Like an insect's antennae, filapodia are the fingerlike projections sent out by a developing nerve cell to detect environmental cues. Scientists at the University of California at San Diego have discovered how the filapodia communicate with the main body of the cell: through a kind of biological Morse code.
In a paper in the March 8 issue of Science, the researchers show how the fingertips of the cell sample the environment and generate tiny bursts of calcium that send back information to the "growth cone" -- the cell body -- and enable it to direct growth in the right direction. Neurons are then able to wire up connections in the brain, spinal cord and other parts of the developing embryo.
"Many kinds of birth defects and a large fraction of spinal cord defects appear to be associated with problems in the formation of nerve connections in the developing embryo," said Nicholas Spitzer, a professor of biology at UCSD. "We want to understand how the normal brain is put together, so we can understand the cases in which it is not put together correctly."
"These bursts, which only occur at the very ends of the filapodia, are extremely brief, about a 300-microsecond pulse, which may explain why they were undetected in earlier studies," said Timothy Gomez, who conducted the research when he was a postdoctoral fellow in Spitzer's laboratory.
"Since many other types of cells have these fingerlike projections, we believe that these brief calcium bursts may be a universal signaling mechanism for all motile, or moving cells, including immune, epithelial and metastatic cells."
Gomez and his colleagues studied frog spinal nerve cells, which are easy to grow in the laboratory. They introduced a fluorescent calcium marker to the cells, and recorded, with a camera attached to a microscope, the rapid bursts of calcium at the tips of the filapodia.
The calcium pulses varied with the chemical environment in the culture dish. They were frequency modulated -- FM pulses, like the information transmitted by an FM radio.
"Just like our fingers on the FM dial on the radio, these cues change the FM dial on the growth cone to produce these highly relevant signals," said Spitzer.