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There is a
macabre brilliance to the machine in Jeff Lichtman’s
laboratory at Harvard University that is worthy of a
Wallace and Gromit film. In one end goes brain. Out the
other comes sliced brain, courtesy of an automated arm
that wields a diamond knife. The slivers of tissue drop
on to a conveyor belt
that zips along with the merry whirr of a cine
projector.
Lichtman’s machine is an automated tape-collecting
lathe ultramicrotome (Atlum). It produces long strips of
sticky tape with brain slices attached, all ready to be
photographed through a powerful electron microscope.
When these pictures are combined into 3D images, they
reveal the inner wiring of the organ, a tangled mass of
nervous spaghetti. The research by Lichtman and his
co-workers has a goal in mind that is unthinkably
ambitious.
If we are ever to
understand the brain in full, they say, we must know how
every neuron inside is wired up.
Though fanciful, the payoff could be profound. Map
out our ‘connectome’ - following other major ‘ome’
projects such as the genome and transcriptome - and we
will lay bare the biological code of our personalities,
memories, skills and susceptibilities. Somewhere in our
brains is who we are.
To use an understatement heard often from scientists,
the job at hand is not trivial. Lichtman’s machine
slices brain tissue into exquisitely thin wafers. To
turn a 1mm thick slice of brain into neural salami takes
six days in a process that yields about 30,000 slices.
But chopping up the brain is the easy part. When
Lichtman began this work several years ago, he
calculated how long it might take to image every slice
of a 1cm mouse brain. The answer was 7,000 years. The human brain is another
story. There are 85bn neurons in the 1.4kg of flesh
between our ears. Each has a cell body (grey matter) and
long, thin extensions called dendrites and axons (white
matter) that reach out and link to others.
Most neurons have lots of
dendrites that receive information from other nerve
cells, and one axon that branches on to other cells and
sends information out. On average, each neuron forms
10,000 connections, through synapses with other nerve
cells. Altogether, Lichtman estimates there are between
100tn and 1,000tn connections between neurons.
Unlike the lung, or the
kidney, where the whole organ can be understood, more or
less, by grasping the role of a handful of repeating
physiological structures, the brain is made of thousands
of specific types of brain cell that look and behave
differently. Their names - Golgi, Betz, Renshaw,
Purkinje - read like a roll call of the pioneers of
neuroscience. |
