FLUORESCENCE MICROSCOPY

Fluorescence microscopy is an essential tool in molecular and cellular biology. It is a technique that allows one to study and visualize the cellular structures and dynamics of tissues and organelles, and macromolecular assemblies inside the cell. It was devised in the early twentieth century by various scientists like Köhler, Lehmann, Reichert and others.

The wide utilization of fluorescent proteins since their discovery have revolutionized the applications and use of the microscope in biological studies.

A fluorescence microscope uses the property of fluorescence to generate an image. It uses a high-intensity light source that excites the fluorescent molecule that may be inherently present in the sample to be studied or may be artificially labelled with a fluorescent molecule. The fluorescent molecule is called the fluorophore which is usually present in the fluorescent dye. 

Therefore, one could say that any microscope that works on the same basis to study the properties of organic or inorganic substances is a fluorescent microscope.See the source image

A fluorescence microscope is a type of optical microscope that uses fluorescence (ability of a substance to emit light on excitation) and phosphorescence (ability of a substance to continue emitting light even after the removal or withdrawal of the excitation factor). It may use these properties instead of or in addition to the properties of scattering, absorption, reflection and attenuation. 

The setup for the microscope may be simple as in an epifluorescence microscope or it may have a complicated design like that of a confocal microscope. A confocal microscope uses optical sectioning to provide a better resolution of the fluorescence image.

Principle:

Fluorescent substances are the substances that absorb light of a particular energy and wavelength and then emit light of a longer wavelength and lesser energy.  

This phenomenon of fluorescent substances can be applied to the working of the fluorescent microscope. Fluorescent dyes (also called fluorochromes or fluorophores) are molecules that have the ability to absorb excitation light at a given wavelength, and then emit light of a comparatively longer wavelength after a delayed time interval.

In practical use, the sample is stained with a fluorescent dye and then illuminated with a blue light. The blue light (short wavelength) is absorbed by the fluorophores of the fluorescent dye, and the green light (which has longer wavelength) is emitted. This change is called the Stokes shift.

The light source that is used in fluorescent microscopy is a high intensity mercury arc lamp. The lamp emits white light when then passes through a device called an ‘exciter filter’. (as shown in the figure) This device filters the emission light to reveal the location of the fluorophores. It allows only the blue component of white light (white light comprises of coloured light of all wavelengths) to pass through and prevents the passage of light of other colors.

.See the source image

The dichroic mirror is used to reflect the blue light and allows the green light to pass. The angle of the mirror is fixed in such a way that the blue light is reflected towards the specimen placed below. It allows the passage of green light.

Finally, when the light reaches the ‘barrier filter’, it blocks out or removes all the remnants of the residual blue light from the specimen which may not have been ideally reflected by the dichroic mirror.

Thus, enabling the observer to perceive the glowing green portions of the specimen against the jet-black background of the dark field condenser that is used. The portions of the specimen that have not been stained remain invisible to the eye and this is how fluorescence microscopy provides a sharp image for the observation of the fine and intricate components of the sample to be studied.

Components:

The essential components of the fluorescence microscope are:

  • Fluorescent dyes (fluorophore): Chemical compounds that have the ability to re-emit light upon excitation. Examples include; nucleic acid stain like DAPI and Hoechst, phalloidin etc.Image result for fluorescence microscope structure
  • Light source: This is provided by a bright mercury vapor arc lamp, xenon lamp or LEDs with a dichroic excitation filter, lasers etc.
  • Heat filter: The lamp produces infrared rays which generate considerable heat. No other major uses of the heat filter exist.)
  • Exciter filter: The light undergoes cooling and passes through the exciter filter which allows the passage of the shorter waves which play a role in excitation of the fluorochrome dye coated sample on the slide and does not allow the other wavelengths to pass through.
  • Dichroic mirror: An accurate colour filter/mirror which selectively allows the passage of light of a particular wavelength and reflects the others. 
  • Condenser: A dark field condenser is usually used because it provides a dark background and it is easy to detect even mild fluorescence exhibited by the sample
  • Barrier filter: It removes all the remnants of the exiting light and is situated in the body tube of the microscope between the objectives and the eye piece. 

See the source image

Applications: 

  • Identify structures in fixed and live biological samples in microbiological studies.
  • Used in food chemistry for the assessment of the structural organization and spatial distribution of the components of food.
  • Used for the study of mineral like coal and graphene oxide in minerology.
  • Used in the textile industry for analysis of fibre dimensions. 

Article By- Shaily Sharma (MSIWM041)

References:

Fluorescence Microscopy (nih.gov)

Immunofluorescence staining – PubMed (nih.gov)

Fluorescence Microscopy – Explanation & Labelled Images (microscopeinternational.com)

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