A certain Nobel prize

Chemists will be green with envy to find out that a Nobel prize (possibly in Chemistry) is almost certain to be won by someone using using nothing fancier than formaldehyde, acrylamide and an ionic detergent (Sodium Dodecyl Sulfate — the SDS of electrophoresis fans everywhere)_. For details see Nature vol. 497 pp. 332 – 337 ’13 (16 May Issue). It’s by the same man (Karl Deisseroth) who already has a Nobel coming for the invention of optogenetics.

First — a bit of history. The tissue of the brain is so tightly packed that it is impossible to see the cells that make it up with the usual stains used by light microscopists. People saw nuclei all right but they thought the brain was a mass of tissue with nuclei embedded in it (like a slime mold). Muscle is like that — long fibers with hundreds of nuclei here and there. It wasn’t until that late 1800’s that Camillo Golgi developed a stain which would now and then outline a neuron with all its processes. Another anatomist (Ramon Santiago y Cajal) used Golgi’s technique and argued with Golgi that yes the brain was made of cells. Fascinating that Golgi, the man responsible for showing nerve cells, didn’t buy it. This was a very hot issue at the time, and the two received a joint Nobel prize in 1906 (only 5 years after the prizes began).

How tightly packed is the brain? The shortest wavelength of visible light is 4000 Angstroms. Cells in the brain are packed far more tightly. To see the space between the brain cell external membranes you need an electron microscope (EM). Just preparing a sample for EM really fries the tissue. Neurons are packed together with less than 1000 Angstroms between them. So how much of this is artifact of preparation for electron microscopy has never been clear to me. One study injected a series of quantum dots of known diameter into the cerebral spinal fluid (CSF) to see the smallest sized dot that could insinuate itself between neurons [ Proc. Natl. Acad. Sci. vol. 103 pp. 5567 – 5572 ’06 ]. The upper limit was around 350 Angstroms. No wonder the issue was contentious when all they had was the light microscopy.

Your brain (and mine) is mostly fat. Light doesn’t get through fat very well at all. Deisseroth figured out a way to remove the fat leaving the other brain structures intact. The technique even works on brains fixed in formaldehyde for years. First they infused formaldehyde and acrylamide into brain tissue at 4 degrees Centigrade. The formaldehyde hardens the tissue, but it also links the acrylamide to the proteins making up the tissue. Then they raised the temperature to 37 Centigrade causing the acrylamide to polymerize. Then they infused sodium dodecyl sulfate into the tissue using electrophoresis. When the SDS was pulled out of the tissue (again by electrophoresis) pulling the fat (lipids) of the brain with it, this left what they call a hydrogel (which light could go through).

Using the technique it is possible to look through slabs of brain tissue 500 microns thick (5 million Angstroms thick) with a light microscope and see cell bodies and nerve fibers in their natural habitat (e.g. whole populations of neurons along with their projections). Even better you can stain the hydrogel with your antibody of choice and see what protein is where. Then you can wash this out and look at something else.

It is an incredible advance and certain to revolutionize our understanding of the brain. Look at the paper. The pictures are amazing and more are sure to follow from other workers. Definitely Nobel caliber work.

It is extremely amusing to me that this work could have been done 50 years ago. It just took someone smarter than you and I to think of it.

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