Pre-natal development of rat nasal epithelia. V. Freeze-fracturing on necklaces of primary and secondary cilia of olfactory and respiratory epithelial cells.

Many cilium types have at their proximal base a particulated membrane structure, the so-called ciliary necklace. Necklaces of primary and secondary cilia of olfactory receptor cells and ciliated respiratory cells, and of primary cilia of olfactory supporting cells were studied as a function of embryonic age. Strand numbers in necklaces of primary cilia of these cell types do not differ, but they differ significantly from those of necklaces of secondary cilia. Primary cilia have 2 to 4, but most commonly 3, necklace strands. This is true for necklaces of primary cilia of 8 different nasal cell types: olfactory epithelial basal and glandular cells, vomeronasal receptor and supporting cells, and microvillous respiratory epithelial cells, in addition to the 3 cell types mentioned above. Comparison with other systems suggests that primary cilia resemble flagella of eukaryotic flagellates and spermatozoa of some invertebrates with respect to their number of necklace strands. Average numbers of necklace strands in secondary olfactory cilia increase from 3-4 at the 16th and 17th gestational days to 6-7 in adults. Those in secondary respiratory cilia increase from 2-3 at the 18th and 19th gestational days to 5-6 in adults. Longer cilia have more strands than shorter ones. Necklaces often have free strand endings, also in primary cilia, suggesting that they spiral. Comparing the present data with those in the literature suggests that necklace features occurring during reciliation differ from those of de novo ciliogenesis. Primary and secondary cilia share the following qualities: 1) Membrane regions above necklace strands can differ quite drastically from those below the strands.(ABSTRACT TRUNCATED AT 250 WORDS)