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This Perspective is an account of my early experience while I studied the dynamic organization and behavior of the mitotic spindle and its submicroscopic filaments using polarized light microscopy. The birefringence of spindle filaments in normally dividing plant and animal cells, and those treated by various agents, revealed (a) the reality of spindle fibers and fibrils in healthy living cells; (b) the labile, dynamic nature of the molecular filaments making up the spindle fibers; (c) the mode of fibrogenesis and action of orienting centers; and (d) force-generating properties based on the disassembly and assembly of the fibrils. These studies, which were carried out directly on living cells using improved polarizing microscopes, in fact predicted the reversible assembly properties of microtubules.
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Download Supplemental Comment 2 (PDF).
Download Supplemental Movie 1 (MOV): Dividing endosperm cells of African blood lily Haemanthus katherinae recorded in phase contrast by Andrew and Wishia Bajer (1951, 1956). The movements and shape change of chromosomes are displayed most strikingly, but spindle fibers and phragmoplast fibrils are not visible. (Movie courtesy of Andrew and Wishia Bajer of the University of Oregon.)
Download Supplemental Movie 2 (MOV): Dividing pollen mother cell of Easter lily Lilium longiflorum observed using polarizing microscope shown in text Fig. 9. The movie shows the chromosomes being brought to the metaphase plate and then led polewards by birefringent spindle fibers. Soon birefringent fibrils re-emerge between the daughter nuclei to form the phragmoplast, and vesicles assemble in its mid-zone to form the cell plate (Inoué 1964). This sequence, first shown publicly at the MBL in 1951, finally established the general reality of spindle fibers and fibrils in living cells.
Download Supplemental Movie 3 (MOV): Reversible disappearance of spindle birefringence in dividing sea urchin eggs exposed to low temperature (Inoué 1952b). Birefringent spindle fibers disappear when a cell is chilled. They soon reappear upon warming, and the cell resumes division. These experiments suggested that fibrils making up spindle fibers depolymerize by cold and repolymerize when the cell is warmed. The observations foretold the assembly properties of in vitro microtubules found two decades later.
Download Supplemental Movie 4 (MOV): “Northern lights flickering” of birefringent spindle fibers in dividing spermatocytes of a grasshopper Chrysocraon japonicas. These spectacular images showing the fluctuating birefringence, distortions of spindle fibers and associated chromosome movements were filmed by Sato and Izutsu in 1974 using a rectified polarizing microscope. The striking northern lights flickering could not be fully explained in 1974 but was thought to reflect tread-milling and the growth and shortening of microtubules which were actively assembling and disassembling. (Movie courtesy of Hidemi Sato, Nagoya University, and Kosaku Izutsu, Mie Medical School.)
Download Supplemental Movie 5 (MOV): Birefringence of crane fly spermatocytes in meiosis-I recorded with LC-PolScope (Oldenbourg 1996). Distribution and activities of the birefringent spindle microtubules and other organelles are captured at high resolution. In the un-flattened cell shown in the first sequence, the optical section in focus shows two pairs of autosomal bivalents and the two late-separating sex chromosomes together with their chromosomal spindle fibers. In the flattened cell shown in the second sequence, all three autosomal bivalents and the two sex chromosomes together with their chromosomal spindle fibers are visible. But the spindle becomes distorted in anaphase due to extreme compression of the cell. As anaphase progresses, strongly birefringent mitochondria form a sheath around the spindle. Scattered, lipid-containing “dyctiosomes” are also strongly birefringent (cf. text Fig. 6). (Unpublished movie courtesy of Jim LaFountain of the University at Buffalo; this movie also appears in Comment 2.)
Literature cited in movie captions:
Bajer, A. 1951. Ciné-micrographic studies on mitosis in endosperm. III. The origin of the mitotic spindle. Exp. Cell Res. 13:493-502.
Bajer A, Bajer JM. 1956. Ciné-micrographic studies on mitosis in endosperm. II. Chromosome, cytoplasmic and Brownian movements. Chromosoma 7:558-607.
Inoué S. 1952b. Effect of temperature on the birefringence of the mitotic spindle. Biol. Bull. 103:316.
Inoué S. 1964. Organization and function of the mitotic spindle. In Primitive Motile Systems in Cell Biology, ed. RD Allen, N Kamiya, pp. 549-98. New York: Academic Press.
Inoué S. 2008. Collected Works of Shinya Inoué: Microscopes, Living Cells, and Dynamic Molecules. Singapore: World Sci. Publ.
Oldenbourg R. 1996. A new view on polarization microscopy. Nature 381:811-812.