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Unmyelinated corticospinal axons in adult rat pyramidal tract. An electron microscopic tracer study
Brain Research, 459 (1988) 173-177
173
Elsevier BRE 23069
Unmyelinated corticospinal axons in adult rat pyramidal tract. An electron microscopic tracer study E.A.J. Joosten and A . A . M . Gribnau Department of Anatomy and Embryology, Facultyof Medicine, University of Nijmegen, Nijmegen (The Netherlands) (Accepted 11 May 1988)
Key words: Pyramidal tract; Corticospinal tract; Unmyelinated axon; Anterograde tracing; Rat
The aim of the present study was to provide experimental ultrastructural evidence for a corticospinal component in the adult rat pyramidal tract (PT). For this purpose, the entire sensorimotor and frontal cortex of the left hemisphere was labelled using the anterograde tracer horseradish-peroxidase (HRP). Six months old rats were sacrificed 24 or 48 h after implantation of 6-8 HRP-gels. The detection of anterogradely transported HRP at the cervical as well as the lumbar intumescence was carried out as described earlier (J. Histochem. Cytochem., 35 [1987] 623-626). Our results demonstrate the occurrence of labelled myelinated as well as labelled unmyelinated axons within the adult rat PT at both spinal cord levels analyzed. This implicates that at least part of the unmyelinated profiles in the adult rat PT belong to fibres originating in the cortex and therefore must be irrterpreted as corticospinal axons. The findings are discussed in the light of their physiological significance.
The p y r a m i d a l tract (PT) is a m a j o r descending fibre tract of the central nervous system and is known to be an i m p o r t a n t m o t o r pathway, which is only present in mammals. It is defined as a set of fibres passing through the m e d u l l a r y pyramids. O n e of its m a j o r c o m p o n e n t s is the corticospinal tract (CST). The CST in the rat is a substantial pathway which, after passing through the m e d u l l a r y pyramids, decussates and subsequently proceeds via the contralateral dorsal column through the whole length of the spinal cord 3'7'8'27. The PT has been shown to contribute to the control of fine m a n i p u l a t o r y m o v e m e n t s of the hand in primates 1,1°,18. I m p a i r m e n t of m o t o r function of the forepaw has been d e m o n s t r a t e d in rats and hamsters following PT lesions 4'14. Electron microscopical observations provided evidence that the PT contains a substantial a m o u n t of unmyelinated profiles, not only in rodents 17,19,26, but also in cat 3° and m o n k e y 29, although the presence of unmyelinated axons in the m o n k e y is questioned by Ralston et al. 25. L e e n e n et al. 2° counted a total number of 43,000 + 2,000 m y e l i n a t e d and 35,000 +
8,000 unmyelinated fibres at the level of the second cervical segment in the rat; at the second sacral segment 2,800 u n m y e l i n a t e d fibres could still be detected 5. The site of their originating neurones still is a matter of speculation and therefore the object of the present study. Three Wistar rats aged 6 months (weighing approximately 250 g) were anesthetized by i.p. injections with Nembutal. The entire s e n s o r i m o t o r and frontal cortex of the left hemisphere was labelled by implantation of 6 - 8 H R P - g e l s 9. Using this technique, a d e p o t of tracer is established for p r o t r a c t e d H R P transport and therefore its enhanced detection along the length of the axons. On the other hand, for quantitative studies, the use of the more sensitive tracer wheat germ agglutinin-conjugated horseradish peroxidase ( W G A - H R P ) is p r o b a b l y m o r e appropriate in o r d e r to label greater numbers of axons. The postimplantation survival times were either 24 or 48 h. A f t e r reanaesthetization, the animals were transcardially perfused with 50 ml 5% sucrose in phosphate buffer (PB) ( p H 7.2), followed by 1%
Correspondence: E.A.J. Joosten, Dept. of Anatomy and Embryology, Faculty of Medicine, University of Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. 0006-8993/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
174 paraformaldehyde and 2% glutaraldehyde in the same buffer. A f t e r perfusion, the brains and spinal cords were immediately r e m o v e d and the spinal cords were further i m m e r s e d for about 1 h before being transferred into cold (4 °C) PB with 5% sucrose. Then 100-#m vibratome sections of the cervical (C5/C6) and lumbar (L2/L3) intumescence were cut' transversely on an Oxford Vibratome. A n t e r o g r a d e l y transported H R P was visualized with the use of the chromogen t e t r a m e t h y l b e n z i d i n e (TMB) in combination with the stabilizing agent amm o n i u m h e p t a m o l y b d a t e ( A H M ) in 0.1 M PB at p H 6.0, as introduced by Olucha et al. 2~ for light microscopy and a d a p t e d for electronmicroscopy 1:. In short, the vibratome sections were p r e s o a k e d for 20 rain in a mixture containing 0.005% TMB (dissolved in absolute ethanol) and 0.25% A H M in 0.1 M PB (pH 6.0). The incubation was started by the addition of 50 ~1 of 33% H2O 2 per 100 ml pre-incubation bath, which was r e p e a t e d every 5 rain for 20 rain. Control sections were processed identically, but the incubation was carried out without TMB. A f t e r the H R P T M B - ( A H M ) reaction, osmication was carried out using 1% OsO4 solution in 0.1 M PB (pH 5.0) for 4 h at room temperature. Osmication was followed by an accelerated dehydration and subsequent e m b e d d i n g in Epon. Transverse or longitudinal ultrathin sections were mounted on 75-mesh formvar ( 0 . 8 % ) - c o a t e d grids contrasted with uranylacetate (20 rain) and lead citrate (5 min) respectively, and were studied in a Philips EM-300 at an accelerating voltage of 60 kV. O u r results d e m o n s t r a t e that the adult rat PT contains many H R P - T M B - ( A H M ) labelled myelinated and a number of unmyelinated axons originating in the cortex, at both the cervical as well as the lumbar intumescence (Figs. 1-4). The occurrence of labelled profiles is restricted to the PT-area of the dorsal fu-
niculus. The results obtained after 24 h survival time were identical to those obtained after 48 h, at least as far as the white matter is concerned. Control sections were always negative. A t the ultrastructural level, the H R P - T M B ( A H M ) reaction product generates a clearly visible and highly discernable intracellular crystalline marker, which makes the identification of H R P - l a b e l l e d fibres rather easy (Figs. 1-4). Besides, TMB proved to be the most sensitive chromogen available for the demonstration of H R P labelling 22. Longitudinal sections clearly d e m o n s t r a t e the uneven distribution of the reaction product along myelinated as well as unmyelinated axons (Fig. 4). Because of the uneven distribution of H R P - T M B - ( A H M ) crystals in labelled profiles, quantification of the results should be carried out very carefully. Transversely sectioned unlabelled profiles might contain the H R P - T M B ( A H M ) reaction product at more distal or proximal levels (Fig. 4). Because of this uneven distribution pattern, it is very easy to underestimate the quantity of labelled unmyelinated CST axons. Whereas the diameters of the labelled unmyelinated corticospinal axons remain fairly constant at about 0.2/~m, labelled myelinated CST axons vary considerably with respect to their diameters, namely between 0.5 and 3/~m. (Figs. 1 and 3). Although previous studies 5'17A9'2° d e m o n s t r a t e d the presence of n u m e r o u s u n m y e l i n a t e d profiles in the adult rat PT at several spinal cord levels, the origin, destination and function of these profiles are still unknown. Based on longitudinal sections, Ralston et al. 25 concluded that most of the profiles which might be interpreted as unmyelinated axons in the primate PT at medullary levels are actually astroglial processes. They stated that less than 1% of the PT in the old world adult primate are unmyelinated axons. O u r results d e m o n s t r a t e that at least part of the unmy-
Fig. 1. Cervical intumescence; 48 h survival time. The labelled CST comprises myelinated as well as unmyelinated axons (arrowhead). Note the typical crystalline appearance of the HRP-TMB-(AHM) reaction product (arrows). Transverse section. Bar = I urn. Fig. 2. Cervical intumescence; 48 h survival time. Detail of Fig. 1. Transverse section. Bar = 0.5 ~m. Fig. 3. Lumbar intumescence; 24 h survival time. Detail of a labelled unmyelinated axon (arrowhead) surrounded by labelled myelinated axons (arrows). Transverse section. Bar = 0.5 ~m. Fig. 4. Lumbar intumescence; 48 h survival time. Longitudinal section of labelled unmyelinated axons. Note the uneven distribution of HRP-TMB-(AHM) crystals within the unmyelinated axons. Bar = 0.5urn
t.,sl
om~
176 elinated axons in the adult rat PT have their origin in the cortex.
elinated axons, which either reach out into the spinal gray or, being recurrent collaterals, might even re-
From physiological studies on the conduction velo-
turn to more proximal destinations. On the other hand, the unmyelinated CST axons might also represent axons which have not yet acquired myelin sheaths. This would implicate that myelination of CST axons still occurs at a low level during adult-
cities of PT axons it was concluded that all measurements are within the range of myelinated axons 21. Based on their different biophysical properties, their different connections as well as their varying conduction velocities, PT cells are subdivided into two distinct populations6"Z8: first, slow-conducting PT cells with conduction velocities below 21 m/s (the slowest velocities reported are 4 - 6 m/s), and second, fast PT cells whose axons conduct at velocities of 2 1 - 9 0 m/s. Fast-conducting PT neurones discharge phasically and are involved in the initiation and control of brief, quick movements. O n the other hand, slowly conducting PT neurones discharge in tonic phase, while
hood, because myelination appears to be completed at postnatal day 28 at the cervical as well as at the lumbar intumescence 13. Unilateral pyramidal lesions in monkeys revealed that recovery occurs by local sprouting of CST fibres of the intact bundle into contralateral areas of spinal neurones with severed homolateral CST connections 15. Probably unmyelinated CST axons in the adult PT account for this recovery function.
they are implicated in the determination of muscle tone and control of small, fractioned movements 2'
A recent study on the corticospinal projection neurones in adult rats 23 using retrograde HRP-tech-
~x.3~. Although L e e n e n et al. 2° suggested that the unmyelinated axons in the adult rat PT might have a
niques revealed, besides great concentrations of HRP-positive neurones in the sensorimotor cortex,
similar function as the slowly conducting PT neurones, the conduction velocities measured 21'28 are not in the range to be expected for u n m y e l i n a t e d axons. Remarkably, even the slowest conduction velo-
less dense concentrations in anterior cingulate and prefrontal cortical areas. Neurones from the latter regions might also account for the u n m y e l i n a t e d CST projections. Hence, we are currently dealing with the ultrastructural visualization of CST axons in the adult rat PT, with their originating n e u r o n e s in (pre-) fron-
cities reported ( 4 - 6 m/s) 6'16"21'28 do not correspond with the category of myelinated axons with fibre diameters of 0 . 5 - 1 . 0 # m in the rat PT 7. Possibly, the u n k n o w n mode of collateralization of CST axons may hamper an accurate determination of conduction velocities. The u n m y e l i n a t e d CST axons in the adult rat PT may, at least in part, represent collaterals of my-
1 Beck, C.H. and Chambers, W.W., Speed, accuracy and strength of forelimb movement after unilateral pyramidotomy in rhesus monkeys, J. Comp. Physiol. Psychol., 70 (1970) 1-22. 2 Biedenbach, M.A., DeVito, J.L. and Brown, A.C., Pyramidal tract of the cat: axon size and morphology, Exp. Brain Res., 61 (1986) 303-310. 3 Brown Jr., L.T., projections and terminations of the corticospinal tract in rodents, Exp. Brain Res., 13 (1971) 432-450. 4 Castro, A.J., Motor performance in rats. The effects of pyramidal tract section, Brain Research, 44 (1972) 313-323. 5 Chung, K. and Coggeshall, R.E., Postnatal development of the rat dorsal funieulus, J. Neurosci., 7 (1987) 972-977. 6 Deschenes, M., Labelle, A. and Landry, P., Morphological characteristics of slow and fast PT-cells in the cat, Brain Research, 178 (1979) 251-274. 7 Dunkerley, G.B. and Duncan, D., A light and electron microscopic study of the normal and degenerating cortico-
tal cortex areas.
We are grateful for technical and photographic assistance from Jos D e d e r e n and Theo Hafmans, respectively.
8
9 I0
11
12
spinal tract in the rat, J. Comp. Neurol., 137 (1969) 155-184. Gribnau, A.A.M., de Kort, E.J.M., Dederen, P.J.W.C. and Nieuwenhuys, R., On the development of the pyramidal tract in the rat. II. An anterograde tracer study of the outgrowth of the corticospinal fibers, Anat. Embryol., 175 (1986) 101-110. Griffin, G., Watkins, L.R. and Mayer, D.J., HRP pellets and slow-release gels: two techniques for greater localization and sensitivity, Brain Research, 168 (1979) 595-601. Hepp-Reymond, M.C.M., Wiesendanger, M., Brumert, A., Mackel, A., UngeL R. and Wespi, J., Effects of unilateral pyramidotomy on conditioned finger movement in the monkey, Brain Research, 24 (1970) 544-551. Humphrey, D.R. and Corrie, W.S., Properties of the pyramidal tract neuron system within a functionally defined subregion of primate motor cortex, J. Neurophysiol., 41 (1978) 216-243. Joosten, E.A.J., Gribnau, A.A.M. and Dederen. P.J.W.C.,
177 Ultrastructural visualization of anterogradely transported horseradish peroxidase in the developing corticospinal tract of rat, J. Histochem. Cytochem., 35 (1987) 623-626. 13 Joosten, E.A.J., Gribnau, A.A.M. and Dederen, P.J.W.C., Postnatal development of the corticospinal tract in the rat. An ultrastructural anterograde HRP study, submitted. 14 Kalil, K. and Schneider, G.E., Motor performance following unilateral pyramidal tract lesions in the hamster, Brain • Research, 100 (1975) 170-174. 15 Ku~era, P. and Wiesendanger, M., Do ipsilateral corticospinal fibers participate in the functional recovery following unilateral pyramidal lesions in monkeys?, Brain Research,. 348 (1985) 297-303. 16 Landry, F., Wilson, C.J. and Kitai, S.T., Morphological and electrophysiological characteristics of pyramidal tract neurons in the rat, Exp. Brain Res., 57 (1984) 177-190. 17 Langford, L.A. and Coggeshall, R.E., Unmyelinated axons in the posterior funiculi, Science, 211 (1981) 176-177. 18 Lawrence, D.G. and Kuypers, H.G.J.M., The functional organization of the motor system in the monkey. I. The effects of bilateral pyramidal lesions, Brain, 91 (1968) 1-14. 19 Leenen, L., Meek, J. and Nieuwenhuys, R., Unmyelinated fibers in the pyramidal tract of the rat: a new view, Brain Research, 246 (1982) 297-301. 20 Leenen, L., Meek, J., Posthuma, P.R. and Nieuwenhuys, R., A detailed morphometrical analysis of the pyramidal tract of the rat, Brain Research, 359 (1985) 65-80. 21 Mediratta, N.K. and Nicoll, J.A.R., Conduction velocities of corticospinal axons in the rat studied by recording cortical antidromic responses, J. Physiol. (Lond.), 336 (1983) 545-561. 22 Mesulam, M.-M. and Rosene, D.L., Sensitivity in horse-
23 24
25
26
27
28
29
30
31
radish peroxidase neurochemistry: a comparative and quantitative study of nine methods, J. Histochem. Cytochem., 27 (1979) 763-773. Miller, M.W., The origin of corticospinal projection neurons in rat, Exp. Brain Res., 67 (1987) 339-351. Olucha, F., Martinez-Garcia, F. and L6pez-Garcia, C., A new stabilizing agent for the tetramethylbenzidine (TMB) reaction product in the histochemical detection of horseradish peroxidase (HRP), J. Neurosci. Methods, 13 (1985) 131-138. Ralston, D.D., Milroy, A.M. and Ralston III, H.J., Nonmyelinated axons are rare in the medullary pyramids of the macaque monkey, Neurosci. Len., 73 (1987) 215-219. Reh, T. and Kalil, K., Development of the pyramidal tract in the hamster. II. An electron microscopic study, J. Comp. Neurol., 205 (1982) 77-88. Schreyer, D.J. and Jones, E.G., Growth and target finding by axons of the corticospinal tract in prenatal and postnatal rats, Neuroscience, 7 (1982) 1837-1853. Takahashi, K., Slow and fast groups of pyramidal tract cells and their respective membrane properties, J. Neurophysiol., 28 (1965) 908-924. Thomas, A.P., An ultrastructural and morphometric study of unmyelinated axons in the pyramidal tract of monkeys, Soc. Neurosci. Abstr., 373 (1985) 18. Thomas, A., Westrum, L.E., DeVito, J.C. and Biedenbach, M.A., Unmyelinated axons in the pyramidal tract of the cat, Brain Research, 301 (1984) 162-165. Wiesendanger, M., The pyramidal tract: its structure and function. Handbook of Behavioral Neurobiology, Vol. 5, Motor Coordination, Plenum, New York, 1981, pp. 401-492.
173
Elsevier BRE 23069
Unmyelinated corticospinal axons in adult rat pyramidal tract. An electron microscopic tracer study E.A.J. Joosten and A . A . M . Gribnau Department of Anatomy and Embryology, Facultyof Medicine, University of Nijmegen, Nijmegen (The Netherlands) (Accepted 11 May 1988)
Key words: Pyramidal tract; Corticospinal tract; Unmyelinated axon; Anterograde tracing; Rat
The aim of the present study was to provide experimental ultrastructural evidence for a corticospinal component in the adult rat pyramidal tract (PT). For this purpose, the entire sensorimotor and frontal cortex of the left hemisphere was labelled using the anterograde tracer horseradish-peroxidase (HRP). Six months old rats were sacrificed 24 or 48 h after implantation of 6-8 HRP-gels. The detection of anterogradely transported HRP at the cervical as well as the lumbar intumescence was carried out as described earlier (J. Histochem. Cytochem., 35 [1987] 623-626). Our results demonstrate the occurrence of labelled myelinated as well as labelled unmyelinated axons within the adult rat PT at both spinal cord levels analyzed. This implicates that at least part of the unmyelinated profiles in the adult rat PT belong to fibres originating in the cortex and therefore must be irrterpreted as corticospinal axons. The findings are discussed in the light of their physiological significance.
The p y r a m i d a l tract (PT) is a m a j o r descending fibre tract of the central nervous system and is known to be an i m p o r t a n t m o t o r pathway, which is only present in mammals. It is defined as a set of fibres passing through the m e d u l l a r y pyramids. O n e of its m a j o r c o m p o n e n t s is the corticospinal tract (CST). The CST in the rat is a substantial pathway which, after passing through the m e d u l l a r y pyramids, decussates and subsequently proceeds via the contralateral dorsal column through the whole length of the spinal cord 3'7'8'27. The PT has been shown to contribute to the control of fine m a n i p u l a t o r y m o v e m e n t s of the hand in primates 1,1°,18. I m p a i r m e n t of m o t o r function of the forepaw has been d e m o n s t r a t e d in rats and hamsters following PT lesions 4'14. Electron microscopical observations provided evidence that the PT contains a substantial a m o u n t of unmyelinated profiles, not only in rodents 17,19,26, but also in cat 3° and m o n k e y 29, although the presence of unmyelinated axons in the m o n k e y is questioned by Ralston et al. 25. L e e n e n et al. 2° counted a total number of 43,000 + 2,000 m y e l i n a t e d and 35,000 +
8,000 unmyelinated fibres at the level of the second cervical segment in the rat; at the second sacral segment 2,800 u n m y e l i n a t e d fibres could still be detected 5. The site of their originating neurones still is a matter of speculation and therefore the object of the present study. Three Wistar rats aged 6 months (weighing approximately 250 g) were anesthetized by i.p. injections with Nembutal. The entire s e n s o r i m o t o r and frontal cortex of the left hemisphere was labelled by implantation of 6 - 8 H R P - g e l s 9. Using this technique, a d e p o t of tracer is established for p r o t r a c t e d H R P transport and therefore its enhanced detection along the length of the axons. On the other hand, for quantitative studies, the use of the more sensitive tracer wheat germ agglutinin-conjugated horseradish peroxidase ( W G A - H R P ) is p r o b a b l y m o r e appropriate in o r d e r to label greater numbers of axons. The postimplantation survival times were either 24 or 48 h. A f t e r reanaesthetization, the animals were transcardially perfused with 50 ml 5% sucrose in phosphate buffer (PB) ( p H 7.2), followed by 1%
Correspondence: E.A.J. Joosten, Dept. of Anatomy and Embryology, Faculty of Medicine, University of Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. 0006-8993/88/$03.50 © 1988 Elsevier Science Publishers B.V. (Biomedical Division)
174 paraformaldehyde and 2% glutaraldehyde in the same buffer. A f t e r perfusion, the brains and spinal cords were immediately r e m o v e d and the spinal cords were further i m m e r s e d for about 1 h before being transferred into cold (4 °C) PB with 5% sucrose. Then 100-#m vibratome sections of the cervical (C5/C6) and lumbar (L2/L3) intumescence were cut' transversely on an Oxford Vibratome. A n t e r o g r a d e l y transported H R P was visualized with the use of the chromogen t e t r a m e t h y l b e n z i d i n e (TMB) in combination with the stabilizing agent amm o n i u m h e p t a m o l y b d a t e ( A H M ) in 0.1 M PB at p H 6.0, as introduced by Olucha et al. 2~ for light microscopy and a d a p t e d for electronmicroscopy 1:. In short, the vibratome sections were p r e s o a k e d for 20 rain in a mixture containing 0.005% TMB (dissolved in absolute ethanol) and 0.25% A H M in 0.1 M PB (pH 6.0). The incubation was started by the addition of 50 ~1 of 33% H2O 2 per 100 ml pre-incubation bath, which was r e p e a t e d every 5 rain for 20 rain. Control sections were processed identically, but the incubation was carried out without TMB. A f t e r the H R P T M B - ( A H M ) reaction, osmication was carried out using 1% OsO4 solution in 0.1 M PB (pH 5.0) for 4 h at room temperature. Osmication was followed by an accelerated dehydration and subsequent e m b e d d i n g in Epon. Transverse or longitudinal ultrathin sections were mounted on 75-mesh formvar ( 0 . 8 % ) - c o a t e d grids contrasted with uranylacetate (20 rain) and lead citrate (5 min) respectively, and were studied in a Philips EM-300 at an accelerating voltage of 60 kV. O u r results d e m o n s t r a t e that the adult rat PT contains many H R P - T M B - ( A H M ) labelled myelinated and a number of unmyelinated axons originating in the cortex, at both the cervical as well as the lumbar intumescence (Figs. 1-4). The occurrence of labelled profiles is restricted to the PT-area of the dorsal fu-
niculus. The results obtained after 24 h survival time were identical to those obtained after 48 h, at least as far as the white matter is concerned. Control sections were always negative. A t the ultrastructural level, the H R P - T M B ( A H M ) reaction product generates a clearly visible and highly discernable intracellular crystalline marker, which makes the identification of H R P - l a b e l l e d fibres rather easy (Figs. 1-4). Besides, TMB proved to be the most sensitive chromogen available for the demonstration of H R P labelling 22. Longitudinal sections clearly d e m o n s t r a t e the uneven distribution of the reaction product along myelinated as well as unmyelinated axons (Fig. 4). Because of the uneven distribution of H R P - T M B - ( A H M ) crystals in labelled profiles, quantification of the results should be carried out very carefully. Transversely sectioned unlabelled profiles might contain the H R P - T M B ( A H M ) reaction product at more distal or proximal levels (Fig. 4). Because of this uneven distribution pattern, it is very easy to underestimate the quantity of labelled unmyelinated CST axons. Whereas the diameters of the labelled unmyelinated corticospinal axons remain fairly constant at about 0.2/~m, labelled myelinated CST axons vary considerably with respect to their diameters, namely between 0.5 and 3/~m. (Figs. 1 and 3). Although previous studies 5'17A9'2° d e m o n s t r a t e d the presence of n u m e r o u s u n m y e l i n a t e d profiles in the adult rat PT at several spinal cord levels, the origin, destination and function of these profiles are still unknown. Based on longitudinal sections, Ralston et al. 25 concluded that most of the profiles which might be interpreted as unmyelinated axons in the primate PT at medullary levels are actually astroglial processes. They stated that less than 1% of the PT in the old world adult primate are unmyelinated axons. O u r results d e m o n s t r a t e that at least part of the unmy-
Fig. 1. Cervical intumescence; 48 h survival time. The labelled CST comprises myelinated as well as unmyelinated axons (arrowhead). Note the typical crystalline appearance of the HRP-TMB-(AHM) reaction product (arrows). Transverse section. Bar = I urn. Fig. 2. Cervical intumescence; 48 h survival time. Detail of Fig. 1. Transverse section. Bar = 0.5 ~m. Fig. 3. Lumbar intumescence; 24 h survival time. Detail of a labelled unmyelinated axon (arrowhead) surrounded by labelled myelinated axons (arrows). Transverse section. Bar = 0.5 ~m. Fig. 4. Lumbar intumescence; 48 h survival time. Longitudinal section of labelled unmyelinated axons. Note the uneven distribution of HRP-TMB-(AHM) crystals within the unmyelinated axons. Bar = 0.5urn
t.,sl
om~
176 elinated axons in the adult rat PT have their origin in the cortex.
elinated axons, which either reach out into the spinal gray or, being recurrent collaterals, might even re-
From physiological studies on the conduction velo-
turn to more proximal destinations. On the other hand, the unmyelinated CST axons might also represent axons which have not yet acquired myelin sheaths. This would implicate that myelination of CST axons still occurs at a low level during adult-
cities of PT axons it was concluded that all measurements are within the range of myelinated axons 21. Based on their different biophysical properties, their different connections as well as their varying conduction velocities, PT cells are subdivided into two distinct populations6"Z8: first, slow-conducting PT cells with conduction velocities below 21 m/s (the slowest velocities reported are 4 - 6 m/s), and second, fast PT cells whose axons conduct at velocities of 2 1 - 9 0 m/s. Fast-conducting PT neurones discharge phasically and are involved in the initiation and control of brief, quick movements. O n the other hand, slowly conducting PT neurones discharge in tonic phase, while
hood, because myelination appears to be completed at postnatal day 28 at the cervical as well as at the lumbar intumescence 13. Unilateral pyramidal lesions in monkeys revealed that recovery occurs by local sprouting of CST fibres of the intact bundle into contralateral areas of spinal neurones with severed homolateral CST connections 15. Probably unmyelinated CST axons in the adult PT account for this recovery function.
they are implicated in the determination of muscle tone and control of small, fractioned movements 2'
A recent study on the corticospinal projection neurones in adult rats 23 using retrograde HRP-tech-
~x.3~. Although L e e n e n et al. 2° suggested that the unmyelinated axons in the adult rat PT might have a
niques revealed, besides great concentrations of HRP-positive neurones in the sensorimotor cortex,
similar function as the slowly conducting PT neurones, the conduction velocities measured 21'28 are not in the range to be expected for u n m y e l i n a t e d axons. Remarkably, even the slowest conduction velo-
less dense concentrations in anterior cingulate and prefrontal cortical areas. Neurones from the latter regions might also account for the u n m y e l i n a t e d CST projections. Hence, we are currently dealing with the ultrastructural visualization of CST axons in the adult rat PT, with their originating n e u r o n e s in (pre-) fron-
cities reported ( 4 - 6 m/s) 6'16"21'28 do not correspond with the category of myelinated axons with fibre diameters of 0 . 5 - 1 . 0 # m in the rat PT 7. Possibly, the u n k n o w n mode of collateralization of CST axons may hamper an accurate determination of conduction velocities. The u n m y e l i n a t e d CST axons in the adult rat PT may, at least in part, represent collaterals of my-
1 Beck, C.H. and Chambers, W.W., Speed, accuracy and strength of forelimb movement after unilateral pyramidotomy in rhesus monkeys, J. Comp. Physiol. Psychol., 70 (1970) 1-22. 2 Biedenbach, M.A., DeVito, J.L. and Brown, A.C., Pyramidal tract of the cat: axon size and morphology, Exp. Brain Res., 61 (1986) 303-310. 3 Brown Jr., L.T., projections and terminations of the corticospinal tract in rodents, Exp. Brain Res., 13 (1971) 432-450. 4 Castro, A.J., Motor performance in rats. The effects of pyramidal tract section, Brain Research, 44 (1972) 313-323. 5 Chung, K. and Coggeshall, R.E., Postnatal development of the rat dorsal funieulus, J. Neurosci., 7 (1987) 972-977. 6 Deschenes, M., Labelle, A. and Landry, P., Morphological characteristics of slow and fast PT-cells in the cat, Brain Research, 178 (1979) 251-274. 7 Dunkerley, G.B. and Duncan, D., A light and electron microscopic study of the normal and degenerating cortico-
tal cortex areas.
We are grateful for technical and photographic assistance from Jos D e d e r e n and Theo Hafmans, respectively.
8
9 I0
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