Ever-changing shapes of nerve cells.

The complex treelike silhouette of a mature nerve cell can vary continuously, with individual branches textending, retracting, disappearing or forming anew, report scientists at Washington University Medical School in St. Louis. Many of the signals from other nerve cells are received along these branches, which are called dendrites. Long-term changes in the nervous sytem occur at specialized communication sites, known as synapses. An important question has been whether the long-term changes involve primarily alterations in the function of existing synapses--for example, those changes described in the marine snail Aplysia (SN: 11/16/85, p. 308) -- or whether there are structural changes in the nerve cell.

A new technique now allows scientists to analyze the shape of an individual nerve cell over days and months. Robert D. Hadley and Dale Purves have examined mouse nerve cells on the surface of a cluster of cells, called the superior cervical ganglion, located in the animal's neck. They anesthetize the mouse and photographically record the location of a nerve cell. Then they inject the cell with a nontoxic fluorescent dye that diffuses into the dendrites, revealing from the anesthesia. A few days to a few months later, the procedure is repeated to provide a second view of the same cell.

"Dendritic arbors contract and extend, but mostly extend," says Hadley. He reports an average dendritic growth in young adult mice of about 10 percent during three to seven days. After longer periods, there were progressively greater changes of dendritic geometry. "The subtle changes in single cells that we describe would have been impossible to discern by looking at populations of neurons with conventional means," Hadley and Purves say. "These morphological changes are almost certainly associated with functional changes in the synaptic circuitry." They conclude, "Such modulations of connectivity may bear on the cellular basis of long-term change in the central nervous system."