CELL ULTRASTRLCTURE L\ MAMMALS 



the parietal cells of the gastric mucosa, they 

 appear in great abundance in the luminal 

 portion of the cell. They seem to migrate 

 towards the cell surface and fuse with the 

 surface plasma membrane (Fig. 9). It may 

 be assumed that they participate in a certain 

 secretory process, involving the transport of 

 large amounts of fluid to the cell surface. 

 Similar vesicles are a prominent feature of 

 the capillary wall, where many such struc- 

 tures are found thi'oughout the endothe- 

 lial cytoplasm as well as in connection with 

 both the surface and the basal plasma 

 membrane (Fig. 10). The theory has been 

 forwarded that they represent the structural 

 evidence of fluid being transported across 

 the capillary wall. In nerve endings and in 

 connection with synapses, similar vesicles 

 have been demonstrated to contain acetyl- 

 choline. The presence of synaptic vesicles 

 has suggested that these might be involved 



in either the formation of the chemical 

 mediator or in its transmission across the 

 synapse. 



Ergastoplasm (rough-surfaced endo- 

 plasmic reticulum). In the light micro- 

 scope, areas with basophilic structures have 

 been called ergastoplasm. The electron 

 microscopical analysis of such areas reveals 

 highly complicated systems of paired mem- 

 branes, each membrane having a thickness of 

 about 40A (Fig. 1). Each pair of membranes 

 surrounds a light space called cisterna. It has 

 been demonstrated in nerve cells that the 

 cisternaeof the Nissl body communicate with 

 each other. The cytoplasmic aspect of each 

 membrane is studded with numerous small 

 granules with a thickness of 150A (Fig. 2). 

 These granules have been separated from the 

 membranes by cell fractionation and subse- 

 quent differential centrifugation. Enzymatic 

 tests prove that they contain ribonucleic acids 



Fig. 10. Part of a lymph capillary in the human epididymis. The cytoplasm of the 

 capillary endothelium (L) contains a large number of microvesicles. One mitochon- 

 drion (M) is seen. At the top is the capillary lumen (Lu) and at the bottom part of a 

 fibroblast (F) as well as several cross sectioned collagen fibrils (C). The lymph capil- 

 laries lack the usual type of basement membrane. Instead, a network of fine fibrils 

 (Re) probably of reticular nature, establish the immediate base upon which the endo- 

 thelial cells rest. Magnification 34,oOOX 



103 



