Mast cells reside in various tissues, such as skin, stomach, mesentery and peritoneal cavity. Stem cell factor is an important growth factor for mast cells, and the expression level of its receptor, KIT, is correlated with the number of mast cells. We examined the number of mast cells in several kinds of homozygous mutant mice at the mi locus, at which a transcription factor named MITF is encoded. MITF regulated the expression of KIT gene, and most homozygous mutants at the mi locus showed a reduced KIT expression. The number of mast cells in these mutants decreased in the skin, and no mast cells were present in stomach, mesentery and peritoneal cavity. In contrast to MITF mutants showing decreased expression of KIT, Miwh/ Miwh mice were normal in KIT expression. In consistent with the previous observation, Miwh/ Miwh mice contained normal number of mast cells in the skin, stomach and mesentery. However, no mast cells were detected in the peritoneal cavity of Miwh/Miwh mice. This indicated that the number of peritoneal mast cells was not correlated with the KIT expression level. Recently, we found that a newly identified mast cell adhesion molecule, SgIGSF, was not expressed in Miwh/ Miwh mice. This suggested the possibility that SgIGSF expression was related to the number of peritoneal mast cells. Here, we examined the number of peritoneal mast cells in a mild homozygous mutant, mivit/mivit In mivit/mivit mice, SgIGSF expression was reduced but apparently detected and KIT expression was normal. The number of peritoneal mast cells was one fifth that of wild-type mice. The number of mast cells in the skin, stomach and mesentery was comparable to that of wild-type mice. These results suggested that the adhesion molecule, SgIGSF, regulated the number of mast cells in the peritoneal cavity, and that KIT regulated the number of mast cells in other tissues, such as skin, stomach and mesentery.
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