The sense organs of the vertebrate head comprise structures as varied as the eye, inner ear, and olfactory epithelium. In the early embryo, these assorted structures share a common developmental origin within the preplacodal region and acquire specific characteristics only later. Here we demonstrate a fundamental similarity in placodal precursors: in the chick all are specified as lens prior to acquiring features of specific sensory or neurogenic placodes. Lens specification becomes progressively restricted in the head ectoderm, initially by FGF and subsequently by signals derived from migrating neural crest cells. We show that FGF8 from the anterior neural ridge is both necessary and sufficient to promote olfactory fate in adjacent ectoderm. Our results reveal that placode precursors share a common ground state as lens and progressive restriction allows the full range of placodal derivatives to form.

Cranial placodes are focal regions of columnar epithelium next to the neural tube that contribute to sensory ganglia and organs in the vertebrate head, including the olfactory epithelium and the crystalline lens of the eye. Using focal dye labelling within the presumptive placode domain, we show that lens and nasal precursors arise from a common territory surrounding the anterior neural plate. They then segregate over time and converge to their final positions in discrete placodes by apparently directed movements. Since these events closely parallel the separation of eye and antennal primordia (containing olfactory sensory cells) from a common imaginal disc in Drosophila, we investigated whether the vertebrate homologues of Distalless (Dll) and Eyeless (Ey), which determine antennal and eye identity in the fly, play a role in segregation of lens and nasal precursors in the chick. Dlx5 and Pax6 are initially co-expressed by future lens and olfactory cells. As soon as presumptive lens cells acquire columnar morphology all Dlx family members are down-regulated in the placode, while Pax6 is lost in the olfactory region. Lens precursor cells that express ectopic Dlx5 never acquire lens-specific gene expression and are excluded from the lens placode to cluster in the head ectoderm. These results suggest that the loss of Dlx5 is required for cells to adopt a lens fate and that the balance of Pax6 and Dlx expression regulates cell sorting into appropriate placodal domains.

Cranial sensory placodes arise as transient thickenings of embryonic head ectoderm (van Wijhe, 1883;von Kupffer, 1891) and invaginate/ingress to form components of the nose, lens and inner ear (Bailey and Streit, 2006; Baker and Bronner-Fraser, 2001;Schlosser, 2006). Of the three sensory placodes, the lens and otic are relatively easy to distinguish and manipulate early in development. Therefore, tissue interactions and signals guiding their induction have been studied extensively (reviewed by Brown et al., 2003;Chow and Lang, 2001;Donner et al., 2006;Grainger, 1992;Noramly and Grainger, 2002;Riley and Phillips, 2003;Saha et al., 1989;Streit, 2001;Torres and Giraldez, 1998).By contrast, development of the olfactory sensory system has been primarily investigated at later stages. An attractive model for studying neurogenesis, its myriad of derivatives include the regenerative odorant sensing olfactory neurons in the olfactory epithelium (Calof et al., 1998;Graziadei and Graziadei, 1979;Graziadei and Metcalf, 1971;Graziadei and Monti Graziadei, 1983), the ingressing gonadotropin-releasing hormone neurons (reviewed by Parhar, 2002;Wray, 2002) [for a contrary view see Whitlock (Whitlock, 2005)], the pheromone-detecting vomeronasal organ (Dulac, 1997) and other neuromodulatory and neuroendocrine cells (Northcutt and Muske, 1994;Tarozzo et al., 1995;Yamamoto et al., 1996). It is the only placode to give rise to glial cells that ingress and migrate along the olfactory nerve towards the brain (Chuah and West, 2002;Ramon-Cueto and Avila, 1998). Cues that guide differentiation along these different pathways and the molecular mechanisms underlying the patterned wiring of olfactory sensory neurons have been examined extensively (reviewed by Baker and Margolis, 2002; Balmer and LaMantia, 2005).Contrasting with this wealth of data, inductive events that initiate olfactory development remain ambiguous, partially because the placode is morphologically invisible until relatively late (HH13+ in birds). Transplantation studies show that precursors straddle the lateral anterior neural folds and adjacent ectoderm in the neurula (Couly and Le Douarin, 1985), but their exact location at intervening stages was unclear. Recent cell marking studies in zebrafish and chick have examined the origin of the olfactory placode at higher resolution and at intervening stages (Bhattacharyya et al., 2004;Whitlock and Westerfield, 2000). Olfactory precursors are initially distributed in a broad region spanning the anterior neural folds and overlapping with lens precursors. Over time, they become progressively restricted to the most anterior ectoderm, finally resolving to a discrete olfactory pit. Knowledge of the location of precursors at different stages has facilitated manipulation of their development.To understand how and when the nasal epithelium is first induced, we examined competence, specification and commitment of ectoderm toward an olfactory fate. This information is a necessary prerequisite for interpreting results of functional perturba...

A relativistic analysis of the rate of spin flips in ionization of an ensemble of Dirac H atoms subjected to intense circularly polarized laser fields is made. A remarkable intensity-dependent asymmetry between the spin up and spin down electron currents is found. It is nonzero even when the retardation effect, hence the magnetic component of the field, as well as the spin-orbit interaction responsible for the well-known Fano effect, is negligible. Transformation properties of the amplitudes show that the sign of asymmetry can be controlled by changing the helicity of the laser photons from outside.

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