Intraflagellar transport is essential for mammalian spermiogenesis but is absent in mature sperm.
This mutation is highly disruptive to ciliary assembly in other organs.Ift88(-/-) mice are completely sterile.The short flagella rarely have axonemes but assemble ectopic microtubules and outer dense fibers and accumulate improperly assembled fibrous sheath proteins.
Affiliation: Program in Molecular Medicine, University of Massachusetts Medical School, Worcester, MA 01605.
No MeSH data available.
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Figure 5: The caudal cytoplasm of Ift88−/− spermatids often lack axonemes and have ectopically assembled outer dense fibers. (A) Electron micrograph of a section tangential to the lumen of a wild-type stage VII seminiferous tubule. This micrograph contains cross-sections through the caudal cytoplasm, or developing midpieces, of several step 16 spermatids (16, asterisks). The midpieces decrease in diameter as the cross-sections are cut closer to the junction with the principal piece. Virtually every midpiece contains an axoneme with outer dense fibers around which the midpiece mitochondria (m) have condensed. Also seen are cross-sections through the principal pieces of several less mature step 7 (7) spermatids, all of which contain axonemes. Scale bar, 5 μm. (B) A comparable section through the caudal regions of several Ift88−/− step 16 spermatids (16) showing cross-sections of what should be developing midpieces (asterisks). In contrast to the wild-type section, not a single normal axoneme is apparent in these “midpieces,” although axonemes were occasionally seen in other sections. Normal principal pieces also are not seen. Scale bar, 5 μm. (C) A higher magnification of cross-sections through the midpiece region of two wild-type step 16 (tubular stage VI) spermatids in which mitochondria (m) have condensed around the outer dense fibers (white arrows), one of which is associated with each of the nine axonemal doublet microtubules. Also visible are flagella of step 6 round spermatids (6), which have not yet assembled outer dense fibers, and a principal piece of a step 16 spermatid, which has a fibrous sheath (fs). Scale bar, 500 nm. (D) A section through the developing “midpiece” of an Ift88−/− step 16 spermatid. No axoneme is present. Numerous outer dense fibers (white arrows) are scattered throughout the cytoplasm and are serving as foci for condensation of midpiece mitochondria (m). Scale bar, 500 nm. (E) Enlarged image of the developing “midpiece” of another Ift88−/− step 16 spermatid. Mitochondria (m) have condensed around several ectopically located outer dense fibers (white arrows). Scale bar, 500 nm. (F) A section through the midpiece of an Ift88−/− step 16 spermatid, showing one of the few instances in which an axoneme was observed. Even in the presence of an axoneme and its associated outer dense fibers, some outer dense fibers have assembled ectopically (white arrows) in the cytoplasm. A microtubule (arrowhead) is associated with one or more of the outer dense fibers. Scale bar, 500 nm.
Cross-sections through the caudal cytoplasm of controls at these stages showed well- organized axonemes, which, at the later stages, were surrounded by outer dense fibers; in the developing midpiece, mitochondria were condensing around the axonemes and associated outer dense fibers (Figure 5, A and C). In contrast to the controls, most sections through the caudal cytoplasm of mutant sperm did not reveal any cross-sections of axonemes (Figure 5, B, D, and E); in these cells, the outer dense fibers formed ectopically in the cytoplasm and often had mitochondria associated with them. Even when an axoneme was present, outer dense fibers, sometimes associated with microtubules, were formed ectopically (Figure 5F).