Confocal and Fluorescence Microscopy

Fluorescence microscopy images specimens by exciting fluorescent molecules and collecting the longer-wavelength light they emit, giving high-contrast, molecularly specific images of cells. Confocal microscopy adds a pinhole that rejects out-of-focus light, producing sharp optical sections that can be assembled into three-dimensional views of the cell.

Definition

Fluorescence microscopy uses the absorption and re-emission of light by fluorophores to image labelled structures; confocal microscopy is a fluorescence technique in which a pinhole excludes out-of-focus light so that only a thin in-focus plane contributes to each image, yielding optical sections.

Scope

The entry covers the principle of fluorescence contrast, the optical-sectioning advantage of confocal imaging, and the broad class of super-resolution methods that push fluorescence imaging below the diffraction limit. It treats these as imaging methods within cell biology and not as clinical instruction.

Core questions

• How does fluorescence provide molecular contrast in a cell?
• How does a confocal pinhole produce optical sections?
• How are three-dimensional images of cells reconstructed?
• How do super-resolution methods exceed the diffraction limit?

Key concepts

• Fluorescence excitation and emission
• Fluorophores and fluorescent proteins
• Confocal pinhole and optical sectioning
• Three-dimensional reconstruction
• Photobleaching and phototoxicity
• Super-resolution (diffraction-unlimited) imaging

Mechanisms

In fluorescence microscopy a fluorophore absorbs excitation light and re-emits at a longer wavelength, which is separated from the excitation by filters to give bright, specific contrast against a dark background, as reviewed by Lichtman and Conchello. A conventional fluorescence image, however, contains blurring light from above and below the focal plane; confocal microscopy places a pinhole conjugate to the focal point so that out-of-focus light is rejected, producing the optical sections described by Conchello and Lichtman that can be stacked into three dimensions. Because emission, like all light microscopy, is still subject to the diffraction limit, super-resolution approaches such as stimulated-emission-depletion microscopy, introduced by Hell and Wichmann, manipulate the fluorescence process itself to resolve detail far below that limit, as surveyed by Schermelleh and colleagues.

Clinical relevance

Confocal and fluorescence microscopy are widely used in pathology, ophthalmology, and biomedical research to localize molecules and image tissue in three dimensions. This entry explains the imaging principles involved and is reference-educational, not a basis for individual diagnostic or treatment decisions.

History

The confocal principle was conceived by Marvin Minsky in the 1950s, but practical laser-scanning confocal microscopes and bright synthetic and genetically encoded fluorophores made fluorescence imaging dominant in cell biology from the late twentieth century. The subsequent breaking of the diffraction limit by stimulated-emission-depletion microscopy (Hell & Wichmann, 1994) and related techniques opened the era of super-resolution fluorescence imaging summarized by Schermelleh and colleagues.

Key figures

• Jeff Lichtman
• Jose-Angel Conchello
• Stefan Hell
• Marvin Minsky

Seminal works

• lichtman-2005
• conchello-2005
• hell-1994
• schermelleh-2010

Frequently asked questions

What does the confocal pinhole do?

It is positioned so that light from outside the focal plane is blocked, leaving only in-focus light to form the image; this produces a thin optical section that can be combined with others into a three-dimensional view.

How can fluorescence microscopy beat the diffraction limit?

Super-resolution methods such as stimulated-emission-depletion microscopy control the fluorescence emission process so that detail well below the classical diffraction limit can be resolved.