Number Of Myelinated Fibers Research Articles

Lateral and longitudinal ultrastructural study of the recurrent laryngeal nerve in adult rats

Despite the importance of the recurrent laryngeal nerve (RLN) in terms of respiration and phonation functions, studies on the normal histology of this nerve in rats are still scanty. The RLN is asymmetric in length, because the different origin on both sides but neither a longitudinal ultrastrucutral study nor information about ultrastructural lateral asymmetry of this nerve is available in rats. Male adult Wistar rats (N=5), had their RLN prepared for transmission electron microscopy and morphometry. Comparisons were made between segments on the same side and same levels between sides. The RLN fascicles are significantly larger proximally, on both sides as is the myelinated fiber number. No other longitudinal differences were observed. Unmyelinated fiber number and density is larger on right, compared to proximal left segment. This lateral difference was not present on the distal segments. No other lateral differences were observed. Our results indicate that the RLN are asymmetric not only macroscopically (origin and length, between sides) but also on its microscopic anatomy, not only between sides but also between proximal and distal segments. This information is crucial for those studies investigating RLN neuropathies since results can vary according to the nerve site studied.Grant Funding Source: FAPESP, CNPq, CAPES and FAEPA

Longitudinal morphometric study of the cervical vagus nerve in young Wistar‐Kyoto rats

Spontaneously hypertensive rats (SHR) are widely used as a model for essential hypertension in humans and the vagus nerve morphology in this model was already investigated [1]. Nevertheless, there is no description of the vagus nerve histological characteristics in the SHR normotensive counterparts, the Wistar‐Kyoto rats (WKY), except for a study of the vagus vascularization [2]. We investigated the cervical vagus nerve in 30 days old WKY. Rats were systemically perfused with a fixative solution and had their cervical vagi nerves prepared for light microscopy study. Proximal, medium and distal segments of the left vagus nerve were morphometrically evaluated with the aid of computer software. Comparisons were made between segments in order to evaluate possible longitudinal changes along the nerve. Differences were considered significant when p < 0.05. Fascicle morphometric parameters evaluated were area, number and density of myelinated fibers and number and density of Schwann cell nuclei. None of these parameters were different between segments. This is the first description of vagus nerve morphometry in WKY and these data will provide morphological basis for further studies involving functional investigations of the efferent arm of the baroreflex in SHR.Grant Funding Source: FAPESP, CNPq and FAEPA

Morphometry of nerves responsible for sensory innervation of the hind limbs in rats

The sensory innervation of the distal hind limb is achieved by branches of the sacral and lumbar plexuses. In rats, the skin of the dorsum of the foot is innervated mainly by the sural and saphenous nerves, with considerable overlapping between them. We aimed to compare the morphology and morphometry of the sural and saphenous nerves in young rats. Female Wistar rats (N=5) aged 30 days were perfused with 2.5% glutaraldehyde and had their right and left sural and saphenous nerves prepared for light microscopy and morphometry. Comparisons were made between segments (proximal and distal) and sides in the same nerves and between nerves of the same side. Both nerves are longitudinally symmetric on both sides. A lateral symmetry is also present on both nerves. In general, no differences were found between nerves, either for the fascicle or for the myelinated fibers morphometry. The similar number of myelinated fibers, associated with their similar size in both sural and saphenous nerves suggests that the innervation territory for both nerves is equally distributed. This information is useful for studies of sensory neuropathies, in particular, studies of pain.Grant Funding Source : FAPESP, CNPq, CAPES and FAEPA

Different multiple regeneration capacities of motor and sensory axons in peripheral nerve

After peripheral nerve injury, axons often project sprouts from the node of Ranvier proximal to the damage site. It is well known that one parent axon can sprout and maintain several regenerating axons. If enough endoneurial tubes in the distal stump are present for the regenerating axons to grow along, then the number of mature myelinated nerve fibers in the distal stump will be greater than the number in the proximal stump. "Multiple regeneration" is used to describe this phenomenon in the peripheral nerve. According to previous studies, a prominent nerve containing many axons can be repaired by the multiple regenerating axons sprouting from another nerve that contains fewer axons. Most peripheral nerves contain a mixture of myelinated motor and sensory axons as well as unmyelinated sensory and autonomic axons. In this study, a multiple regeneration animal model was developed by bridging the proximal common peroneal nerve with the distal common peroneal nerve and the tibial nerve. Differences in the multiple regeneration ratio of motor and sensory nerves were evaluated using histomorphometry one month after ablating the dorsal root ganglion (DRGs) and ventral roots, respectively. The results suggest that the motor nerves have a significantly larger multiple regeneration ratio than the sensory nerves at two different time points.

Sural nerve involvement in experimental hypertension: morphology and morphometry in male and female normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR)

BackgroundThe sural nerve has been widely investigated in experimental models of neuropathies but information about its involvement in hypertension was not yet explored. The aim of the present study was to compare the morphological and morphometric aspects of different segments of the sural nerve in male and female spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Rats aged 20 weeks (N = 6 in each group) were investigated. After arterial pressure and heart rate recordings in anesthetized animals, right and left sural nerves were removed and prepared for epoxy resin embedding and light microscopy. Morphometric analysis was performed with the aid of computer software, and took into consideration the fascicle area and diameter, as well as myelinated fiber number, density, area and diameter.ResultsSignificant differences were observed for the myelinated fiber number and density, comparing different genders of WKY and SHR. Also, significant differences for the morphological (thickening of the endoneural blood vessel walls and lumen reduction) and morphometric (myelinated fibers diameter and G ratio) parameters of myelinated fibers were identified. Morphological exam of the myelinated fibers suggested the presence of a neuropathy due to hypertension in both SHR genders.ConclusionsThese results indicate that hypertension altered important morphometric parameters related to nerve conduction of sural nerve in hypertensive animals. Moreover the comparison between males and females of WKY and SHR allows the conclusion that the morphological and morphometric parameters of sural nerve are not gender related. The morphometric approach confirmed the presence of neuropathy, mainly associated to the small myelinated fibers. In conclusion, the present study collected evidences that the high blood pressure in SHR is affecting the sural nerve myelinated fibers.

Histopathological examination of nerve samples from pure neural leprosy patients: obtaining maximum information to improve diagnostic efficiency

Nerve biopsy examination is an important auxiliary procedure for diagnosing pure neural leprosy (PNL). When acid-fast bacilli (AFB) are not detected in the nerve sample, the value of other nonspecific histological alterations should be considered along with pertinent clinical, electroneuromyographical and laboratory data (the detection of Mycobacterium leprae DNA with polymerase chain reaction and the detection of serum anti-phenolic glycolipid 1 antibodies) to support a possible or probable PNL diagnosis. Three hundred forty nerve samples [144 from PNL patients and 196 from patients with non-leprosy peripheral neuropathies (NLN)] were examined. Both AFB-negative and AFB-positive PNL samples had more frequent histopathological alterations (epithelioid granulomas, mononuclear infiltrates, fibrosis, perineurial and subperineurial oedema and decreased numbers of myelinated fibres) than the NLN group. Multivariate analysis revealed that independently, mononuclear infiltrate and perineurial fibrosis were more common in the PNL group and were able to correctly classify AFB-negative PNL samples. These results indicate that even in the absence of AFB, these histopathological nerve alterations may justify a PNL diagnosis when observed in conjunction with pertinent clinical, epidemiological and laboratory data.

Analysis of myelinated fibers in human skin biopsies of patients with neuropathies

Given the availability of effective but costly treatment for acquired demyelinating neuropathies, biomarkers for these disorders are urgently needed. Here we aimed to quantify morphological abnormalities of myelinated fibers in skin biopsies from the proximal leg of patients with neuropathies to determine a potential diagnostic role of this method. We used double immunofluorescence to detect myelinated and unmyelinated fibers in thigh skin from 81 patients with polyneuropathy, 19 patients with small fiber neuropathy, and 25 controls. Dermal myelinated fibers were reduced 6.8-fold in patients with polyneuropathy (p < 0.0001). The number of dermal nerve bundles with myelinated fibers was reduced 2.7-fold (p = 0.0025). In small fiber neuropathy, myelinated fibers in dermal nerve bundles were only reduced in the length-dependent type, indicating that this subgroup may represent an early stage of generalized polyneuropathy. Elongated nodes of Ranvier were detectable in demyelinating neuropathies only. Our data suggest that changes in the number and morphology of myelinated fibers in the proximal leg can confirm the diagnosis of neuropathy, and may help to distinguish between demyelinating and axonal neuropathy, and to differentiate pure small fiber neuropathy from early polyneuropathy.

Segmentation of nerve fibers using multi-level gradient watershed and fuzzy systems

This paper presents an algorithm based on multi-level watershed segmentation combined with three fuzzy systems to segment a large number of myelinated nerve fibers in microscope images. The method can estimate various geometrical parameters of myelinated nerve fibers in peripheral nerves. It is expected to be a promising tool for the quantitative assessment of myelinated nerve fibers in related research. A novel multi-level watershed scheme iteratively detects pre-candidate nerve fibers. At each immersion level, watershed segmentation extracts the initial axon locations and obtains meaningful myelinated nerve fiber features. Thereafter, according to a priori characteristics of the myelinated nerve fibers, fuzzy rules reject unlikely pre-candidates and collect a set of candidates. Initial candidate boundaries are then refined by a fuzzy active contour model, which flexibly deforms contours according to the observed features of each nerve fiber. A final scan with a different set of fuzzy rules based on the a priori properties of the myelinated nerve fibers removes false detections. A particle swarm optimization method is employed to efficiently train the large number of parameters in the proposed fuzzy systems. The proposed method can automatically segment the transverse cross-sections of nerve fibers obtained from optical microscope images. Although the microscope image is usually noisy with weak or variable levels of contrast, the proposed system can handle images with a large number of myelinated nerve fibers and achieve a high fiber detection ratio. As compared to manual segmentation by experts, the proposed system achieved an average accuracy of 91% across different data sets. We developed an image segmentation system that automatically handles myelinated nerve fibers in microscope images. Experimental results showed the efficacy of this system and its superiority to other nerve fiber segmentation approaches. Moreover, the proposed method can be extended to other applications of automatic segmentation of microscopic images.

Transplantation of undifferentiated bone marrow stromal cells improves sciatic nerve regeneration and functional recovery through inside-out vein graft in rats

To study the effect of undifferentiated bone marrow stromal cells (BMSCs) on peripheral nerve regeneration using a rat sciatic nerve regeneration model. In recent years, cell transplantation has become the focus of attention, and reliable outcomes have been achieved in regeneration of the sciatic nerve. Materials and methods: A 10-mm sciatic nerve defect was bridged using an inside-out vein graft (IOVG) filled with undifferentiated BMSCs (2 × 107 cells/mL). In the control group, the vein was filled with phosphate buffered saline alone. The regenerated fibers were studied 4, 8, and 12 weeks after surgery. Assessment of nerve regeneration was based on functional (walking track analysis), histomorphometric, and immunohistochemical (Schwann cell detection by S-100 expression) criteria. Results: The functional study confirmed significant recovery of regenerated axons in the IOVG/BMSC group (P < 0.05). Quantitative morphometric analyses of regenerated fibers showed that the number and diameter of myelinated fibers in the IOVG/BMSC group were significantly higher than in the control group (P < 0.05). Conclusion: This study demonstrates the potential of using undifferentiated BMSCs in peripheral nerve regeneration without limitations of donor-site morbidity associated with isolation of Schwann cells. It also reduces costs due to the simplicity of laboratory procedures compared to those for differentiated BMSCs and the reduction in the interval from tissue collection until cell injection.

The Effect of Severing L6 Nerve Root of the Sacral Plexus on Lower Extremity Function

Nerve transfer is a valid surgical procedure for lower-extremity function restoration after lumbosacral plexus avulsion. To evaluate the impact of severing the L6 nerve root on the functions of the healthy limb in rhesus monkeys and the feasibility of using the contralateral L6 nerve root as a donor nerve to repair lumbosacral plexus root avulsion. Twenty-four rhesus monkeys were randomly assigned into 2 groups. In the experimental group, the right L6 nerve root was explored and severed, whereas in the control group animals underwent a sham operation. Electrophysiology, muscle mass, histology, and ultrastructure of the target muscles were examined. Three weeks after transection, reduced amplitude and prolonged latency of compound muscle action potential were observed in the medial gastrocnemius, extensor digitorum brevis, peroneus longus, and abductor hallucis muscles of the experimental group, as well as reduction in muscle mass and myofiber cross-sectional area of these muscles. The number of myelinated nerve fibers of the sciatic nerve in the experimental group was significantly less than that of the control group. Abnormal ultrastructure of motor end plates of these muscles was also observed in the experimental group. Eight weeks postoperatively, all of these parameters were similar between the experimental and control groups. Severing the L6 nerve root does not damage the healthy limb as far as electrophysiology, muscle mass, histology, and ultrastructure of the target muscles are concerned. L6 (analogous to S1 in humans) nerve root may be used as a donor nerve to repair lumbosacral plexus root avulsion.

The use of undifferentiated bone marrow stromal cells for sciatic nerve regeneration in rats

In recent years, cell transplantation has become a focus of attention and reliable outcomes have been achieved in regeneration of the sciatic nerve. The effect of undifferentiated bone marrow stromal cells (BMSCs) on peripheral nerve regeneration was studied using a rat sciatic nerve regeneration model. A 10-mm sciatic nerve defect was bridged using an inside-out vein graft (IOVG) filled with undifferentiated BMSCs (2×107cells/ml). In the control group, the vein was filled with phosphate buffer saline alone. The regenerated fibres were studied 4, 8 and 12 weeks after surgery. Assessment of nerve regeneration was based on functional (walking track analysis), histomorphometric and immuohistochemical (Schwann cell detection by S100 expression) criteria. The functional study confirmed significant recovery of regenerated axons in the IOVG/BMSC group (P<0.05). Quantitative morphometric analyses of regenerated fibres showed the number and diameter of myelinated fibres in the IOVG/BMSC group were significantly higher than in the control group (P<0.05). This demonstrates the potential for using undifferentiated BMSCs in peripheral nerve regeneration without the limitations of donor-site morbidity associated with isolation of Schwann cells. It also reduces costs because the interval between tissue collection and cell injection is reduced and the laboratory procedures are simpler compared to undifferentiated BMSCs.

Can Wistar rats be used as the normotensive controls for nerve morphometry investigations in spontaneously hypertensive rats (SHR)?

We compared the sural nerve morphology among Wistar (WR), Wistar-Kyoto (WKY) and Spontaneously hypertensive (SHR) rats, including the nerve fascicles and myelinated fibers morphometry. Age matched (20 weeks) female WR (N=6), WKY (N=6) and SHR (N=7) had their right and left sural nerves removed, embedded in epoxy resin, and observed by light microscopy. Morphometric analysis was performed with the aid of computer software. Despite presenting the same age, WR were heavier than WKY and SHR, as were SHR compared to WKY. Systolic arterial pressure was higher in SHR compared to WR, but no differences between SHR and WKY or WR and WKY were observed. The sural nerves were morphometrically symmetric between proximal and distal segments on the same side and between sides in all strains with no differences in the myelinated fiber number. Schwann cell number and density were smaller in SHR and G ratio was larger in SHR, indicating that SHR have thinner myelinated fibers. Sural nerve morphology is similar between WKY and WR, allowing the use of WR as the SHR controls in morphological investigations involving peripheral neuropathies.

Functional Motor Nerve Regeneration Without Motor-Sensory Specificity Following End-to-Side Neurorrhaphy: An Experimental Study

To evaluate the quality of regenerating myelinated axons and motor-sensory specificity in an end-to-side nerve repair model. We divided 20 rats into 3 groups: (1) end-to-side neurorrhaphy using the ulnar nerve as donor nerve and the musculocutaneous nerve as recipient nerve; (2) normal control; and (3) transected nerve with the stumps buried. At 5 months, we monitored the grooming test, the electrophysiological response, and the histologic changes in nerve and muscle. Grooming recovered successfully, and electrophysiological investigations revealed that the target muscles had been reinnervated in the end-to-side group. The mean wet weight of the reinnervated biceps brachii muscle was 72% of the normal muscle, and the mean muscle fiber cross-sectional area of the reinnervated muscle was similar to the normal muscle. The implanted musculocutaneous nerve contained varying but satisfactory numbers of axons (end-to-side group: 596 ± 348 vs normal group: 1,340 ± 241). Acetylcholinesterase staining revealed a similar percentage of myelinated fibers in the musculocutaneous nerve (39%) and the biceps brachii branch of the musculocutaneous nerve (38%) in the end-to-side group. This was similar to the number of myelinated fibers in the donor ulnar nerve (37%). The present study confirms that limited but functional reinnervation can occur on the basis of collateral sprouting of intact axons from the ulnar nerve. The motor-sensory specificity is not important.

Chemoarchitecture of Layer 4 Isocortex in the American Water Shrew (Sorex palustris)

We examined the chemoarchitecture of layer 4 isocortex and the number of myelinated nerve fibers of selected cranial nerves in the American water shrew (Sorex palustris). This study took advantage of the opportunity to examine juvenile brain tissue, which often reveals the most distinctive cortical modules related to different sensory representations. Flattened cortical sections were processed for the metabolic enzyme cytochrome oxidase, revealing a number of modules and septa. Subdivisions related to sensory representations were tentatively identified by performing microelectrode recordings in a single adult shrew in this study, combined with microelectrode recordings and anatomical findings from a previous investigation. Taken together, these results suggest that characteristic chemoarchitectonic borders in the shrew neocortex can be used to delineate and quantify cortical areas. The most obvious subdivisions in the water shrew include a relatively small primary visual cortex which responded to visual stimuli, a larger representation of vibrissae in the primary somatosensory cortex, and a prominent representation of oral structures apparent in the more rostral-lateral cortex. A presumptive auditory area was located in the far caudal cortex. These findings for the cortex are consistent with counts from optic, auditory and trigeminal nerves, suggesting that somatosensory inputs dominate the shrew’s senses whereas visual and auditory inputs play a small role in navigation and in finding prey. More generally, we find that shrews share unusual features of cortical organization with moles, supporting their close taxonomic relationship.

Transfer of the Distal Terminal Motor Branch of the Extensor Carpi Radialis Brevis to the Nerve of the Flexor Pollicis Longus

In tetraplegics, thumb and finger motion traditionally has been reconstructed via orthopedic procedures. Although rarely used, nerve transfers are a viable method for reconstruction in tetraplegia. To investigate the anatomic feasibility of transferring the distal branch of the extensor carpi radialis brevis (ECRB) to the flexor pollicis longus (FPL) nerve and to report our first clinical case. We studied the motor branch of the ECRB and FPL in 14 cadaveric upper limbs. Subsequently, a 24-year-old tetraplegic man with preserved motion in his shoulder, elbow, wrist, and finger extension, but paralysis of thumb and finger flexion underwent surgery. Seven months after trauma, we transferred the brachialis muscle with a tendon graft to the flexor digitorum profundus. The distal nerve of the ECRB was transferred to the FPL nerve. The branch to the ECRB entered the muscle in its anterior and proximal third. After sending out a first collateral, the nerve runs for 2.4 cm alongside the muscle and bifurcates intramuscularly. A main branch from the anterior interosseous nerve, which entered the muscle 3 cm from its origin on the radius, innervated the FPL. The ECRB and FPL nerves had similar diameters (∼1 mm) and numbers of myelinated fibers (∼180). In our patient, 14 months after surgery, pinching and grasping were restored and measured 2 and 8 kg strength, respectively. Transfer of the ECRB distal branch to the FPL is a viable option to reconstruct thumb flexion.

Enhancement of sciatic nerve regeneration after vascular endothelial growth factor (VEGF) gene therapy

Recent studies have emphasized the beneficial effects of the vascular endothelial growth factor (VEGF) on neurone survival and Schwann cell proliferation. VEGF is a potent angiogenic factor, and angiogenesis has long been recognized as an important and necessary step during tissue repair. Here, we investigated the effects of VEGF on sciatic nerve regeneration. Using light and electron microscopy, we evaluated sciatic nerve regeneration after transection and VEGF gene therapy. We examined the survival of the neurones in the dorsal root ganglia and in lumbar 4 segment of spinal cord. We also evaluated the functional recovery using the sciatic functional index and gastrocnemius muscle weight. In addition, we evaluated the VEGF expression by immunohistochemistry. Fluorescein isothiocyanate-dextran (FITC-dextran) fluorescence of nerves and muscles revealed intense staining in the VEGF-treated group. Quantitative analysis showed that the numbers of myelinated fibres and blood vessels were significantly higher in VEGF-treated animals. VEGF also increased the survival of neurone cell bodies in dorsal root ganglia and in spinal cord. The sciatic functional index and gastrocnemius muscle weight reached significantly higher values in VEGF-treated animals. We demonstrate a positive relationship between increased vascularization and enhanced nerve regeneration, indicating that VEGF administration can support and enhance the growth of regenerating nerve fibres, probably through a combination of angiogenic, neurotrophic and neuroprotective effects.

An anatomical study of the myelination of human laryngeal nerves

To determine the differences in myelination between the human recurrent laryngeal nerve and superior laryngeal nerve. Fifteen confirmed laryngeal nerve specimens were harvested from five cadavers. Cross-sections were examined under a photomicroscope and morphometric analysis performed. There was a significantly greater number of myelinated fibres than unmyelinated fibres, in both the recurrent laryngeal nerve (p = 0.018) and the superior laryngeal nerve (p = 0.012). There was a significantly greater number of myelinated fibres in the superior laryngeal nerve, compared with the recurrent laryngeal nerve (p = 0.028). However, there was no significant difference in the number of unmyelinated fibres, comparing the two nerves (p = 0.116). These findings support those of previous studies, and provide further evidence against the historical plexus theory of laryngeal nerve morphology. The differences in the degree of myelination, both within and between the human laryngeal nerves, may have clinical consequence regarding recovery of function following nerve injury.

English

The peripheral nervous system has the ability to regenerate after injury. Peripheral nerve injuries are caused by penetrating injury, crush, traction and ischemia compression. However, the availability of various nerve coaptation and other techniques for the attainment of functional nerve regeneration is still inadequate. The objective of this study was to compare the effectiveness of bone marrow stromal cells (BMSCs) implantation and epineural nerve suture on peripheral nerve regeneration in a rabbit model. Ten male New Zealand white rabbits were divided into two groups. In the primary epineural repair group (control group), the left sciatic nerve was skeletonized from the sciatic notch to the point of bifurcation, with the nerve been transected at the mid-shaft of the femoral bone and repaired with six epineural sutures. In the treated group, the epineural repaired nerve was implanted with BMSCs in the proximal and distal segments of the transected sciatic nerve. Assessment of the nerve regeneration was based on functional (motor and sensory), histological and morphometric criteria, including the number of myelinated nerve fibers, nerve fiber diameter, axon diameter, myelin sheath thickness, g ratio and relative gastrocnemius muscle weight. The results of the examination showed that the treated group had the best regeneration and functional recovery. &nbsp; Keyword:&nbsp;Bone marrow stromal cells (BMSCs), histopathology, morphometric, peripheral nerve, regeneration.

Neuroprotective and neuroregenerative effects of low-intensity aerobic exercise on sciatic nerve crush injury in mice

Here, we established a program of low-intensity aerobic exercise and compared the effects of exercise preoperative, postoperative, and a combination of both pre- and postoperative protocols on recovery from sciatic nerve crush injury in mice using behavioral, biochemical, and morphological assays. Sciatic nerve crush was performed in adult male mice. The animals were submitted to preoperative (for 2 weeks), postoperative (for 2 weeks), and a combination of preoperative-postoperative (for 4 weeks) training protocols. During the training period, functional recovery was monitored using the Sciatic Functional Index, the Sciatic Static Index, and mechanical and cold hypersensitivity analyses. Morphological and biochemical alterations were analyzed on the 14th day post-crushing. The functional recovery values of all of the exercised groups were significantly better than the nonexercised group. Biochemically, all of the exercise groups showed a reduction in the increase of interleukin-1β (IL-1β) in the sciatic nerve and in the IL-1β and interleukin-6 receptor (IL-6R) levels in the spinal cord. However, the levels of tumor necrosis factor alpha (TNF-α) decreased only in the postoperative group and in the combination exercise protocols. In the morphological analysis, the combination exercise subjects presented an increase in fiber and axon diameter, in the myelination degree and in the number of myelinated fibers. The present study showed that pre- and postoperative exercise achieved values for functional and morphological sciatic nerve regeneration that were significantly better than either the preoperative or postoperative protocols. This experimental study suggests that physical exercise can restore motor and nerve function to a substantial degree when performed using a prophylactic and therapeutic approach.

Functional and morphometric differences between the early and delayed use of phototherapy in crushed median nerves of rats

This study evaluated the functional and quantitative differences between the early and delayed use of phototherapy in crushed median nerves. After a crush injury, low-level laser therapy (GaAs) was applied transcutaneously at the injury site, 3 min daily, with a frequency of five treatments per week for 2 weeks. In the early group, the first laser treatment started immediately after surgery, and in the delayed group, after 7 days. The grasping test was used for functional evaluation of the median nerve, before, 10, and 21 days after surgery, when the rats were killed. Three segments of the median nerve were analyzed histomorphometrically by light microscopy and computer analysis. The following features were observed: myelinated fiber and axon diameters, myelin sheath area, g-ratio, density and number of myelinated fibers, and area and number of capillaries. In the proximal segment (site of crush), the nerves of animals submitted to early and delayed treatment showed myelinated fiber diameter and myelin sheath area significantly larger compared to the untreated group. In the distal segment, the myelin sheath area was significantly smaller in the untreated animals compared to the delayed group. The untreated, early, and delayed groups presented a 50, 57, and 81% degree of functional recovery, respectively, at 21 days after injury, with a significant difference between the untreated and delayed groups. The results suggest that the nerves irradiated with low-power laser exhibit myelinated fibers of greater diameter and a better recovery of function.