While some cells divide, live, and die all in the same tissue locale, others travel freely throughout the body, originating in one site and taking up residence at another. Many of these cells are targeted to specific locations. Embryonic cells travel in precise routes as the embryo develops. Inflammatory cells, such as neutrophils and monocytes, travel in the circulation to specific inflammatory sites, bypassing other normal tissues. Similarly, lymphocytes travel in the circulation and selectively emigrate through specific channels in lymph nodes or Peyers patches. Metastatic tumor cells also migrate in a selective manner from their primary site of origin to selected secondary organs.
Until recently, information concerning cellular targeting was generally descriptive, consisting primarily of patterns of migration without any molecular detail as to the mechanisms of the targeting process. In the past several years, advances in cell and molecular biology techniques along with the development of monoclonal antibodies have enabled researchers to identify many of the molecules that mediate cellular homing and of metastatic tumor cell targeting to secondary sites. Consequently these two fields comprise the bulk of the chapters in this volume. In general, however, the contributions made in one field have not been rapidly transferred to other fields in which cell targeting has been studied, Although more descriptive information exists concerning pattern development and cell migration in the embryo, the study of the molecular mechanisms of such patterns has not developed as rapidly as that concerning mechanisms of leukocyte homing, for example. One important reason for assembling the diverse contributions to this volume is to bridge that gap and allow researchers and students of these different fields to compare the progress that has been made in each area, and to compare the molecules that have been discovered. This can often be quite productive as it appears that many of the adhesion molecules involved in inflammatory cell homing may also play a role in the dissemination of tumor cells to specific distant organs.