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("Axonal") Flow: Neurofilaments

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( Proc. Nat. Acad. Sci. USA Vol. 68, No. 4, pp. 846-850, April 1971 Organelles in Neuroplasmic ("Axonal") Flow: Neurofilaments PAUL A. WEISS AND ROBERT MAYR* The Rockefeller University, New York 10021 Communicated January 26, 1971 ABSTRACT The electronmicroscopically conspicuous graphed at identical magnifications under the electronmicro- "neurofilaments" in axons manifest a marked constancy of mutual distance at all levels of a nerve fiber regardless scope. of the wide variation of cross-sectional area along the On first inspection (Figs. 1 and 2), we were struck (a) by the course of a single axon or between different axons. The regularity of the spacing between the individual filaments and filaments conform to the rheological flow patterns of the (b) by their being grouped in bundles of parallel lines, often axonal flow and must be viewed as discontinuous, meta- aligned in rows, predominantly in the general direction of the stable, and transitory linear assemblies of subunits present in the axonal matrix. The grid-like distribution of the fila- nerve fiber axis, but each bundle following a detailed course of ments in the matrix suggests properties akin to those of its own, rather sharply set off from the path of its neighbors. liquid crystals of heterogeneous composition. As a result, slightly oblique sections show filament bundles in various slants, some transversal, others in lateral profile "Axoplasmic flow" refers to the fact (1) that the "axis cyl- (Fig. 3). inder" of the mature neuron keeps growing forth throughout Measurements of 227 interfilament distances yielded the life from its base in the cell body, its macromolecular substance histogram of Fig. 4, showing a rather sharp peak at 428 A, i.e., being perpetually reproduced there and then conveyed as a an average domain per filament of roughly 2 X 105 A2, or cohesive semi-solid mass toward the distal ending of the nerve about 500 filaments per square micron. From this rough esti- fiber, where its content is in part dissolved, in part discharged. mate of average filament density, we then proceeded to a more The whole column moves forward at a standard rate of the direct determination of the ratio between the number of order of 1 mm per day, driven by a microperistaltic wave gen- filaments and the actual area of axonal matrix occupied by erated in the axonal surface. There is also evidence for a pro- cess of much more rapid transfer of substance from center to periphery by means and routes that have not yet been identi- ilk . w ... , , a fied [see Dahlstr6m, ref. 2, and recent reviews by myself (3,4)]. Electronmicroscopic study has revealed that the mito- chondria move with the flow (5, 6) and the neuronal micro- tubules ("neurotubules") advance as single, continuous, and undivided constituents embedded in the axonal column (7), FIG.1.Axonalcross section with clusters of neurotubules the number of tubukar cross sections in single branched axons (e~g.,atarrows)andneurotilaments.X 58,000 being the same in the common stem and in the sum of the Oz'~ branches. ~~~~~~~~-JN~~~~~~~~ In sharp contrast to this numerical invariance of the neuro- tubules, the number of filaments varies in direct proportion to .~~. 0~A the actual local cross-sectional area of the axon. This distinc- -141, ~ 'I, tion, added to that of protein composition (8), called for a ~~~~~~~~~~~~~~~ ~ ~ ~ ~ ~ ~ 4 critical reassessment of the electronmicroscopic features of those two systems. Objects and Methods were the same as in the preceding article on neurotubules (7): small motor nerve fiber bundles; 150-250 g rats, perfused by buffered glutaraldehyde; post- fixed in osmium tetroxide; embedded in araldite or epon; thin-sectioned at consecutive levels; stained in uranyl acetate and lead citrate; sample sections from different levels, micro- scopically identified as of one and the same axon, photo- (e~lg.at arrows) an euoiamns X *,000 This paper is part III of a series on organelles in axoplasmic flow. Part II is ref. 7. * Permanent address: Anatomisches Institut der UniversitAt, Wien IX, Austria. 846 Downloaded by guest on September 29, 2021 Proc._ sNat. Or_,Acad. Sci.zWIrUSA w68 $(1971) $ Neurofilaments in Axoplasmic Flow 847 ,#. S;;§ ..w. I* ;!¢tloSw,_f3,,#; P.0| ..vx,* * a _ At_ So r$wt Ado- w -0 8 S .- ., f f ,; A_ _ A< 9'i'.4 ,*'.4' X Jo:_: ,! _ Jet f .R .S . 4. - ,_ # ^i*ft-t X 44 14~~~~~~~~~~~~ ¢ e w f '" .,, .,: < tF e He All d. "kg% y :4c.X:-i .. _it .- '. An 4*~~~~~~~~~~~~~~~~~~~~~~~~4 As,, J An.% . s _ 4 A; -' .-,, A: / ... v-i' 5 _ ^ Serf .' FIG. 2. Longitudinal axonal section with neurotubules ',_ 5 '9 ,. (arrows)-A. '' Sand - ;filaments. t Is. S.X58,000 > 6 v ; *,- re,' .61I them, the latter measured as total axonal cross section reduced by the space (about 15%) occupied by mitochondria, endo- plasmic reticulum, and neurotubules. N~~~~~~~~~~~~~~~~~~~~~~~~~~~~N The diameter of a single axon fluctuates considerably along its axis. This variability extends way beyond the familiar narrowing at each node of Ranvier over all internodal sections. The following laborious counts and measurements were car- ried out on three single axons, referred to as A, B, and C. Cross sections were measured by planimetry. Filaments were FIG. 3. "Streamlined" fascicles of filaments in transverse counted either over the entire section (case A), or over a (single arrows), lengthwise (feathered arrow), and oblique courses. representative unit sampling area (cases B and C); in the X58,000 latter procedure, the count of the sample unit was then multi- plied by the ratio between the cross-sectional area of the axon (corrected downward for the area occupied by mitochondria I I i ~~~~i i 200 300 400 500 600 700 800 iooo A 100 TABLE 1. Proportionality between number offilaments and local axonal area SO 45, x 428 A Axonal area (Am') No. of s * 77A 5 Total Relevant No. of filaments No. of 40 SR: A n a 227 Case Level area area* filaments per um2 tubules 35. A a 1.84 1.54 340 230 17 b 1.62 1.36 231 185 18 30 c 0.53 0.49 111 260 18 25 B a 10.7 1,600 150 b 1.7 175 103 20 c 6.3 930 148 15 d 5.5 880 160 1 unit = 16.93A C a 3.5 650 186 10 b 2.6 570 218 c 12.5 2,800 224 5' .* I . q3. q3- 1 3 3S5.7.917~9 111~3252 72931 * Axonal cross-sectional area, reduced by cross sections of mito- 13XiS171922l 133353739414345 50 chondria and endoplasmic reticulum. FIG. 4. Histogram of interfilament distances. Downloaded by guest on September 29, 2021 848 Cell Biology: Weiss and Mayr Proc. Nat. Acad. Sci. USA 68 (1971) a a C 0 E 'U S I~- E le U. Axonal cross section FIG. 5. Plot of number of filaments (circles) and tubules (squares) against relevant cross-sectional areas of single axon A. Axonal cross section FIG. 6. Plots of numbers of filaments against relevant cross- sectional areas of axons B and C and reticulum) and the sampled area (minus microtubular (full circles) (open circles). space). Table 1 gives the respective values. Case A. The relevant axonal area at level a is 3.14 times as large as that at c, and the number of filaments counted in a is (total number of filaments divided by total relevant matrix 3.07 times as high as that in c; that is, the numbers of filaments area) is 180 filaments/,gm2; the separate values are, for the are roughly proportional to the areas of matrix they populate. fully counted case A, 227, and for the extrapolated estimates of The respective proportionality factors between the three B and C, 140 and 209, respectively, thus fluctuating about the levels are a: c = 1.25, a: b = 0.80, b: c = 1.33, for an average of total average of 180 by no more than 26%. It is obvious that 1.13. The graph in Fig. 5, plotting number of filaments against such a 26% deviation becomes quite insignificant if one com- corrected axonal areas, illustrates this relation. Contrastingly, pares it with the range of variation of 2500% of the popula- the count of neurotubules was essentially the same at all three tions of filaments and of the sizes of areas over which it held levels (17, 18, and 18; broken line in Fig. 5) (7). true. Therefore, the proportionality between any local area of Cases B and C. [Table 1 and Fig. 6; the point for level c organelle-free axonal matrix and the number of filaments pass- of case C, lying beyond the limits of printing space, is repre- ing through it is of standard value both within a given axon sented in the figure by a point (broken contour), both co- and for axons of different nerves, at least those treated by the ordinates of which have been proportionately scaled down by same histological procedure. The higher value of filament one fourth.] Given the uncertainties of sampling error, and density (500/,Am2), calculated from Fig. 4, but less compar- errors of planimetry and counting, it is all the more striking able because of different osmolarity of the fixing fluid in that that the number of filaments per unit area of matrix, i.e., the case, is still of the same order of magnitude. Figs. 5 and 6 domain of matrix belonging to a single filament, has a near- document (a) the linear proportionality within each of the constant value regardless of the actual size of the total area in three studied axons and (b) the essential identity of the pro- which they are counted.
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