Ulstrastructural changes in the salivary gland of the female cattle tick, Haemaphysalis longicornis, during feeding

In the cattle tick, Haemaphysalis longicornis, an ultrastructural study of sequential changes of the salivary gland was carried out during feeding. The salivary gland of the female tick comprised three kinds of acini: one agranular and two granule-secreting types. Type I agranular acini, having extensive infoldings of the basal plasma membrane, did not show much in the way of structural changes during feeding. The agranular cell of Type II and III acini changed their volume and structure during feeding to a remarkable extent; the e-cell (vacuolar cell) and the abluminal interstitial cell (water cell) of Type III acini in particular developed greatly at the late stage of feeding, ending up with an elaborate system of membranous infoldings. Thus, it is concluded that Type I acini play an important role in uptake of water vapour from the atmosphere at the unfed stage, and that the agranular cell of Type II and III acini may contribute to elimination of the excess water and electrolytes. Four kinds of granule-containing cells were recognized in the salivary gland, i.e. the a- and b-cells of Type II acini, and the c- and d-cells of Type III acini; the a-, c- and d-cells appear to secrete attachment cement precursors and the b-cell is a most likely candidate for secretion of hydrolytic enzymes. Summary There have been many ecological and physiological studies on the cattle tick Haemaphysalis longicornis, because of the economic importance of this species as the most dominant tick in pastures of Japan and as the vector for theileriosis. However, there have been few ultrastructural studies on this species. Therefore, the findings in this study not only revealed the structure of the salivary gland but also supported the past physiological knowledge from the viewpoint of the functional morphology. It has been known that the salivary gland of H. longicornis functions in maintaining osmotic and ionic balance; however, the type of acini (or cells) which play important roles in the function have not been identified so far. Type I acini, which had an elaborate system of membranous infoldings, a common feature of fluid-transporting epithelia, showed the highest cell activity at the unfed stage and did not change so much structurally during feeding. Thus, Type I acini are concerned with uptake of water vapour from the atmosphere at the unfed stage. On the other hand, elimination of the surplus water and electrolytes, which are produced by concentration of blood meal, is an important physiological event for feeding females. The excretory function is attributed to the interstitial cell of Type II acini, the e-cell and abluminal interstitial cell of Type III acini, all of which develop conspicuously after attachment. These agranular cells exhibit extensive infoldings of the plasma membrane, and the abundant e-cells and abluminal interstitial cells may contribute to the excretion of the bulk of excess water and electrolytes, while the interstitial cell of Type II acini, which is small in comparison with the former two kinds of cells, appears to have a supplemental function. The adluminal interstitial cell of Type III acini, containing numerous microtubules, seems to function in supporting the e-cell and abluminal interstitial cell, both of which change their volume and structure strikingly during feeding. Therefore, it is concluded that the osmotic balance in female H. longicornis is regulated by Type I acini at the unfed stage, and by the agranular cells of Type II and III acini at the feeding stage. Moreover, four kinds of granule-containing cells, i.e. the a- and b-cells of Type II acini, and the c- and d-cells of Type III acini, were recognized in the salivary gland of this species. Both the a-cell of Type II acini and the c-cell of Type III acini resembled each other in ultrastructure of the granules, and showed a high secretory activity at the early stage of feeding, as did the d-cell of Type III acini. It is assumed, by comparison with histochemical and ultrastructural findings on other ixodid salivary glands, that the above three kinds of granule-containing cells are closely related to secretion of attachment cement precursors. The remaining b-cell of Type II acini continued to keep granules even at the late stage of feeding, and consequently it seems to secrete hydrolytic enzymes such as esterase and aminopeptidase during feeding.