DIFFERENTIATION OF THE OLFACTORY EPITHELIUM DURING DEVELOPMENT

Maturation of the olfactory epithelium was studied in the rat. As a de velopmental index of the olfactory epithelium, quantitation was based on the thickness and the number of cell layers of the epithelium. Mean epithelial thicknesses were 84 mu m on embryonic day (E) 19, 94 mu m on postnatal day (P) 0, 98 mu m on P10 and 57 mu m in adult. Average n umbers of cell layers were 7 (E19), 8 (P0), 10 (P10) and 7 (adult). Th us the olfactory epithelium progressively increased in thickness betwe en P0-P10, and later decreased in thickness as well as in number of ce ll layers toward adult. To clarify the number of mature receptor cells sending their axons to the olfactory bulb, the percentages of recepto r cell labeling (number of labeled cells/total number of receptor cell s) were estimated with an aid of retrograde fluorescent tracer injecte d into the olfactory bulb. As a result, their percentages were 34% (P1 -P3), 50% (P8-P10) and 78% (adult). The ablation experiment of the bul b on P1 pups and adult rats revealed that the percentages of receptor cell degeneration were 31% in neonates (P1) and 76% in adults, confirm ing the results obtained from the retrograde tracing. Thus the present study shows the substantial increase of mature receptor cells during development and may provide a morphological basis in understanding of the functional maturity of olfaction.