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 6: Principal pieces of Ift88−/− spermatids have truncated axonemes, lack fibrous sheath longitudinal columns, accumulate fibrous sheath rib material, and have numerous ectopically assembled outer dense fibers and microtubules. (A) Longitudinal section through the developing flagellum of a wild-type step 15 spermatid; the annulus (white diamond) marks the boundary between the midpiece (mp) and the principal piece, in which can be seen fibrous sheath ribs (black arrows) and a fibrous sheath longitudinal column (bracket) on either side of the axoneme. (B) A cross-section through the principal piece of a wild-type step 15 spermatid showing the fibrous sheath longitudinal columns (arrowheads) associated with doublets 3 and 8 and the fibrous sheath ribs that are attached to the longitudinal columns and surround the axoneme. (C) A longitudinal section through a Ift88−/− step 15 spermatid, comparable to the section of the wild-type spermatid shown in A; the boundary between the midpiece (mp) and the principal piece is marked by the annulus (white diamond). The end of the flagellum is swollen and filled with what appears to be unorganized fibrous sheath rib material. (D) Cross-section through the principal piece of an Ift88−/− step 15 spermatid showing an axoneme, which has two extra outer dense fibers but otherwise appears to be normal. The fibrous sheath ribs are present, but the longitudinal columns, which normally are associated with doublets 3 and 8, are missing (arrowheads). (E) Cross-section through a swollen principal piece of an Ift88−/− step 15 spermatid. No axoneme is present; numerous ectopically assembled outer dense fibers are visible, and a large amount of fibrous sheath rib material has accumulated around the periphery of the swollen flagellum. (F) A higher magnification of the boxed region in E. The outer dense fibers (arrows) are associated with singlet (arrowhead) or doublet (bracket) microtubules. (G) Another cross-section through the principal piece of an Ift88−/− spermatid. Although an axoneme is present, the fibrous sheath material remains associated with the distended flagellar membrane. (H) A higher magnification of the boxed region in G showing the axoneme, which lacks fibrous sheath longitudinal columns (arrowheads) and has supernumerary outer dense fibers. (I) Cross-section through a flagellum of a wild-type step 6 spermatid. The anlagen of the fibrous sheath longitudinal columns (black arrowheads) is visible adjacent to doublets 3 and 8. (J, K) Cross-sections through flagella of Ift88−/− probable step 6 spermatids. The anlagen of the fibrous sheath longitudinal columns are missing. The mutant flagellum in K has supernumerary singlet and doublet microtubules (white arrowheads). Scale bars, 1 μm (A, C, E, G), 100 nm (B, D, F, H–K).
To better understand how the reduction in IFT affected assembly of the axoneme and its accessory structures, we examined the principal pieces of the relatively rare ift88−/− spermatids that had a flagellum (Figure 6). In principal pieces of controls at step 6, outer dense fibers have not yet initiated, but the longitudinal columns of the fibrous sheath are formed and attached to doublets 3 and 8 (Figure 6I). At step 15, nine outer dense fibers are being assembled along the axoneme and will be completed in the next step. The fibrous sheath is completed at step 15 and consists of two longitudinal columns attached to outer doublets 3 and 8 and transverse ribs that form an arc around the axoneme, bridging the longitudinal columns (Figure 6, A and B). In principal pieces of the mutant, fibrous sheath assembly is defective from the earliest stages, and the longitudinal columns appear either to never form or never attach to the doublets (Figure 6, J and K). In some cases, the fibrous sheath rib material surrounded the outer dense fibers normally (Figure 6D), but in most cases, it condensed into large aggregates that were associated with the membrane (Figure 6, C, E, and G). Axonemes often were surrounded by supernumerary outer dense fibers (Figure 6, D and H); ectopic microtubules also were observed (Figure 6K). In some cross-sections of tails lacking an organized axoneme, large numbers of outer dense fibers were associated with singlet and doublet microtubules (Figure 6, E and F). In some Ift88−/− spermatids, the distal end of the flagellum was greatly swollen, suggesting an accumulation of unassembled axonemal precursors (Figure 6, C and G). Axonemes in the mutant flagella appeared to have a normal complement of inner and outer dynein arms and radial spokes (Figure 6, D, H, J, and K).