CELL ULTKASnaCTLRE IN MAMMALS 



most cellular enzymes and one believes that 

 the membranous struct m'es of their interior 

 represent either the enzymes proper or the 

 surfaces upon which the enzymes and the 

 metabolites interact. 



Microbodies. The microbodies are spheri- 

 cal and usually smaller than the mitochon- 

 dria. They are surrounded by a single mem- 

 brane, about 50A thick. The matrix has the 

 same appearance as that of the mitochondria 

 but lacks the inner membranes of the latter 

 (Fig. 3). They have been demonstrated so 

 far only in kidney, liver, cortical adrenal 

 cells, and in the delta cells of the endocrine 

 portion of the pancreas. They may represent 

 the precursors of mitochondria although this 

 has not been convincingly proved. It is still 

 a mystery how mitochondria develop. Some 

 people believe they can arise de novo from 

 the ground substance of the cytoplasm, 

 where possibly the microbodies would con- 

 stitute an intermediate stage. Others con- 

 sider a splitting or budding of already exist- 

 ing mitochondria to be more likely, as is 

 known to occur during cell mitosis. 



Large Granules. The large granules en- 

 countered in cells of various tissues are 

 usually very dense; this has led most in- 

 vestigators to believe that they contain 

 lipids. They have, therefore, often been 

 called cell lipid granules. However, the den- 

 sity varies from time to time and it is diffi- 

 cult to predict if this is due to a certain func- 

 tional variation or if it mainly reflects a 

 difference in structures. Most granules in 

 cells represent a secretory product and as such 

 will be dealt with below (Secretion). The re- 

 maining large granules are of two types 

 - — spherical granules and granules with ir- 

 regular outlines. The large spherical granules 

 are surrounded by a distinct single mem- 

 brane and display a medium dense structure- 

 less matrix (Fig. 3). It is most likely that 

 they represent end products of substances 

 taken in by the cells by means of a micro- 

 pinocytotic activity through the tubular 



invaginations of the plasma membrane. By 

 a certain metabolic process, the engulfed 

 fluid, and therein dissolved substances, 

 become concentrated and now appear as 

 dense granules. This is surely the case 

 with macrophages and has also been dem- 

 onstrated in connection with uptake of 

 proteins by the proximal convoluted cells 

 of the nephron and of the intestinal cells. 

 The granules with irregular outlines most 

 likely represent lipid granules which the 

 cell handles as part of its metabolism 

 (Fig. 8). Structural evidence is at hand 

 for a certain interaction between the lipid 

 granules and the mitochondria, as for 

 instance in liver cells. The intense blacken- 

 ing of lipid granules by osmium tetroxide 

 indicates that these granules contain un- 

 saturated sulfhydryl groups. Saturated fats 

 do not take stain with osmic acid; this can 

 be clearly demonstrated in fat cells where 

 the fat globules are convincingly stained 

 with Sudan III for light microscopy but in 

 the electron microscope show up as un- 

 stained vacuoles with a bordering thin 

 membrane. 



Pigments. The pigments represent an- 

 other type of spherical granule which can be 

 encountered in a number of cells. Similar to 

 lipid granules, they stain intensely with 

 osmium tetroxide (Fig. 8). The pigments of 

 the retina are surrounded by a single mem- 

 brane. Their matrix is homogeneous. The 

 pigments encountered in the cells of the 

 epidermis (often called melanin granules) do 

 not have a limiting membrane but unveil an 

 abundance of small pigment micelles each of 

 which has a diameter of about 75A. The pig- 

 ments of the epidermis are supposedly 

 formed within special cells, the melanocytes, 

 and migrate into the basal cells of the 

 epidermis. 



Golgi ApiJaratus. The Golgi apparatus is 

 located near the nucleus mostly surrounding 

 its one pole like a halo. It consists of a sys- 

 tem of paired membranes, small vesicles and 



101 



