Fine structure of the fat body and nephrocytes in the life-stages of Dermacentor variabilis.

The fine structure of the fat body and associated nephrocytes of the American dog tick, Dermacentor variabilis (Say), was described in unfed larvae, unfed nymphs, and in unfed and fed adults of both sexes. The fat body consisted of one type of cell, the trophocyte. Morphological changes that occurred in the trophocytes of both sexes were dependent on feeding. The ultrastructure of feeding male trophocytes was distinct from trophocytes of feeding females. In the feeding female, the trophocyte developed an ultrastructure characteristic of cells that produce secretory proteins. A type of scalariform cell junction was found associated with rough endoplasmic reticulum of the trophocytes. Nephrocytes were closely associated with trophocytes but were not part of the fat body. Nephrocyte ultrastructure was unaltered throughout the life-stages we examined, except at the end of oviposition. Organelles in the nephrocytes were not randomly distributed, but were found in distinct regions of the cytoplasm. Slit diaphragms at the surface of the nephrocytes were extracellular specializations that had a periodic ultrastructure.

Antibodies against a 23Kd heparin binding fragment of thrombospondin inhibit platelet aggregation.

Antiserum against a 23Kd heparin binding fragment of thrombospondin inhibits the aggregation of platelets in response to ADP, collagen or thrombin. The antiserum inhibits the secretion-dependent second phase, but not the primary phase of aggregation of platelets responding to ADP. Although immune serum added during the second phase of ADP-induced aggregation causes some inhibition of secretion, it also causes reversal of aggregation to the level produced during primary aggregation. Since thrombospondin is the endogenous lectin of human platelets, these results support the conclusion that the endogenous lectin mediates, at least in part, the secretion-dependent aggregation of platelets. Our data suggest that the region of thrombospondin which contains the heparin binding domain(s) present in the 23Kd fragment play(s) a critical role in secretion-dependent aggregation of platelets.