Some of the fantastic innovations of science which brought
about a tremendous change are often outcomes of ingenuity and out of box of
thinking. Here is one such classical example which opened up new vistas of
microscopy. Edward Boyden a neuroengineer of Massachusetts Institute of
Technology (MIT) in Cambridge with his colleagues developed a novel and subtle way
of observing living tissues. The technique called as expansion microscopy uses
the material used in baby diapers to observe the living tissues.
Conventional Optical microscope suffers from the major
impediment of distinguishing the objects which are closer together than about
200 nanometres or roughly half the wavelength of visible light. These objects appear
blurred or as a blob. This resolution limit is referred to as Abbe’s diffraction
limit after the scientist Ernst Abbe who first identified it in 1873. Hence the
optical microscope which is otherwise desirable for identifying the cell
organelles is not suitable for observing structural details. Microscopes which
use electron beams instead of light have finer resolution but they can be used
in vacuum hence dead tissues alone can be used. Abbe’s limit couldn’t be
overcome according to laws of physics. But scientists using fluorophores or fluorescent
molecules and hitting them with lasers of specific wavelength engineered new
ways to resolve proteins as close as 20 nanometres in living cells. These
techniques developed by three scientists independently- Stephan Hell, William
Moerner and Eric Betzig were awarded the Nobel Prize in Chemistry in 2014 for
the pioneering work on super-resolution microscopy. These techniques are very
useful in observing the vesicle moments across the nerve synapses, cell protein complexes and spaces between rows of skeletal microtube filaments etc but these
cumbersome techniques demand expertise in handling expensive and specialised
equipment.
In contrast to the
super-resolution microscopy, Boyden employed the super absorbent material acrylate
used in diapers for resolving the structural details of the brain tissues.
Acrylate has two advantages. It is capable of forming a dense mesh capable of
holding the proteins in place and it swells in presence of water. Acrylate
salts form water lock and gives the diapers sponginess. They have a capacity to
expand four and half times the original size. In order to locate the proteins
which are closely packed, the living tissues before swelling are soaked in a
chemical cocktail which makes them transparent and then infused with fluorescent
molecules that anchor specific proteins to acrylate. Upon adding water, the
acrylate would cause the living tissues to swell uniformly in all directions. In their experiment with
brain tissues of mice, Boyden and his colleagues demonstrated that proteins
which were too close and probably difficult to distinguish could be identified
under visual-light microscope. With this technique proteins as close as 60nm
were resolved. In future with further fine tuning of the techniques the resolution can be improved.
In this new technique of expansion microscopy the original
living tissues are increased in size upon absorption of water. Crucially it
also maintains the relative orientation and interconnections of the proteins
and other cellular structures intact. But the position of position is shifted
by 1-4%. The results of the expansion microscopy were comparable to
super-resolution microscopy techniques.
This novel technique was feted by scientists all around who commended
its ingenuity. These kinds of unique scientific feats would lay foundation for
a genre of science which circumvents the use of highly sophisticated and
complicated equipment. It also opens new frontiers in science where a judicious
combination of the ingenious techniques with the hardwired innovative equipment
could unravel the mysteries of life.
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