Zou and colleagues studied the guidance system used to assemble this complex network in newborn mice, where corticospinal axon growth is still underway. Before birth, axons grow out from the cell body of a nerve cell in the motor cortex. The axons follow a path back through the brain to the spinal cord.
By the time of birth, the axons are just growing into the cord. During the first week after birth they grow down the cervical and thoracic spinal cord until they reach their proper position, usually after seven to ten days.
From previous studies, Zou and colleagues knew that a gradient of various Wnt proteins, including Wnt4, formed along the spinal cord around the time of birth. Here they show that two other proteins, Wnt1 and Wnt5a are produced at high concentrations at the top of the cord and at consecutively lower levels farther down.
They also found that motor nerves are guided by Wnts through a different receptor, called Ryk, that mediates repulsion by Wnts. Antibodies that blocked the Wnt-Ryk interaction blocked the downward growth of corticospinal axons when injected into the space between the dura and spinal cord in newborn mice.
This knowledge, coupled with emerging stem cell technologies, may provide the most promising current approach to nervous system regeneration. If Wnt proteins could be used to guide transplanted nerve cells -- or someday, embryonic stem cells -- to restore the connections between the body and the brain, "it could revolutionize treatment of patients with paralyzing injuries to these nerves," Zou suggests.
"Although half the ba
Source:University of Chicago Medical Center