PubMed Health⌕ Search

Biomedical subjects

T Elsdale

Publications and source records attributed to T Elsdale.

13 recordsLinked to original sources
PubMed1987

Timekeeping by frog embryos, in normal development and after heat shock.

(1) Timekeeping refers to the uniformity of development in time. The precision of timekeeping is measured by the extent to which embryos, within an initially synchronous population, come to diverge in the course of their development. (2) Divergence is measured as the variation in the stage of development reached between embryos allowed to develop for a fixed period of time. The lower the variation the better the timekeeping. (3) Divergence among frog embryos that started development at the same time is hardly measurable after approx. 100 h of development. This striking uniformity indicates good timekeeping. (4) Timekeeping is not impaired among the survivors following heat shocks that retard development and disturb and curtail morphogenesis. (5) The immediate effect of heat shock is a stoppage of development, the duration of which is the same for all embryos in the same treatment batch. The embryos react to heat shock by rescheduling their development with the interpolation of a rest, the duration of which is controlled to the same precision as normal development. The postponement of development, without impairment of timekeeping, implies dis-engagement of the processes of morphogenesis from, and their subsequent re-engagement with, an enduring rate-determining activity unaffected by heat shock. (6) We have searched for embryos whose rate of development was disturbed by heat shock to run slower or faster than the norm. We have found none. It seems that the (temperature-compensated) rate of development is invariant up to the moment of failure, or a change is immediately lethal.

Animals↗
PubMed1986

Growth regulation in multilayered cultures of human diploid fibroblasts: the roles of contact, movement and matrix production.

Early subcultures of human embryonic lung fibroblasts are exceptional, as they grow far beyond confluence before growth ceases: the stationary dish may well contain 3-10 monolayer equivalents. Maximal growth rates, however, occur at about one-sixth confluence when doubling times are 15-20 hr; a density at which cell contacts begin to become frequent. The fact that a slowing down of growth is first apparent at such low densities argues against this regulation being due to diffusion effects. Confirmation of the role of short-range or contact interactions in growth regulation comes from an experiment using mixed cultures of fibroblasts: this shows that growth inhibition is not carried by medium-borne influences but depends on short-range (less than 1 mm) interactions. Evidence that cells can escape the effects of such contact interactions and so divide comes from time-lapse studies of dense cultures: there is a burst of motility soon after a fresh-medium change, which is followed by a burst of mitosis approximately 20 hr later. A medium change to conditioned medium supplemented with 10% foetal calf serum leads to neither the burst of motility nor the subsequent burst of mitosis, although this medium is better able to support the growth of sparse cells than is fresh medium. Data are also presented to show that the amount of collagen deposited in superconfluent cultures affects their growth: the stimulation of collagen production with ascorbic acid leads to an unexpectedly low stationary cell density and rather less movement in the culture. This result suggests that the collagen stabilizes cell contacts that are responsible for growth inhibition. The question of why these cells grow more slowly as density increases cannot be answered directly by these experiments; nevertheless, the results suggest that cell contact affects the permeability of the cell membrane to medium.

Cell Communication↗
PubMed1983

Somitogenesis in amphibia. IV. The dynamics of tail development.

Following neurulation, the frog segments c.40 somites and concurrently undergoes a striking elongation along the anteroposterior axis. This elongation (excluding the head) is largely the result of a presegmental extension of posterior tissue with a lesser contribution from the extension of segmented tissue. Presegmental extension is entirely the result of activity within a narrow zone of extension that occupies the central region in the tail bud. Within the zone of extension, a minimum of six prospective somites undergo an eight- to ten-fold extension along the axis. The zone passes posteriorly across the tissue of the tail tip. The anterior of the tail bud contains three extended prospective somites in the course of segmentation. The anterior boundary of the zone of extension coincides in space exactly with the anterior boundary of the zone of abnormal segmentation that results from temperature shock. This means that extension ceases immediately before the sudden tissue change associated with segmentation.

Animals↗
PubMed1980

Somitogenesis in amphibian embryos. III. Effects of ambient temperature and of developmental stage upon pattern abnormalities that follow short temperature shocks.

Temperature shocks of a few minutes duration at 37 degrees C to tail-bud embryos of Rana induce zones of abnormal segmentation along the somite files subsequently produced. The immediate result of a temperature shock is a temporary arrest of development as a whole, following which the schedule of somite determination and formation is resumed at the normal rate. It is during the period immediately following this that the zone of abnormal somite pattern is determined. Thus the length of the abnormal zone reflects the total time taken by the morphogenetic system to recover from the disturbance, and might depend upon variables affecting both the duration of the initial arrest and the duration of the recovery period itself. Observations are presented demonstrating how the length of abnormal zones, caused by a temperature shock of any particular severity, are affected by three variables; (1) the ambient temperature to which the embryos were adapted before shock, (2) the ambient temperature of post-shock development, (3) the stage in somitogenesis, i.e. the number of somites already formed at the time of shock. The data (in this and previous papers of the series) support models postulating that the spatial periodicity in cell behaviour, that is somite morphogenesis, reflects a normal interaction between two hidden aspects of development, one a wavefront of cellular activation passing down the body axis, and the other having the character of a temporal periodicity throughout the tissue. Temperature shock, as well as halting the wavefront (i.e. stopping development) temporarily, leads to a subsequent period during which there is only gradual recovery of normal co-ordination between the periodicity within cells of the tissue and the wavefront progress. It is the relative rate of this recovery, alone, that is responsible for variation in the length of the abnormal zone.

Animals↗
PubMed1979

Somitogenesis in amphibian embryos. I. Experimental evidence for an interaction between two temporal factors in the specification of somite pattern.

Somitogenesis is described in two species of anuran amphibians, Xenopus laevis and Rana temporaria, in which the cellular mechanics of somite formation are distinctly different. Heat shocks are employed to demonstrate a wave of cellular change which precedes somite formation down the body axis. This prior wave is shown to be kinematic. It is not a propagated wave. It is a consequence of the temporal activities of the cells laid out in space, but there is no evidence that these activities depend upon an interpretation of their position. Heat shocks cause characteristic segmental abnormalities over a zone of somites which is formed several hours after the shock. Evidence from double heat shock experiments suggests that the pattern of abnormality is the result of (i) a disturbance of co-ordination between pre-somitic cells, and (ii) the time available to those cells for recovery before they are recruited into a segmental pre-pattern at the time of passage of the prior wave. It is a temporal co-ordination that is disturbed and subsequently recovered following a heat shock. This temporal co-ordination of pre-somitic cells does not depend upon position along the axis. The evidence for two physiologically independent temporal patterns of cellular processes, which interact to specify the segmental pattern of somites (their size, shape and number), gives experimental support for the theoretical account of somitogenesis proposed by Cooke & Zeeman (1976).

Animals↗
PubMed1979

Somitogenesis in amphibia. II. Origins in early embryogenesis of two factors involved in somite specification.

A somite pre-pattern is established shortly before visible segmentation. The pre-pattern results from the interaction of two components: a wave of cell behavioural change that passes along the axis, and, an underlying co-ordination of the cells that is the basis for their association into large somite-sized groupings. The evidence is derived from studies of the zones of abnormal segmentation that follow temperature shocks delivered between the neurula and tail-bud stages (Pearson & Elsdale, 1979). Temperature shock given earlier at the mid-gastrula stage is however ineffective in inducing abnormalities in somitogenesis. Shocks given before the mid-gastrula stage reveal a prior period of sensitivity stretching back into the blastula. Thus early and late sensitive periods can be defined separated by a short refactory period. Quite different patterns in the distribution of somite abnormalities characterize the results of shock during the two sensitive periods, suggesting different aetiologies. It is concluded that the wave of rapid cell change is set up early in embryogenesis during the blastula stage, and each cell of the prospective paraxial mesoderm carries a determination to change after a specific length of time, i.e. a countdown is set in each cell. As a result of the movements of gastrulation, the prospective paraxial mesoderm cells become laid out along the axis of the neurula in the order (antero-posterior sequence) in which they will change. The achievement of the correct redistribution of the cells depends crucially on the conservation of the sequence in the blastula by the maintenance of topological integrity throughout gastrulation. It is suggested that early shock disturbs gastrulation movements, causing some mixing up of the cells resulting in incoherence of the wavefront. Whereas early shocks are thus assumed to affect the wave, the evidence suggests that late shock undergoes co-ordination. It is concluded therefore that co-ordination is established later, after the refractory period, around the late gastrula stage.

Animals↗
PubMed1976

Abnormalities in somite segmentation following heat shock to Xenopus embryos.

The typical abnormality induced by a 15 min shock at 37degreesC is a single discrete length along the somite file within which segmental boundaries are absent or irregular. The two sides of the same embryo present a similar but not necessarily identical appearance. Usually all the embryos in a treated batch show abnormalities of similar severity. Survival of treated embryos, the details of the visible malformations, and temporal aspects of the phenomenon have been studied. The results indicate a temperature sensitive period that traverses the neurula, from head to tail at about the same rate as the somites form, but some hours beforehand. The temperature sensitive process is not associated with cell determination and differentiation, and there are reasons for thinking that the specification of the normal somite number occurs independently. The results are discussed in relation to Cooke & Zeeman's model of a wave front interacting with an oscillator.

Animals↗
PubMed1972

Collagen substrata for studies on cell behavior.

A simple technique is described for the preparation of collagen substrata containing 0 1% of collagen by weight, in the form of native bundles with a 640 A period, the substrata are similar in these respects to soft-tissue matrices These substrate are hydrated collagen lattices (HCLs) in which the watery milieu is held within a fibrous collagen net mainly by capillary forces. HCLs have been characterized in terms of the course of collagen precipitation and aggregation, ultrastructure, and their stability under various conditions. The ways in which HCLs can be employed as both two- and three-dimensional substrata in cell behavioral studies are illustrated with some preliminary observations on the form, motility, adhesion, and growth of human diploid cells and two lines of malignant cells.

Animals↗
PubMed1969

Morphogenetic aspects of multilayering in Petri dish cultures of human fetal lung fibroblasts.

Randomly seeded Petri dish cultures of embryonic human lung fibroblasts generate, in the course of their growth, highly ordered cellular arrangements. Thick, bilaterally symmetrical ridges with an axial polarity and an orthogonal, multilayered internal organization are observed within stationary cultures. The generation of these structures has been investigated. Ridges result from the spontaneous aggregation of cells in postconfluent cultures brought about by directed cell movements. These movements are promoted by the localized production of extracellular matrix sheets containing collagen, which provide new substrates for cellular colonization. Cells that have colonized one matrix substrate may secrete another above themselves, which will in turn be colonized. By a continuation of this cycle, thick stacks consisting of alternate layers of cells and matrix are produced to yield the observed aggregations. The distribution and shape of ridges in a culture imply that matrix substrates are confined to specific locations. The suggested control hypothesis assumes that all the cells in fibroblast cultures are potential producers of a single species of matrix. The serviceability of this matrix as a substrate for cellular colonization, however, is destroyed if the producer cells are motile. Matrix substrates, therefore, are only made by nonmotile cells.

Cell Aggregation↗