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Microscopia a fluorescenza a super risoluzione
La microscopia a fluorescenza a super risoluzione (SRFM) fornisce una risoluzione migliore rispetto alla microscopia a fluorescenza convenzionale ridu…
La microscopia a fluorescenza convenzionale può risolvere solo strutture più distanti di 200 nm. Questo perché al microscopio, la diffrazione della luce fa apparire le molecole fluorescenti come macchie sfocate invece che come punte acuminate. Di conseguenza, quando gli oggetti sono molto vicini l'uno all'altro, le loro immagini si sovrappongono.
Pertanto, gli scienziati hanno sviluppato tecniche di super-risoluzione che forniscono una migliore risoluzione per l'immagine anche di piccole strutture subcellulari. Queste tecniche superano i limiti di diffrazione utilizzando specifici modelli di luce per eccitare i fluorofori nel campione.
Ad esempio, la microscopia a illuminazione strutturata utilizza un modello a strisce che viene poi ruotato per acquisire più immagini. Le immagini vengono combinate, creando un modello di interferenza che viene elaborato computazionalmente per creare l'immagine finale.
La microscopia a deplezione di emissione stimolata crea un modello utilizzando due laser. Un laser primario eccita il campione mentre un raggio a forma di ciambella sopprime la fluorescenza attorno ad esso, facendo apparire il punto fluorescente più piccolo.
Altre tecniche evitano la sovrapposizione dei segnali utilizzando sonde fluorescenti che possono essere accese e spente. Per ogni immagine vengono attivate solo alcune sonde, riducendo al minimo la sovrapposizione del segnale e consentendo la localizzazione di una singola molecola fluorescente.
Molte di queste immagini possono essere combinate in un'unica immagine composita ad alta risoluzione.
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Q1: Why does conventional fluorescence microscopy have a 200 nm resolution limit?
Conventional fluorescence microscopy cannot resolve structures closer than 200 nm due to light diffraction. When light passes through a microscope, it diffracts and causes fluorescent molecules to appear as blurry spots rather than sharp points. When objects are very close together, their blurred images overlap, making it impossible to distinguish them as separate structures.
Q2: What is the point spread function and how does it affect microscopy images?
The point spread function (PSF) is the light intensity distribution from a single point that causes it to appear blurred under a microscope. PSF makes each fluorescing point appear larger than its actual size. When nearby fluorophores have overlapping PSFs, their images merge into a single blurred spot, reducing image clarity and resolution.
Q3: How does structured illumination microscopy improve resolution?
Structured illumination microscopy uses a striped pattern of light to excite fluorophores in the sample. The pattern is rotated to capture multiple images from different angles. These images are computationally combined to create an interference pattern that reveals finer details, effectively overcoming the diffraction limit and producing higher-resolution images.
Q4: What role do photo-switchable probes play in stochastic optical reconstruction microscopy?
In stochastic optical reconstruction microscopy (STORM), photo-switchable probes can be activated and deactivated using different wavelengths of light. Only a small subset of probes is activated at any time, minimizing signal overlap. The center of each activated fluorophore is precisely located, and the process repeats until all probes are recorded, then images are superimposed to create a high-resolution composite.
Q5: How does stimulated emission depletion microscopy create smaller fluorescent points?
Stimulated emission depletion (STED) microscopy uses two lasers to achieve super-resolution. A primary laser excites the sample, while a donut-shaped beam from a second laser suppresses fluorescence around the excited area. This suppression makes the fluorescent point appear much smaller, allowing visualization of structures that would otherwise appear as overlapping blurs.
Q6: What is photoactivated localization microscopy and when is it useful?
Photoactivated localization microscopy (PALM) uses single-molecule detection to resolve closely spaced fluorophores. A variant of green fluorescent protein with high fluorescence is activated in small groups and imaged with high precision. Step-by-step activation of all GFPs across the specimen, followed by computational processing, generates a high-resolution image suitable for live-cell imaging applications.
Q7: How do super-resolution techniques combine multiple images into a final high-resolution image?
Super-resolution techniques capture multiple images of the same specimen using different excitation patterns or sequential activation of fluorophores. These individual images are computationally processed and combined through interference pattern analysis or superimposition. The result is a single composite image with resolution far exceeding that of imaging biological samples with optical microscopy alone.