Immunohistochemistry and nerve lesion experiments on the methionine-enkephalin immunopositive neurons in the small intestine of the bullfrog (Rana catesbeiana)

Summary Nerve elements in the small intestine of the bullfrog. Rana catesbeiana, were studied by immunohistochemistry with anti-methionine enkephalin antisera and by nerve lesion experiments, using laser irradiation. Methionine-enkephalin immunopositive nerve fibers occur in the myenteric plexus, circular muscle layer, submucosa, and mucosa. Immunopositive nerve cell bodies in the myenteric plexus have dendrite-like and a long axon-like processes. In the froglet (3 months after metamorphosis), these axon-like processes lead posteriorly in the nerve strand of the myenteric plexus. Some bifurcate, one branch continuing posteriorly, the other doubling back to lead anteriorly; both form terminal varicose fibers in the circular muscle layer. Nerve lesion experiments, in the adult bullfrog, resulted in accumulations of methionine-enkephalin immunoreactivity at the oral and hinder edges of the laser-irradiated necrotic area; there were sprouting and nonsprouting immunopositive stumps. It is suggested that bidirectional flow of methionine-enkephalin in the myenteric plexus is mediated via the anterior and posterior branches of the axon-like process. The difference in sprouting behavior of immunopositive nerve fiber stumps, after nerve lesion, is discussed with reference to regional differences of the axon-like process.     

Histochemical and electron microscopical demonstration of the sympathetic nerve fibers joining to the fourth and the sixth cranial nerves in rats

The localization of sympathetic fibers on the floor of the cranium was studied in rats using amine fluorescence histochemistry, neuropeptide-Y (NPY) immunohistochemistry, and electron microscopy. The vast majority of amine fluorescent fibers joined the abducent nerve and were localized in the peripheral zone under the perineurium. After advancing along this nerve for some distance, the fibers diverged into many bundles that converged to form the cavernous plexus at a rostral end of the trigeminal ganglion. On the dorsal surface of the trigeminal ganglion, one or two medium-calibered fluorescent bundles ran inside or in close proximity to the trochlear nerve, while many small-calibered, brightly fluorescent bundles also extended longitudinally in the epidural connective tissue. In rats that had undergone nerve severance, NPY-immunoreactive fibers were detected at the cut ends of the abducent and trochlear nerve. The differing amounts of NPY accumulated at the rostral and the caudal stumps indicated the direction of the NPY-bearing fibers. Electron microscopy confirmed the presence of unmyelinated fibers in both the abducent and trochlear nerves.     

Complementary Effects of Two Growth Factors in Multifunctionalized Silk Nanofibers for Nerve Reconstruction

With the aim of forming bioactive guides for peripheral nerve regeneration, silk fibroin was electrospun to obtain aligned nanofibers. These fibers were functionalized by incorporating Nerve Growth Factor (NGF) and Ciliary NeuroTrophic Factor (CNTF) during electrospinning. PC12 cells grown on the fibers confirmed the bioavailability and bioactivity of the NGF, which was not significantly released from the fibers. Primary neurons from rat dorsal root ganglia (DRGs) were grown on the nanofibers and anchored to the fibers and grew in a directional fashion based on the fiber orientation, and as confirmed by growth cone morphology. These biofunctionalized nanofibers led to a 3-fold increase in neurite length at their contact, which was likely due to the NGF. Glial cell growth, alignment and migration were stimulated by the CNTF in the functionalized nanofibers. Organotypic culture of rat fetal DRGs confirmed the complementary effect of both growth factors in multifunctionalized nanofibers, which allowed glial cell migration, alignment and parallel axonal growth in structures resembling the ‘bands of Bungner’ found in situ. Graftable multi-channel conduits based on biofunctionalized aligned silk nanofibers were developed as an organized 3D scaffold. Our bioactive silk tubes thus represent new options for a biological and biocompatible nerve guidance conduit.     

Histochemical and electron microscopical demonstration of the sympathetic nerve fibers joining to the fourth and the sixth cranial nerves in rats

Summary The localization of sympathetic fibers on the floor of the cranium was studied in rats using amine fluorescence histochemistry, neuropeptide-Y (NPY) immunohistochemistry, and electron microscopy. The vast majority of amine fluorescent fibers joined the abducent nerve and were localized in the peripheral zone under the perineurium. After advancing along this nerve for some distance, the fibers diverged into many bundles that converged to form the cavernous plexus at a rostral end of the trigeminal ganglion. On the dorsal surface of the trigeminal ganglion, one or two medium-calibered fluorescent bundles ran inside or in close proximity to the trochlear nerve, while many small-calibered, brightly fluorescent bundles also extended longitudinally in the epidural connective tissue. In rats that had undergone nerve severance, NPY-immunoreactive fibers were detected at the cut ends of the abducent and trochlear nerve. The differing amounts of NPY accumulated at the rostral and the caudal stumps indicated the direction of the NPY-bcaring fibers. Electron microscopy confirmed the presence of unmyelinated fibers in both the abducent and trochlear nerves.Dedicated to Professor Dr. T. H. Schiebler on the occasion of his 65th birthday.     

Arylsulfatase A and sphingomyelinase activities in the normal sciatic nerve of the rat, and in proximal and distal fragments]

We measured the protein, arylsulfatase A and sphingomyelinase activities in the total normal rat sciatic nerve and in the proximal and distal stumps. The protein level of the whole sciatic nerve (about 8 % of fresh weight) is similar to the levels of the stumps. Arylsulfatase A activity, in the total sciatic nerve as well as in the stumps is more important by gram of fresh weight than the sphingomyelinase activity. Both enzymes activities, by gram of fresh weight, are more important in the proximal stumps than in the distal ones.     

PRP-1 Protective Effect against Central and Peripheral Neurodegeneration following n. ischiadicus Transection

We investigated the action of the new hypothalamic proline-rich peptide (PRP-1), normally produced by neurosecretory cells of hypothalamic nuclei (NPV and NSO), 3 and 4 weeks following rat sciatic nerve transection. The impulse activity flow of interneurons (IN) and motoneurons (MN) on stimulation of mixed (n. ischiadicus), flexor (n. gastrocnemius – G) and extensor (n. peroneus communis – P) nerves of both injured and symmetric intact sides of spinal cord (SC) was recorded in rats with daily administration of PRP-1 (for a period of 3 weeks) and without it (control). On the injured side of SC in control, there were no responses of IN and MN on ipsilateral G and P stimulation, while responses were elicited on contralateral nerve stimulation. The neuron responses on the intact side of SC were revealed in a reverse ratio. Thus, there were no effects upon stimulation of the injured nerve distal stump in the control because of the absence of fusion between transected nerve stumps. This was also testified by the atrophy of the distal stump and the absence of motor activity of the affected limb. In PRP-1-treated animals, the responses of SC IN and MN in postaxotomy 3 weeks on the injured side of SC at ipsilateral nerve stimulation and on the intact side at contralateral nerve stimulation were recorded because of the obvious fusion of the severed nerve stumps. The histochemical data confirmed the electrophysiological findings. Complete coalescence of transected fibers together with restoration of the motor activity of the affected limb provided evidence for reinnervation on the injured side. Thus, it may be concluded that PRP-1 promotes nerve regeneration and may be used clinically to improve the outcome of peripheral nerve primary repair.     

Tonic activity of parasympathetic efferent nerve fibers hyperpolarizes the resting membrane potential of frog taste cells

We investigated the relationship between the membrane potential of frog taste cells in the fungiform papillae and the tonic discharge of parasympathetic efferent fibers in the glossopharyngeal (GP) nerve. When the parasympathetic preganglionic fibers in the GP nerve were kept intact, the mean membrane potential of Ringer-adapted taste cells was -40 mV but decreased to -31 mV after transecting the preganglionic fibers in the GP nerve and crushing the postganglionic fibers in the papillary nerve. The same result occurred after blocking the nicotinic acetylcholine receptors on parasympathetic ganglion cells in the tongue and blocking the substance P neurokinin-1 (NK-1) receptors in the gustatory efferent synapses. This indicates that the parasympathetic nerve (PSN) hyperpolarizes the membrane potential of frog taste cells by -9 mV. Repetitive stimulation of a transected GP nerve revealed that a -9-mV hyperpolarization of taste cells maintained under the intact GP nerve derives from an approximately 10-Hz discharge of the PSN efferent fibers. The mean frequency of tonic discharges extracellularly recorded from PSN efferent fibers of the taste disks was 9.1 impulses/s. We conclude that the resting membrane potential of frog taste cells is continuously hyperpolarized by on average -9 mV by an approximately 10-Hz tonic discharge from the parasympathetic preganglionic neurons in the medulla oblongata.     

Hyaluronate accumulation and nerve-dependent growth during regeneration of larval Ambystoma limbs

Hyaluronate-mediated expansion of the extracellular matrix has been suggested as an important element of growth and morphogenesis in several developing systems. In vitro, various growth factors have been shown to stimulate hyaluronate synthesis as well as cell proliferation. A similar link between proliferation and hyaluronate production during in vivo growth is difficult to demonstrate, because in most systems the source of growth-promoting factors is either not known or not amenable to experimental manipulation. During amphibian limb regeneration, cell proliferation depends upon paracrine release of factors from axons in the limb stump, and the nerve supply can be eliminated or augmented experimentally for study of growth in this system. Denervated and amputated limbs of larval salamanders do not begin to regenerate until distal areas of the limb stumps are reinnervated. We have used such limbs to examine the effect exerted by the reappearance of nerves on the amount of hyaluronate in the tissue undergoing the growth response. Hyaluronate was demonstrated by the metachromatic dye Ethyl Stains-all, which stains hyaluronate blue while sulfated glycosaminoglycans (GAGs) and proteins in the extracellular matrix stain various shades of violet, and by microspectrophotometry of alcian-blue-stained GAGs in serial sections pretreated with buffer or with Streptomyces hyaluronidase (SH) to remove hyaluronate specifically. Both methods showed little hyaluronate in the distal region of limb stumps prior to reinnervation, while reinnervated stumps had amounts of hyaluronate similar to those of control blastemas. Autoradiography of ³H-glucosamine-labeled limbs indicated that hyaluronate in the blastemas of reinnervated limb stumps included material newly synthesized by cells throughout the growing tissue. The microspectrophotometric study revealed that the relative concentration of hyaluronate increased during the time distal limb areas were undergoing reinnervation, which was monitored by staining of nerve fibers. The increase in hyaluronate concentration was followed immediately by an increase in mitotic activity and a decrease in mesenchymal cell density, two changes leading to blastema formation that others have shown to be associated with reinnervation in this system. These observations indicate that the growth-promoting influence of nerves includes stimulation of hyaluronate production, an effect similar to that of serum or purified mitogens on many cultured cells. Hyaluronate synthesis appears to promote expansion of the limb stump, which occurs when denervated-amputated larval limbs are reinnervated.     

Electrical Stimulation Promotes Regeneration of Defective Peripheral Nerves after Delayed Repair Intervals Lasting under One Month

Background

Electrical stimulation (ES) has been proven to be an effective means of enhancing the speed and accuracy of nerve regeneration. However, these results were recorded when the procedure was performed almost immediately after nerve injury. In clinical settings, most patients cannot be treated immediately. Some patients with serious trauma or contaminated wounds need to wait for nerve repair surgery. Delays in nerve repair have been shown to be associated with poorer results than immediate surgery. It is not clear whether electrical stimulation still has any effect on nerve regeneration after enough time has elapsed.

Methods

A delayed nerve repair model in which the rats received delayed nerve repair after 1 day, 1 week, 1 month, and 2 months was designed. At each point in time, the nerve stumps of half the rats were bridged with an absorbable conduit and the rats were given 1 h of weak electrical stimulation. The other half was not treated. In order to analyze the morphological and molecular differences among these groups, 6 ES rats and 6 sham ES rats per point in time were killed 5 days after surgery. The other rats in each group were allowed to recover for 6 weeks before the final functional test and tissue observation.

Results

The amounts of myelinated fibers in the distal nerve stumps decreased as the delay in repair increased for both ES rats and sham ES rats. In the 1-day-delay and 1-week-delay groups, there were more fibers in ES rats than in sham ES rats. And the compound muscle action potential (CMAP) and motor nerve conduction velocity (MNCV) results were better for ES rats in these two groups. In order to analyze the mechanisms underlying these differences, Masson staining was performed on the distal nerves and quantitative PCR on the spinal cords. Results showed that, after delays in repair of 1 month and 2 months, there was more collagen tissue hyperplasia in the distal nerve in all rats. The brain-derived neurotrophic factor (BDNF) and trkB expression levels in the spinal cords of ES rats were higher than in sham ES rats. However, these differences decreased as the delay in repair increased.

Conclusions

Electrical stimulation does not continue to promote nerve regeneration after long delays in nerve repair. The effective interval for nerve regeneration after delayed repair was found to be less than 1 month. The mechanism seemed to be related to the expression of nerve growth factors and regeneration environment in the distal nerves.     

Nerve Cross-Bridging to Enhance Nerve Regeneration in a Rat Model of Delayed Nerve Repair

There are currently no available options to promote nerve regeneration through chronically denervated distal nerve stumps. Here we used a rat model of delayed nerve repair asking of prior insertion of side-to-side cross-bridges between a donor tibial (TIB) nerve and a recipient denervated common peroneal (CP) nerve stump ameliorates poor nerve regeneration. First, numbers of retrogradely-labelled TIB neurons that grew axons into the nerve stump within three months, increased with the size of the perineurial windows opened in the TIB and CP nerves. Equal numbers of donor TIB axons regenerated into CP stumps either side of the cross-bridges, not being affected by target neurotrophic effects, or by removing the perineurium to insert 5-9 cross-bridges. Second, CP nerve stumps were coapted three months after inserting 0-9 cross-bridges and the number of 1) CP neurons that regenerated their axons within three months or 2) CP motor nerves that reinnervated the extensor digitorum longus (EDL) muscle within five months was determined by counting and motor unit number estimation (MUNE), respectively. We found that three but not more cross-bridges promoted the regeneration of axons and reinnervation of EDL muscle by all the CP motoneurons as compared to only 33% regenerating their axons when no cross-bridges were inserted. The same 3-fold increase in sensory nerve regeneration was found. In conclusion, side-to-side cross-bridges ameliorate poor regeneration after delayed nerve repair possibly by sustaining the growth-permissive state of denervated nerve stumps. Such autografts may be used in human repair surgery to improve outcomes after unavoidable delays.     

The virtue of being too early: Paul A. Weiss and 'axonal transport'

The essay introduces how Paul A. Weiss (1898-1989) analyzed his data on neuronal outgrowth and axonal transport, supported by constriction experiments of thousands of living mature nerve fibers. At the University of Chicago his group measured the steady proximo-distal flow of nerve fibers. To visualize the data he used tissue culturing, light microscopy, radioactive tracers, time-lapse motion pictures and electronmicroscopy. The work resulted in the discovery of fasciculation of outgrowing nerves and a computation of the rate of axonal transport, published in a classical article in 1948. He stated that the outgrowth of nerve fibers occurs from nerve centers in their nucleated cell bodies by fasciculation and protein synthesis. However, at that time one suspected the published microphotograph to be an idealized image and, therefore, did not accept the analysis, or the 'new' knowledge. In the mid 1960s the improved technique of autoradiography confirmed in an indirect way Weiss data and analysis of axonal transport. The objective here is to show (1) how Weiss employed the visibility of micrographs and drawings to corroborate his observations and analysis on axonal dynamics, and (2) to offer some tentative suggestions why his colleagues did not accept the 'neuro-images' as an evidence of new knowledge.     

周期性单轴拉伸力学刺激对哺乳动物细胞有丝分裂方向影响的研究

哺乳动物细胞的有丝分裂过程与细胞的增殖、分化以及生物体发育、组织器官形成、损伤组织的修复和疾病的发生有关．广泛存在的力学刺激能否对细胞有丝分裂方向产生影响,以及其影响有丝分裂定向的途径尚未完全阐明．采用小鼠成纤维细胞作为模型,研究周期性单轴拉伸力学刺激对细胞应力纤维排布和有丝分裂方向的影响．结果表明,周期性单轴拉伸诱导细胞有丝分裂与应力纤维垂直于拉伸方向排布．而阻断应力纤维的两种基本组成成分(微丝和肌球蛋白Ⅱ),会造成在周期性单轴拉伸条件下的应力纤维和有丝分裂方向重排．特别是,Y27632 (10 μmol/L) 和低浓度的ML7 (50 μmol/L)、Blebbistatin (50 μmol/L)可以诱导细胞有丝分裂与应力纤维平行于拉伸方向排布．统计结果表明,在不同实验条件下,应力纤维排布和有丝分裂方向均具有高度相关性．Western blot实验表明,肌球蛋白轻链磷酸化水平与周期性单轴拉伸刺激下的应力纤维排 布和有丝分裂方向密切相关．上述结果提示：周期性单轴拉伸力学刺激通过诱导应力纤维的排布,决定了细胞的有丝分裂方向．     

Thallium transport and the evaluation of olfactory nerve connectivity between the nasal cavity and olfactory bulb

Little is known regarding how alkali metal ions are transported in the olfactory nerve following their intranasal administration. In this study, we show that an alkali metal ion, thallium is transported in the olfactory nerve fibers to the olfactory bulb in mice. The olfactory nerve fibers of mice were transected on both sides of the body under anesthesia. A double tracer solution (thallium-201, (201)Tl; manganese-54, (54)Mn) was administered into the nasal cavity the following day. Radioactivity in the olfactory bulb and nasal turbinate was analyzed with gamma spectrometry. Auto radiographic images were obtained from coronal slices of frozen heads of mice administered with (201)Tl or (54)Mn. The transection of the olfactory nerve fibers was confirmed with a neuronal tracer. The transport of intranasal administered (201)Tl/(54)Mn to the olfactory bulb was significantly reduced by the transection of olfactory nerve fibers. The olfactory nerve transection also significantly inhibited the accumulation of fluoro-ruby in the olfactory bulb. Findings indicate that thallium is transported by the olfactory nerve fibers to the olfactory bulb in mice. The assessment of thallium transport following head injury may provide a new diagnostic method for the evaluation of olfactory nerve injury.     

High-Frequency Electrical Stimulation Can Be a Complementary Therapy to Promote Nerve Regeneration in Diabetic Rats

The purpose of this study was to evaluate whether 1 mA of percutaneous electrical stimulation (ES) at 0, 2, 20, or 200 Hz augments regeneration between the proximal and distal nerve stumps in streptozotocin diabetic rats. A10-mm gap was made in the diabetic rat sciatic nerve by suturing the stumps into silicone rubber tubes. Normal animals were used as the controls. Starting 1 week after transection, ES was applied between the cathode placed at the distal stump and the anode at the proximal stump every other day for 3 weeks. At 4 weeks after surgery, the normal controls and the groups receiving ES at 20, and 200 Hz had a higher success percentage of regeneration compared to the ES groups at 0 and 2 Hz. In addition, quantitative histology of the successfully regenerated nerves revealed that the groups receiving ES at a higher frequency, especially at 200 Hz, had a more mature structure with more myelinated fibers compared to those in the lower-frequency ES groups. Similarly, electrophysiology in the ES group at 200 Hz showed significantly shorter latency, larger amplitude, larger area of evoked muscle action potentials and faster conduction velocity compared to other groups. Immunohistochemical staining showed that ES at a higher frequency could significantly promote calcitonin gene-related peptide expression in lamina I-II regions in the dorsal horn and recruit a higher number of macrophages in the diabetic distal sciatic nerve. The macrophages were found that they could stimulate the secretion of nerve growth factor, platelet-derived growth factor, and transforming growth factor-β in dissected sciatic nerve segments. The ES at a higher frequency could also increase cutaneous blood flow in the ipsilateral hindpaw to the injury. These results indicated that a high-frequency ES could be necessary to heal severed diabetic peripheral nerve with a long gap to be repaired.     

Encoding of a spectrally-complex communication sound in the bullfrog's auditory nerve

1. A population study of eighth nerve responses in the bullfrog, Rana catesbeiana, was undertaken to analyze how the eighth nerve codes the complex spectral and temporal structure of the species-specific advertisement call over a biologically-realistic range of intensities. Synthetic advertisement calls were generated by Fourier synthesis and presented to individual eighth nerve fibers of anesthetized bullfrogs. Fiber responses were analyzed by calculating rate responses based on post-stimulus-time (PST) histograms and temporal responses based on Fourier transforms of period histograms. 2. At stimulus intensities of 70 and 80 dB SPL, normalized rate responses provide a fairly good representation of the complex spectral structure of the stimulus, particularly in the low- and mid-frequency range. At higher intensities, rate responses saturate, and very little of the spectral structure of the complex stimulus can be seen in the profile of rate responses of the population. 3. Both AP and BP fibers phase-lock strongly to the fundamental (100 Hz) of the complex stimulus. These effects are relatively resistant to changes in stimulus intensity. Only a small number of fibers synchronize to the low-frequency spectral energy in the stimulus. The underlying spectral complexity of the stimulus is not accurately reflected in the timing of fiber firing, presumably because firing is 'captured' by the fundamental frequency. 4. Plots of average localized synchronized rate (ALSR), which combine both spectral and temporal information, show a similar, low-pass shape at all stimulus intensities. ALSR plots do not generally provide an accurate representation of the structure of the advertisement call. 5. The data suggest that anuran peripheral auditory fibers may be particularly sensitive to the amplitude envelope of sounds.     

周围神经再生过程中靶器官的诱向性作用机制

以细胞培养技术与自然凝胶电泳系统方法证明,在周围神经再生过程中,损伤的坐骨神经远侧端,即起衍生的靶器官诱导神经突起的定向生长。分析探讨与神经诱向性再生相关的活性因子,其结果提示在远侧端神经组织中出现的90kDa蛋白组分具有很强的诱向性作用,诱导神经突起在神经再生过程中能够准确地到达靶器官。由此说明雪旺细胞在神经再生过程中扮演着重要角色。     

Studies on the Distribution of Factor I and Acetylcholine in Crustacean Peripheral Nerve

Extracts of whole nerve (chelipeds of Cancer magister) cause inhibition of impulse generation of the crayfish stretch receptor preparation, similar to that produced by gamma-aminobutyric acid (GABA). This is not found with extracts containing only sensory or sensory and motor fibers. Extracts of inhibitory fibers inhibit the stretch receptor discharge—indicating an inhibitory action equivalent to that of up to 30,000 micrograms of GABA per gm. wet weight of inhibitor fiber. This high value is taken as an indication that the inhibitory substance in crab inhibitory fibers is not identical with gamma-aminobutyric acid. Whole nerves were found to contain 1.7 to 6.7 µg. acetylcholine per gm. nerve tissue (clam ventricle and frog rectus abdominis muscle). No acetylcholine could be detected in extracts of motor and inhibitory fibers. The acetylcholine content of sensory fibers can account for the acetylcholine activity of whole nerve extract. It is concluded that the factor I of crustacean nerve is an exclusive property of the inhibitory fibers. The results support the assumption that factor I is the transmitter substance of inhibitory neurons in these animals. The absence of acetylcholine in motor fibers indicates that this substance does not function as a transmitter of motor impulses in Crustacea, and explains the previously observed failure of the substance to elicit motor responses in these animals. The function of acetylcholine in sensory fibers is not yet clarified.     

An approximation method for the determination of diffusion and permeability coefficients in nerve trunks

A recently introduced approximation method is applied in order to obtain an expression for the amount of a substance remaining within a nerve at any time, the nerve having been soaked for a long time in a solution containing the substance until the time zero when it is bathed in the same solution but without the substance. The case of a uniform nerve without a sheath leads to substantially the same results as previously obtained by A. V. Hill (1928) for this case. A solution is given for the case of a nerve without sheath but having fibers which are permeable. In this case it is shown how an effective diffusion coefficient for the interstitial fluid can be obtained, as well as the effective inward and outward fiber permeabilities. A solution is given for the case of a nerve with a sheath in which the substance considered does not penetrate the fibers, and it is shown how the effective diffusion coefficients of the sheath and the interstitial fluid can be obtained.     

A kinematic model of stretch-induced stress fiber turnover and reorientation

A kinetic model based on constrained mixture theory was developed to describe the reorganization of actin stress fibers in adherent cells in response to diverse patterns of mechanical stretch. The model was based on reports that stress fibers are pre-extended at a “homeostatic” level under normal, non-perturbed conditions, and that perturbations in stress fiber length destabilize stress fibers. In response to a step change in matrix stretch, the model predicts that stress fibers are initially stretched in registry with the matrix, but that these overly stretched fibers are gradually replaced by new fibers assembled with the homeostatic level of stretch in the new configuration of the matrix. In contrast, average fiber stretch is chronically perturbed from the homeostatic level when the cells are subjected to cyclic equibiaxial stretch. The model was able to describe experimentally measured time courses of stress fiber reorientation perpendicular to the direction of cyclic uniaxial stretch, as well as the lack of alignment in response to equibiaxial stretch. The model also accurately described the relationship between stretch magnitude and the extent of stress fiber alignment in endothelial cells subjected to cyclic uniaxial stretch. Further, in the case of cyclic simple elongation with transverse matrix contraction, stress fibers orient in the direction of least perturbation in stretch. In summary, the model predicts that the rate of stretch-induced stress fiber disassembly determines the rate of alignment, and that stress fibers tend to orient toward the direction of minimum matrix stretch where the rate of stress fiber turnover is a minimum.