Developing a sense of scents: Plasticity in olfactory placode formation

Abstract

The sense organs of the vertebrate head arise predominantly from sensory placodes. The sensory placodes have traditionally been grouped as structures that share common developmental and evolutionary characteristics. In attempts to build a coherent model for development of all placodes, the fascinating differences that make placodes unique are often overlooked. Here I review olfactory placode development with special attention to the origin and cell movements that generate the olfactory placode, the derivatives of this sensory placode, and the degree to which it shows plasticity during development. Next, through comparison with adenohypophyseal, and lens placodes I suggest we revise our thinking and terminology for these anterior placodes, specifically by: 1) referring to the peripheral olfactory sensory system as neural ectoderm because it expresses the same series of genes involved in neural differentiation and differentiates in tandem with the olfactory bulb, 2) grouping the anterior placodes with their corresponding central nervous system structures and emphasizing patterning mechanisms shared between placodes and these targets. Sensory systems did not arise independent of the central nervous system; they are part of a functional unit composed of peripheral sensory structures and their targets. By expanding our analyses of sensory system development to also include cell movements, gene expression and morphological changes observed in this functional unit, we will better understand the evolution of sensory structures.

Development of the olfactory system in turbot (Psetta maxima L.)

“…Although more cells are added to the emergent placode during embryonic stages, both the appearance and size of the placode remain almost invariable throughout this period. When the turbot olfactory placode begins its differentiation, most of the placodal cells are PCNA-immunonegative, supporting recent evidences that indicate that early cell recruitment in this sensory placode is not a result of increased mitotic activity, but the incoming of new cells that move from a specific field of the anterior neural plate (Whitlock and Westerfield, 2000;Whitlock, 2008).…”

Competence, specification and commitment to an olfactory placode fate

“…More direct evidence for an olfactory placode origin for these neurons have come from dye labeling (Murakami and Arai, 1994), transplantation (Yamamoto et al, 1996) and ablation studies (Akutsu et al, 1992;Norgren and Gao, 1994). However, it has been argued that cells with non-olfactory placode origins could have mixed in with olfactory placode cells in these studies (Whitlock, 2008). Additional evidence in support of an olfactory placode origin has come from analysis of the Sey mutant, which lacks olfactory placodes and has no GnRH neurons (Dellovade et al, 1998;Skynner et al, 1999).…”

The loss of scents: Do defects in olfactory sensory neuron development underlie human disease?

“…Using zebrafish as a model system, it was shown through single cell fate map analysis of the highly expressed dlx3 domain that the olfactory placodes arise from a region continuous with the neural plate through cell movements and not localized cell division (Whitlock and Westerfield, ; Whitlock, ). The lack of an obvious morphological border separating the precursors of the peripheral and central olfactory structures led to the suggestion that the olfactory system may be more similar to the development of the retina of eye and its target in the diencephalon (Whitlock, ). With the zebrafish model system, cells were tracked in the anterior neural plate, starting at the end of gastrulation/initiation of somitogenesis (Fig.…”