TEM of swirls of connective tissue fibers and fiber-producing cells surrounding a mast cell create an articitc pattern as seen in this ultra-thin section TEM. A mast cell (also known as a mastocyte or a labrocyte) is a type of white blood cell. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a part of the immune and neuroimmune systems and contains many granules rich in histamine and heparin.