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The bird olfactory system has a simple structure and affords an attractive developmental model for the study of olfactory morphogenesis and differentiation. We have cloned and characterized several chick olfactory receptor (COR) genes belonging to the superfamily of seven-transmembrane domain proteins. In situ hybridization analysis of their spatiotemporal patterns of expression during development reveals several important characteristics. COR expression starts early in placodal cells (Embryonic Day 5, E5). Changes in their expression pattern then correlate with the onset of synaptogenesis (E8). The adult pattern, achieved before hatching, shows that cells expressing a particular COR are not regionalized within the epithelium. By double-label in situ hybridization, we clearly demonstrate that a single cell does not coexpress different COR genes (or subsets of CORs) at any stage of development. Following bulbar deafferentation, COR expression ceases more rapidly than expected from previous axotomy experiments. Concomitantly, a reactivation of the Cash-1 gene, which is involved in early neuronal specification, could be an early sign of olfactory neuronal regeneration. Modulation of COR and Cash-1 expression points to a simultaneous process of neuronal degeneration and regeneration in the olfactory epithelium after axotomy. COR expression is restricted to the olfactory epithelium except during early stages (before synaptogenesis). At that time, cells distributed along the olfactory nerve, from the placode to the anterior telencephalon, also express CORs. This cell population is different from the luteinizing hormone releasing hormone neurons migrating from the placode. Our results show that the olfactory neurons or neuroblasts choose to express one COR before establishing functional connections with the bulb. Later on, bulboepithelial connections seem important not only for olfactory neuron survival but also for stimulation of COR expression. In addition, beyond their implication in functional odor detection, CORs could be involved, at early stages, in processes of olfactory morphogenesis, including the establishment of a bulbar chemotopy.

Expression of β-Nerve Growth Factor Receptor mRNA in Sertoli Cells Downregulated by Testosterone TESTICULAR FUNCTION DEPENDS ON local cellular interactions that are influenced by pituitary secretions of luteinizing hormone (LH) [...]

Olfactory embryogenesis was studied using an anti-chick keratin antibody on chick embryo sections as well as in vitro. Olfactory placodes form at embryonic day 3 (ED3) in the anterior facial ectoderm and invaginate to form the nasal pits. At ED5, the epidermal ectoderm and respiratory epithelium show the same dense cytokeratin immunoreaction. In contrast, absence of keratin expression in the basal part of olfactory epithelial primordium, in the deeper nasal pit area, coincides with one of the critical first steps of olfactory neurogenesis. However, beginning with periphero-central olfactory synaptogenesis at ED8, a new basal cell population starts to express keratin in the olfactory epithelium. Keratin positive cells appear to correspond, by their epithelial localisation and morphology, to sustentacular and basal cells. This interpretation was confirmed in vitro with ED14 chick primary olfactory cultures where TrKA immunoreactivity was used as a marker of horizontal basal cells (HBCs). After ED15, late keratin expression was detected in forming Bowman's glands. The density of keratin expressing basal cells was measured between ED10 and ED20, and appeared highest in the median part of the olfactory epithelium, the area of most active olfactory neurogenesis and neuronal maturation. Thus, keratin expression corresponds to a specialisation of horizontal basal cells as active neuronal stem cells.