ELECTRON MICROSCOPY 



until the whole luminal part of the cell (the 

 goblet) is filled by a muhittide of mucous 

 granules which at all limes are discharged 

 into the lumen. It is believed that either 

 type of exocrine cell has the ability to re- 

 generate new secretory granules. Accord- 

 ing to earlier theories, the cell would disin- 

 tegrate after the secretory cycle is completed. 

 Endocrine Secretion. The endocrine 

 granules are usually smaller than their 

 exocrine relatives. They are also more elec- 

 tron-dense and are mostly surrounded by a 

 single membrane. Few high resolution stud- 

 ies have so far been performed on endocrine 

 glands, but judging from the data available, 

 the formation of the endocrine granules is 

 similar to what is known about the exocrine. 

 A varying amount of rough-surfaced endo- 

 plasmic reticulum (ergastoplasm) as well as 

 an abundance of RNA-particles seem to 

 contribute the necessary prerequisites for 

 the production of the early stages of endo- 

 crine granules. In the beta-cells of the endo- 

 crine portion of the pancreas, it has been 

 quite convincingly demonstrated that the 

 granules first appear in the Golgi zone. The 

 matter of bringing these products in contact 

 with the capillaries of the gland is still not 

 fully explained but evidence is at hand for a 

 migration of the endocrine granules toward 

 that part of the cell which faces the capillary. 

 In some instances, it has also been possible 

 to demonstrate that the membrane of the 

 granule fuses with the plasma membrane, 

 thus giving the dense granule the oppor- 

 tunity to be discharged into the small extra- 

 cellular space existing between the plasma 

 membrane and the basement membrane 

 which surrounds the endocrine cells. From 

 here, the content of the granule may quite 

 easily diffuse into the interstitial space and 

 from there into the capillary. Further studies 

 are, however, needed to prove that this 

 occurs in relation to all endocrine organs. 



Ground Substance of the Cytoplasm 



In light microscopy, the term "ground 

 substance" referred to that part of the cyto- 



plasm which was not organized as mito- 

 chondria, Golgi apparatus, specialized cell 

 inclusions like zymogen granules, pigments, 

 or structures such as myofibrils. With the 

 introduction of the ultrastructural era, it 

 was demonstrated that the ground substance 

 does contain particular well-defined struc- 

 tures like cytoplasmic membranes of various 

 kinds, vesicles, RNA-particles, osmiophilic 

 large granules, and various fibrillar struc- 

 tures. These ultrastructures may, of course, 

 still be regarded as being part of the "ground 

 substance" of the cytoplasm. However, it 

 may also be justifiable today to call that 

 W'hich is as yet not defined as discrete struc- 

 tural entities as representing the ground sub- 

 stance awaiting new techniciues to be de- 

 veloped before this portion of the cytoplasm 

 can be described. In most electron micro- 

 graphs, a certain homogeneous background 

 characterized by a certain electron density 

 can always be recorded. This "background" 

 represents the ground substance in our 

 present concept of the cytoplasm. It is con- 

 ceivable that this background contains a 

 great variety of salts and ions as well as 

 carbohydrates, proteins and fats w^hich 

 available staining techniques and resolving 

 power of the electron microscope fail to 

 bring out. Until these are further developed, 

 let us consider the homogeneous background 

 of our electron micrographs as being the true 

 ground substance. In doing so, we may create 

 the necessary challenge to explore the un- 

 known structural world of the atoms of the 

 cytoplasm. 



REFERENCES 



General Reviews 



Selby, C. C, "Microscopy. II. Electron micro- 

 scopy: a review," Cancer Research, 13, 753 

 (1953). 



Sjostrand, F. S., "Electron microscopy of cells 

 and tissues," in "Physical Techniques in 

 Biological Research," 3, 241 (1956). Eds. G. 

 Oster and A. W. Pollister, Academic Press, 

 Inc., New York. 



Sjostrand, F. S., "The ultrastrvicture of cells as 

 revealed by the electron microscope", Int. 

 Rev. Cytol., 5, 455 (1956). 



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