Paris, May 28, 2009
The axon is a part of the neuron through which nerve impulses are transmitted, and at the end of which is located the synapse, which connects it to another neuron. In the event of a lesion, the axon is the component which must be regenerated in order to restore the connections between the different neurons and re-form the nerve.
The regeneration capacity of axons within the central nervous system, of which the spinal cord is part, has until now been much debated. Axons can regenerate toward the muscles, whereas in the opposite direction inhibiting factors prevent regrowth toward the nerve centers. The observation made by Geneviève Rougon's team at IBDML shows that the axons also regrow in the direction of the spinal cord within a short lapse of time after the injury. Moreover, this regrowth is encouraged by post-traumatic angiogenesis, in other words by the process of formation of new blood vessels in the damaged tissue.
After injury to a mouse's spinal cord, extensive and extremely active angiogenesis is observed, peaking in intensity one week after the lesion. At the same time, regrowth of the axons takes place preferentially and more rapidly in the vicinity of the blood vessels. These observations suggest that stimulating and prolonging angiogenesis could open up new prospects for treatment and encourage functional recovery after, for instance, lesion of the spinal cord.
This spatio-temporal interaction was described by combining two new techniques in imaging: the use of mice whose cell populations can be observed thanks to their fluorescent properties, and 2-photon microscopy. This new imaging protocol makes it possible to display in situ and in 3D the cell phenomena that come into play under traumatic or pathological conditions, and to characterize their dynamics by means of repeated observations of the same mouse. In this way, cell interactions can be described dynamically, over space and time, in a live animal, something that is impossible to do with conventional histology(1) techniques, which require the sacrifice of several animals at each relevant stage. This non-invasive technique drastically reduces the number of mice used and, since the experiment can be reproduced using the same animal, considerably improves the result.
In addition to its importance for fundamental research, such a combination opens up new prospects for preclinical research. In the field of pharmacology, for instance, this kind of dynamic imaging could make it possible to precisely define application protocols for medicines, and better control their effects and adjust dosage.
© Franck Debarbieux, IBDML
Fluorescence images obtained in vivo by 2-photon microscopy, showing the comparative organization of axons (green) and blood vessels (red) at the level of the dorsal column of the spinal cord of a mouse after lesion. The axons were imaged in the same mouse on the 3rd and 7th days after the lesion. As shown by the arrows that indicate the end of the regrowing axon, the injured axons in direct contact with blood vessels (middle two images) are elongating much more quickly than the axons located far from blood vessels (two images at left) On the 14th day after the lesion (image at right), regrowing axons extend through and past the site of the lesion shown by an asterisk, contradicting the dogma that states that axons are not able to regenerate in the spinal cord.
1) Histology is the branch of biology which studies tissue.
Quantitative analysis by in vivo imaging of the dynamics of vascular and axonal networks in injured mouse spinal cord, PNAS online, 21 May 2009
Cyril Dray, Geneviève Rougon, Franck Debarbieux
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