This filter absorbs any ultraviolet radiation not absorbed by the speci- 

 men, but transmits the fluorescent light. 



The nucleus of the unstained animal cell is usually nonfluorescent, 

 while the cytoplasm shows a weak bluish fluorescence. A relatively 

 strong yellow fluorescence is associated with the mitochondria, which 

 probably is due to the presence of vitamins (e.g., riboflavin) in these 

 structures. The more or less specific colors of fluorescence of substances 

 such as the vitamins and hormones (yellow, green, or blue), the porphy- 

 rins (red), and certain minerals (calcium, white fluorescence) has made 

 it possible to follow their distribution in cells undergoing changes in 

 metabolic activity. The autofluorescence of most plant cells is sufficiently 

 strong to permit the recognition of cell structures and tissues in sections. 

 This is especially true of the chloroplasts, which fluoresce a deep red 

 color due to their content of chlorophyll. The staining of microorganisms 

 with fluorochromes has been used to identify strains of bacteria in in- 

 fected tissues. Certain microorganisms, such as the acid-fast bacteria, are 

 autofluorescent and can be identified in tissues by virtue of this property. 



X-ray Microscopy 



X-ray microscopy, specifically applied to the study of biologic materials, 

 includes a number of methods designed to visualize the object through 

 the use of X rays. These methods include ( 1 ) contact microradiography, 

 (2) projection microradiography, and (3) X-ray diffraction. 



The simplest kind of X-ray microscopy is direct contact microradiog- 

 raphy. In this method, the specimen is placed in close contact with a 

 fine-grained photographic emulsion and exposed to soft X rays which 

 range in wavelength from 1 to 10 A (Figure 11-30). A metal diaphragm 

 is generally used to delimit the area of the specimen exposed to the 



Target 

 X-rays 



im 



V ^ry 



■Figure 11-30. Schematic Diagram Showing the Principles of Contact 

 (left) and Projection Microradiography (right). (From Engstrom, A.. 1959. 

 "X-ray Microscopy," in Mellors, R. C. (Ed.), "Analytical Cytology," 2nd 

 ed., McGraw-Hill Book Co., New York, N. Y., Fig. 5.1, p. 345.) " 



252 / CHAPTER 11 



