Granule changes of human skin mast cells characteristic of piecemeal degranulation and associated with recovery during wound healing in situ.

Human skin mast cells (HSMC) in situ were examined by electron microscopy of surgical biopsy specimens obtained from a wide variety of circumstances. From these studies, it is apparent that the general ultrastructural morphology of normal HSMC is similar to that of human mast cells from other sites, except that the most prevalent granule pattern is that of crystal granules. Scroll granules, particle granules, and mixed granules can also occur in HSMC. Cytoplasmic lipid bodies occur in HSMC extremely rarely, unlike mast cells from lung and gut. Circumstances in which piecemeal degranulation (PMD) of HSMC occurs, such as bullous pemphigoid, examined at high magnifications, revealed typical, focal geographic losses from cytoplasmic granules, often leaving completely empty granule containers in the cytoplasm. Crystal portions of mixed granules were uniquely susceptible to granule losses typified by PMD. Cytoplasmic smooth vesicles were prominent in PMD. These structures were either empty or contained granule-like dense materials and were free in the cytoplasm or attached to granules. HSMC present in wound healing revealed recovery from PMD losses. Typically these granules contained numerous irregular foci of markedly dense new granule products within empty or partially empty granule containers. The morphology of PMD and recovery of HSMC in vivo is contrasted with the morphology of anaphylactic degranulation (AND) and recovery of human mast cells ex vivo.