Fine structure of shark leucocytes during chemotactic migration.

It has previously been demonstrated that peripheral blood leucocytes of the nurse shark (Ginglymostoma cirrhatum) are capable of in vitro chemotaxis. In the present study we have identified the chemotactic cells as the shark granulocyte and monocyte-macrophage. Chemotaxis assays were set up in blind well chambers. Cells migrating into the filter in a chemotaxis assay were identified morphologically by transmission electron microscopy. Migration, in response to LPS-activated rat serum, was accompanied by change in cell shape and reorganization of the cytoskeleton. Ultrastructural changes included formation of lamellipodia with cell polarization, submembranous accumulation of microfilaments at points of plasma membrane contact with filter surfaces, densification of the membrane at these points of engagement, appearance of bundles of intermediate filaments extending into the protruded lamellipodia, and presence of oriented microtubules in the cytoplasma (cellular cortex). The phagocytic and chemotactic properties of the shark granulocyte and monocyte-macrophage points to their being structural and functional equivalents of the mammalian neutrophil and macrophage.