Mast cells (MCs) are constitutively present in most tissues and a distinct subset of MCs can also be induced upon host responses to inflammation. The hematopoietic lineage development of tissue MCs is unique compared to other myeloid-derived cells because it is early lineage progenitors, undetectable by histochemistry, that leave the bone marrow to enter the circulation. These immature lineage MCs immediately undergo transendothelial recruitment into peripheral tissues wherein the appearance of secretory granules with a particular protease phenotype is regulated by the peripheral tissue. In this Perspective, we discuss our current understanding of how these unique immunocytes arise, traffic to various sites, and may or may not mature into tissue-directed granulated phenotypes and query whether a granulated end stage is their only intended role.
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