CELL CLASSIFICATION 



56 



CELL SHAPE 



myeloblasts and myelocytes in leuco- 

 cytogenesis. But the first differentiat- 

 ing intermitotic in any line of differen- 

 tiation is produced by division of a 

 vegetative intermitotic. One of the 

 daughter cells of this division, or in 

 some instances both daughter cells from 

 mitosis of a dividing vegetative inter- 

 mitotic, achieve no further differentia- 

 tion than their parent cells, for 

 otherwise the reservoir of vegetative 

 intermitotics would not be maintained 

 but would differentiate itself out of 

 existence. 



Postmitotic cells, on the other hand, 

 are cells whose lives are postmitotic in 

 the sense that they perform their duty, 

 age and die. They are the culminations 

 of the various lines of differentiation. 

 Again, two sorts are recognizable: 

 First the reverting postmitotics, which 

 are capable of full functional activity 

 and usually go on to death, yet, on 

 occasion, some of which can revert and 

 divide. Hepatic and renal cells are 

 examples. Second, the fixed postmi- 

 totics, which are different insofar that 

 they are incapable of mitosis so that 

 aging and death is for them inevitable 

 as for instance nerve cells of adults, 

 sperms and polymorphonuclear neutro- 

 phile leucocytes. In contrast with the 

 other 3 kinds these fixed postmitotics 

 have lost the potentiality of malignant 

 transformation (Cowdry, E. V., Prob- 

 lems of Aging. Baltimore, Williams 

 & Wilkins, 1942, 626-629). 



Cell Components can be examined by tech- 

 niques too numerous to list including 

 Staining, Supravital and Vital Staining, 

 Impregnation, Microdissection, Micro- 

 manipulation, Microinjection, Centrif- 

 ugation, many Microchemical Reac- 

 tions, and Indicators by at least 6 differ- 

 ent kinds of Microscopes. Methods for 

 many of these components are given 

 under Capsule Stains, Mitochondria, 

 Zymogen, Nissl Bodies, etc. 



Cell Division, see Mitosis, Ainitosis and 

 series of papers on chemistry of cell 

 division (Mauer, M. E. and Voegtlin, 

 C, Am. J. Cancer, 1937, 29, 483-502). 



Cell Enlargement, see Giant Cells. 



Cell Injury detected by fluorescence 

 (Herick, F., Protoplasma, 1939, 32, 

 527-535). See Dead Cells. 



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 mji and 

 that the whole membrane is between In 

 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 

 0.25^. 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, Pinocytosis. 

 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 

 morphology 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 

 layer 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-rayed not four- 

 rayed as might be the case if the cross- 

 sections were squares. Mechanically 

 this is a great advantage . When the ep- 

 ithelium is stratified provision must be 

 made for contact with cells on all sides. 



