Human mast cells.

The ultrastructural complexities of human mast cells are reviewed. Numerous investigators have provided ultrastructural descriptions of human mast cells in a variety of tissues in situ. We have reviewed these contributions and provide here unified information necessary for the recognition of the variable images presented by human mast cells and the substructural patterns of their granules. These studies in aggregate provide sufficient and necessary morphologic information for the identification of human mast cells found in tissues as contrasted with the necessary ultrastructural criteria for the identification of human basophilic leukocytes present either in bone marrow, peripheral blood, or tissues. Recent technical advances have provided rich sources of isolated, purified human mast cells from several organ sites. We have studied isolated, purified human lung mast cells in depth and present a review of these studies. Specifically, these in vitro studies of human mast cells have made possible the delineation of morphologic criteria for the distinction between two important subcellular organelles in human mast cells--secretory granules and lipid bodies. These organelles differ in structure, in content, in mechanism(s) of formation, in behavior during degranulation, and in behavior during recovery from degranulation. Secretory granules and their associated performed mediator(s) of inflammation are released together from appropriately stimulated mast cells. Lipid bodies contain large amounts of arachidonic acid. Ultrastructural autoradiographic analysis and biochemical determination of secreted, labeled products of arachidonic acid oxygenation by stimulated human mast cells demonstrate the release of small amounts of these labeled products in the same time course. We used an ultrastructural, morphometric, organelle aggregate volume analysis to show that the mechanism(s) of generation of human mast cell scroll granules differed from the mechanism of generation of lipid bodies. The morphologic kinetics of anaphylactic degranulation in vitro were correlated with histamine release kinetics in replicate samples of isolated, purified human lung mast cells. This noncytotoxic process is characterized by sequential morphologic changes. These include initially the swelling and alteration of intragranular materials, followed by granule membrane fusions to form elongated cytoplasmic degranulation channels filled with altered matrix materials. These closed channels open to the cells' surface through multiple narrow pores at peak release values for histamine. The aggregate volume of cytoplasmic granules decreases dramatically, whereas there is no significant changes i