Apoptosis in the development of the mouse olfactory epithelium.

Apoptotic cells were detected in the mouse olfactory epithelium (OE) at different embryonic and postnatal stages by in situ nick translation (ISNT) and Tdt-mediated dUTP nick end-labeling (TUNEL) techniques. During development, the apoptotic process presented two peaks. One at E12 during the invagination of the olfactory placode and the second at E16 corresponding to olfactory axon synaptogenesis. Then, from E18, a sharp decrease in the number of apoptotic cells was observed and at E19 the apoptotic index reached low values that were maintained in postnatal stages, P1 and P8, and in the adult. Apoptotic nuclei belonged to mature as well as immature olfactory receptor neurons (ORNs). Indeed, double-labeling experiments evidenced apoptotic neurons immunopositive for olfactory marker protein (OMP), carnosine and GAP-43. According to our data, two apoptotic phases occur during early development. One is involved in the morphogenesis of the OE when this last is not yet, or poorly, connected to its target, the olfactory bulb (OB). The second peak of apoptosis is more closely dependent on the interplay between OE and OB.