Developmental changes of keratin expression in chick embryo olfactory epithelium in relation to cellular differentiation and neurogenesis in vivo and in vitro.

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.