Mast cells (MCs) play a pivotal role in the hypersensitivity reaction by regulating the innate and adaptive immune responses.Humans have two types of MCs.The first type,termed MCTC,is found in the skin and other connective tissues and expresses both tryptase and chymase,while the second,termed MCT,which only expresses tryptase,is found primarily in the mucosa.MCs in-duced from human adult-type CD34+ cells are reported to be of the MCT type,but the development of MCs during embryonic/fetal stages is largely unknown.Using an efficient coculture system,we identified that a CD34+c-kit+ cell population,which appeared prior to the emergence of CD34+CD45+ hematopoietic stem and progenitor cells (HSPCs),stimulated robust production of pure Tryptase+Chymase+ MCs (MCTCs).Single-cell analysis revealed dual development directions of CD34+c-kit+ progenitors,with one lineage developing into enythro-myeloid progenitors (EMP) and the other lineage developing into HSPC.Interestingly,MCTCs derived from early CD34+c-kit+ cells exhibited strong histamine release and immune response functions.Particularly,robust release of IL-17 suggested that these early developing tissue-type MCTcs could play a central role in tumor immunity.These findings could help elucidate the mechanisms controlling early development of MCTCs and have significant therapeutic implications.