20 The Anatomy of the Salivary Glands 



THE ULTRASTRUCTURE OF THE SALIVARY GLANDS 



The introduction of the electron microscope has already revolu- 

 tionized our ideas of the structure of cells and has placed an 

 entirely new set of facts at our disposal which must be made to 

 harmonize with the physiology of the secretory process. Up till the 

 present few satisfactory studies of the salivary glands have 

 appeared, but a great deal of detailed study has been made of the 

 pancreas. In all essential details the ultrastructure of the salivary 

 cells of the rat appear to resemble the pancreatic cells. The differ- 

 ences are merely of degree. We will therefore draw heavily on the 

 description of the pancreatic cell, adding such details as are 

 known of the salivary glands. 



The plasma membranes of both apical and basal faces of secret- 

 ing cells have a three-layered structure with a total thickness of 

 60-70 A (Sjostrand, 1956; Robertson, 1959). In material fixed in 

 Os0 4 this appears as two dark bands separated by a light band 

 about 25-30 A wide. It is still uncertain which structures are stained 

 by Os0 4 , but the consensus of opinion is that this reagent has a 

 strong affinity for spread protein layers and a rather poor affinity 

 for lipids. The three layers may therefore be considered as a lipoid 

 layer sandwiched between spread protein layers. This morpho- 

 logical interpretation agrees well with the physiological concept of 

 a cell membrane built up from studies on cell permeability, and the 

 physicochemical and electrical properties of the cell membrane (see 

 Danielli and Davson, 1943). At the areas of contact between adja- 

 cent cells the membranes are distinct and may follow a relatively 

 straight path as in the pancreas (Sjostrand and Hanzon, 19540) or 

 be tortuous and interlocking as in the rat submaxillary acini (Lee- 

 son and Jacoby, 1959). In the striated ducts of the rat submaxillary 

 an intermediate picture is seen, the adjacent membranes interlock 

 in the basal one-third of the cells and then run straight towards the 

 apical poles. 



At both free surfaces of the cell foldings of the cell membrane 

 occur. At the apical surface these are mainly seen as microvilli pro- 

 truding into the lumen of the acinus (Sjostrand and Hanzon, 

 1954a; Pease, 1956; Siekevitz and Palade, 1958; Leeson and 

 Jacoby, 19596). These microvilli are found in all stages of develop- 

 ment between simple protrusions of the plasma membrane and the 

 pinching off of droplets of "secretion" (Fig. 2.8). 



