Salivary gland of the tick vector (R. appendiculatus) of East Coast fever. II. Cellular basis for fluid secretion in the type III acinus.

Fluid balance is a major physiological problem for hematophagous ticks. To maintain osmotic balance they must conserve water for prolonged periods while seeking a mammalian host, and they must eliminate a very large volume of excess fluid taken in during a relatively short period of feeding. This is accomplished in part by modification of the salivary gland during 7-10 days of feeding to secrete a copious saliva which is pumped into the bovine host. This function has previously been attributed to certain interstitial epithelial cells of the type III acinus which differentiate in the course of feeding into cells reminiscent of those of the avian salt gland. The ultrastructural changes in the type III acinus during the blood-meal were studied. In addition to the differentiation of the interstitial cells, this paper describes a remarkable sequence of changes in external form and internal organization of the e-cells. This results in their transformation from typical protein-secreting glandular cells to transport cells having myriad basolateral processes interdigitating with those of the ablumenal interstitial cells to form a basal labyrinth comparable to that of other fluid-transporting epithelia. The findings are discussed in relation to various postulated mechanisms for fluid and electrolyte transport.