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Biomedical subjects

C R Phillips

Publications and source records attributed to C R Phillips.

At least 19 recordsLinked to original sources
PubMed1992

Planar induction of anteroposterior pattern in the developing central nervous system of Xenopus laevis.

It has long been thought that anteroposterior (A-P) pattern in the vertebrate central nervous system is induced in the embryo's dorsal ectoderm exclusively by signals passing vertically from underlying, patterned dorsal mesoderm. Explants from early gastrulae of the frog Xenopus laevis were prepared in which vertical contact between dorsal ectoderm and mesoderm was prevented but planar contact was maintained. In these, four position-specific neural markers (engrailed-2, Krox-20, XlHbox 1, and XlHbox 6) were expressed in the ectoderm in the same A-P order as in the embryo. Thus, planar signals alone, following a path available in the normal embryo, can induce A-P neural pattern.

Animals↗
PubMed1992

Properties of 220Rn progeny (212Pb) in the presence of trace gases.

The charge neutralization of 220Rn progeny (212Pb) was studied in nitrogen and argon environments containing trace concentrations of ethylene, propylene and propane. The diffusion coefficient of 212Pb atoms in the presence and in the absence of oxygen and hydrocarbons was determined using a diffusion tube method which measured the penetration fraction of the 220Rn decay products. The results are explained in terms of the formation of lead dioxide with an ionization potential in the range of 10.5-11.1 eV. The ionization potential of 212PbO2 was found to be higher than that of 218PoO2. Charge neutralization was found to occur in a gas mixture of argon and oxygen containing 600 ppm ethylene (C2H4) and also in a mixture of nitrogen and oxygen containing 600 ppm propylene (C3H6). Charge neutralization was not found to occur in a mixture of argon and oxygen containing up to 5000 ppm propane (C3H8) and in a mixture of argon and nitrogen containing 600 ppm ethylene. In pure oxygen, nitrogen, argon, dry air and a mixture of nitrogen and oxygen, no neutralization was observed to occur through charge transfer from neutral molecules. The 220Rn concentration was found to influence the neutralization mechanism. Diffusion coefficients for partially neutralized and neutralized 212Pb ranged from 0.050 to 0.067 cm2 s-1 at a 220Rn concentration of 3.7 x 10(4) atoms cm-3 and a relative humidity of less than 5%.

Alkenes↗
PubMed1991

Response of the unattached 218Po activity size distribution in H2O-SO2-N2 atmospheres.

The response of the unattached 218Po activity size distribution to variations in H2O and SO2 concentration was investigated in a high-purity N2 atmosphere. Lognormal size distributions were reconstructed from screen diffusion battery data using a random search and optimization procedure. For relative humidities of 1% to 90%, SO2 concentrations of 0-6 ppm, and residence times of 38 s, median diameters of the unattached mode ranged from 0.50 (+/- 0.04) to 0.80 (+/- 0.06) nm, corresponding to diffusion coefficients of 0.1-0.05 cm2 s-1. Increases in water vapor concentration resulted in an enhanced rate of ion neutralization and suppression of ion cluster formation. Decreases in cluster diameter with increasing humidity were therefore observed. Addition of SO2 at constant relative humidity resulted in an initial increase in diameter, with a subsequent decrease. Geometric standard deviations averaged 1.05 (+/- 0.01) to 1.06 (+/- 0.01) in all cases, with the unattached fraction comprising 83% to 88% of the total distribution. Results suggest that increases in water vapor concentration enhance neutralization in the immediate vicinity of the recoil path through water radiolysis. Conversion of OH to H2OSO4 on addition of SO2 provides a binary condensable phase capable of clustering around the ion at low relative humidities, increasing particle size. High H2O and SO2 concentrations enhance neutralization through electron scavenging and subsequent ion collision near the end of the recoil path, decreasing the number of available cluster sites and suppressing cluster formation. This reduction is more significant than the increasing size of the stable ion cluster, with increasing H2O and H2SO4 concentrations as predicted by Raes, resulting in decreases in both median diameter and distribution spread.

Air Pollutants, Radioactive↗
PubMed1991

Neural induction.

The study of neural induction in Xenopus can be approached from two broad perspectives. One can study the competence of the ectoderm to respond to neural induction signals and any potential prepattern within the ectoderm. The second area of study involves the neural induction signals, in terms of the chemical nature of the signals, their sources, and their method of delivery. The neural competence of ectoderm has been studied by either grafting the ectoderm into areas which normally form neural tissue or by grafting tissues which normally induce neural structures onto the ectoderm to be tested. In general, it appears that ectoderm loses neural competence by midgastrula. However, there is some experimental evidence from various amphibian studies that the loss of neural competence in ectoderm does not occur simultaneously throughout all regions. It is not yet known if this phenomenon is also true in Xenopus. There appear to be several signaling events involved in the process of neural induction. Molecular probes have made it possible to study early steps in the neural induction and patterning processes which were not possible to study using only the development of neural morphology as a marker for neural induction. Antibodies directed against early epidermal versus neural epithelium indicate that the dorsal animal blastomeres are biased toward a neural pathway during early cleavage. Another signaling event occurs at early gastrula and the resulting dorsal ectoderm now responds more readily to some neural induction events than does the ventral ectoderm. The source of the early gastrula signals has been studied by a variety of methods, including exogastrula embryos, Keller sandwiches, and grafting a blastopore lip to the edge of competent ectoderm. The blastopore lip can send signals through the plane of the ectoderm capable of inducing competent ectoderm to become neural tissue. There are several issues relative to the process of neural induction which are not yet resolved. The major issue involves the mechanism of establishing pattern within the neural plate. Ectoderm appears to lose neural competence prior to the time when involuted dorsal mesoderm comes to underlie the anterior neural plate region. Several investigators have shown that information for expression of spatially restricted neural-specific molecules can travel through the ectoderm, independent of underlying dorsal mesoderm.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Signals from the dorsal blastopore lip region during gastrulation bias the ectoderm toward a nonepidermal pathway of differentiation in Xenopus laevis.

Epi 1, a monoclonal antibody, was generated against an epidermal specific epithelial antigen; it does not stain neural epithelium. We have used Epi 1 as a marker to determine when the spatial patterns delineating neural from nonneural epithelium become established. We used ventral ectoderm in a sandwich assay to show that signals from the central blastopore lip region, passing through the plane of the ectoderm sheet, define the pattern and boundary characteristics of Epi 1 expression. The dorsal blastopore lip at stages 10 and 12 are the strongest in inhibiting Epi 1 expression. The involuted chordamesoderm has only a limited inhibitory effect on Epi 1 expression in ventral ectoderm recombinates and does not appear to establish pattern boundaries. We suggest that the blastopore lip region establishes a preneural bias in the adjacent ectoderm prior to the interaction of the latter with chordamesoderm.

Animals↗
PubMed1986

Expression of an epidermal antigen used to study tissue induction in the early Xenopus laevis embryo.

A monoclonal antibody (Epi 1) has been produced that recognizes an antigen expressed in epidermal cells of Xenopus laevis embryos. The Epi 1 antigen appears in embryonic epidermis at the end of gastrulation and is not expressed in nonepidermal structures derived from ectoderm (for example, neural tube or cement gland). The capacity to express the Epi 1 antigen is restricted to cells of the animal hemisphere prior to the midblastula stage of development (stage 8), and tissue interactions during gastrulation inhibit the expression of the Epi 1 antigen in neural ectoderm. This epidermal antigen will be a valuable marker for studies of ectodermal commitment.

Animals↗
PubMed1986

A simple two-count method for routine monitoring of 222Rn and 220Rn progeny working levels in U mines.

In the case of U mines containing Th, the total alpha energy will contain contributions from both 222Rn progeny and 220Rn progeny. A simple routine grab sampling monitoring scheme is therefore needed for separate measurement of the working levels of 222Rn and 220Rn progeny. The scheme presented here is based on a short delay period after sampling (2 min) for 222Rn progeny and a long delay period (340 min) before counting 220Rn progeny. An advantage of this scheme is that no corrections need to be made to the 222Rn progeny working level for the presence of 220Rn progeny. Such corrections must be made in the commonly used Kusnetz scheme.

Mathematics↗
PubMed1985

Spatial changes in poly(A) concentrations during early embryogenesis in Xenopus laevis: analysis by in situ hybridization.

The dorsal-ventral axis of the Xenopus embryo is approximately determined by the point of sperm entry, which occurs randomly about the animal-vegetal axis of the egg. Two cytoplasmic components were studied whose spatial arrangements and local concentration patterns appear to reflect the forming dorsal-ventral axis. First, three-dimensional maps depicting the rearrangements of different size classes of yolk platelets were reconstructed from serial sections. These maps provide information on the time when movements occur and the intracellular directions and distances components can travel within the early embryo. Second, local poly(A) concentrations were observed using a [3H]poly(U) in situ hybridization probe. Poly(A) concentrations were observed to change reproducibly from fertilization to late cleavage. Two cytoplasmic areas were found to undergo major, stage-specific increases in poly(A) concentration.

Animals↗
PubMed1985

Dependence of electrostatic diffusion of Rn progeny on environmental parameters.

In previous work it was established that electrostatic diffusion of Rn progeny attached to aerosols can be used for passive dosimetry by replacing the pump-filter assembly in an active dosimeter by an electret. In the present work, results are presented on the dependence of activity collected by an electret on aerosol concentration, aerosol diameter, attachment rate of Rn progeny to aerosols, unattached fraction of Rn progeny, and Rn progeny concentrations. The electrostatic flow rate of Rn progeny is defined and all relationships analyzed in terms of this flow rate. The electrostatic flow rate is found to be reasonably constant for 0.03 micron less than particle diameter less than 0.09 micron, unattached fraction of 218Po less than 0.40 and 70,000/cm3 less than aerosol concentration less than 180,000/cm3; experimentally obtained data outside these limits are too few to draw any definite conclusions. The results are useful from a practical point of view because typical U mine conditions are described by these limits.

Aerosols↗
PubMed1985

The application of electrets to passive Rn progeny dosimeters.

The theoretical and experimental bases are presented for development of a passive electret dosimeter for Rn progeny. The mechanism of aerosol collection is described, and experiments to develop a suitable aerosol collecting element (electret) for a passive Rn progeny dosimeter are reported.

Air Pollutants↗