Mast cells are found primarily in the connective tissue of the skin and mucous membranes. Their location gives a hint of their function: protection against foreign elements through quick distribution of chemicals which help eliminate those elements (4).
Mast cells are part of the immune system and are reactive to antibodies produced by plasma cells against foreign elements. The granules in mast cells contain histamine (dilates blood vessels allowing more blood to the affected area) and heparin (prevents blood clotting allowing flushing of tissue), among other things. When you take an anti-histamine you are using a drug that blocks receptors on mast cells and prevents their activation (4).
Those who are allergic to bees and other stinging insects have a super-sensitive mast cell system which all release their contents at once causing anaphylaxis (4).
There two major sub-populations of mast cell, which differ considerably in their stainability, depending on the staining protocol and the fixative used (3). Bryan Hewlett (3) has found that connective tissue mast cells are probably the easiest to stain with traditional dye technology. Mucosal mast cells can be more problematic, particularly with formaldehyde fixation.
Several respondents have suggested the local vet to be a good source of control tissue (1,7). John Kiernan (6) suggests that the dura mater of the rat would be a good positive control as well as rat or mouse external ear or tongue. A cross-section of rat ear has wonderful mast cells lined-up along either side of the cartilage (7).
Mastocytomas of skin are fairly common in both canine and feline. Some mastocytomas are loaded with granules while poorly differentiated tumours may show only focal granularity in some cells (1).
In humans, mast cells are found primarily in the connective tissue of the skin and mucous membranes. They are also found in the thymus and the capsules of most organs. Tim Morken suggests to look around small blood vessels (4).
Bryan Hewlett (3) uses small bowel as a control ( with a variety of fixatives) because it contains both connective tissue and mucosal mast cells. Other tissues to consider, for the same reason, would include lung and nasal polyp (3).
Jeff Silverman(1) suggests that in common human material, neurofibroma, myxoid fibroadenomas and allergic sinonasal polyps are good bets to contain scattered mast cells. Look for ovoid to dendritic cells with blue granules and small oval central nucleus.
The Wenks (2) use a bladder that was removed due to very bad cystitis. Someone once mentioned to them that fallopian tubes have mast cells in them, and that's what they use as their mast cell control. They haven't tested this out yet, so they don't have first hand knowledge.
There was some disagreement as to whether mast cells are found in the CNS (4,5,6). Geoff McAuliffe (5) cites two references demonstrating mast cells in the CNS:
John Kiernan (6) explains that Silver et al were writing about a bird's brain. In rats there are mast cells around some cerebral blood vessels (eg. Devel. Neurosci. 4:220-224, 1981; Acta Anat. 124:149-158, 1985), but perhaps these are not quite truly "in" the brain. You do get truly intraparenchymal mast cells in parts of the brains of some mammals, in the habenular nuclei (as in the pigeon) and also in nearby dorsal and medial parts of the thalamus (eg. Nature 210:756-757, 1966; J. Anat. 121:303-311, 1976; Acta Anat. 75:443-452, 1970). The only mammals (to my knowledge) that have these truly in-the-brain mast cells, in contact with neurones and glia, are the hedgehog, the tree-shrew and the slow loris.
John (6) goes on to report that the dura mater of the rat has been much used for experiments with mast cells. See big book, "The Mast Cells" by Hans Selye (Butterworths, 1963).
It seems that the distribution of mast cells will vary depending on the species being studied.
Bryan Hewlett (3) reports that the number of mast cells demonstrable varies with the technology used. In human material, immunohistochemistry for CD117 or Mast cell tryptase will reveal the greatest number( in rat RMCPI or RMCPII). Dye technology, independent of fixative, will only stain approximately 60-90% of mast cells ( paper submitted). He suggests that you read chapter 24, p695, by Lennart Enerbach, in "Histochemistry in Pathological Diagnosis", editor Samuel S. Spicer, 1987, Marcel Dekker Inc. NY. ISBN 0-8247-7408-6.
John Kiernan (6) says that rodent mast cell granules survive pretty well any fixation. For other species an alcoholic fixative will give the best results (Carnoy and alcoholic Bouin are both OK).