bluuebunny/crossref_metadata_2025_split huggingface dataset:
The spatial pattern of aggregation centres in the cellular slime mould
Authors: Waddell David R. 
Published: April 2021

ABSTRACTThe spatial pattern of aggregation centres of the slime mould Dictyostelium discoideum was analysed by using nearest-neighbour distributions. Both undisturbed cultures, and cultures that were initiated from cells dissociated from cultures that had already aggregated, formed non-randomly spaced patterns. However, the minimal distance between aggregates in undisturbed cultures was approximately ten times that observed in dissociated cultures.In undisturbed cultures the aggregate size is regulated as a function of cell density (Bonner & Hoffman, 1963): As cell density increases aggregate density decreases and the aggregate size consequently increases. At the same cell density more and smaller aggregates were formed by dissociated cultures than by undisturbed cultures. Nevertheless, the same cell-densitydependent regulation of aggregate density existed in the dissociated cultures.Here a model is developed to account for both the non-random spacing and the celldensity-dependant regulation of aggregate density. In this model, distance-dependent competition occurs between points in a random prepattern to generate patterns very similar to those observed in experiments. The cell-density-dependent regulation of the aggregate density can be explained by assuming that a constant fraction of the cell population has the capacity to initiate centres at the time of pattern determination regardless of the cell density. As the cell density is increased the fraction of potential centres that survive distance-dependent competition decreases and hence the aggregate size increases. These results suggest that distance-dependent competition may be a mechanism that evolved to control aggregate size at high cell densities.Results obtained with this model indicate that the decrease in aggregate size in dissociated cultures is due primarily to an increase in the fraction of the cell population capable of initiating centres. This implies that as morphogenesis progresses a greater fraction of the cell population acquire a capacity which they will not normally express. Although this increase may have a function in later stages of morphogenesis, it may also provide a way to ensure aggregation in small populations of amoebae and at low cell densities.
Effects of ultraviolet light on the activity of an avian limb positional signalling region
Authors: Honig Lawrence S. 
Published: April 2021

ABSTRACT
Cells from a region on the posterior margin of the developing avian limb bud (the polarizing region) can signal positional information to responding anterior cells. Polarizing activity may be assayed by disaggregation of the tissue into single cells, followed by reaggregation and grafting of cell pellet. This method enables treatments of the cells while in single-cell suspension. The effects of ultraviolet radiation (254 nm) were studied to determine the role of nucleic acids in polarizing activity. Ultraviolet radiation eliminated quail polarizing activity over the same range of doses as it reduced cell spreading (20-60 J/m2). In contrast to the published effects of γ-radiation in which polarizing activity is affected only at doses much higher than those that are cell lethal, ultraviolet apparently abolishes cell survival and polarizing activity at comparable doses. Compared to other biological systems, polarizing activity is quite sensitive to ultraviolet light: the D37 is 18 J/m2; the extrapolation number, n, is 3·6. From comparison of the effects of ultraviolet radiation to those published for ionizing X- or γ-radiation, one may conclude that ultraviolet and y-radiation abolish limb polarizing activity at equivalent nucleic acid dosages, not at equivalent cell lethal doses.
A quantitative study of hair growth using mouse and rat vibrissal follicles
Authors: Ibrahim Lamees , Wright E. A. 
Published: April 2021

ABSTRACT
The volumes of the dermal papilla, matrix and fully grown hair of vibrissal follicles of rats and mice were measured.
Dermal papillae were surgically removed from rats’ vibrissal follicles and a small new dermal papilla was always reformed. The lower third of rats and mice follicles, containing the whole of the dermal papilla was amputated and very small dermal papillae were reformed.
Follicles containing small dermal papillae produced small hairs and a constant positive linear relationship of 7·0 was found between the volume of the dermal papilla and the volume of the full-grown hair for all cases studied.
Changes in the Protein Content of Yolk during Chick Embryogenesis
Authors: Mcindoe W. M. 
Published: April 2021

ABSTRACTThe original purpose of the present investigation was to determine the rate of degradation of phosphoprotein in the yolk at different times during embryogenesis as a preliminary to examining the mechanism of its assimilation, and to see if phosphoprotein and other proteins were assimilated at different times or at markedly different rates. The early work on the assimilation of yolk protein has been reviewed by Needham (1931) and few pertinent papers have appeared since then (Schenck, 1932; Rupe & Farmer, 1955; Walter & Mahler, 1958). Recently Holoubek & Brada (1956) have made an extensive examination of the distribution of phosphorus compounds in the egg during embryogenesis.
Cortical Grafting inXenopus laevis
Authors: Curtis A. S. G. 
Published: April 2021

ABSTRACTA technique for the grafting of cortical material from one embryo to another at early cleavage stages is described. This method may also be used for the grafting of cortical material from infertile eggs to early cleavage stages. Grafts transplanted to embryos during first cleavage to the same site as that from which they were derived, resulted in normal development. Grafts obtained from grey crescent material and transplanted to the ventral margin induced the appearance of a secondary dorsal lip and subsequent secondary axis. Those of animal pole or ventral margin material placed in the grey crescent resulted in the failure of invagination during gastrulation at the graft site, and this causes the splitting of the dorsal lip into two and the appearance of double axes. Animal pole material placed at the vegetal pole produces a peculiar form of exogastru-lation. Cortical material from infertile eggs if grafted into embryos during their first cleavage inhibits further development. Cortical material from fertile uncleaved eggs, if placed in embryos in the midst of their first cleavage, appears to cause a lag in cleavage in that half of the embryo which received the graft. These results show that the cortical material definitely possesses morphogenetic properties which may be transferred with it. These are concerned with gastrulation, and the experiments are discussed in relation to the ‘cortical field’ theory of Dalcq & Pasteels. This technique of grafting allows investigation of the organization of the cortical field.
Oocytes and early embryos of Xenopus laevis contain intermediate filaments which react with anti-mammalian vimentin antibodies
Authors: Godsave Susan F. , Anderton Brian H. , Heasman Janet , Wylie Christopher C. 
Published: April 2021

ABSTRACT
Previous studies have shown that Xenopus oocytes possess a cortical shell, which includes actin-containing microfilaments and cytokeratin-containing intermediate filaments. In this paper we show that oocytes of Xenopus laevis also contain filaments throughout their cytoplasm which are stained by several anti-vimentin antibody preparations. We also show that dramatic changes in pattern of these filaments occur during oocyte differentiation, first during vitellogenesis, and then during maturation of the oocyte to form an egg.
Intercellular relationships during cavitation of aggregates of extraembryonic endoderm cells from gastrulating chick embryos
Authors: Milos Nadine , Zalik Sara E. , Sanders Esmond J. , Ledsham Irene M. 
Published: April 2021

ABSTRACT
Extraembryonic endoderm cells from gastrulating chick embryos undergo epiboly and change from a multilayered cell group to a single cell layer surrounding the yolk. Single cell suspensions from this cell layer can aggregate in vitro to form aggregates that cavitate. To study the stages of cavitation aggregates were harvested after different times in culture, and fixed and processed for light and electron microscopy. In aggregates harvested at 75 min of culture cell contact consisted of areas of parallel and close membrane apposition and interdigitation. Desmosomes were occasionally observed. Aggregates in the early stages of cavitation (24 h) contained numerous intercellular spaces bordered by irregularly shaped cells which appeared to be digesting their yolk and releasing material extracellularly. Long cytoplasmic projections were extended into these spaces. In addition to regions of parallel membrane apposition and interdigitation, desmosomes and adherens junctions were observed. Cells closer to the periphery of the aggregates displayed fewer cell projections and also showed signs of release of material extracellularly. After 48 h of culture, a single smooth-walled central cavity was present and cells still exhibited signs of extracellular release of material. These same cell shapes and intercellular junctions were also observed when area opaca tissue dissected from gastrulating embryos was examined. Aggregates of different sizes were created and cultured. The results suggest that a critical tissue mass may be important for cavitation.
Switch genes and sex determination in the nematode C. elegans
Authors: Hodgkin Jonathan 
Published: April 2021

ABSTRACT
The development of the nematode C. elegans is highly invariant, and has been described in great detail. Many developmental mutations have been isolated and analysed; some of these identify switch genes, i.e. genes that control the choice between two developmental alternatives. It appears that the genetic controls of development in this animal are discrete, hierarchical and relatively simple. The control of sexual dimorphism provides an example of how a series of switch genes are organized in a regulatory cascade.
Polarity, calcium and abscission: molecular bases for developmental plasticity in plants
Authors: Trewavas A. J. , Sexton R. , Kelly P. 
Published: April 2021

ABSTRACT
This article considers novel aspects to plant development. It starts by outlining the concept of plastic development. It then discusses polarity in plants arguing for the vascular tissue and surrounding cells as a major internal polar axis. The requirement for a change in polarity in this system when regeneration occurs is discussed and molecular mechanisms for specification and changing of polarity are outlined. The process of abscission is then discussed as an example of plastic development and it is concluded that many factors control abscission.
Molecular changes in abscission zone cellulase are summarized and the notion of two phases in abscission are developed and generalized to other systems of development in plants.
Models for positional signalling, the threefold subdivision of segments and the pigmentation pattern of molluscs
Authors: Meinhardt Hans 
Published: April 2021

ABSTRACT
Models of biological pattern formation are discussed. The regulatory features expected from the models are compared to those observed experimentally. It will be shown that: Stable gradients appropriate to supply positional information can be produced by local autocatalysis and long-range inhibition.Spatially ordered sequences of differentiated cell states can emerge if these cell states mutually activate each other on long range but exclude each other locally. Segmentation results from the repetition of three such cell states,S,A and P (and not of only two, as is usually assumed). With a repetition of three states, each segment has a defined polarity. The confrontation of P cells and S cells lead to the formation of a segment border (… P/SAP/ SAP/S …) while the A-P confrontation is a prerequisite for appendage formation. Mutations of Drosophila affecting larval segmentation are discussed in terms of this model.The two models for the generation of sequences of structures in space (positional information including interpretation versus mutual activation) lead to different predictions with respect to intercalary regeneration. This allows a distinction between the two models on the basis of experiments.The pigmentation patterns of certain molluscs emerge from a coupled oscillation of cells (that is, a lateral inhibition in time, instead of space). The oblique lines result from a chain of triggering events.
Subroutines in the programme of Chlamydomonas gene expression induced by flagellar regeneration
Authors: Schloss Jeffery A. 
Published: April 2021

ABSTRACT
The unicellular green alga Chlamydomonas reinhardtii possesses two anterior flagella that are rapidly replaced if they are lost. A cytoplasmic pool of flagellar precursors supports regeneration of partial length flagella, but complete regeneration requires de novo synthesis of flagellar proteins. This increase in protein synthesis is transient and is programmed by changes in the physical abundance of a set of RNAs. These changes were measured using cloned cDNAs. The curves that were generated with probes for a number of different RNAs were variations-on-the-theme of a rapid accumulation following deflagellation of the cells, followed by similarly rapid degradation. Differences in the characteristics of the accumulation curves suggest that several ‘subroutines’ for RNA metabolism appear to run concurrently during flagellar regeneration. A significant portion of the mRNA abundance regulation occurs at the transcriptional level. A group of cDNAs that encode mRNAs whose abundance remains at constant levels during regeneration provides an important internal control for the abundance and transcription measurements. Flagella contain over 200 different proteins, so the possibility exists that the cells coordinately and rapidly alter the transcription of the same number of genes. Chlamydomonas flagellar regeneration thus provides an opportunity to study cellular mechanisms for coordinating the expression of a large set of genes, and correlating these changes with an easily monitored morphogenetic process.
Allocation of mesendodermal cells during early embryogenesis in the starfish, Asterina pectinifera
Authors: Kominami Tetsuya 
Published: April 2021

ABSTRACT
The volume of archenteron tissue (mesendoderm) in the early-to-middle gastrula of the starfish Asterina pectinifera was nearly one-quarter of whole embryo. Treatment with LiCl during 7–10 h increased this volume ratio by about 30 %, whereas the total volume and the total number of cells of whole embryo remained unchanged. Such a relative increase in mesendodermal part and simultaneous reduction in ectodermal part by LiCl treatment was confirmed by counting the number of constituent cells of these parts at early bipinnaria stage. Pulse treatment with LiCl revealed that the effective period of the treatment is limited from 7 to 10 h of development, when tightly packed blastulae are formed through increase in adhesiveness of blastomeres. These results indicate that a fraction of presumptive ectodermal cells can change its fate to mesendoderm during 7–10 h of development. Cellular interactions during a specific stage of development are suggested to be involved in the determination of mesendodermal tissue.
The stages of flank ectoderm capable of responding to ridge induction in the chick embryo
Authors: Carrington Jill L. , Fallon John F. 
Published: April 2021

ABSTRACT
Reports on the stages when chick flank ectoderm can respond to ridge induction are contradictory. Different results have been obtained using presumptive wing or leg bud mesoderm as the inducing tissue with flank ectoderm as the responding tissue. In addition, although incomplete outgrowths have been obtained from recombinants with stage-19 flank ectoderm in a small percentage of cases, no complete outgrowths have been obtained from recombinants with ectoderm older than stage 18. We reinvestigated when chick flank ectoderm can respond to ridge induction and promote outgrowth of complete limbs. To do this, we combined flank ectoderm with in situ chick presumptive wing bud mesoderm using a pre-limb bud recombinant technique. When presumptive wing bud ectoderm was removed from the host and not replaced, wing development was suppressed. When host ectoderm was replaced with stage-15 through -18 chick flank ectoderm, limbs grew out in all cases; 86·4% of these recombinant limbs were distally complete. Stage-19 flank ectoderm formed a ridge and promoted limb outgrowth in 80·9% of recombinants; 52·9% of these were distally complete limbs. Recombinants made by grafting early stage-20 (40-somite donor) flank ectoderm to stage-15 hosts resulted in outgrowths in 60% of the cases and 33·3% of these were distally complete. Graft ectoderm from older donors did not respond to inductive mesoderm.
Our results demonstrate that chick flank ectoderm from stage-15 through early stage-20 donors can respond to inductive signals from presumptive wing bud mesoderm to form an apical ridge. This ridge can promote outgrowth of distally complete wings in a substantial proportion of recombinants. This is two stages beyond when the ability to promote outgrowth of distally complete wings appeared to be lost using other methods.
Microtubules influence compaction in preimplantation mouse embryos
Authors: Maro Bernard , Pickering Susan J. 
Published: April 2021

ABSTRACT
The role of microtubules during compaction of the 8-cell-stage mouse embryo was investigated using the drugs Taxol (which leads to a non-controlled polymerization of tubulin) and Nocodazole (which causes depolymerization of microtubules). Taxol inhibits compaction in most non-compacted embryos and reverses it in already compacted embryos. These effects were observed on both cell flattening (as judged by phase-contrast microscopy) and on cell surface polarization (as judged by scanning electron microscopy and the surface binding of fluorescent concanavalin A). In contrast Nocodazole does not inhibit cell flattening, but rather accelerates its completion. Nocodazole influences the detailed organization of the surface pole and appears to reduce the incidence of surface polarization but does not reverse polarity once established to a significant extent. We conclude that microtubules exercise a constraining role during compaction, influencing cell shape, cell organization and the time at which compaction takes place.
Restoration of the antibody response to sheep erythrocytes in thymectomized Xenopus implanted with MHC-compatible or MHC-incompatible thymus
Authors: Gearing A. J. H. , Cribbin Frances A. , Horton J. D. 
Published: April 2021

ABSTRACT
These experiments make use of an amphibian model system for investigating the role of the thymus in T helper cell education. Clawed toads (Xenopus laevis), thymectomized at 7 days, are unable to mount an antibody response to thymus-dependent antigens, such as sheep red blood cells (SRBC). When thymectomized larvae are implanted with larval thymuses (either irradiated or non-irradiated), incompatible at the major histocompatibility complex (MHC) or with MHC-compatible or -incompatible ‘adult’ thymuses, their splenic plaque-forming cell response and serum haemolytic antibody production to SRBC are both restored, to some extent. However, levels of mercaptoethanol-resistant antibody were extremely poor in those animals implanted with MHC-incompatible ‘adult’ thymus. Larval thymus implants were shown, by ploidy-labelling studies, to become repopulated with host-derived lymphocytes. Whether or not these lymphocytes acquire their MHC restriction specificities in the thymus awaits clarification.
Formation of the notochord in living ascidian embryos
Authors: Miyamoto David M. , Crowther Robert J. 
Published: April 2021

ABSTRACT
The dynamic behaviour of cells during formation of the notochord in the ascidian, Ciona intestinalis, was examined by means of Differential Interference Contrast (DIC) microscopy and time-lapse videorecording. The initial rudiment is formed in part as a consequence of the pattern of mitotic divisions as the blastopore shifts posteriorly. Vertical and horizontal rearrangements produce an elongate rod of disc-shaped cells stacked end to end. Further elongation is accompanied by a cell shape change. Some cell growth or swelling is indicated to occur later in development, but this growth appears to contribute mostly to an increase in the diameter, and only insignificantly to the length of the notochord. Intracellular vacuoles that appear around 13 h after fertilization increase in size and fuse at about 16 h to form intercellular ones. These in turn merge to form the central matrix core of the notochord at around 18 to 20 h. As the notochord elongates and cells change in shape, the basal surfaces bleb actively. This surface activity may be related to formation of the perinotochordal sheath.
Cautery-induced colour patterns in Precis coenia (Lepidoptera: Nymphalidae)
Authors: Nijhout H. F. 
Published: April 2021

ABSTRACT
Cautery of the dorsal hind wing in the butterfly, Precis coenia, induces the formation of a concentric colour pattern around the site of injury. The induced pattern is identical in pigmentation to the eyespots that normally develop on this wing surface. This response to cautery also occurs, though much less dramatically, on the ventral forewing. In addition to the peculiar response to cautery, the dorsal hindwing of Precis also develops a series of unique pattern aberrations in response to coldshock. These consist of irregular elongation of the anterior eyespot along the proximodistal axis of the wing. In the most dramatic aberrations the eyespot field covers the entire anterior half of the wing surface. An analysis is presented that attempts to reconcile the effects of cautery on the Precis hindwing with the very different morphological effects of cautery on the colour pattern of Ephestia kühniella, described by Kühn & Von Engelhardt. Computer simulations reveal that the finding presented in this paper, as well as the classical work on Ephestia, can both be explained by assuming that the site of cautery becomes a sink for one of the morphogens involved in colour pattern determination. The experimental findings furthermore indicate that minor perturbations of the wing epidermis can evoke the physiological conditions that attend normal eyespot determination. It is shown that this interpretation also helps to explain the unusual pattern modifications following coldshock.
Maps of strength of positional signalling activity in the developing chick wing bud
Authors: Honig Lawrence S. , Summerbell Dennis 
Published: April 2021

ABSTRACT
Tissue from the posterior margin of the developing limb bud, when grafted to the anterior margin, evokes the formation of a mirror-image limb duplication from the host tissue. We present maps of the spatial and temporal distribution of this signalling activity in the chick wing bud based on a bioassay that provides a quantitative measure of the completeness of the additional structures (the strength of activity index). Activity is first detected prior to the initial appearance of the limb primordium as early as Hamburger & Hamilton stage 14. It reaches a maximum during early outgrowth of the bud at stages 19 to 25. It then declines as the limb starts to differentiate into its final morphological pattern. The design of the experiment provides serendipitous data showing that two operators can consistently perform grafts with high reproducibility between them while variability between embryos is somewhat higher. The maps of activity are of particular practical value in precisely defining for the experimental embryologist and molecular biologist those positions and stages at which peak signalling activity resides.