365-374, 2005 Local regulation of the axonal phenotype, a

365COURT & ÁLVAREZ Biol Res 38, 2005, 365-374Biol Res 38: 365-374, 2005 BRLocal regulation of the axonal phenotype, a case ofmerotrophism

FELIPE A. COURT1 and JAIME ÁLVAREZ2

1. San Raffaele Scientific Institute, DIBIT, Milan, Italy; 2. Facultad de Ciencias Biológicas, PontificiaUniversidad Católica. Santiago, Chile

ABSTRACT

In this essay, we show that several anatomical features of the axon, namely, microtubular content, caliber andextension of sprouts, correlate on a local basis with the particular condition of the glial cell, i.e., the anatomyof axons is dynamic, although it is seen usually in its ‘normal’ state. The occurrence of ribosomes andmessenger RNAs in the axon suggests that axoplasmic proteins are most likely synthesized locally, atvariance with the accepted notion that they are supplied by the cell body. We propose that the supporting cell(oligodendrocyte or Schwann cell) regulates the axonal phenotype by fine-tuning the ongoing axonal proteinsynthesis.

Key terms: axon, protein synthesis, nerve regeneration, Schwann cell, Wlds.

Corresponding author: Felipe Court, E-mail: [email protected]

Received: June 4, 2005. Accepted: June 13, 2005.

The nerve has a dual effect upon theskeletal muscle. It regulates what themuscle does (contraction) and what themuscle is (phenotype). Contraction is areversible short-term change of thecytoplasm triggered by membranephenomena, without involvement of thenucleus. In contrast, the phenotype resultsof genetic programs and the phenotypemodifications are measured in days. Forexample, fibrillation of the muscle, which isa typical trophic phenomenon, occurs a fewdays after denervation and is reduced byinhibition of transcription or translation(Llados and Zapata, 1974).

The phenomena relating to the control ofthe phenotype have been embraced underthe heading of trophism. A contact of thesynaptic type is not required. In the nervoussystem, a non-synaptic trophic effect isillustrated by the development of themyelin sheath, a dependence of theSchwann cell, which is triggered by theaxon. We will present evidence that theconverse also occurs, namely, that the glialcell regulates the phenotype of theensheathed axon, which is effected on a

local basis. We will consider themicrotubular content of axons, theircalibers, and their ability to extend sproutsand to synthesize proteins. We also willoutline a tentative model for this glia-to-axon control of the phenotype (Fig 1).

GLIAL CELLS CONTROL MICROTUBULES AND

CALIBER OF AXONS

Neurofilaments and microtubules are themajor components of the cytoskeleton of theaxon. The microtubular density(microtubules/µm2) of axons is an inversefunction of their caliber (Fig 2), andindependent of the myelin sheath (Fadic et al.,1985; Vergara et al., 1991). For a givenaxonal size, the density is the sameirrespective of the length of the axon, thenerve examined, the age of the animal, thedeveloping or regenerating condition of thenerve, the nutritional status, or the speciessurveyed (Álvarez and Zarour, 1983; Espejoand Álvarez, 1986; Faúndez and Álvarez,1986; Faúndez et al., 1989; Saitua andÁlvarez, 1988). Nonetheless, we found one

COURT & ALVAREZ Biol Res 38, 2005, 365-374366

exception: the spinal root. Motor and sensoryaxons span from the cord through the root tothe peripheral nerve. For axons of equal size,the microtubular density in the root is onehalf that of intracord or peripheral domains.This holds for axons regardless of whetherthey are sensory or motor, myelinated orunmyelinated, spinal or cranial (Fadic et al.,1985; Saitua and Álvarez, 1989; López andÁlvarez, 1990). Therefore, axoplasmicproteins are arrayed differently in the root ascompared to intracord or peripheraltrajectories, i.e., the axonal phenotypecorrelates with the environment, while theneuronal class (motor, sensitive, orsympathetic) is irrelevant.

This correlation requires that the axon“senses” its environment. This hypothesiswas tested in the cat using the nodosalganglion of the vagal nerve. The centralbranches of the T-shaped axons of nodosalneurons are homologous to dorsal rootfibers. They extend about one cm in theneck before entering into the skull. Themicrotubular density of these centralbranches was half that of the peripheralbranches of the same neurons, i.e., theasymmetry of these T-axons about itsbifurcation was comparable to that ofordinary sensory axons. These centralbranches were allowed to regenerate alongthe hypoglossal nerve by means of asurgical anatomosis or, as a control, alongits anatomical trajectory. When the centralbranches regenerated along their anatomicaltrajectory, the original low microtubulardensity was found, while upon regenerationalong the foreign peripheral nerve, a highdensity was found (Serra and Álvarez,1989). Therefore, the phenotype of the axoncorrelated with the environment, not withthe central nature of the branch.

Thus, in the correlation between axonalphenotype and environment, the latterseems to have the upper hand, and theneuronal process gives way. The Schwanncell stands as the best candidate to effectthis “environmental” regulation (cf. Courtet al. , 2004). We assumed that aconstitutive axonal program deals with themicrotubular density and that the Schwanncell controls the set point of the program.To test this conjecture, Schwann cells weredestroyed selectively along a 4mm segmentof sciatic nerve by inhibiting transcriptionwith actinomycin D. A few days later, onlyremnants of Schwann cells were seen in thenerve, the axons looked healthy but with anapparent increase of microtubules (Fig 3B,C). The quantitative study revealed that aconstant density replaced the inversecorrelation between size and microtubulardensity across the spectrum of sizes, whichmatched the uppermost value of controlaxons (smallest class) (Fig 2). Thisphenomenon also occurred on a strictlylocal basis, as within a few mm central anddistal to the treated site, the fibers werenormal (Bustos et al., 1991). We conclude

Figure 1. Model axon. The axon (lower part ofthe diagram) embodies a sprouting program(fork), a destruction program (bomb), and themachinery for protein synthesis (beaded string);in addition, the axon regulates the Schwann cell(↑). Typically, the axon induces the Schwanncell to differentiate. The Schwann cell (upperpart of the diagram), in turn, controls the axon(↓). The Schwann cell represses the sproutingprogram and down-regulates the microtubularcontent of the axoplasm. In this paper, we willnot deal with the destruction program. Modifiedfrom Alvarez, 2001.

367COURT & ÁLVAREZ Biol Res 38, 2005, 365-374

that Schwann cells down-regulate the setpoint of the microtubular program of axons,and for the same reason, the high andconstant microtubular density across thesize spectrum is the intrinsic set point. Thesubstantial increase of microtubules in arestricted axon segment requires asupplement of proteins. The view that thecell body supplies all axonal proteinscannot explain the local rise ofmicrotubules in a straightforward manner,since this model allows only forredistribution of existing proteins, i.e.,accumulation at one point entails depletionat another, which was not observed.

We further studied whether microtubulescan be forced to increase exaggeratedly ona local basis. To this end, we administeredtaxol to the nerve, a drug known to stabilizemicrotubules. A few days later, themicrotubular mass was six fold the originalmass, also on a strictly local basis (cf. Fig

3D) (Bustos et al., 1991). Therefore, a localsupply of axonal proteins seemedmandatory to account for this newphenotype.

The caliber of the axon is alsocontrolled, on a local basis by the glial cell.In culture, neurons extend neurites that donot surpass a certain size, but uponmyelination, they do so (Windebank et al.,1985). In the heart, myelinated fiberspresent occasionally unmyelinatedsegments. In this domain, the size of theaxon is smaller than the adjacentmyelinated domains (Yokota, 1994). Theoptic nerve axons illustrate the samephenomenon in the central nervous system.They are unmyelinated in the eyeball andup to the lamina cribosa in the optic nerve;thereafter, axons are myelinated. Uponmyelination, the cross-sectional area of theoptic nerve axons increases by 59%.Interestingly, as the axon enlarges, the

Figure 2. Microtubule quantification in unmyelinated fibres of the sural nerve. Actinomycin D andcycloheximide were administered locally. Ordinates: in A, microtubular density; in B, microtubulesper axon. Abscissas: cross sectional area of axons. (0.1µm2 interval). Symbols indicateexperimental condition. Values are mean plus or minus SEM. In control axons, the microtubulardensity exhibits an inverse correlation with the size of the axon (A) despite the increase of thenumber of microtubules with the size (B). Actinomycin destroys the Schwann cell and themicrotubular density becomes constant and adjusted to the uppermost value observed in controlaxons (the smallest class). Cycloheximide reduces the microtubular density across the wholespectrum in the same proportion (A, B). Modified from Bustos et al., 1991.


microtubular content increases in such away that the correlation density versus sizeremains unaffected by myelination(Hernández et al., 1989).

The microtubules and size of axons alsoare affected by electrical activity. A highand prolonged discharge increases bothmicrotubular content and caliber of axons(Álvarez and Ramírez, 1979; Álvarez et al.,1982; Vergara et al., 1992).

Summing up, the fine anatomy of theaxoplasm varies in relation to its historyand its neighbors. Connections in thenervous system are often described as“wiring” or “circuitry.” These words takenfrom electrical engineering are misleadingas they connote that the elements arepassive and static whereas the biological“wire” (axon) is a dynamic element thatchanges its properties in relation to use andenvironmental cues.

SPROUTING PROGRAM OF AXONS

An outstanding feature of neurons is thatthey extend a process, the axon, which mayattain enormous lengths. In largevertebrates, a nerve cell may be severalmeters long. This process elongates,decides its trajectory, i .e., navigates,branches at certain points and, finally,contacts a target. Development andregeneration are the conditions most used tostudy phenomena relating to growth in vivo.

We observed that uninterrupted axons ofa peripheral nerve extended sprouts when theSchwann cell was destroyed selectively withactinomycin D (Bustos et al., 1991). Both,myelinated (Fig 3C) and unmyelinated axonssprouted. From this, we conclude that theaxon has an intrinsic and distributedsprouting program, i.e., the ability to extendsprouts anywhere when the appropriateconditions arise, that is repressed locally,tentatively, by the Schwann cell. In theinitial sprouting response, the cell body isnot specifically involved.

In our studies with extracellularinhibitors of serine proteases, we observeda sprouting response of unmyelinated andmyelinated axons (Álvarez et al., 1992;Moreno et al., 1996). In addition, Schwann

cells of the treated segment proliferated(Fig 4A) and destroyed their myelin(Álvarez et al., 1995). Interestingly, axonssprouted even before the destruction of themyelin sheath (Fig 4B). We conjecturedthat the differentiated Schwann cellrepresses the sprouting program, while itbecomes permissive upon proliferation ordedifferentiation.

In a further step, we considered nerveregeneration under the notions of thesprouting program and its regulation bySchwann cel ls . A severed nerveregenerates, i.e., the blind end of axons,extend sprouts that lengthen and mature toeventually re-establish the anatomical andfunctional connections. Here, we will beconcerned with the initial event, theextension of sprouts. The growth of axonsfollowing a nerve crush presents a delay of1-2 days. This delay has been ascribed tothe time taken by signals going from thelesion up to the cell body, which developsin turn a regenerative response. We willshow that this view is l ikely to beincorrect.

We conjectured that the delay ofregeneration could be accounted for by theinterplay of the sprouting program ofproximal axons and repressive action of theterritory to be invaded. In fact, once thenerve is severed, the widow Schwann cellstake their t ime to proliferate anddedifferentiate. In this scenario, the delayof regeneration could reflect the fadingaway of the repressive action of Schwanncells. Thus, we performed a crush on asegment already treated with proteaseinhibitors to induce proliferation ofSchwann cells and found that the ensuingregeneration had no delay. The same wasobserved when the distal segment wasfrozen to destroy all cells in the area (Tapiaet al., 1995). Interestingly, the rate ofaxonal elongation remained normal. Theseresults indicate that the delay ofregeneration correlates with the conditionof the environment hence the involvementof the cell body becomes an unnecessaryconstruct to account for this delay. On theother hand, the rate of axonal growth,which does belong to the biology of thenerve cell, is unaffected.


In the Wallerian degeneration slow(Wlds) mice, severed axons present aprotracted survival or prolonged delay ofdegeneration, 3-4 weeks, rather than theusual 1-2 days, and regeneration also isdelayed (Lunn et al., 1989). We consideredthe nerve of the Wlds strain under thenotion of the axonal sprouting programand its repression by Schwann cells. Inthis scenario, the protracted survival ofsevered Wlds axons prevent thededifferentiation of the Schwann cells ofthe distal stump, whereby they remainrepressive for the in-growth of axons. Wetested this conjecture by crushing a nerveacross a segment whose Schwann cellswere previously destroyed withactinomycin D. Regeneration of the Wlds

nerve normalized (Court and Álvarez,2000). Thus in the Wlds s train, theprolonged delay of regeneration is aconsequence of the abnormal environmentoffered to regrowing axons, while theability of neurons to grow seems normal.

This far, the sprouting program emergesas an attribute intrinsic to the axon, whichis launched as soon as the environmentbecomes permissive. Since the cell bodyseems unnecessary to trigger the sproutingresponse of axons, we conjectured, thus,that axons fully disconnected from their cellbodies should sprout at their blind end, asthey embody a sprouting program ready togo. However, ordinary axons degenerate ina day or two after a lesion, which precludesan experimental study of their ability togrow. In contrast, a Wlds nerve, owing toits long survival in situ after a lesion, is asuitable model. In the sciatic nerve of Wlds

mice, a piece of nerve was excised and thedistal stump was fitted with a 5mm crush,thus, the distal nerve had two domains insuccession: (i) the extended crushed zone,which contained no living Schwann cells,hence it was a territory permissive foraxonal growth; followed by (ii) thesurviving axons, which embodied thesprouting program (Fig 5, diagram). A few

Figure 3. Microtubules and sprouts of nerve fibers treated with actinomycin D and taxol. Diagramshows the actors: The Schwann cell is affected by actinomycin including its regulatory activity (↓).Microtubules, stabilized by taxol, accumulate, calling for a source of microtubular proteins. A.Control unmyelinated bundle: Schwann cell processes surround every axon. Arrow points to amicrotubule. B. Unmyelinated bundle: Actinomycin destroyed the Schwann cell (debrisinterspersed between axons; compare with control Schwann cells in A); axons touch each otherextensively, and microtubules (arrow) are abundant (compare with A). C. Actinomycin destroyedthe Schwann cell; the myelin sheath, though, remains as there is no Schwann cell to digest it; acrown of sprouts lies between myelin and basal lamina (arrowheads); microtubules (thick arrow)are abundant in the axon; notice a lipid droplet in the extracellular space (thin arrow). D.Unmyelinated bundle after treatment with taxol. Microtubules are crowded in the axoplasm to thepoint of coalescence. Arrowheads point to Schwann cell processes. Calibration, 0.5µm. Modifiedfrom Bustos et al., 1991.


days later, in the extended crush, sproutswere observed with the electronmicroscope, horseradish peroxidaseadministered to the surviving axons madeits way to the sprouts, and DiI, a dye thatdoes not leave membranes, stainedcontinuously from the site of application tothe blind end of axons and to the sproutsemerging from them (Fig 5) (Iñiguez andÁlvarez, 1999). Since these axons weredisconnected from their cell bodies, theoccurrence of sprouts shows beyond anyreasonable doubt that axons have a built-insprouting program that is triggered whenthe external conditions are appropriated.

In our view, nerve regeneration is aparticular case in which the sproutingprogram of axons is expressed. Theinterplay of the sprouting program of axonsand the regulatory action of glial cells may

lead to a wide range of manifestations inphysiological conditions and pathologicaldisorders (cf Schmidt et al., 1996; Nien etal., 1998; Court, 2004).

PROTEIN SYNTHESIS IN AXONS

The phenotype of a cell is the macroscopicarray of proteins and othermacromolecules. The fact that thephenotype of axons is locally controlledraises the question, among others, for thesource of the requisite proteins. There aretwo competing views concerning thesource of axonal proteins. The first is thetextbook notion that the cell body suppliesmost, if not all, proteins to the axon by aslow transport mechanism. This view isuntenable unless ad hoc assumptions are

Figure 4. Effect of aprotinin, a protease inhibitor, on axons and Schwann cells. Diagram shows theactors. Aprotinin induce Schwann cells to proliferate and dedifferentiate while the axon extendssprouts. The reciprocal regulation between axon and Schwann cell is lost. A. Myelinated fiber. Inthe cytoplasm of the Schwann cell, the dark masses are chromosomes, i.e., the cell is proliferatingbut its myelin sheath is still intact. B. Myelinated fiber. The axon inside the myelin sheath looksnormal, while a second axonal profile with a considerable number of mitochondria, a sprout of theformer, lies between the myelin and the basal lamina. Modified from Álvarez et al., 1995.


Figure 5. Centralward growth of decentralized Wlds axons. Diagram is experimental condition. Cellbody is to the left; (g) gap in the nerve; broken line is a 5mm nerve crush (c); continuous line,undamaged nerve. Arrow indicates the site illustrated by the micrograph, the boundary betweencrushed and undamaged segments. Nerve examined 5 days after the lesion. DiI was administered toa fresh cut through the undamaged segment after fixation. Inset, whole nerve, extended crush is tothe left; calibration, 0.5mm. Micrograph shows a surviving fiber stained up to the blind end and 3thin sprouts (arrows) that emerge from it. Below, sprouts also are present (arrow), but themicrograph does not show the parent axon; calibration, 50µm. Modified from Iñiguez and Álvarez,1999.

added t ime and again. For example,transported proteins should spend years intransit , t ime enough for completedegradation before reaching theirdestination (Álvarez and Torres, 1985). Theother view claims that the axoplasmsynthesizes its own proteins, a view largelyignored –despite the evidence accumulatedfor fifty years– on the ground thatribosomes have not been observed in theaxoplasm. (For a thorough discussion ofthese views, see Álvarez et al., 2000.)

We have shown above that themicrotubular content may increase in arestricted segment, without modification ofthe content in the adjacent segments. Since

the slow transport view allows only forlocal redistribution of existing proteins,which was not observed, we explored thepossibility of local synthesis of proteins asa source to account for these observations.The local administration of two proteinsynthesis inhibitors, cycloheximide andemetine, to a short span of nerve reduced byhalf the microtubular content within a week(Fig 2), while the adjacent segmentsremained normal. Moreover, this reductionwas reversible (Bustos et al. , 1991).Together, the studies on axonalmicrotubules strongly suggest that they arelocally controlled through regulation of thelocal supply of protein.


Studying Wlds nerves sectioned for overa week under the electron microscope, wefound unexpectedly enormous amounts ofribosome-like particles embedded in theaxoplasm (Álvarez and Court, 2000). Theseparticles had the size and shape ofribosomes and were observed dispersed inthe axoplasm, arrayed in strings aspolyribosomes do, attached to membranouscisternae resembling rough endoplasmicreticulum, crowded in membranous sacs(Fig 6A); finally, they incorporated labeleduridine administered after the lesion(Álvarez and Court, 2000; Álvarez, 2001).Further studies with probes for specificribosomal constituents confirmed theribosomal nature of these particles (Fig 6B;Court, Álvarez and van Minnen,unpublished). Therefore, axons do containribosomes. The cell body could not supplythese ribosomes since the nerve wassevered while a local RNA source wassupported positively by their labeling with aradioactive uridine.

At this point, we want to mention twoconditions in which the bulk of the

axoplasm increases and its correlation tolocal synthesis of protein. The elongation ofregenerating axons, which imply accretionof axoplasm, is impaired by local inhibitionof protein synthesis (Gaete et al., 1998). Wementioned (see above) that hyperactivityincreases size and microtubular content ofaxons; it also increases the incorporation ofamino acids into the axoplasm (Eugeninand Álvarez, 1995). This correlationsuggests that protein synthesis in axonsmay be controlled, not only locally, but alsoalong its entire length.

The axoplasm has been shown already tocontain tRNAs and mRNAs and toincorporate amino acids into proteins (cfÁlvarez et al., 2000) and now ribosomes,hence axons have the machinery for proteinsynthesis. The transfer of RNAs proposedhere sets a new frame for understanding thebiology of the nervous system. It becomesclear that the phenotype of the axon isspecified incompletely by the geneticprograms of the neuron; it has to be fine-tuned by the associated glial cells. Proteinsynthesis in the axoplasm becomes a

Figure 6. Ribosomes in Wlds axons decentralized for a week. A. Electron micrograph of amyelinated axon. Myelin and axoplasm are well preserved. In the cortical zone of the axoplasm,abundant particles are present. The morphological features correspond to that of ribosomes. Upperinset, a polyribosome. Lower inset, a membranous sac loaded with particles. Calibration, 0.1µm;upper inset, 50nm; lower inset; 0.1µm. B. A myelinated axon stained with an antiserum anti-Pprotein of ribosomes (red), antibody anti-P0 protein of the myelin (green), and antibody anti-neurofilament (blue). In the longitudinal axon above, many ribosomal clusters (red puncta) are seenin a field of neurofilament (blue). Lower panels are Z projections of the longitudinal axon at thecorresponding numbers. They show in the transverse plane that the ribosomes (red puncta) are alsoin a field of neurofilaments (blue), with a preference for the cortex of the axoplasm (Court, vanMinnen, unpublished). Calibration, 5mm.


pivotal mechanism. We have coined theword “merotrophism” (meros, part) todenote the regulation of the phenotype of apiece of cytoplasm without involvement ofthe nucleus (Álvarez et al., 2000), asopposed to “trophism.” Activity has beenshown to be a regulator of the anatomy ofthe axon and we add merotrophism. Weforesee that both will be important for theunderstanding of the physiology as well asthe degenerative disorders of the nervoussystem and will shed light onto theunexplained functional recoveries followinglesions of the central nervous systems.

ACKNOWLEDGEMENTS

We dedicate this article to Dr. PatricioZapata. FAC holds an EMBO long-termfellowship.

REFERENCES

ÁLVAREZ J (2001) The autonomous axon: A model basedon local synthesis of proteins. Biol Res 34: 103-109

ÁLVAREZ J, COURT F (2000) Sprouting, growth andregeneration: A novel model of the axon. J Physiol523P: 51-52

ÁLVAREZ J, RAMÍREZ BU (1979) Axonal microtubules:Their regulation by the electrical activity of the nerve.Neurosci Lett 15: 19-22

ÁLVAREZ J, TORRES JC (1985) Slow axoplasmictransport: A fiction? J Theor Biol 112: 627-651

ÁLVAREZ J, ZAROUR J (1983) Microtubules in short andlong axons of the same caliber: Implications for themaintenance of the neuron. Exp Neurol 79: 283-286

ÁLVAREZ J, ARREDONDO F, ESPEJO F, WILLIAMS V(1982) Regulation of axonal microtubules: Effect ofsympathetic hyperactivity elicited by reserpine.Neuroscience 7: 2551-2559

ÁLVAREZ J, MORENO RD, LLANOS O, INESTROSANC, BRANDAN E, COLBY T, ESCH FS (1992)Axonal sprouting induced in the sciatic nerve by theamyloid precursor protein (APP) and otherantiproteases. Neurosci Lett 144: 130-134

ÁLVAREZ J, MORENO RD, INESTROSA NC (1995)Mitosis of Schwann cells and demyelination areinduced by the amyloid precursor protein and otherprotease inhibitors in the rat sciatic nerve. Eur JNeurosci 7: 152-159

ÁLVAREZ J, GIUDITTA A, KOENIG E (2000) Proteinsynthesis in axons and terminals: Significance formaintenance, plasticity and regulation of phenotype.With a critique of slow transport theory. Progress inNeurobiology 62: 1-62

BUSTOS J, VIAL JD, FAÚNDEZ V, ÁLVAREZ J (1991)Axonal microtubules decrease after local inhibition ofprotein synthesis and increase after inhibition oftranscription: A morphometric study in rat suralnerves. Eur J Neurosci 3: 1123-1133

COURT FA (2004) Cellular organisation of peripheralnerve and the neuromuscular junction. Doctoral thesis,Edinburgh University, Edinburgh, UK

COURT F, ÁLVAREZ J (2000) Regeneration of Wlds

nerves is normalized by actimomycin D. BrainResearch 867: 1-8

COURT FA, SHERMAN DL, PRATT T, GARRY EM,RIBCHESTER RR, COTTRELL DF, FLEETWOOD-WALKER SM, BROPHY PJ (2004) Restricted growthof Schwann cells lacking Cajal bands slows conductionin myelinated nerves. Nature 431(7005): 191-5

ESPEJO F, ÁLVAREZ J (1986) Microtubules and calibersin normal and regenerating axons of the sural nerve ofthe rat. J Comp Neurol 250: 65-72

EUGENÍN J, ÁLVAREZ J (1995) Incorporation of aminoacids into the axoplasm is enhanced by electricalstimulation of the fiber. Brain Res 677: 319-325

FADIC R, VERGARA J, ÁLVAREZ J (1985) Microtubulesand caliber of central and peripheral processes ofsensory axons. J Comp Neurol 236: 258-264

FAÚNDEZ V, ÁLVAREZ J (1986) Microtubules andcalibers in developing axons. J Comp Neurol 250: 73-80

FAÚNDEZ V, CORDERO ME, ROSSO P, ÁLVAREZ J(1989) Calibers and microtubules of nerve fibers:Differential effect of undernutrition in developing andadults rats. Brain Res 509: 198-204

GAETE J, KAMEID G, ÁLVAREZ J (1998) Regeneratingaxons of the rat require a local source of proteins.Neurosci Lett 251: 197-200

HERNÁNDEZ C, BLACKBURN E, ÁLVAREZ J (1989)Calibre and microtubules of the nonmedullated andmyelinated domains of optic nerve axons. Eur JNeurosci 1: 654-658

IÑIGUEZ A, ÁLVAREZ J (1999) Isolated axons of Wld(s)mice regrow central ward. Neurosci Lett 268: 108-110

LLADOS F, ZAPATA P (1974) Effects of blockers ofprotein synthesis upon fibrillation due to denervation.Acta Physiol Lat Amer 24: 48-56

LÓPEZ JMA, ÁLVAREZ J (1990) The microtubularpattern changes at the spinal cord-root junction andreverts at the root-peripheral nerve junction in sensoryand motor fibres of the rat. Eur J Neurosci 2: 873-878

LUNN ER, PERRY VH, BROWN MC, ROSEN H,GORDON S (1989) Absence of Wallerian degenerationdoes not hinder regeneration in peripheral nerve fibres.Eur J Neurosci 1: 27-33

MORENO RD, INESTROSA NC, CULWELL AR,ÁLVAREZ J (1996) Sprouting and abnormal contacts ofnonmedullated axons, and deposition of extracellularmaterial induced by the amyloid precursor protein (APP)and other protease inhibitors. Brain Res 718: 13-24

NIEN JK, SCHMIDT J, CARTIER L, ÁLVAREZ J (1998)Cerebrospinal fluid of HTLV-1 associated myelopathypatients induces axonal sprouting and Schwann cellproliferation in the rat sciatic nerve. J Neurol Sci 159:17-24

SAITUA F, ÁLVAREZ J (1988) Do axons grow duringadulthood? A study of caliber and microtubules in suralnerve axons of young, mature and aging rats. J CompNeurol 269: 203-209

SAITUA F, ÁLVAREZ J (1989) Microtubular packingvaries along the course of motor and sensory axons:Possible regulation of microtubules by environmentalcues. Neurosci Lett 104: 249-252

SCHMIDT J, NIEN JK, SCHERSON A, CAMPOS EO,CARTIER L, ÁLVAREZ J (1996) Segmentaldemyelination induced by cerebrospinal fluid ofprogressive spastic paraparesis: correlation with alteredproteolytic parameters. Neurosci Lett 214: 1-4


SERRA M, ÁLVAREZ J (1989) On the asymmetry of theprimary branching of vagal sensory axons: Possiblerole of the supporting tissue. J Comp Neurol 284: 108-118

TAPIA M, INESTROSA NC, ÁLVAREZ J (1995) Earlyregeneration: Repression by Schwann cells and aprotease. Exper Neurol 131: 124-132

VERGARA J, REPETTO G, ÁLVAREZ, J (1992) Theaxonal microtubular density is higher than normal infibres innervating spastic muscles. J Submicrosc CytolPathol 24: 129-134

VERGARA J, SERRA J, SAITUA F, ITURRIAGA R,

ÁLVAREZ J (1991) Axonal microtubules:Comparative anatomy in vertebrates, including man. JSubmicrosc Cytol Pathol 23: 357-363

WINDEBANK AJ, WOOD P, BUNGE RP, DYCK PJ(1985) Myelination determines the caliber of dorsalroot ganglion neurons in culture. J Neurosci 5: 1563-1569

YOKOTA R (1984) Occurrence of long non-myelinatedaxonal segments intercalated in myelinated,presumably sensory axons: Electron microscopicobservations in the dog atr ial endocardium. JNeurocytol 13: 127-143

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365-374, 2005 Local regulation of the axonal phenotype, a