We see objects because light either passes through them or is reflected off them and enters our eye, where it is translated into an image. However, we are limited in what we can see with the unaided eye. Without the aid of magnifying lenses, the smallest object we can resolve is approximately 100 µm or 0.1 mm in diameter, which is approximately the size of a fine pencil dot. Resolution is a measure of the ability to see small objects. For example, we must use a microscope to resolve structures less than 100 µm in diameter.

There are two basic types of microscopes: light microscopes and electron microscopes. As their names imply, light microscopes use light to produce an image, and electron microscopes use beams to electron. Light microscopes usually use transmitted light, which is light that passes through the object being examined, but some light microscopes are equipped to use reflected light. Glass lenses are used in light microscopes to magnify images, and either images can be observed directly by looking into the microscope, or the light from the images can be used to expose photographic film to make a photomicrograph of the images. Video cameras are sometimes used to record images. The resolution of light microscopes is limited by the wave-length of light, and the lower limit is approximately 0.1 µm, which is approximately the size of a small bacterium.

A biopsy is the removal of tissue from living patients for diagnostic examination. For example, changes in tissue structure allow pathologists to identify tumors and to distinguish between noncancerous (benign) and cancerous (malignant) tumors. Light microscopy is used on a regular basis to examine biopsied tissues. Light microscopy is used instead of electron microscopy because less time and effort are required to prepare materials for examination and the resolution is adequate to diagnose most conditions that cause changes in tissue structure.

Because images usually are produced using transmitted light, tissues to be examined must be cut very thinly to allow the light to pass through them. Sections are routinely cut between l to 20 µm thick to make them thin enough for light microscopy. To cut these thin sections, the tissue must be fixed or frozen, which is a process that preserves the tissue and makes it more rigid. Fixed tissues are then embedded in some material, such as wax or plastic, that makes the tissue rigid enough for cutting into sections. The frozen sections, which can be prepared rapidly, are rigid enough for sectioning, but tissue embedded in wax or plastic can be cut much thinner, which makes the image seen through the microscope clearer. Because most tissues are colorless and transparent when thinly sectioned, the tissue must be stained with a colored dye so that the structural details can be seen. As a result, the colors seen in color photomicrographs are not the true colors of the tissue but are the colors of the stains used. The color of the stain can provide specific information about the tissue, because special stains color only certain structures.

For objects much smaller than a cell, such as cell organelles, an electron microscope, which has a limit of resolution of approximately 0.1 nm, must be used; 0.1 nm is the size of some molecules. In objects viewed through an electron microscope, a beam of electrons either is passed through objects using a TEM or is reflected off the surface of objects using an SEM. The electron beam is focused with electromagnets. For both processes the specimen must be fixed, and for TEM the specimen must be embedded in plastic and thinly sectioned (0.01 to 0.15 µm thick). Care must be taken when examining specimens in an electron microscope because a focused electron beam can cause most tissues to quickly disintegrate. Furthermore, the electron beam is not visible to the human eye; thus it must be directed onto a fluorescent or photographic plate on which the electron beam is converted into a visible image. Because the electron beam does not transmit color information, electron micrographs are black and white unless color enhancement has been added using computer technology.

The magnification ability of SEM is not as great as that of TEM. However, depth of focus of SEM is much greater, allowing for the production of a clearer three-dimensional image of the tissue structure.

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