CELL MEMBRANES 



65 



CELLOIDIN IMBEDDING 



volume of the average body. Accord- 

 ing to experience, eccentricities are 

 placed in several classes, from 1.0 to 

 0.4, say, with class intervals of 0.2. 

 From the first class, 1.0 to 0.8, an esti- 

 mate of the eccentricity C/B can be 

 made, since ellipses of this class are 

 primarily of the type C, B, particularly 

 if B and C are appro.ximately equal and 

 somewhat less than A. Similarly eccen- 

 tricities of the last class interval 0.6 

 to 0.4 will provide an estimate of C/A. 

 Combining these two estimates the 

 average ratio B/A may be determined. 

 The minor axes of sections of the 

 first and last eccentricity groups are 

 representative of the C or minor axes 

 of the space figure. Hence an estimate 

 of the average value of C may be made, 

 and using the eccentricitj^ ratios al- 

 ready determined the average values of 

 A and B can be calculated. The aver- 

 age volume V equals iir ABC can then 

 be calculated. 

 Cell Membranes do not require any special 

 technique for their demonstration. Al- 

 most any good fixative will do and they 

 can be stained a host of different colors. 

 There is however some difference in the 

 interpretation of what we see with the 

 microscope. The essential component 

 of the walls of all cells is called the 

 plasma membrane. This conditions per- 

 meability and its integrity is essential 

 to the life of the cell. It is said to con- 

 sist of a continuous layer of lipoid 

 molecules (phosphatides, sterols, fats) 

 not more than 2-4 molecules thick on 

 which proteins are adsorbed, the lipoids 

 give permeability and the proteins 

 elasticity and great mechanical strength. 

 The evidence is critically presented by 

 Danielli (Bourne, pp. 68-98). He says 

 that it is improbable that the lipoid 

 layer is ever thicker than 10 m/x and 

 that the whole membrane is between Ifx 

 and 1 m/i thick. Consequently in many 

 cases we cannot expect to visualize the 

 plasma membrane itself directly with 

 visible light because the theoretical 

 limit of visibility is a particle size of 

 0.25m. However the position of the 

 plasma membrane is made clear by the 

 difference in properties of the cytoplasm 

 which it limits and the fluid without 

 and also in the dark field by the light 

 reflected from its surface. In addition 

 it is often backed internally by a thin 

 layer of cytoplasmic cortex (ectoplasm) 

 which is typically free from cytoplasmic 

 granules. The plasma membrane may 

 be supplemented externally by special 

 membranes such as the myelin sheaths 

 about nerve fibers. There are many 

 special techniques for its investigation. 

 Some are briefly referred to under 



Lysis, Permeability, Surface Tension 

 and Wetting Properties, Nuclear Mem- 

 brane, PinocytosiB. 

 Cell Shape. The shape of epithelial cells, 

 and of all cells for that matter, is deter- 

 mined by perfectly definite causes. 

 Obviously those suspended in fluid tend 

 to be spherical (lymphocytes) unless 

 their internal organization conditions 

 some other shape (erythrocytes) . Con- 

 tact with a surface generally promotes 

 flattening on that surface. Epithelial 

 cells are sessile. The study of their 

 rnorphology is not complicated by mo- 

 tility. When disposed in a single layer 

 and subjected to lateral pressure from 

 their neighbors they take a distinctive 

 shape which has been analyzed in a 

 convincing way by F. T. Lewis (Am. 

 Scientist, 1946, 34, 357-369, and many 

 earlier papers). In sections of the 

 laj^er parallel to the surface it may be 

 seen that most of the cells are six-sided, 

 or hexagonal. They form a mosaic, 

 the character of which can easily be re- 

 membered by students forced to dream 

 of the benzene "ring" with its 6 carbon 

 atoms. By drawing many such chemi- 

 cal symbols side by side a similar mosaic 

 is formed. As Lewis points out, the 

 intersections are three-raj'ed not four- 

 rayed as might be the case if the cross- 

 sections were squares. Mechanically 

 this is a great advantage. When the de- 

 ithelium is stratified provision must be 

 made for contact with cells on all sides. 

 Nature adheres to the same three-rayed 

 intersection and molds the cells in that 

 shape which provides the smallest sur- 

 face area for closely crowded bodies. 

 Lewis found that this could be deter- 

 mined mathematically as a 14-sided 

 figure and by careful reconstruction of 

 actual cells proved that they were all 

 primarily tetrakaidecahedral in shape. 

 Examination of his clear illustrations 

 will be more helpful than pages of de- 

 scription. The same architectural prin- 

 ciples apply to many other cell 

 aggregates, like fatty tissue for ex- 

 ample. No longer is the histologist 

 justified in vaguely referring to such 

 cells as polyhedral. Evidently in the 

 construction of epithelial surfaces the 

 cells are fitted together in a much more 

 effective way than bricks in the building 

 of a wall. Except for the reference, 

 the above paragraph is quoted from the 

 Second Edition of Cowdry's Histology, 

 Philadelphia : Lea & Febiger , 1938. See 

 technique for three-dimensional study 

 of cell shape in plants, Holtzman, D. H., 

 Am. J. Bot., 1951, 38, 221-234. 

 Celloidin Imbedding. Celloidiu is a kind 

 of generic term covering various cellu- 

 lose compounds, nitrocellulose, soluble 



