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M K Richardson

Publications and source records attributed to M K Richardson.

31 records · Page 2Linked to original sources
PubMed1997

Symposium on neural crest development.

The following review articles are based on presentations given at a Symposium on Neural Crest Development, held in December 1996 at St George's Hospital Medical School, London. The Symposium formed part of the Winter Meeting of the Anatomical Society of Great Britain and Ireland. There were 6 invited speakers, 5 selected short presentations, and posters. The topics ranged widely, from autocrine regulation of Schwann cell development through clinical and animal disorders involving the melanocyte lineage, the heart and other organs, to the regulation of neural crest cell differentiation by cytokines.

Journal Article↗
PubMed1997

The neural crest population responding to endothelin-3 in vitro includes multipotent cells.

The peptide endothelin 3 (EDN3) is essential for normal neural crest development in vivo, and is a potent mitogen for quail truncal crest cells in vitro. It is not known which subpopulations of crest cells are targets for this response, although it has been suggested that EDN3 is selective for melanoblasts. In the absence of cell markers for different precursor types in the quail crest, we have characterised EDN3-responsive cell types using in vitro colony assay and clonal analysis. Colonies were analysed for the presence of Schwann cells, melanocytes, adrenergic cells or sensory-like cells. We provide for the first time a description of the temporal pattern of lineage segregation in neural crest cultures. In the absence of exogenous EDN3, crest cells proliferate and then differentiate. Colony assay indicates that in these differentiated cultures few undifferentiated precursors remain and there is a low replating efficiency. By contrast, in the presence of 100 ng/ml EDN3 differentiation is inhibited and most of the cells maintain the ability to give rise to mixed colonies and clones containing neural crest derivatives. A high replating efficiency is maintained. In secondary culture there was a progressive decline in the number of cell types per colony in control medium. This loss of developmental potential was not seen when exogenous EDN3 was present. Cell type analysis suggests two novel cellular targets for EDN3 under these conditions. Contrary to expectations, one is a multipotent precursor whose descendants include melanocytes, adrenergic cells and sensory-like cells; the other can give rise to melanocytes and Schwann cells. Our data do not support previous claims that the action of EDN3 in neural crest culture is selective for cells in the melanocyte lineage.

Animals↗
PubMed1996

Overexpression of BMP-2 and BMP-4 alters the size and shape of developing skeletal elements in the chick limb.

Bone morphogenetic proteins are members of the transforming growth factor beta (TGF beta) superfamily which are involved in a range of developmental processes including modelling of the skeleton. We show here that Bmp-2 is expressed in mesenchyme surrounding early cartilage condensations in the developing chick limb, and that Bmp-4 is expressed in the perichondrium of developing cartilage elements. To investigate their roles during cartilage development, BMP-2 and BMP-4 were expressed ectopically in developing chick limbs using retroviral vectors. Over-expression of BMP-2 or BMP-4 led to a dramatic increase in the volume of cartilage elements, altered their shapes and led to joint fusions. This increase in volume appeared to result from an increase in the amount of matrix and in the number of chondrocytes. The latter did not appear to be due to increased proliferation of chondrocytes, suggesting that it may result from increased recruitment of precursors. BMP-2 and BMP-4 also delayed hypertrophy of chondrocytes and formation of the osteogenic periosteum. These data provide insights into how BMP-2 and BMP-4 may model and control the growth of skeletal elements during normal embryonic development, suggesting roles for both molecules in recruiting non-chondrogenic precursors to chondrogenic fate.

Animals↗
PubMed1995

Heterochrony and the phylotypic period.

There has been a resurgence of interest in comparative embryology. It is now important to be able to compare gene expression in different species at similar developmental stages. One phenomenon which may make it difficult to compare embryos in this way is heterochrony--a change in developmental timing during evolution. It is not clear whether heterochrony can affect the intermediate stages of embryonic development, when many important genes involved in pattern formation are expressed. A prevalent view is that these so-called phylotypic stages are resistant to evolutionary change because they are when the body plan is laid down. Haeckel's famous drawings, which show different vertebrates developing from virtually identical somite-stage embryos, are still used to support this idea. I have reexamined the morphological data relating to developmental timing in somite-stage embryos. The data reveal striking patterns of heterochrony during vertebrate evolution. These shifts in developmental timing have strongly affected the phylotypic stage, which is therefore poorly conserved and is more appropriately described as the phylotypic period. This is contrary to the impression created by Haeckel's drawings, which I show to be inaccurate and misleading. The study of gene expression in embryos which show heterochrony could give important insights into evolutionary and developmental mechanisms.

Animals↗
PubMed1995

Gene expression, polarising activity and skeletal patterning in reaggregated hind limb mesenchyme.

The developing chick limb has two major signalling centres; the apical ectodermal ridge maintains expression of several important genes and outgrowth of the limb, and the polarising region specifies the pattern of skeletal elements along the anteroposterior axis. We have used reaggregated leg grafts (mesenchyme dissociated into single cells, placed in an ectodermal jacket and grafted to a host) to study patterning in a system where the developmental axes are severely disrupted. Reaggregates from different regions of leg mesenchyme developed correspondingly different digits, giving a system in which skeletal phenotype could be compared with the expression of genes thought to be important in patterning. We found that posterior third and whole leg reaggregates gave rise to different digits, yet expressed the same combination of HoxD, Bmp-2 and shh genes throughout their development. Anterior thirds initially only express the 3' end of the HoxD cluster but activate the more 5' members of the cluster sequentially over a period of 48 hours, a period during which Bmp-2 is activated but no shh or Fgf-4 expression could be detected. Our results suggest that there are two independent mechanisms for activating the HoxD complex, one polarising region-dependent and one independent, and that shh expression may not be necessary to maintain outgrowth and patterning once a ridge has been established.

Animals↗
PubMed1994

Bone morphogenetic proteins and a signalling pathway that controls patterning in the developing chick limb.

We show here that bone morphogenetic protein 2 (BMP-2) is involved in patterning the developing chick limb. During early stages of limb development, mesenchymal expression of the Bmp-2 gene is restricted to the posterior part of the bud, in a domain that colocalizes with the polarizing region. The polarizing region is a group of cells at the posterior margin of the limb bud that can respecify the anteroposterior axis of the limb when grafted anteriorly and can activate expression of genes of the HoxD complex. We dissect possible roles of BMP-2 in the polarizing region signalling pathway by manipulating the developing wing bud. Retinoic acid application, which mimics the effects of polarizing region grafts, activates Bmp-2 gene expression in anterior cells. This shows that changes in anteroposterior pattern are correlated with changes in Bmp-2 expression. When polarizing region grafts are placed at the anterior margin of the wing bud, the grafts continue to express the Bmp-2 gene and also activate Bmp-2 expression in the adjacent anterior host mesenchyme. These data suggest that BMP-2 is part of the response pathway to the polarizing signal, rather than being the signal itself. In support of this, BMP-2 protein does not appear to have any detectable polarizing activity when applied to the wing bud. The pattern of Bmp-4 gene expression in the developing wing bud raises the possibility that BMP-2 and BMP-4 could act in concert. There is a close relationship, both temporal and spatial, between the activation of the Bmp-2 and Hoxd-13 genes in response to retinoic acid and polarizing region grafts, suggesting that expression of the two genes might be linked.

Amino Acid Sequence↗
PubMed1993

Distribution of pluripotent neural crest cells in the embryo and the role of brain-derived neurotrophic factor in the commitment to the primary sensory neuron lineage.

Many early migratory neural crest cells are pluripotent in the sense that their progeny are able to generate more than one differentiated phenotype (Sieber-Blum and Cohen, 1980, Dev. Biol. 80:95-106; Baroffio, Dupin, and Le Douarin, 1988, Proc. Natl. Acad. Sci. USA 85:5325-5329; Bronner-Fraser and Fraser, 1988, Nature 335:161-164; Sieber-Blum, 1989a, Science 243:1608-1611; Ito and Sieber-Blum, 1991, Dev. Biol. 148:95-106). At trunk levels, the neural crest contains two classes (Sieber-Blum and Cohen, 1980) and at posterior rhombencephalic levels, three different classes of pluripotent cells (Ito and Sieber-Blum, 1991). We investigated cell differentiation by in vitro clonal analysis to determine when in development the pool of pluripotent neural crest cells becomes exhausted. The data suggest that different classes of pluripotent cells, precursor cells with more restricted developmental potentials, and apparently committed cells, exist at sites of advanced migration (posterior branchial arches) and even at target sites of neural crest cell differentiation [posterior branchial arches, dorsal root ganglia (DRG), sympathetic ganglia (SG), and epidermal ectoderm]. Some putative classes of pluripotent cells persist well into the second half of embryonic development. These observations have implications for our understanding of the mechanisms that control neural crest cell migration and differentiation. They support the idea that cues originating from the microenvironment affect differentiation of pluripotent neural crest cells. One such signal appears to be brain-derived neurotrophic factor (BDNF). In the presence of BDNF, but not nerve growth factor (NGF), there is a significant increase in the number of neural crest cells per colony that express a sensory neuron-specific marker. Because this increase is not accompanied by a corresponding increase in the total number of cells per colony, this suggests that BDNF plays a role in cell type specification.

Animals↗
PubMed1993

Pluripotent neural crest cells in the developing skin of the quail embryo.

The neural crest, a migratory population of embryonic cells, gives rise to a wide range of differentiated cell types in the mature vertebrate organism, including the melanocytes of the skin. Little is known about the developmental potentials of neural crest cells toward the end of their migratory phase. We have therefore used in vitro analysis to examine the developmental potential of mesenchymal cells derived from explants of trunk epidermal ectoderm of the quail embryo. Melanocytes which differentiated in the cultures could be identified by their content of melanin granules. To detect different neuronal cells, the cultures were stained with antibodies including anti-dopamine-beta-hydroxylase (anti-DBH), which characterizes sympathoadrenal cells, and AC4, an antibody which recognizes the stage-specific embryonic antigen-1 (SSEA-1) that is expressed by cells in the sensory neuron lineage of the quail embryo, but not by sympathoadrenal cells. Seventy-eight percent of the population of neural crest-derived cells seeding the ectoderm around stage 21 gave rise to colonies containing melanocytes only. Twenty percent, however, generated mixed colonies that contained melanocytes, DBH+ cells, SSEA-1+ cells, and unidentified, unpigmented cells. Small numbers of colonies containing fewer phenotypes were also seen. With increasing embryonic age, the number of colonies containing multiple phenotypes declined, until by stage 30 all neural crest colonies contained melanocytes only. Some colonies had been marked at the single-cell stage, and this provided additional confirmation that each colony-type could be generated from a single cell. Thus the significant finding in this study is that a substantial fraction of the neural crest cells arriving early in the ectoderm are pluripotent cells that are able to give rise to pigment cells, to sympathoadrenal cells, to primary sensory neuron precursors, and possibly to other cells which were not identified here. This observation may have implications for our understanding of the mechanisms that control neural crest cell migration and differentiation.

Animals↗
PubMed1991

Pigment patterns in neural crest chimeras constructed from quail and guinea fowl embryos.

The pattern of pigmentation in bird embryos is determined by the spatial organization of melanocyte differentiation. Some of the results from recent, neural crest transplantation experiments support a model based on a prepattern in the feathers; others could be interpreted in terms of a nonspecific pattern resulting from a failure of the crest cells to read the positional values in another species. To distinguish between these possibilities, the crucial test is to construct chimeras from two species with different pigment patterns. We have examined the wing plumage of quail and guinea fowl embryos. The quail has a characteristic pattern of pigmented and unpigmented feather papillae, whereas the guinea fowl shows uniform pigmentation. Chimeras were constructed by grafting wing buds isotopically between embryos. The wing buds were transplanted before they had become invaded by neural crest cells. Quail wing buds grafted to the guinea fowl developed, in most cases, a pigment pattern resembling that of the quail and not that of the guinea fowl. A few cases became uniformly pigmented and appeared to represent nonspecific patterns. The reciprocal grafts (guinea fowl wing buds grafted to the quail) became pigmented all over. We found evidence that the timing of melanocyte differentiation is controlled by cues in the feather papillae. Some cases developed a severe inflammatory response. The model which best accounts for these findings--and which can account for inconsistencies in previous reports--is the following. A prepattern is present in the feathers and this can control the differentiation of melanoblasts, even if they come from a different species. The local cues which constitute the prepattern are not positional values. In some chimeras melanoblasts fail to respond to the prepattern and so a nonspecific pattern of uniform pigmentation is produced.

Animals↗
PubMed1991

In vitro clonal analysis of progenitor cell patterns in dorsal root and sympathetic ganglia of the quail embryo.

The possible presence of pluripotent cells in dorsal root (DRG) and sympathetic ganglia (SG) of the quail embryo has been investigated by in vitro clonal analysis. Both types of ganglia originate from the neural crest. At, or soon after, initiation of emigration from the neural tube, the neural crest appears as a mixed population of pluripotent cells and cells with more restricted developmental capacities. In the present study it was determined that pluripotent cells and precursor cells with restricted developmental potentials are also present in early DRG and SG, and that their proportions change with progressing age of the embryo. As in the neural crest, cells in one class are at least tripotent, able to give rise to pigment cells, sensory neurons, and cells in the sympathoadrenal lineage. Cells in the other class appear to have lost the melanogenic potential, but generate cells in the sensory neuron and sympathoadrenal lineages. In addition, there are two types of cells that seem to be committed to the melanogenic and sensory neuron lineages, respectively. Apparently committed melanogenic cells within the DRG are not detected after the first third of embryonic development, whereas precursor cells that are at least bipotent and generate both types of neurons persist in both DRG and SG at least through the first half of embryonic development. Neurogenic cells that are apparently committed to the sensory neuron or sympathoadrenal lineages were observed in the appropriate type of ganglion only, suggesting that location-specific cues influence the choice of phenotype generated by pluripotent neural crest cells.

Animals↗
PubMedDate not supplied

Genomic annotation and transcriptome analysis of the zebrafish (Danio rerio) hox complex with description of a novel member, hox b 13a.

The zebrafish (Danio rerio) is an important model in evolutionary developmental biology, and its study is being revolutionized by the zebrafish genome project. Sequencing is at an advanced stage, but annotation is largely the result of in silico analyses. We have performed genomic annotation, comparative genomics, and transcriptional analysis using microarrays of the hox homeobox-containing transcription factors. These genes have important roles in specifying the body plan. Candidate sequences were located in version Z v 4 of the Ensembl genome database by TBLASTN searching with Danio and other vertebrate published Hox protein sequences. Homologies were confirmed by alignment with reference sequences, and by the relative position of genes along each cluster. RT-PCR using adult Tübingen cDNA was used to confirm annotations, to check the genomic sequence and to confirm expression in vivo. Our RT-PCR and microarray data show that all 49 hox genes are expressed in adult zebrafish. Significant expression for all known hox genes could be detected in our microarray analysis. We also find significant expression of hox 8 paralogs and hox b 7 a in the anti-sense direction. A novel gene, D. rerio hox b 13 a, was identified, and a preliminary characterization by in situ hybridization showed expression at 24 hpf at the tip of the developing tail. We are currently characterizing this gene at the functional level. We argue that the oligo design for microarrays can be greatly enhanced by the availability of genomic sequences.

Animals↗