Uni - directional. One Way Only. Saliva is always secreted into the lumen of the gland (and then onwards down the ducts). Crudely speaking, material is taken up at one side of the cell and dumped out the other which means that one end of a salivary gland acinar cells is specialised for inlux and the other for efflux. In short, these cells are polarised. Striated cells actively transport electrolytes in the opposite direction, these cells are also polarised... but differently

The unidirectionality of secretion is achieved by the barrier function of the acinar and duct cells in separating blood from saliva and, at a cellular level, by polarisation of structure (figure 12) and function. Every cell type involved in salivary secretion is polarised in one way or another. Acinar and duct cells are connected together by tight junctions, which also form the division between the apical membrane which faces into the lumen of the gland and the basolateral membrane which faces the blood. The different properties of these two membranes are fundamental to the polarisation of cell function necessary for unidirectional secretion.

Striated ducts are so called because in longitudinal section, their basolateral side has a striped appearance. The stripes are caused by many infoldings of the basal membrane, crammed full of mitochondria (figure 12). A high density of mitochondria, close to the plasma membrane is usually indicative of primary active transport, in this case the Na⁺/K⁺ ATPase. The most obviously defining feature of acinar cells is the apical pole of the cell, densely packed with secretory vesicles.

From a functional perspective, the apical pole of the acinar cells is where all the most critical events occur. Secretory vesicles are directed by the actin cytoskeleton towards the apical pole of the cell and exocytosis occurs almost exclusively at the apical pole.

The key event in fluid secretion, activation of the Ca²⁺-dependent anion channel also occurs at the apical pole. There is growing evidence to indicate that the controlling Ca²⁺ signal originates at the apical pole of the cell and under certain circumstances, may be restricted to this pole of the cell

Acinar and striated ducts are very obviously polarised. Acinar cells have a high density of secretory vesicles at the apical pole and striated duct cells have basal infoldings and a high density of mitochondria.