cutaneous nerves they can terminate anywhere over the skin surface. The return of correctly localized reflex behaviour therefore demands a restructuring of the central nervous system in response to local position-specific signals, presumably of developmental origin, that are supplied to the sensory nerves by the skin. The re-arrangement of the central nervous connections made by the central processes of the sensory neurons probably uses the same competitive mechanism of enabling and disabling formed synaptic connections as is used in sorting out the correct site of functional termination of the peripheral processes of motoneurons.

Animals↗
PubMed2001

Canadian Aboriginal communities and medical service patterns for the management of injured patients: a basis for surveillance.

Growing attention has been placed on injury as a major public health problem which has served to highlight the need for relevant injury data for preventive purposes at the community level. In the case of reserve-based Aboriginal communities in Canada, available injury data, from large datasets, often has little or no relevance at the community level. In addition, the availability of local data is complicated by unique health service and community infrastructures. As such, a prerequisite to establishing injury surveillance requires an understanding of Medical Service Patterns (MSPs) for injured patients intrinsic to a community's health service infrastructure. In determining patterns, cultural and environmental contexts are integral to methodological considerations as historically, Canada's Aboriginal population has been 'controlled' by others in the areas of health, education and social services. The objective of the study was to investigate MSPs in a Canadian Aboriginal community, specific to the management of injured patients, for the purpose of identifying data sites, sources, and collectors. The method relied on a four-step qualitative process designed explicitly for the study community, comprising: (1) semi-structured interviews with key informants; (2) a flow diagram process; (3) focus group discussions; and (4) a summary matrix diagram. This methodology was later replicated with three additional pilot communities. Three major MSPs were identified from nine original patterns generated through the initial data collection process. MSPs were found to be most directly impacted by severity of injury and the proximity of health service providers. Data collection practices were inconsistent, sporadic and poorly coordinated. Data was exclusive to respective data sources and off-reserve documentation was not reported back to the community. MSPs identified key data sites, sources, and collectors relevant to the study population. In conclusion, the four-step qualitative methodology employed in the study was found to be reliable and feasible in identifying community MSPs. Empirical findings confirm the need to investigate MSPs in communities considering surveillance activities, as intra-national differences may be considerable given social inequalities, geographic uniqueness and cultural factors. The use of sophisticated methodologies may detract rather than promote collaborative efforts.

Alberta↗
PubMed1996

Different protein patterns derived from follicular fluid of mature and immature human follicles.

The purpose of our study was to compare the protein patterns originating from fluids of mature and immature human follicles in order to gain further insight into their biochemical composition. A total of 10 patients were stimulated for in-vitro fertilization (IVF) using different stimulation protocols. Follicular fluids were aspirated transvaginally and analysed microscopically for the presence of oocytes. Follicular fluids were stored at -18 degrees C. Samples of 500 microliters were processed for two-dimensional gel electrophoresis. Up to 60 proteins in various groups could be detected. Seven protein spots were selected for chemical analysis by cutting them out of the gels and subjecting them to internal amino acid sequencing procedures. Our results can be summarized as follows: (i) major differences were not detected between the protein patterns from the various mature follicles of a particular patient, nor were significant differences observed in the proteins derived from follicular fluids collected from the seven patients with mature follicles; (ii) considerable differences were observed in the protein patterns derived from fluids of immature compared with mature follicles. Fluid from the three patients with immature follicles contained many fewer proteins, some of which were expressed at low levels. We conclude that the observed variations in protein composition of follicles of different developmental age reflect their physiological condition and serve as biomedical markers for follicular maturity.

Amino Acid Sequence↗
PubMed1993

Effects of a magnesium-deficient diet on sleep organization in rats.

The influence of magnesium, one of the most important cations in the vertebrate body, on the sleep-wakefulness cycle and ECoG patterns in chronically implanted rats recorded during the light period over a 6-hour period was investigated. Two groups of rats were studied. Group 1 (6 rats): after a control period of 2 weeks, the rats were maintained for 9 weeks on a Mg(2+)-deficient diet. Group 2 (5 rats): after a control period of 2 weeks, the rats were maintained for 7 weeks on a Mg(2+)-deficient diet followed by 4 weeks on a normal diet (recovery period: weeks 8-11). Mg(2+)-deficient diet for 9 weeks induced sleep and ECoG time-dependent alterations. After 6-7 weeks on a Mg(2+)-deficient diet (n = 11) sleep analysis showed a significant increase of wakefulness (+50%) at the expense of slow wave sleep (-24%) but paradoxical sleep was not significantly modified. After 9 weeks of a Mg2+ deficient diet, sleep was disorganized: light sleep and polyspikes occurred indicating an increase in neuronal excitability. When Mg2+ was reintroduced in food and water, sleep organization and ECoG recordings were restored to their original patterns. Our findings which are in line with previous clinical and pharmacological observations provide conclusive arguments for the neuroprotective effect of magnesium ions in neurologic disorders and epileptiform activity. Mg2+ deficiency induces ECoG alterations in the rat which bear some similarities with those seen in neurogenic spasmophilic syndromes in man.

Animals↗
PubMed1998

Beyond black, white and Hispanic: race, ethnic origin and drinking patterns in the United States.

This study used data on 42,862 U.S. adults, including 18,352 past-year drinkers, to describe differentials by race and national origin in U.S. drinking patterns. Age-sex standardized estimates were presented within 21 categories of ethnic origin for whites and within five categories each for individuals of black and other races. Of the three racial groups, whites were the most likely to drink, but blacks had the highest volume of intake and frequency of heavy drinking. Differences by ethnic origin within racial categories were as marked as differentials between races. Compared to whites of European origin, those of Hispanic and native American origin were less likely to drink but consumed more alcohol on days when they drank. Whites of Southern and Eastern European origin drank proportionately more wine and demonstrated more moderate drinking patterns (lower intake per drinking day and/or less frequent heavy drinking) than those of Northern or Central European origin. Hispanics of Caribbean origin were less prone to heavy drinking than other white Hispanics; similarly, blacks from the English-speaking Caribbean showed more moderate drinking patterns than other blacks. Individuals of Asian origin, in particular those of non-Japanese origin, had the most moderate drinking patterns within the category of other race. Although the black/white differentials in volume of intake and frequency of heavy drinking disappeared after adjusting for marital status, education and income, most of the differences by ethnic origin retained their statistical significance if not their original magnitudes. These findings indicate that cultural forces exert a strong effect on drinking behavior. Differences among European whites with respect to prevalence of drinking, beverage preference and frequency of heavy drinking suggest that the association between ethnic origin and drinking behavior may persist even after many generations of presumed acculturation.

Acculturation↗
PubMed1997

Reestablishment of the olivocerebellar projection map by compensatory transcommissural reinnervation following unilateral transection of the inferior cerebellar peduncle in the newborn rat.

It is unclear whether reparative processes in the injured mammalian brain are able to restore the topographic organisation of neuronal connections. To address this question, we have investigated the plasticity of the olivocerebellar system. This pathway has a precise topographic arrangement, in which subsets of inferior olivary neurons project to parasagittally oriented Purkinje cell compartments. Following unilateral transection of the inferior cerebellar peduncle in newborn rats, axons from the contralateral projection cross the cerebellar midline and reinnervate the deafferented hemicerebellum. By this experimental approach, we first analysed the behaviour of calcitonin gene-related peptide (CGRP)-immunoreactive climbing fibres. This marker is transiently expressed by a subset of developing inferior olivary axons, which terminate in the cerebellar cortex into several parasagittal strips. We show that transcommissural axons reestablish the original pattern of climbing fibre bands within a few days after lesion. Then, in adult animals injured at birth, we assessed whether the newly formed climbing fibre bands align with zebrin II+/- Purkinje cell compartments, as in normal conditions. The newly formed projection is organised in parasagittally oriented strips which mirror the distribution of their counterparts on the intact side and are precisely aligned to the heterogeneous Purkinje cell compartments. In addition, the patchy distribution of olivo-nuclear fibres suggests that specific reinnervation is also achieved in the deep nuclei. Thus, transcommissural olivocerebellar reinnervation is not random, but it is regulated by selective interactions between distinct subsets of olivocerebellar axons and target neurons aimed at reestablishing the correct projection map.

Animals↗
PubMed1996

Expression and regulation of brain-derived neurotrophic factor and neurotrophin-3 mRNAs in distinct avian motoneuron subsets.

We performed a detailed study of the expression of neurotrophin-3 and brain-derived neurotrophic factor transcripts in spinal motoneurons using in situ hybridization of serially sectioned chick embryos aged 3 to 8 days (E3 to E8). Neurotrophin-3 mRNA is detected in motoneuron subsets from E3.5 to E4 only in brachial segments of the neural tube and from E5 in both brachial and lumbar regions. Expression of brain-derived neurotrophic factor mRNA is first evident on E5 in a subset of brachial level motoneurons and from E6 also in motoneurons located in the rostral-most portion of the lateral motor column, as well as in the tail-innervating region of the spinal cord. Analysis along the rostrocaudal extent of the brachial lateral motor column reveals an overlap zone of expression of both neurotrophins of about two segments. In transverse sections of this region, it is observed that neurotrophin-3-positive motoneurons preferentially occupy the lateral part of the column, whereas brain-derived neurotrophic factor-producing motoneurons are localized in a more medial position. These results show that the two factors are synthesized at discrete axial levels of the spinal cord by distinct motoneuron subpopulations. Since brain-derived neurotrophic factor mRNA is expressed within the brachial but not the lumbar lateral motor column, we tested the possibility that brain-derived neurotrophic factor expression is regulated by the type of peripheral target, that is, the wing or the leg. Unilateral transplantation of a wing bud instead of a leg bud and vice versa, prior to the onset of peripheral innervation, failed to alter the original pattern of brain-derived neurotrophic factor mRNA observed in either level of the axis. Thus, the early synthesis of brain-derived neurotrophic factor by subsets of spinal motoneurons is independent of the type of peripheral target and may instead reflect intrinsic differences between motoneuron populations.

Animals↗
PubMedDate not supplied

Fragile X mental retardation: prevalence in a group of institutionalized patients in Italy and description of a novel EEG pattern.

We have studied the prevalence of the fra (X) and of the autosomal fragile sites fra (10) (q25) and fra (16) (q22) in patients from an institute for the mentally retarded in Italy. We found six cases (1.9%) of fra (10) (q25) and 9 (2.9%) of fra (16) (q22). The study of the fra (X) was restricted to a subgroup of 91 males who did not have other chromosome anomalies or variants, and led to the discovery of 4 fra (X) cases. These 4 had the Martin-Bell syndrome; 3 of them were epileptic and had a characteristic EEG pattern originating during sleep from the temporal lobe not previously described in fra (X) mental retardation.

Adolescent↗
PubMed1993

Is segmentation generic?

When two populations of cells within a tissue mass differ from one another in magnitude or type of intercellular adhesions, a boundary can form within the tissue, across which cells will fail to mix. This phenomenon may occur regardless of the identity of the molecules that mediate cell adhesion. If, in addition, a choice between the two adhesive states is regulated by a molecule the concentration of which is periodic in space, or in time, then alternating bands of non-mixing tissue, or segments, can form. But temporal or spatial periodicities in concentration will tend to arise for any molecule that is positively autoregulatory. It is therefore proposed that segmentation is a 'generic' property of metazoan organisms, and that metamerism would be expected to have emerged numerous times during evolution. A simple model of segmentation, based solely on differential adhesion and periodic regulation of adhesion, can account for segment properties as disparate as those seen in long and short germ band insects, and for diverse experimental results on boundary regeneration in the chick hind brain and the insect cuticle. It is suggested that the complex, multicomponent segment-forming systems found in contemporary organisms (e.g., Drosophila) are the products of evolutionary recruitment of molecular cues such as homeobox gene products, that increase the reliability and stability of metameric patterns originally templated by generic self-organizing properties of tissues.

Animals↗