10 CYTOLOGICAL TECHNIQUE 



proteins, lipoproteins, and lipids) that have been prepared for 

 electron-microscopy in the same way as pieces of tissue. This kind 

 of work is still in its infancy, but a most interesting start has 

 been made with phospholipids.^^® These appear in electron- 

 micrographs in the form of parallel membranes, often arranged 

 concentrically. Similar membranes are often seen in electron- 

 micrographs of cells.®" » ** They are called 'Golgi apparatus' by 

 many electron-microscopists. It seems better to regard them 

 provisionally as phospholipids.^"' ^^ These substances sometimes 

 occur free in the cytoplasm of the living cell, but sometimes they 

 are bound up in lipoprotein complexes. Certain fixatives and 

 other reagents used in microtechnique 'unmask' the phospho- 

 lipids of these complexes: that is to say, they set them free from 

 combination with protein.®^ In an electron-micrograph there may 

 be nothing to indicate in which form the phospholipid was 

 present in the living cell.^°' ^^ 



Reaction-products of two kinds occur repeatedly in the cyto- 

 plasm of very diverse cells. They appear to represent two distinct 

 objects, which may perhaps be of universal or almost universal 

 occurrence. These are the endoplasmic reticulum and the 'small 

 particles' of Palade (fig. 2). 



The endoplasmic reticulum is a three-dimensional network 



Fig. 2 {plate). A, electron-micrograph representing part of the 

 cytoplasm of a large phagocytic cell {macrophage) derived from 

 the blood of the domestic fowl. The micrograph is very exceptional 

 in representing not a section, but a whole mount of an extremely 

 thin cell. 



f, fold in cell membrane; gc, ground cytoplasm {in the strict 

 sense); 1, lipid droplet; m, mitochondrion {unusual in being 

 branched); r, rounded component of the endoplasmic reticulum; 

 t, trabecular component of the endoplasmic reticulum. 



B, electron-micrograph representing a section of a small part of 

 the basal region of an acinar cell from the pancreas of the rat. 



cm, cell membrane ; qv, flattened components of the endoplasmic 

 reticulum, seen in transverse section; sp, small particles of Palade. 



Reproduced from the Journal of Experimental Medicine^*' and the Journal of 

 Biophysical and Biochemical Cytology, '*" by kind permission of the Editors of 

 the journals and of Dr G. E. Palade and Dr K. R. Porter (authors of the papers 

 in which the micrographs first appeared). The lettering on the micrographs has been 



altered. 



