Genesis of cilia and microvilli of rat nasal epithelia during pre-natal development. II. Olfactory epithelium, a morphometric analysis.

Rat foetuses from intra-uterine days E14 through E22 (day before parturition) and adults were used for a quantitative scanning electron-microscopic examination of ciliogenesis in olfactory receptor cells and microvillogenesis in olfactory supporting cells. Four developmental stages in olfactory ciliogenesis can be discerned. Two of these are characterized by the presence of primary cilia only, the other two concern outgrowth in number and length of secondary cilia. (1) Primary cilia on undifferentiated cells; this stage occurs up to E14. (2) Primary cilia on differentiating olfactory receptor and also olfactory supporting cells. This stage begins at E14 and lasts, for the olfactory receptor cells, at least up to E22. On the supporting cells primary cilia are rarely observed after E18. Virtually all primary cilia are about 1 micron long. Up to E21 dendritic endings with primary cilia occur more frequently than those with any other number of cilia; all endings have a transitional stage in which they bear primary cilia only. (3) Secondary olfactory cilia increase in number. From E16 onwards the cells become multiciliated. Beginning at this stage and continuing up to E22 an average of one cilium per day is added to the endings. At E22 the average number of cilia observed per ending is about 70% of that in adults; more than 90% of the endings are multiciliated. From E15 to E22 the exchange rate between receptor cells with only primary cilia and multiciliated cells is about 0.5 X 10(6) cells/cm2 per day. When considered in the light of electrophysiological data on developing rats, our data suggest that when the cells have just primary cilia, they may respond indiscriminately to all odorants, whereas multiciliated cells display odorant specificity. (4) Secondary olfactory cilia increase in length. From E14 to E19 and over the whole population of receptor cells the cilia grow at an average rate of about 0.5 micron/day. Proximal parts of olfactory cilia are longer than primary cilia; olfactory cilia begin to taper in increasing numbers around E18. At E19 the receptive membrane surface, i.e. regions of the cells facing the nasal lumen, of individual cells is about 8%, and the increase in epithelial surface due to sprouting of cilia is 5% of such values in adult animals. Concomitant with the onset of tapering of olfactory cilia, i.e. around E18, microvilli of supporting cells show a spurt in growth from about 0.4 micron to about 1.3 micron. Unlike olfactory cilia they show no growth, on average, after E19.(ABSTRACT TRUNCATED AT 400 WORDS)