Changes in Thrombocyte Ultrastructure during Clot Retraction

The contraction of the entire thrombocyte appears to be a primary factor in the retracting mechanism of dogfish ( Mustelus canis) in vitro blood clots. Microtubules probably account for the condensation and lobation of the nucleus, and the impetus for cytoplasmic contraction appears to be dependent upon the grouping of microfibres of dimensions difficult to measure. These microfibres show a striking resemblance to materials stored within adjacent vesicles. A second type of fibre (60 A in diameter) extends from the bases of pseudopods into the adjacent matrix. There also appears to be partial engulfment of an extracellular amorphous material by the plasma membrane. The ultrastructure of the thrombocyte during clot retraction shows some striking similarities to the mammalian platelet under similar conditions, and leads one to believe a phylogenetic relationship exists between these haemostatic cells. This work was performed primarily at the Marine Biological Laboratories, Woods Hole, Massachusetts, U.S.A.