Examples of autoradiography used in light and electron microscopy. a. Photomicrograph of a lymph node section from an animal injected with tritiated (3H) thymidine. Some of the cells exhibit aggregates of metallic silver grains, which appear as small black particles (arrows). These cells synthesized DNA in preparation for cell division and have incorporated the |3H|thymidine into newly formed DNA. Over time, the low-energy radioactive particles emitted from the pHjthymidine strike silver halide crystals in a photographic emulsion covering the specimen (exposure) and create a latent image (much like light striking photographic film in a camera). During photographic development of the slide with its covering emulsion, the latent image, actually the activated silver halide in the emulsion, is reduced to the metallic silver, which then appears as black grains in the microscope, x 1,200. (Original slide specimen courtesy of Dr. Ernst Kallenbach.) b. Electron microscopic autoradiograph of the apical region of an intestinal absorptive cell. In this specimen, l25l bound to nerve growth factor (NGF) was injected into the animal, and the tissue was removed 1 hour later. The specimen was prepared in a manner similar to that for light microscopy. The relatively small size of the silver grains aids precise localization of the 1251-NGF complexes. Note that the silver grains are concentrated over apical invaginations (inv) and early endosomal tubular profiles (tub), x32,000. (Electron micrograph courtesy of Dr. Marian R. Neutra.)

Autoradiography can also be carried out by using thin plastic sections for examination with the EM. Essentially the same procedures are used, but as with all TEM preparation techniques, the processes are much more delicate and difficult; however, they also yield much greater resolution and more precise localization (Fig. 1.5b).


Historadiography is the production of an x-ray photograph (microradiograph) of a specimen on a slide

A historadiograph displays mass just as a regular x-ray does. Although x-rays can be used to examine soft tissues, their greatest utility is in the examination of ground sections of bone or other mineralized tissue. In practice, the ground section of bone is placed in contact with a photographic emulsion on a glass slide and exposed to a beam of x-rays. The photographic emulsion is then developed and viewed with a microscope (Fig. 1.6). Standards of known mass can be added to the slide or to a similarly treated

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