After almost a century of research, we have yet to solve the riddle of the mast cell. Whereas little was known about this cell for most of this time span, more facts have been gathered recently than seem to be reconcilable within a single unified concept of mast cell function.
Anatomical observations have contributed to the confusion by demonstrating incontrovertibly that mast cells can discharge their granules ("degranulate") in response to a bewildering number of agents, both chemical and physical, e.g., polycations of all sorts, antigen-antibody reactions, mechanical or other traumas. It has been argued that degranulation reflects a secretory function, a viewpoint seemingly supported by many physiological, biochemical, and pharmacological studies in which mechanisms of granule release are examined in ever more detail. Other anatomical findings, by contrast, demonstrate just as convincingly that mast cell granules normally are not released from the cell, and among the plethora of biochemical studies some can only be interpreted as also denying a secretory function. In a n attempt to resolve these contradictory viewpoints, it even has been suggested that "degranulation" can occur intracellularly and without granule release, and that it may be reversible. Semantics aside, this is a rather startling and unique proposition for a "secretory" process.
The thrust of this editorial, in line with the anatomist's tenet that structure and function are intimately interrelated, will be to attempt to reconcile and extend these and some other seemingly paradoxical observations into a morphological-functional framework that might suggest new avenues for experimental testing.
In well differentiated mast cells, the usual complement of organelles is present, though these are normally few and not well developed, a fact that suggests a low-keyed resting metabolism and little synthetic activity. The cell can be rejuvenated and its organelles greatly reactivated if the granules are incapacitated i n situ or lost through degranulation. This suggests that fully differentiated (granulated) mast cells are nonsecretory elements that endocytize extracellular fluid, percolate it over their granules for processing and possible binding of some component(s) and then excrete the mix back to the external milieu via pores at the cell surface.
The plasmalemmal surface of the mast cell is folded extensively, and