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R A Cameron

Publications and source records attributed to R A Cameron.

At least 37 records · Page 2Linked to original sources
PubMed1995

Origin of bilaterian body plans: evolution of developmental regulatory mechanisms.

An argument is proposed to explain the origin of large metazoans, based on the regulatory processes that underlie the morphogenetic organization of pattern in modern animals. Genetic regulatory systems similar to those used in modern, indirectly developing marine invertebrates are considered to indicate the Precambrian regulatory platform on which were erected innovations that underlie the development of macroscopic body plans. Those systems are genetic regulatory programs that produce groups of unspecified "set-aside cells" and hierarchical regulatory programs that initially define regions of morphogenetic space in terms of domains of transcription factor expression. These ideas affect interpretation of the development of arthropods and chordates as well as interpretation of the role of the genes of the homeotic complex in embryogenesis.

Animals↗
PubMed1994

Development of sibling inbred sea urchins: normal embryogenesis, but frequent postembryonic malformation, arrest and lethality.

Inbred lines of Strongylocentrotus purpuratus descended from a single pair of wild animals were constructed by sibling mating. We describe results from a systematic series of crosses in which eggs from F2 and from F3 females were fertilized respectively with sperm from their sibling males. Observations were also made on self-fertilized cultures derived from several naturally occurring hermaphrodites. Morphological development, survival efficiency, and expression of three territorial embryonic markers were assayed in the embryos developing from these crosses. Unexpectedly, out of > 90 controlled crosses, we observed no developmental failures whatsoever, up to the end of embryogenesis (i.e., onset of feeding) that could be attributed to homozygous, zygotically acting recessive genes. However, during postembryonic larval development, lethality, morphological malformation, and arrest are observed in inbred cultures at a high frequency. The incidence of these zygotic developmental failures is such that it appears that there is at least one recessive genetic defect affecting larval development per haploid parental genome. The relative imperviousness of the basic embryonic process to defects arising from homozygosity is consistent with other evidence implying that territorial specification in sea urchin embryogenesis is controlled by maternally rather than zygotically expressed gene products.

Animals↗
PubMed1994

Ligand-dependent stimulation of introduced mammalian brain receptors alters spicule symmetry and other morphogenetic events in sea urchin embryos.

Zygotes of S. purpuratus were injected with synthetic mRNAs encoding rodent brain neurotransmitter receptors, and specific developmental phenotypes were produced on addition to the sea water of the respective ligands. Most of these experiments were carried out with a mouse serotonin receptor (5HT-R) mRNA, though exactly comparable results were obtained with a rat muscarinic acetylcholine receptor (MAChR) mRNA; these receptors are expected to couple to the same endogenous signal transduction system. We show by whole mount in situ hybridization that the injected mRNAs diffuse to all of the early blastomeres, and that they are translated in vivo. Three specific phenotypes were reproducibly observed. The most severe, occurring at highest levels of injected mRNA, was a cleavage arrest phenotype in which no overtly differentiated cells ever appear, though the embryos remain alive for at least 72 h. A gastrular arrest (GA) phenotype is generated in appreciable fractions of embryos developing from eggs injected with lower levels of mRNA. In GA embryos the blastocoel is filled with disorganized mesenchyme cells, including pigment cells and skeletogenic cells; there is no archenteron; and the entire ectoderm expresses an oral ectoderm cell surface marker. The least severe phenotype that we recognized displays an altered arrangement of spiculogenic foci (RSE phenotype), generating a ring of extra spicules that are properly positioned with respect to the animal/vegetal axis, but that lack any reference to the oral/aboral axis. However, use of cytological and molecular markers demonstrates that RSE embryos retain normal spatial patterns of aboral and oral ectoderm. They develop a fully formed archenteron, but fail to form either a stomodaeum or a ciliated band. RSE embryos can be produced in embryos expressing the 5HT-R by exposure to serotonin, beginning as late as 12 h postfertilization (pf). All of the morphogenetic processes affected in RSE embryos depend in normal embryos on intercellular interactions occurring at the blastula-gastrula stages of development.

Animals↗
PubMed1993

The embryonic ciliated band of the sea urchin, Strongylocentrotus purpuratus derives from both oral and aboral ectoderm.

The ciliated band of the Strongylocentrotus purpuratus embryo consists of a columnar epithelium, 3-5 cells wide, which gives rise to a small number of neuroblasts. It arises late in development, as a border separating the squamous epithelial cells of the oral and aboral ectoderm. To determine the lineage origins of this structure, we performed double labeling experiments at the 2-cell stage and the 16-cell stage, which were designed to reveal clonal boundaries in the ciliated band. The ciliated band forms in a region of the ectoderm derived from descendants of the following blastomeres: No, VO, Na1u, Na2u, right and left NL1u, and right and left NL2u. In contrast to the lineage contributions of the embryonic territories established early in development, lineage origins of the ciliated band are variable. Specification of the ciliated band thus depends on intercellular interaction rather than lineage.

Animals↗
PubMed1992

Territorial expression of three different trans-genes in early sea urchin embryos detected by a whole-mount fluorescence procedure.

We have developed a new procedure for detection of the protein product of chloramphenicol acetyltransferase (CAT) reporter genes in whole mounted sea urchin embryos. The position of a commercially available anti-CAT antibody is visualized by video or confocal microscopy, and thus the spatial domains of exogenous reporter gene expression can be determined with regard to the intact three-dimensional structures of the embryo. We show that in pluteus stage embryos CAT protein expression patterns for SM50 . CAT or CyIIIa . CAT reporter genes are similar to those previously obtained by in situ hybridizations with radioactive probes. Taking advantage of the superior resolution of cellular CAT expression patterns using the antibody visualization method, we found for the first time that, in addition to the expression in aboral ectoderm, some cells in the ciliated band of the pluteus express CyIIIa . CAT. The expression of a new fusion construct, CyIIa . CAT, was also examined. As expected from the localization of endogenous CyIIa mRNA, CAT protein was expressed under control of the CyIIa promoter in gut and skeletogenic mesenchyme cells.

Animals↗
PubMed1991

Cell type specification during sea urchin development.

Recent discoveries indicate that cell lineages and fates play a key role in the establishment of spatially restricted gene expression during sea urchin development. Unique sets of founder cells generate five territories of gene expression by means of an invariant pattern of complete cleavage. Cell lineage analysis demonstrates that the second embryonic axis, the oral-aboral axis, is specified with reference to the first cleavage plane. In the undisturbed embryo, clones that contribute to one territory or another begin to appear at the third cleavage, and founder cell segregation to all five territories is completed by the sixth cleavage. Founder cell segregation is a key feature of mechanisms that establish the spatially defined gene activity of sea urchin embryogenesis.

Animals↗
PubMed1991

Macromere cell fates during sea urchin development.

This paper examines the cell lineage relationships and cell fates in embryos of the sea urchin Strongylocentrotus purpuratus leading to the various cell types derived from the definitive vegetal plate territory or the veg2 tier of cells. These cell types are gut, pigment cells, basal cells and coelomic pouches. They are cell types that constitute embryonic structures through cellular migration or rearrangement unlike the relatively non-motile ectoderm cell types. For this analysis, we use previous knowledge of lineage to assign macromeres to one of four types: VOM, the oral macromere; VAM, the aboral macromere, right and left VLM, the lateral macromeres. Each of the four macromeres contributes progeny to all of the cell types that descend from the definitive vegetal plate. Thus in the gut each macromere contributes to the esophagus, stomach and intestine, and the stripe of labeled cells descendant from a macromere reflects the re-arrangement of cells that occurs during archenteron elongation. Pigment cell contributions exhibit no consistent pattern among the four macromeres, and are haphazardly distributed throughout the ectoderm. Gut and pigment cell contributions are thus radially symmetrical. In contrast, the VOM blastomere contributes to both of the coelomic pouches while the other three macromeres contribute to only one or the other pouch. The total of the macromere contribution amounts to 60% of the cells constituting the coelomic pouches.

Animals↗
PubMed1990

Segregation of oral from aboral ectoderm precursors is completed at fifth cleavage in the embryogenesis of Strongylocentrotus purpuratus.

A specific set of founder cells uniquely gives rise to the oral and aboral ectoderms in the regularly developing sea urchin Strongylocentrotus purpuratus. We showed earlier that the polar No and Na (animal oral and animal aboral) blastomeres are specified by third cleavage, while the respective oral and aboral lineage contributions of the left and right NL (animal lateral) blastomeres have not yet segregated from one another at third cleavage. Here we demonstrate by iontophoretic injection of lysyl rhodamine dextran lineage tracer that segregation of oral vs aboral cell fates in the lineages of the NL blastomeres has still not occurred by fourth cleavage, but at fifth cleavage there arise from the NL sublineages founder cells whose progeny contribute exclusively to the aboral ectoderm. The sister cells of these fifth cleavage blastomeres are founder cells that contribute exclusively to oral structures. The aboral ectoderm tracts to which NL derivatives give rise occupy lateral regions of the anterior aboral ectoderm, while the oral structures deriving from the NL blastomeres are the lateral sectors of the ciliated bands. The cells of the ciliated bands do not express aboral ectoderm markers and are considered to constitute the border of the oral region. With these new findings we complete our knowledge of the origins, identities, and fates of the 11 founder cells, the progeny of which exclusively give rise to the aboral ectoderm, and of the 5 founder cells, the progeny of which exclusively produce the oral ectoderm and its derivatives.

Animals↗
PubMed1990

Locale and level of bindin mRNA in maturing testis of the sea urchin, Strongylocentrotus purpuratus.

This first study of the onset of spermatogenesis in the sea urchin, Strongylocentrotus purpuratus, was undertaken using individuals reared in the laboratory. Spermatogenesis commences about 11-12 months after metamorphosis in these animals. Bindin message accumulates in late spermatocytes and early spermatids which lie in the luminal germinal layer. Bindin message accumulates later than does the testis-specific histone, H2b-1, suggesting that different classes of genes are sequentially activated during the differentiation of sperm. We correlate the number of bindin mRNA molecules with morphological structure and with quantitative aspects of gonad maturation including the number of nuclei and of sperm. The results suggest that the bindin mRNA concentration in total RNA from testis at different stages of maturation reflects the change in the proportion of expressing cells in the total cell population of the testis.

Animals↗
PubMed1989

Expression of two actin genes during larval development in the sea urchin Strongylocentrotus purpuratus.

We report the first measurements of cell number, total RNA, and transcript accumulations for two actin genes during larval development of the sea urchin Strongylocentrotus purpuratus. At 5 weeks of feeding, when development of laboratory-raised larvae is completed, the cell number has increased about 100-fold with respect to the pluteus-stage embryo to about 150,000 +/- 50,000, and the total RNA has increased 46-fold to about 130 ng per larva. The transcripts of the Cylla cytoskeletal actin gene, which is expressed in adult tissues, continue to accumulate throughout larval development. A contrasting pattern of transcript accumulation is observed for Cyllla, a different cytoskeletal actin gene that in the embryo is expressed only in aboral ectoderm. These transcripts increase in number early in larval development, when the larval epidermis is differentiating, and then decline in quantity. It is known that at metamorphosis the larval epidermis is largely histolyzed and that the Cyllla gene is not expressed in the juvenile or adult.

Actins↗
PubMed1989

The oral-aboral axis of a sea urchin embryo is specified by first cleavage.

Several lines of evidence suggest that the oral-aboral axis in Strongylocentrotus purpuratus embryos is specified at or before the 8-cell stage. Were the oral-aboral axis specified independently of the first cleavage plane, then a random association of this plane with the blastomeres of the four embryo quadrants in the oral-aboral plane (viz. oral, aboral, right and left) would be expected. Lineage tracer dye injection into one blastomere at the 2-cell stage and observation of the resultant labeling patterns demonstrates instead a strongly nonrandom association. In at least ninety percent of cases, the progeny of the aboral blastomeres are associated with those of the left lateral blastomeres and the progeny of the oral blastomeres with the right lateral ones, respectively. Thus, ninety percent of the time the oral pole of the future oral-aboral axis lies 45 degrees clockwise from the first cleavage plane as viewed from the animal pole. The nonrandom association of blastomeres after labeling of the 2-cell stage implies that there is a mechanistic relation between axis specification and the positioning of the first cleavage plane.

Animals↗
PubMed1987

Correct cell-type-specific expression of a fusion gene injected into sea urchin eggs.

A fusion gene construct containing the bacterial chloramphenicol acetyltransferase (CAT) gene under the control of CyIIIa actin gene regulatory sequences was injected into unfertilized eggs of the urchin Strongylocentrotus purpuratus, and early pluteus stage embryos that developed from these eggs were fixed and sectioned for analysis by in situ hybridization. A [3H]RNA antisense probe for CAT mRNA was hybridized to 5-micron embryo sections. Autoradiographic signal denoting the presence of CAT mRNA was detected only over aboral ectoderm cells, in which the CyIIIa gene is normally expressed, and not over any recognizable regions of gut or oral ectoderm included in the same sections.

Acetyltransferases↗
PubMed1987

Lineage and fate of each blastomere of the eight-cell sea urchin embryo.

A fluoresceinated lineage tracer was injected into individual blastomeres of eight-cell sea urchin (Strongylocentrotus purpuratus) embryos, and the location of the progeny of each blastomere was determined in the fully developed pluteus. Each blastomere gives rise to a unique portion of the advanced embryo. We confirm many of the classical assignments of cell fate along the animal-vegetal axis of the cleavage-stage embryo, and demonstrate that one blastomere of the animal quartet at the eight-cell stage lies nearest the future oral pole and the opposite one nearest the future aboral pole of the embryo. Clones of cells deriving from ectodermal founder cells always remain contiguous, while clones of cells descendant from the vegetal plate (i.e., gut, secondary mesenchyme) do not. The locations of ectodermal clones contributed by specific blastomeres require that the larval plane of bilateral symmetry lie approximately equidistant (i.e., at a 45 degree angle) from each of the first two cleavage planes. These results underscore the conclusion that many of the early spatial patterns of differential gene expression observed at the molecular level are specified in a clonal manner early in embryonic sea urchin development, and are each confined to cell lineages established during cleavage.

Animals↗
PubMed1985

Demonstration of the granular layer and the fate of the hyaline layer during the development of a sea urchin (Lytechinus variegatus).

Employing electron-microscopic methods that help retain polyanionic materials, we describe the extracellular coverings of a sea urchin (Lytechinus variegatus) throughout ontogeny. The surface of the embryo is covered by a two-layered cuticle (commonly called the hyaline layer), which in turn is covered by a granular layer. The granular layer is retained after addition of alcian blue to the fixative solutions, and has not been previously described for any sea urchin. After hatching, the granular layer disappears, but the hyaline layer continues to cover most of the larval surface until settlement and metamorphosis. A few days before metamorphosis, the hyaline layer lining the vestibular invagination of the competent pluteus larva is replaced by a three-layered cuticle resembling that of the adult sea urchin. The hyaline layer covering the rest of the larva is evidently lost at metamorphosis during the involution of the general epidermis.

Animals↗
PubMed1983

Electron microscopy of extracellular materials during the development of a sea star, Patiria miniata (Echinodermata: Asteroidea).

The fine structure of conspicuous extracellular materials during the life history of a sea star (Patiria miniata) is described. The outer surface of the developing sea star is covered by two morphologically different cuticles that appear sequentially during ontogeny. The primary cuticle, which is about 120 nm thick and two-layered, is present from mid-blastula through the end of the larval stage. The secondary cuticle, which is about 1 micron thick and three-layered, first appears on the epidermis of the rudiment region of the larva and, after metamorphosis, covers the entire epidermis of the juvenile and adult stages. During ontogeny, there are only two conspicuous gut cuticles: the first lines the newly invaginated archenteron at the start of the gastrula stage, and the second lines the esophagus during the larval stage. A blastocoelic basal lamina first appears at mid-blastula and persists as subectodermal and subendodermal basal laminae. Ruthenium red-positive granules are detectable between the lateral surfaces of adjacent ectodermal cells during part of the gastrula stage; this transient intercellular material may possibly aid in lateral adhesion between cells.

Animals↗