Evidence for in vivo mitosis by granule-containing mast cells from canine mastocytomas.

Mast cell accumulations are generally considered to arise almost exclusively from the recruitment of non-granulated, bone-marrow-derived, precursor cells, with the stem cell factor (SCF) reported to play a crucial role in the growth, development and maturation of granulated mast cells within specific tissue sites. In this study dog mastocytoma specimens have been examined by both immunohistochemical and ultrastructural techniques, to demonstrate that fully granulated mast cells are capable of mitotic activity. Observations showing the formation of mitotic spindles, chromosome separation and cytokinesis all support the concept that granulated mast cells are capable of proliferative activity. The ability of mature granulated mast cells to replicate provides an alternative process for local increases in mast cell numbers, at least in canine mast cell tumours. Such observations suggest the possibility that normal or neoplastic human mast cells, fully granulated, have the potential to proliferate in specific tissue sites.