Pre-natal development of rat nasal epithelia. IV. Freeze-fracturing on apices, microvilli and primary and secondary cilia of olfactory and respiratory epithelial cells, and on olfactory axons.

UNLABELLED: Olfactory axons and apical structures of olfactory epithelia and of nasal respiratory epithelia of rat embryos were studied with the freeze-fracture technique; adult tissue samples of the same sources were used for comparison. At the onset of epithelial differentiation (14th gestational day) intramembranous particle densities are the same for all structures in both epithelial types. During further development, particle densities in membranes of primary cilia remain lower than those in membranes of other apical structures. Otherwise, I found the following from the 14th to the 19th day of gestation. a. Olfactory receptor cells of embryos of all age groups have axons wherein the membrane particle densities are about half those of adults. These densities are always lower than those of dendritic ending structures. Dendritic endings with primary cilia have lower densities than endings with secondary cilia; densities mainly increase when the endings sprout secondary cilia. Adult values are reached at the 18th day of gestation. b. Olfactory supporting cells with only globular particles in their apices gradually transform into, or are replaced by, supporting cells which also have dumbbell-shaped particles in their apices. Particle densities are always higher in apical structures of supporting cells than in apical structures of receptor cells. Adult values are reached at the 17th day of gestation. c. Putative ciliated and ciliated respiratory epithelial cells have considerably lower particle densities in membranes of their apical structures than do olfactory epithelial cells. Of special interest is that this is also true for secondary respiratory and olfactory cilia; as soon as genesis of secondary cilia in either epithelial type begins, their membrane features differ. Also, in contrast to apical structures of the olfactory epithelium, particle densities in apical structures of the respiratory epithelium do not systematically change during pre-natal development, and resemble the density values of adults. An exception are the microvilli of the respiratory cells with secondary cilia, membranes of which have considerably higher particle densities in adults than in embryos. IN
CONCLUSION: Transformations of olfactory receptor cell dendritic endings with primary cilia into endings with secondary cilia, and of olfactory supporting cells with globular particles in their apices into cells with dumbbell-shaped particles in their apices are accompanied by increases in the densities of their intramembranous particles. These developmental changes parallel the electrophysiological onset of olfactory receptor cell specificity.