66,646 research outputs found
A new transgenic reporter line reveals Wnt-dependent Snai2 re-expression and cranial neural crest differentiation in Xenopus
During vertebrate embryogenesis, the cranial neural crest (CNC) forms at the neural plate border and subsequently migrates and differentiates into many types of cells. The transcription factor Snai2, which is induced by canonical Wnt signaling to be expressed in the early CNC, is pivotal for CNC induction and migration in Xenopus. However, snai2 expression is silenced during CNC migration, and its roles at later developmental stages remain unclear. We generated a transgenic X. tropicalis line that expresses enhanced green fluorescent protein (eGFP) driven by the snai2 promoter/enhancer, and observed eGFP expression not only in the pre-migratory and migrating CNC, but also the differentiating CNC. This transgenic line can be used directly to detect deficiencies in CNC development at various stages, including subtle perturbation of CNC differentiation. In situ hybridization and immunohistochemistry confirm that Snai2 is re-expressed in the differentiating CNC. Using a separate transgenic Wnt reporter line, we show that canonical Wnt signaling is also active in the differentiating CNC. Blocking Wnt signaling shortly after CNC migration causes reduced snai2 expression and impaired differentiation of CNC-derived head cartilage structures. These results suggest that Wnt signaling is required for snai2 re-expression and CNC differentiation
Application of the Titius-Bode Rule to the 55 Cancri System: Tentative Prediction of a Possibly Habitable Planet
Following the notion that the Titius-Bode rule (TBR) may also be applicable
to some extra-solar planetary systems, although this number could be relatively
small, it is applied to 55 Cancri, which is a G-type main-sequence star
currently known to host five planets. Following a concise computational
process, we tentatively identify four new hypothetical planetary positions
given as 0.081, 0.41, 1.51 and 2.95 AU from the star. The likelihood that these
positions are occupied by real existing planets is significantly enhanced for
the positions of 1.51 and 2.95 AU in the view of previous simulations on planet
formation and planetary orbital stability. For example, Raymond et al. (2008)
[ApJ 689, 478] argued that additional planets would be possible between 55 Cnc
f and 55 Cnc d, which would include planets situated at 1.51 and 2.95 AU. If
two additional planets are assumed to exist between 55 Cnc f and 55 Cnc d, the
deduced domains of stability would be given as 1.3-1.6 and 2.2-3.3 AU. The
possible planet near 1.5 AU appears to be located at the outskirts of the
stellar habitable zone, which is however notably affected by the stellar
parameters as well as the adopted model of circumstellar habitability. We also
compute the distance of the next possible outer planet in the 55 Cnc system,
which if existing is predicted to be located between 10.9 and 12.2 AU, which is
consistent with orbital stability constraints. The inherent statistical
significance of the TBR is evaluated following the method by Lynch (2003)
[MNRAS 341, 1174]. Yet it is up to future planetary search missions to verify
or falsify the applicability of the TBR to the 55 Cnc system, and to attain
information on additional planets, if existing.Comment: 9 pages, 3 figures, 9 tables; accepted by Publications of the
Astronomical Society of Japan; to be published on 2012 August 25; Ref.: PASJ
64 (4
Lorentz invariance violation and charge (non--)conservation: A general theoretical frame for extensions of the Maxwell equations
All quantum gravity approaches lead to small modifications in the standard
laws of physics which lead to violations of Lorentz invariance. One particular
example is the extended standard model (SME). Here, a general phenomenological
approach for extensions of the Maxwell equations is presented which turns out
to be more general than the SME and which covers charge non--conservation
(CNC), too. The new Lorentz invariance violating terms cannot be probed by
optical experiments but need, instead, the exploration of the electromagnetic
field created by a point charge or a magnetic dipole. Some scalar--tensor
theories and higher dimensional brane theories predict CNC in four dimensions
and some models violating Special Relativity have been shown to be connected
with CNC and its relation to the Einstein Equivalence Principle has been
discussed. Due to this upcoming interest, the experimental status of electric
charge conservation is reviewed. Up to now there seem to exist no unique tests
of charge conservation. CNC is related to the precession of polarization, to a
modification of the --Coulomb potential, and to a time-dependence of the
fine structure constant. This gives the opportunity to describe a dedicated
search for CNC.Comment: To appear in Physical Review
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