40 



Cellular Structure and Activity 



tion to or near the molecular range (15 to 

 200 A), depending on the resolving power 

 of the instrument and the nature of the 

 specimen. Ultrathin sectioning makes the 

 method applicable to a study of cells and 

 tissues. The necessity that the specimen be 

 dry requires previous fixation or freeze-dry- 

 ing. References: Schmitt ('49), Drummond 

 ('50), Cosslett ('51), Hall ('53). 



X-ray Absorption Techniques. Permit de- 

 termination of the content and location of 

 particular elements in cells and of the mass 

 of small objects within cells. References: 

 Engstrom ('50), Brattgard and Hyden ('52). 



Interference Microscopy. May be used to 

 follow changes in dry weight of undamaged, 

 living cells. Is more sensitive than x-ray ab- 

 sorption technique. References: Barer ('52), 

 Davies, Engstrom and Lindstrom ('53). 



Autoradiography. Reveals location in cells 

 of elements introduced as radioactive iso- 

 topes. Resolution relatively low and only 

 roughly quantitative results so far achieved. 

 References: Glick ('49), Doniach ('53). 



Phase Contrast Microscopy. By converting 

 small differences of refractivity into changes 

 of intensity this method permits visual- 

 ization of objects having refractive index 

 similar to that of the surrounding medium. 

 Continuous (rather than discrete) variation 

 of phase offers promise of detection of very 

 small objects within cells. References: Ben- 

 nett, Jupnik, Osterberg and Richards ('51). 



Fluorescence Microscopy. Reveals the pres- 

 ence of fluorescent substances normally pres- 

 ent or introduced into cells. References: 

 Sjostrand ('44), Hamley and Sheard ('47). 



Reflected Light Techniques. Permit study 

 of opaque objects and very thin objects, 

 such as cell membranes, one dimension of 

 which may be submicroscopic. References: 

 Waugh and Schmitt ('40), Pfeiffer ('49). 



STRUCTURE ANALYSIS (INDIRECT 

 METHODS) 



Polarization Microscopy. Reveals orienta- 

 tion and state of subdivision of molecules and 

 submicroscopic constituents. Provides some 

 information about chemical composition of 

 cellular constituents. Being applicable to 

 living cells it avoids some of the indeter- 

 minacies of fixation and is a valuable adjunct 

 to electron microscopy. References: Schmidt 

 ('37), Frey-Wyssling ('48), Schmitt ('50a), 

 Bennett ('50), Seeds ('53). The study of natu- 

 ral and artificial dichroism gives information 

 about the orientation of molecular species 

 which absorb in the ultraviolet (especially 



valuable in studying structures, like chrom- 

 osomes, which contain nucleic acid). Dichro- 

 ism studies in the infrared have been em- 

 ployed to deduce the orientation of particular 

 bonds, such as peptide and hydrogen bonds, 

 in proteins. Reference: Perutz, Jope and 

 Barer ('50). 



X-ray Diffraction. Permits deduction of in- 

 tra- and intermolecular structure of biolog- 

 ical materials, the degree of regularity of 

 structure, orientation and particle size. X-ray 

 diffraction can, under certain conditions, be 

 applied to undried materials, e.g., fibrous 

 tissues, crystalline proteins and the like. 

 Though x-ray studies have been concerned 

 chiefly with small, interatomic separations, 

 small-angle techniques now permit investi- 

 gating separations as large as 700 to 1000 A 

 (e.g., large axial repeating patterns of fibrous 

 proteins). References: Perutz ('49), Hodgkin 

 ('50). 



Electron Diffraction. Electron microscope 

 techniques make it possible to obtain electron 

 diffraction data from particvdar, small re- 

 gions located and observed in the electron 

 microscope. Reference: Drummond ('50). 



CYTOCHEMISTRY (HISTOCHEMISTRY) 



As originally employed, the terms cyto- 

 chemistry or histochemistry referred to the 

 recognition, localization and quantitation of 

 chemical entities, such as certain enzymes, 

 steroids, polysaccharides, proteins and min- 

 erals in cells and tissues. Localization was the 

 chief purpose and this was accomplished by 

 reactions, usually in sections of fixed mate- 

 rial, which characterize the pure substance 

 in vitro. Subsequently, many other types of 

 procedures, including isolation of particulates 

 by differential centrifugation of fragmented 

 tissues, were included in the field of histo- 

 chemistry by some authors. The writer will 

 continue to use the original more restricted 

 definition, reserving the term "analytical 

 cytology" for the more inclusive field. 



Perhaps the chief pitfalls are the uncritical 

 assumption that chemical entities react in 

 the complex colloidal systems of protoplasm 

 in the same way they do in simple in vitro 

 systems and the assumption that, during the 

 over-all procedure, the substance in question 

 remains localized exactly as it was in life. 

 The latter point is particularly to be con- 

 sidered in the inevitable application of the 

 electron microscope to the problem of local- 

 ization. References: Glick ('49), Gomori 

 ('50), Glick, Engstrom, and Malmstrom 

 ('51). rX 



