The development of the olfactory mucosa in the mouse: electron microscopy.

The development of the olfactory epithelium from the 10th day of gestation of postnatal life has been examined electron miscroscopically in the mouse. At 10 days' gestation the epithelium is already differentiated into dark and pale cells, the former representing embryonic stem cells and the latter the developing receptors. Axons are also visible at this stage. At 11 days the first signs of dendrite formation appear, and at 12 days spheroidal terminal swellings containing numerous microtubules are present at the apices of receptor dendrites. Centriole clusters also appear in the receptor cell bodies and dendrites. From the 12th to the 16th day of gestation a few cilia are formed on the receptor endings. Final steps in the maturation of differentiating receptors begin on the 17th day of gestation, when membranous organelles and lysosomes increase greatly in numbers. However, immature receptors can still be found in the base of the epithelium in postnatal life. Supporting cells are first recognizable on the 17th day of gestation, derived apparently from the remaining stem cells. At the same time differentiated basal cells and glands of Bowman begin to appear. In the early develoment of the olfactory nerve bundles the axons have large and varying diameters, but later on axonal sizes are progressively reduced and the adult size range is achieved at about 18 days of gestation. The significance of these findings is discussed.