ELECTRON MICROSCOPY 



displays small tubular invaginations which membrane close to the free surface of all 

 penetrate about half a micron into the cell epithelial cells (Fig. 8). In a sense, they are 

 (Fig. 8). They are found mostly in cells with reminiscent of the coopering bands of a 

 a high rate of fluid uptake. It has been as- barrel, although located inside the barrel, 

 sumed that they represent another means by The terminal bar of one cell is always located 

 which fluid and substances can be taken in opposite a similar strvicture in a neighbor- 

 by a cell without first penetrating the plasma ing cell. Furthermore, the intervening space 

 membrane. Their activity has been com- between the cells is filled with a denser struc- 

 pared with the uptake of water by an ameba ture and is smaller than usually recorded in 

 (pinocytosis). The word "micropinocytosis" other places, undoubtedly indicating the 

 (or membrane flow) has been suggested, firmness by which the cells are attached, 

 indicating that once the fluid or substance Desmosomes. The desmosomes are in a 

 has been taken in by the microtubules, it section having almost the same appearance 

 can be entirely surrounded by the tubular as the terminal bars. However, they repre- 

 membrane, forming a vesicle which can be sent only local points of attachment and are 

 transported elsewhere in the cell (cross actuafly paired button-like structures with 

 reference: kidney ultrastructure, ciliated one ''button" attached to the intracellular 

 epithelia ultrastructure). aspect of the plasma membrane of either 



Cell Border. The plasma membrane cell (Fig. 8). In addition, the intercellular 



which faces a neighboring cell is called cell space between the two "buttons" is larger 



border. than in other places and is occupied by a 



Lateral inter digitations. This portion of the substance of high density which frequently 

 plasma membrane frequently displays small contains even denser structures of lamellated 

 lateral projections of the cell cytoplasm nature. The desmosomes are most abundant 

 which penetrate into the cell in juxtaposi- in the cells of the epidermis (Fig. 14) but are 

 tion (Fig. 14). In some instances, the pro- also found in other epithelial cells. Struc- 

 jections become quite numerous, as is the tures of identical appearance are also demon- 

 case between the cells of the ciliary epithe- strated in striated muscle and in the specific 

 hum of the eye or in the intestine. It was first tissue of the heart (the impulse conducting 

 believed that the lateral interdigitations system). Besides their adhesive function in 

 helped in maintaining cell cohesion; how- these tissues, they probably also serve as 

 ever, it has been suggested recently that they points of less resistance across which the 

 rather are the result of a certain compression impulse of contraction can travel with much 

 or expansion during different functional higher speed than elsewhere, 

 stages of the cells, much like what happens Basal Surface. The basal plasma mem- 

 to the bellows of an accordion. The varia- brane which faces the basement membrane 

 tion of volume does not occur in the cell is from time to time elaborately infolded, 

 itself to any large extent, but rather to the Epithelial cells. This is particularly the 

 intercellular space, which is expanded by case in the cells of the nephron, of the ciliary 

 fluid from time to time. Cell cohesion is, on epithelium of the eye, and of the choroid 

 the other hand, definitely accomplished by plexus of the ventricles of the brain. The 

 two peculiar structures which are closely infoldings sometimes reach to the depth of 

 related to the cell border — the terminal bars half the cell and may also be seen inter- 

 and the desmosomes. digitating laterally with each other. Un- 



Terminal bars. The terminal bars are ring- doubtedly, they serve to increase the basal 



like reinforcements of the cell border at- surface of the plasma membrane upon which 



tached to the inner aspect of the plasma enzymatic activity can more readily occur. 



98 



