Cell Constitution 



57 



side and the environment on the outer side 

 (which may include connective tissue con- 

 stituents, "cement" substances and other 

 poorly defined materials), it has been diffi- 

 cult to obtain reliable evidence of a direct 

 nature as to its composition, structure and 

 function. Many of our concepts originated 

 from indirect evidence, chiefly from the lore 

 of permeability studies, from a consideration 

 of the limiting envelope of the mammalian 

 erythrocyte (which may lack important as- 

 pects of the membrane of tissue cells) and 

 from a consideration of the properties of thin 

 surface and interfacial films as studied by 

 physical chemists. 



For detailed discussions of the literature 

 on the cell membrane see Ponder ('48), 

 Waugh ('50), and Davson ('51). It will be 

 our purpose in this section to draw attention 

 to a few aspects of the subject which may 

 be particularly significant for the student of 

 growth and development and to pose a few 

 problems worthy of further investigation. 



First, to what extent is the limiting en- 

 velope or cell membrane an entity having 

 characteristic ultrastructvire, composition and 

 function and to what extent does it repre- 

 sent merely a non-specific interfacial struc- 

 ture — a repository of all the various mate- 

 rials merely adsorbed upon it from the 

 underlying cytoplasm and external environ- 

 ment? Unfortunately, direct evidence is 

 meagre; one can only present a point of view 

 with the hope that it may stimulate further 

 investigation. 



The polarization optical evidence suggests 

 that the ultrastructure of the cell membrane, 

 the nuclear membrane and the membrane 

 surrounding cytoplasmic vacuoles have the 

 common property of being composed of pro- 

 tein layers thin with respect to the wave- 

 length of light. The membranes show nega- 

 tive uni-axial form birefringence with optic 

 axes normal to the plane of the membrane. 

 Estimates of the thickness of the red cell en- 

 velope range from 50 A (Hoffman and 

 Hillier, '52) to several hundred Angstrom 

 units (Waugh and Schmitt, '40) depending 

 on the method used. Since this includes the 

 lipid components it is obvious that the pro- 

 tein layers must be very thin. The polariza- 

 tion optical evidence gives no clue as to 

 whether the protein or the lipid component 

 is external, whether they are interleafed or 

 arranged in a mosaic. It is consistent with 

 the view that the protein may be a meshwork 

 of very thin filaments lying in the plane of 

 the membrane. Rather ill-defined filaments 

 have been observed with the EM in fixed 



preparations of red cell envelopes by some 

 authors, although Latta ('52) found the sur- 

 face free of discontinuities within the reso- 

 lution of his preparations (60 A). No signi- 

 ficant fine structure has thus far been ob- 

 served with the EM in the plane of the cell 

 membrane of tissue cells.* 



In the red cell envelope and in the cell 

 membrane of several other types of cells 

 which have been studied, the lipid molecules 

 are oriented with paraffin chains normal to 

 the plane of the membrane. The intrinsic 

 birefringence is uniaxially positive with 

 optic axes perpendicular to the plane of the 

 membrane. It seems probable that the lipids 

 occur as bimolecular leaflets of mixed lipids 

 (chiefly phospholipids, galactolipids and 

 steroids). However, there is little direct evi- 

 dence for or against the view that such lipid 

 bimolecular layers are continuous over the 

 surface of the cell — a matter of considerable 

 importance in permeability theory. 



The proteins of the red cell envelope, 

 though still poorly understood, seem charac- 

 teristic of this type of cell. Chief among these 

 is stromatin, which has not yet been isolated 

 in pure form but which appears to have 

 similar amino acid composition in a variety 

 of mammalian species (see Ponder, '48). 

 Evidence has been presented by Moskowitz 

 et al. ('50, '52) for another protein, "elinin," 

 which is said to be an elongate macromole- 

 cule and to contain the Rh antigens. No 

 doubt other proteins, present in relatively 

 small quantities in the red cell envelope, 

 remain to be discovered. However it seems 

 probable that the bulk of the protein moiety 

 is composed of complex molecules which are 

 characteristic of this envelope and which re- 

 semble each other in various animal forms 

 (as do the muscle proteins). 



Almost nothing is known about the pro- 

 teins of the membranes of embryonic cells 

 and of various tissue cells. One good reason 

 for this is that, unlike the mammalian eryth- 

 rocyte, it is very difficult to separate the 

 limiting envelope of tissue cells from the re- 

 mainder of the cell material. It is therefore 

 impossible to say whether there is a class of 

 proteins or macromolecular complexes which 

 is characteristic of the cell membrane gen- 

 erally. It is also impossible to say whether 



* On the other hand, considerable structure has 

 been observed in the nuclear membrane. In the case 

 of the amphibian egg, Callan and Tomlin ('50) 

 found that the nuclear membrane consists of an 

 outer porous layer and continuous inner layer. The 

 pores in the outer layer have a diameter of 400 A 

 and are regularly arranged. 



