Age changes in axon number along the cervical ventral spinal nerve roots in rats

Re-utilization of Schwann cells during ingrowth of ventral root afferents in perinatal kittens

Ventral roots in all mammalian species, including humans, contain significant numbers of unmyelinated axons, many of them afferents transmitting nociceptive signals from receptive fields in skin, viscera, muscles and joints. Observations in cats indicate that these afferents do not enter the spinal cord via the ventral root, but rather turn distally and enter the dorsal root. Some unmyelinated axons are postganglionic autonomic efferents that innervate blood vessels of the root and the pia mater. In the feline L7 segment, a substantial proportion of unmyelinated axons are not detectable until late in perinatal development. The mechanisms inducing this late ingrowth, and the recruitment of Schwann cells (indispensable, at this stage, for axonal survival and sustenance), are unknown. We have counted axons and Schwann cells in both ends of the L7 ventral root in young kittens and made the following observations. (1) The total number of axons detectable in the root increased throughout the range of investigated ages. (2) The number of myelinated axons was similar in the root's proximal and distal ends. The increased number of unmyelinated axons with age is thus due to increased numbers of small unmyelinated axons. (3) The number of separated large probably promyelin axons was about the same in the proximal and distal ends of the root. (4) Schwann cells appeared to undergo redistribution, from myelinated to unmyelinated axons. (5) During redistribution of Schwann cells they first appear as aberrant Schwann cells and then become endoneurial X-cells temporarily free of axonal contact. We hypothesize that unmyelinated axons invade the ventral root from its distal end, that this ingrowth is particularly intense during the first postnatal month and that disengaged Schwann cells, eliminated from myelinated motoneuron axons, provide the ingrowing axons with structural and trophic support.

Automated characterization of nerve fibers labeled fluorescently: determination of size, class and spatial distribution

Morphological classification of nerve fibers could help interpret the assessment of neural regeneration and the understanding of selectivity of nerve stimulation. Specific populations of myelinated nerve fibers can be investigated by retrograde tracing from a muscle followed by microscopic measurements of the labeled fibers at different anatomical levels. Gastrocnemius muscles of adult rats were injected with the retrograde tracer Fluoro-Gold. After a survival period of 3 days, cross-sections of spinal cords, ventral roots, sciatic, and tibial nerves were collected and imaged on a fluorescence microscope. Nerve fibers were classified using a variation-based criterion acting on the distribution of their equivalent diameters. The same criterion was used to classify the labeled axons using the size of the fluorescent marker. Measurements of the axons were paired to those of the entire fibers (axons+myelin sheaths) in order to establish the correspondence between so-established axonal and fiber classifications. It was found that nerve fibers in L6 ventral roots could be classified into four populations comprising two classes of Aalpha (denoted Aalpha1 and Aalpha2), Agamma, and an additional class of Agammaalpha fibers. Cut-off borders between Agamma and Agammaalpha fiber classes were estimated to be 5.00+/-0.09 microm (SEM); between Agammaalpha and Aalpha1 fiber classes to be 6.86+/-0.11 microm (SEM); and between Aalpha1 and Aalpha2 fiber classes to be 8.66+/-0.16 microm (SEM). Topographical maps of the nerve fibers that innervate the gastrocnemius muscles were constructed per fiber class for the spinal root L6. The major advantage of the presented approach consists of the combined indirect classification of nerve fiber types and the construction of topographical maps of so-identified fiber classes.

Morphometric analysis of the fiber populations of the rat sciatic nerve, its spinal roots, and its major branches

Correspondence between the nerve composition and the functional characteristics of its fiber populations is not always evident. To investigate such correspondence and to give a systematic picture of the morphology of the rat hind limb nerves, extensive morphometric study was performed on the sciatic nerve, its founding dorsal and ventral spinal roots, and its major branches. Nerve histology was examined in semithin sections via microscopic image analysis. Variation in the density of myelinated fibers, fiber interspace, and nerve cross-sectional area was studied in individual roots and nerves. In the dorsal roots, fiber numbers and cross-sectional areas were directly linearly proportional to the spinal root level number. Constituent fiber populations were identified using multicomponent lognormal models, and an optimal model for every nerve or root was selected by using an information theoretic approach. For the dorsal and ventral roots and the sciatic and peroneal nerves, optimal fiber population models consisted of three components, whereas, for the tibial and sural nerves, two components were optimal. Functional identities of the revealed fiber populations were established by using calculations of corresponding conduction velocities according to Arbuthnott et al. (J. Physiol. [1980] 308:125-157) and anatomical considerations. It is anticipated that morphological parameters established in this study would advance the development of neural prostheses in humans. The proximodistal correspondences among the fiber populations of different nerves were established by parametric statistical comparisons. The proposed approach provides a conceptual framework for understanding the comparative anatomy of the peripheral nerves and spinal roots and can be further applied in other species.

Motor pudendal nerve characterization in the female rat

The aim of our study was to provide quantitative data on pudendal motor neuron cell bodies and axons in the female rat. To confirm earlier studies, fluorescent retrograde tracers were used to label the motor neurons for correlation with myelinated axon counts along the length of the motor pudendal nerve. The external urethral sphincter of female rats was injected with diamidino yellow and the external anal sphincter with fast blue. The L(6) spinal cord revealed labeled motor neurons. Those in the dorsolateral column (60.8 +/- 10.6) had nuclei labeled yellow from the external urethral sphincter and those in the dorsomedial column (31.7 +/- 8.5) had cytoplasm labeled blue from the external anal sphincter. Double labeling was not present, suggesting that pudendal motor neurons in each column innervate separate sphincters. The motor pudendal nerve in the ischiorectal fossa was also characterized by light microscopy. The mean myelinated axon count (151.4 +/- 17.0) was highly correlated (r = 0.995) in the proximal fascicles and the sum of distal fascicles. This indicated that myelinated axons do not branch at the point where the main motor pudendal nerve branches into separate fascicles. Axon counts between sides were not as well correlated (r = 0.883). The ratio of motor neurons to myelinated axons is 56%, suggesting that some myelinated axons either innervate other muscles or are sensory. This reproducible characterization of the normal pudendal nerve anatomy provides an excellent basis for experimental studies associated with pudendal nerve denervation as a model for neurogenic incontinence.

Long-term observation of the effect of peripheral nerve injury in neonatal and young rats

The purpose of this study was to observe functional recovery and motoneuron death after nerve transection-and-repair in neonatal versus young animals. One hundred nine Lewis rats underwent posterior tibial nerve transection-and-repair at 6 or 22 days of age. Fifty-two and fifty-seven nerves at the 6- and 22-day times were used for endpoint analysis at 1, 3, 10, and 14 months. These assessments included serial functional walking track analysis, electrophysiologic studies, muscle mass evaluation, motoneuron counts with retrograde horseradish peroxidase tracing, and histologic and morphometric nerve analysis. Walking track analysis and nerve conduction velocity indicated significantly poorer functional regeneration in the 6-day-old group than in the 22-day-old group. Muscle mass in the 6-day-old group did not recover as well as in the 22-day-old group. Motoneuron numbers stained with horseradish peroxidase were less in the 6-day-old group than in the 22-day-old group. In contrast, morphometric analysis did not reach significance. This study suggests that the same nerve injury sustained in a neonatal rat is less likely to demonstrate functional recovery than one sustained in a young rat.

A strong myelin thickness-axon size correlation emerges in developing nerves despite independent growth of both parameters

The axon determines whether or not it is myelinated by the Schwann cell. At maturity there is a positive correlation between sheath thickness and axon calibre. This correlation is initially very low or absent, but gradually strengthens during development. This increase could come about because the axon continuously controls Schwann cell myelinating activity, so that a given axon calibre is associated with a particular myelin sheath thickness, an interaction which would entail the Schwann cell continuously monitoring and responding to axon size. This seems unnecessarily complex. This theoretical study shows that the strong correlation between the 2 parameters within a given myelinated fibre population may come about in a much simpler way than outlined above. This is demonstrated by modelling the growth and myelination of a hypothetical population, utilising data from earlier studies on cervical ventral motoneuron axon development. The hypothesis tested shows that the only instructive interactions by the axon on the Schwann cell necessary for the strong correlation between the 2 parameters to emerge are for the initiation of myelination, its continuation and its termination. These could result from a single stimulus being switched on, persisting for a time and being switched off. Under this influence, the Schwann cell is assumed to proceed to form the myelin sheath at a constant rate which it itself inherently determines, in the absence of any quantitative influence exerted by the axon. This continues until the stimulus for myelination ceases to emanate from the axon. The validity of the hypothesis is demonstrated, because the resulting myelin-axon relationships correspond closely to those observed during development.

Vitamin E affects quantitative age changes in lumbar motoneurons and in their peripheral projections

Vitamin E deficiency was previously found to induce plastic changes in the number of primary sensory neurons and in motoneuron peripheral field projections. In this work, quantitative changes in motoneurons of lumbar segments, in nerve fibres constituting ventral roots and in innervating leg motor fibres were studied in normal and vitamin E deficient rats from 1 to 5 months of age. The number of lumbar motoneurons was found to decrease, while there were no changes in the number of ventral root fibres. An increase in the number of innervating leg motor fibres was observed during ageing in control rats; in vitamin E deficient rats the number of fibres in the ventral roots did not change, as occurred in controls, but the decrease in the number of motoneurons was smaller and the number of innervating leg motor fibres increased further in comparison to the controls. The findings are consistent with the idea that vitamin E deficiency causes a decrease in motoneuron death or, alternatively, that it induces some process partially compensating naturally occurring motoneuron death.

Ganglionic axons in motor roots and pia mater

In addition to motor axons and preganglionic axons, ventral roots contain unmyelinated or thin myelinated sensory axons and postganglionic sympathetic axons. It has been said that ventral roots channel sensory axons to the CNS. However, it now seems that these axons end blindly, shift to the pia or loop and return towards the periphery and that these units reach the CNS via dorsal roots. Sensory ventral root axons project from a variety of somatic or visceral receptors; some of them are third branches of dorsal root afferents and some seem to lack a CNS projection. Many ventral root afferents contain substance P (SP) and/or calcitonin gene-related peptide (CGRP). These fibres are not affected by neonatal capsaicin treatment and they cannot induce radicular or pial extravasation. Some thin ventral root axons are sympathetic and relate to blood vessels. Afferents containing SP and/or CGRP and sympathetic axons also occur in the spinal pia mater. The sensory axons mediate pain. They might also have vasomotor, tissue-regulatory and/or mechanoreceptive functions. The motor roots of cranial nerves IV, VI and XI contain unmyelinated axons arranged like in ventral roots outside the autonomic outflow. However, the motor root of cranial nerve V channels some unmyelinated axons into the CNS. The occurrence of thin axons in ventral roots and pia mater changes during development and ageing. After peripheral nerve injury, ipsilateral ventral roots and pia are invaded by new sensory and postganglionic sympathetic axons.

Selective elimination of transient corticospinal projections in the rat cervical spinal cord gray matter

In the present paper a description is given of the development of the rat corticospinal tract (CST) in the lower cervical spinal cord. This area contains, among other cells, the motoneurons innervating the distal forelimb muscles. HRP gels were implanted in the sensorimotor cortex of Wistar rats varying in age from postnatal day 0 (P0) to P60. After a survival period of 48 h, the rats were transcardially perfused, the spinal cords transversely sectioned at 30 microns and the sections reacted for HRP. Labelled CST axons in the dorsal funiculus were first detected at P2, and after a delay of 2 days the first fibres were found in the adjacent gray matter (P4). More labelled fibres were gradually added until maximal number and extension was reached at P10. By then the entire gray matter and large parts of the white matter were covered by labelled CST axons. From P10 onwards, the number of labelled CST fibres as well as their extension decreased. In the adult rat, some areas such as the lateral part of the ventral and dorsal horn and large parts of the ventral and lateral white matter ultimately became devoid of labelled CST axons. It is concluded that a massive overshoot occurs during the development of the terminal field of the rat CST. The results are discussed in conjunction with our previous findings on the development of the motoneurons innervating the rat distal forelimb muscles. The concurrent selective elimination of both CST axons and motoneuron dendrites is suggested to be correlated with progressively more mature, coordinated movements and with high digital skills especially.

Dependence of postnatal motoneurones on their targets: review and hypothesis

Motoneurones are known to die (1) during embryonic development (naturally occurring cell death), (2) early in postnatal development after axonal injury, and (3) as a consequence of disease, such as spinal muscular atrophy or (in later life) amyotrophic lateral sclerosis. Naturally occurring motoneurone death has been extensively investigated, and interaction with the target muscle has emerged as an important factor for survival of embryonic motoneurones. Evidence that this target dependence of motoneurones continues postnatally is discussed in this review, as is the possible nature of the retrograde signal from the muscle. An explanation for the role of the muscle in motoneurone survival is also proposed, which may be applicable in situations where motoneurone death occurs postnatally. This proposal takes into account the changing functional demands imposed on motoneurones as a result of the gradual maturation of the CNS, and suggests that during development the muscle induces the motoneurones to become competent to carry out these requirements.

Receptive properties of pial afferents

The blood vessels and the pial surface of the brain, spinal cord and its roots are innervated by primary afferent neurones. Here, we have electrophysiologically characterized the functional properties of a subpopulation of these afferent fibres that supply the ventral roots of the cat sacral spinal cord. We have taken advantage of the unique anatomical arrangement of these primary afferent neurones which have their central axon in the dorsal root and project with their peripheral process into the segmental ventral root. In 10 experiments, 14 units were recorded in the dorsal root S2 which responded to electrical stimulation of the segmental ventral root. As judged by their conduction velocity ranging from 0.1 to 2.3 m/sec, all fibres were unmyelinated. In 4 cases a spot-like receptive field was located on the root where the units were reproducibly activated by mechanical stimuli, the most effective being a slight stretch. In two units tested, topical application of hypertonic saline onto the receptive field, but not at other portions of the axon elicited a long-lasting vigorous discharge with intermittent bursts. There was no obvious association of the receptive field with small blood vessels. In 5 of the 14 units including 2 with a mechanosensitive receptive field we observed latency jumps of the action potential with electrical stimulation of the ventral root close to the dorsal root ganglion. In some of these units latency jumps were also observed at other positions when the stimulation electrodes were moved centrally towards the spinal cord. We conclude that a subpopulation of unmyelinated fibres in the spinal ventral root are primary afferents innervating the root proper or its sheath.(ABSTRACT TRUNCATED AT 250 WORDS)