2,123 research outputs found
Putative intermediates in the nerve cell differentiation pathway in hydra have properties of multipotent stem cells
We have investigated the properties of nerve cell precursors in hydra by analyzing the differentiation and proliferation capacity of interstitial cells in the peduncle of Hydra oligactis, which is a region of active nerve cell differentiation. Our results indicate that about 50% of the interstitial cells in the peduncle can grow rapidly and also give rise to nematocyte precursors when transplanted into a gastric environment. If these cells were committed nerve cell precursors, one would not expect them to differentiate into nematocytes nor to proliferate apparently without limit. Therefore we conclude that cycling interstitial cells in peduncles are not intermediates in the nerve cell differentiation pathway but are stem cells. The remaining interstitial cells in the peduncle are in G1 and have the properties of committed nerve cell precursors (Holstein and David, 1986). Thus, the interstitial cell population in the peduncle contains both stem cells and noncycling nerve precursors. The presence of stem cells in this region makes it likely that these cells are the immediate targets of signals which give rise to nerve cells
The properties of nerve cell precursors in hydra
Two signals, the head activator and an injury stimulus, control differentiation of nerve cells from uncommitted stem cells in hydra [Th. Holstein, H. C. Schaller, and C. N. David, (1986) Dev. Biol. 115, 9–17]. The time of action of these signals in the precursor cell cycle was determined. Methanol extracts of hydra containing 10−13 M head activator cause nerve cell commitment in S phase of the precursor cell cycle. Committed precursors complete the cell cycle, divide, and arrest in G1. Injury relieves the G1 block and precursors differentiate nerve cells. Under these conditions the time from commitment to nerve differentiation is 12 hr, the time from the end of S phase to nerve differentiation is 9 hr, and the time from the G1 block to nerve differentiation is 4 hr. Committed precursors blocked in G1 are unstable, decaying with a half-life of 12 hr if not stimulated to differentiate by an injury stimulus
Tentacle morphogenesis in hydra
Stimulation of tentacle-specific cell differentiation by the neuropeptide head activator was investigated in Hydra magnipapillata. Tentacle-specific sensory nerve cells were identified by a monoclonal antibody, NV1. Treatment of hydra with 1pM head activator for 18h stimulated differentiation of NV1+ nerve cells and tentacle epithelial cells in tissue from the distal gastric region. Head tissue and tissue from the proximal gastric region did not respond to head activator treatment with increased NV1+ differentiation. Hence the distal gastric region appears to be the site of tentacle formation in hydra. Tentacle precursors in head tissue seem to be committed since they fail to respond to head activator or to changes in tissue size with altered amounts of tentacle formation. We suggest that NV1 precursors form a complex with tentacle epithelial cell precursors, which then moves distally through the head region into the tentacles. The signal for NV1+ differentiation appears to be formation of this complex
On the parameterization dependence of the energy momentum tensor and the metric
We use results by Kirilin to show that in general relativity the nonleading
terms in the energy-momentum tensor of a particle depends on the
parameterization of the gravitational field. While the classical metric that is
calculated from this source, used to define the leading long-distance
corrections to the metric, also has a parameteriztion dependence, it can be
removed by a coordinate change. Thus the classical observables are
parameterization independent. The quantum effects that emerge within the same
calculation of the metric also depend on the parameterization and a full
quantum calculation requires the inclusion of further diagrams. However, within
a given parameterization the quantum effects calculated by us in a previous
paper are well defined. Flaws of Kirilin's proposed alternate metric definition
are described and we explain why the diagrams that we calculated are the
appropriate ones.Comment: 8 pages, 2 figure
Mini-Collagens in Hydra Nematocytes
We have isolated and characterized four collagen-related c-DNA clones (N-COL 1, N-COL 2, N-COL 3, N-COL 4) that are highly expressed in developing nematocytes in hydra. All four c-DNAs as well as their corresponding transcripts are small in size (600-1,000 bp). The deduced amino acid sequences show that they contain a central region consisting of 14 to 16 Gly-X-Y triplets. This region is flanked amino-terminal by a stretch of 14-23 proline residues and carboxy-terminal by a stretch of 6-9 prolines. At the NH2- and COOH-termini are repeated patterns of cysteine residues that are highly conserved between the molecules. A model is proposed which consists of a central stable collagen triple helix of 12-14 nm length from which three 9-22 nm long polyproline II type helices emerge at both ends. Disulfide linkage between cysteine- rich segments in these helices could lead to the formation of oligomeric network structures. Electrophoretic characterization of nematocyst extracts allows resolution of small proline-rich polypeptides that correspond in size to the cloned sequences
Phase diagram of the one dimensional anisotropic Kondo-necklace model
The one dimensional anisotropic Kondo-necklace model has been studied by
several methods. It is shown that a mean field approach fails to gain the
correct phase diagram for the Ising type anisotropy. We then applied the spin
wave theory which is justified for the anisotropic case. We have derived the
phase diagram between the antiferromagnetic long range order and the Kondo
singlet phases. We have found that the exchange interaction (J) between the
itinerant spins and local ones enhances the quantum fluctuations around the
classical long range antiferromagnetic order and finally destroy the ordered
phase at the critical value, J_c. Moreover, our results show that the onset of
anisotropy in the XY term of the itinerant interactions develops the
antiferromagnetic order for J<J_c. This is in agreement with the qualitative
feature which we expect from the symmetry of the anisotropic XY interaction. We
have justified our results by the numerical Lanczos method where the structure
factor at the antiferromagnetic wave vector diverges as the size of system goes
to infinity.Comment: 9 pages and 9 eps figure
Critical Property of Geometric Phase in the Dicke Model
We obtain the ground-state energy level and associated geometric phase in the
Dicke model analytically by means of the Holstein-Primakoff transformation and
the boson expansion approach in the thermodynamic limit. The non-adiabatic
geometric phase induced by the photon field is derived with the time-dependent
unitary transformation. It is shown that the quantum phase transition
characterized by the non-analyticity of the geometric phase is remarkably of
the first-order. We also investigate the scaling behavior of the geometric
phase at the critical point, which can be measured in a practical experiment to
detect the quantum phase transition.Comment: 1 figur
Polaron cross-overs and d-wave superconductivity in Hubbard-Holstein model
We present a theoretical study of superconductivity of polarons in the
Hubbard-Holstein model. A residual kinematic interaction proportional to the
square of the polaron hopping energy between polarons and phonons provides a
pairing field for the polarons. We find that superconducting instability in the
d-wave channel is possible with small transition temperatures which is maximum
in the large to small polaron cross-over region. An s-wave instability is found
to be not possible when the effective on-site interaction between polarons is
repulsive
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