Every year, hundreds of thousands of people suffer from paralyzed limbs as a result of peripheral nerve injury. Recently, implantation of artificial nerve grafts has become the method of choice for repairing damaged peripheral nerves. Grafts can lead to some degree of functional recovery when a short segment of nerve is damaged. But they are of little use when it comes to regenerating nerves over distances greater than a few millimeters, and such injuries therefore often lead to permanent paralysis.
Now though, surgeons from Germany have made what could be a significant advance in nerve tissue engineering. They have developed artificial nerve grafts made from hollowed-out pig veins filled with spider silk fibres and, in a series of animal experiments, showed that the grafts can enhance the regeneration of peripheral nerves over distances of up to 6cm. Their findings have just been published in the open access journal PLoS One.
Peripheral nerves have a greater regenerative capacity than those in the central nervous system, but regenerating them properly is challenging. The individual nerve fibres must not only regrow into the damaged area, but also find their proper targets. Furthermore, the regenerated nerve will not function properly unless it is populated by Schwann cells, which produce myelin. This fatty tissue is essential for full recovery, as it wraps itself around the nerve fibres at regular intervals (a process called myelination), facilitating the conductance of nervous impulses along their length.
Conventionally, damaged peripheral nerves are treated either by suturing or by implantation of nerve grafts. The two ends of a severed nerve can be surgically re-attached to each other, as long as the nerve is not stretched in the process. This is not possible for gaps longer than about 5mm, in which case a short length of nerve from elsewhere in the patient’s body can be grafted into the damaged area. But this often causes causes pain in the donor area, and it can be difficult to find a nerve segment that has the same diameter as the damaged nerve. Nerves can be obtained from another person, but they can be rejected by the recipient’s immune system, so drugs that suppress the immune response are usually administered.
An alternative approach, which has emerged in the past ten years or so, is the use of artificial nerve grafts made from silicon or synthetic polymers such as polyethylene. These form scaffolds which bridge the gap in the damaged nerve and serve as conduits through which the nerve fibres can regrow. Artificial grafts can lead to some degree of functional recovery, but they can become toxic with time, or they can constrict the nerve.
These problems can potentially be overcome by using nerve grafts made from biodegradable materials. Five years ago, Peter Vogt and his colleagues in the Department of Plastic, Hand and Reconstructive Surgery at Hannover Medical School reported that Schwann cells readily ensheath spider silk fibres when grown on them, and that nerve grafts made of de-cellularized veins filled with spider silk can be maintained in culture for periods of up to a week. More recently, they showed that spider silk vein grafts can be used to regenerate 20mm gap in the sciatic nerve of rats, either alone or when supplemented with Schwann cells.
In the new study, Vogt’s group dissected 6cm lengths from the small veins in pigs’ legs, washed them and stripped away most of the endothelial cells from their inner walls. They then harvested dragline silk from the golden silk spider Nephila clavipes and pulled the silk through the de-cellularized veins, until it filled about one quarter of their diameter. Using adult sheep, the researchers removed a 6cm length of the tibial nerve in the leg. In one group of animals, the gap was bridged with the spider silk constructs; in another, the section of nerve that had been removed was replaced in reverse orientation.
Defects in the animals’ gait became apparent immediately after the surgery – the hind limb was partially paralyzed and flexed abnormally. But within three weeks there was a significant improvement, with both groups of animals being able to stand properly. By four months, the animals could stand upright on both hind limbs, the hind limbs moved in co-ordination with one another during walking, and there was no obvious difference in strength between the operated and unoperated limbs.
Ten months after surgery, the sheep were killed and their regenerated nerves examined under the microscope. In both groups of animals, the severed nerve fibres had regrown into the nerve grafts to bridge the 6cm gap; Schwann cells had migrated into the grafts and wrapped themselves around the entire length of the regenerated nerves; and the sodium channels required for generating nerve impulses were distributed irregularly along the fibres. This shows that myelination had occurred properly, with the formation of Nodes of Ranvier, the regular gaps in the myelin sheath at which the sodium channels normally cluster. No trace of residual spider silk was detected in the experimental animals, and there was no sign of inflammation at the repair site, indicating that the silk fibres were absorbed subtly without adverse effects.
These findings could have important applications in reconstructive nerve surgery. This is the first time that a large animal model has been used to study nerve regeneration, and the study is the first in which a defect longer than 2cm in length has been successfully repaired. The spider silk constructs enhanced nerve regeneration at least as effectively as the sheeps’ own nerves, and would be advantageous in the clinic, because transplanting large lengths of a patient’s own nerves is unfeasible.
More work will be needed before the technique can be applied to humans. Meanwhile, the regeneration reported here could be further enhanced in a number of ways. The spider silk constructs could, for example, be loaded with substances such as Nerve Growth Factor, or they could be grafted together with Schwann cells, to speed up nerve regrowth. But ultimately, engineering fully functional peripheral nerves will probably require a combination of advanced microsurgery, transplantation of both cells and tissues, advances in materials science and, possibly, gene transfer for the effective delivery of growth factors.
Radtke, C. et al. (2011). Spider Silk Constructs Enhance Axonal Regeneration and Remyelination in Long Nerve Defects in Sheep. PLoS ONE, 6 (2) DOI: 10.1371/journal.pone.0016990.
An article on vagus nerve.
The vagus nerve is the tenth cranial nerve. It has the most extensive distribution of any of the cranial nerves and contains sensory, motor and parasympathetic fibers. The vagus emerges from the brain stem at the medulla oblongata, between the olive and the inferior cerebellar peduncle. It exits the cranium through the jugular foramen with the glossopharyngeal and accessory nerves. The vagus nerve has two ganglia, the superior and inferior ganglia. The superior ganglion lies within the jugular foramen. The inferior ganglion is situated just below. Just below the inferior ganglion, the vagus is joined by the cranial part of the accessory nerve. The vagus then passes downwards within the carotid sheath and enters the thorax at the root of the neck.
An article on sympathetic nerves, phrenic and splanchnic nerves.
The sympathetic ganglia are swellings along the length of the pair of sympathetic trunks running longitudinally on either side of the vertebral column. The sympathetic trunks are located anterior to the cervical transverse processes, anterior to the heads of the ribs, on the antero- lateral aspects of the lumbar vertebral bodies, on the anterior aspect of the sacrum (medial to the ventral sacral foramina) and on the front of the coccyx. They are located at the site of synapses between the preganglionic and postganglionic neurons. There are a variable number of ganglia, approximately two (or three) cervical, eleven thoracic, four lumbar, four sacral and one coccygeal. The ventral rami of all spinal nerves are connected to the sympathetic trunk by gray rami communicantes. The ventral rami of T1 to L2 (L3) are also connected to the sympathetic trunk by white rami communicantes.
An article on superior cervical ganglia.
The superior cervical ganglion is the largest of the cervical ganglia and consists of the fused ganglia of C1 to C4. It is situated at the level of the second and third cervical vertebrae, anterior to the longus capitis muscle and posterior to the internal carotid artery and its carotid sheath. It is connected to the middle cervical ganglion inferiorly by the sympathetic trunk. It gives rise to lateral, medial and anterior branches. The lateral branches of the superior cervival ganglion consist of gray rami communicantes, which pass to the four upper cervical spinal nerves, the inferior vagal ganglion and hypoglossal nerve (XII cranial nerve). The jugular nerve ascends to the base of the skull and divides to join the inferior glossopharyngeal and superior vagal ganglia; other fibers reach the superior jugular bulb and meninges of the posterior cranial fossa.
A short article on the course of ulnar nerve.
The ulnar nerve (C7, 8, T1) is the continuation of the medial cord of the brachial plexus. It is usually joined in the axilla by fibers from C7. The ulnar nerve is motor to most of the small muscles of the hand, to flexor carpi ulnaris, and to the ulnar half of the flexor digitorum profundus. It provides sensibility to the ulnar aspect of the hand.
A short article on the course of radial nerve.
The radial nerve is the direct continuation of the posterior cord (C5-T1) of the brachial plexus. It supplies the muscles of the extensor compartments, the skin overlaying them and the skin over the dorusm od the hand.
A short article on the course of median nerve.
The median nerve (C5, 6, 7, 8, T1) is motor to most of the long flexors of the forearm and muscles of the thenar eminence. It supplies sensibility to the skin of the palm, (usually) radial three and half digits, elbow, wrist, and hand joints.