Abstract: The mammary gland is a highly active organ whose epithelium changes dramatically during puberty and reproductive cycles.These changes are driven by specialized stem and progenitor cells.The research of mammary stem cells is of great significance to animal mammary gland development,lactation and breast cancer.Today,significant advances have been made in the biology of mammary stem cells.Mammary stem cells are histologically undifferentiated epithelial cells that can divide symmetrically to produce two identical stem cells,or asymmetrically to produce one kind of stem cell and one kind of luminal epithelial progenitor cell or basal/myoepithelial progenitor cell.A variety of different types of mammary stem/progenitor cells can be identified by labeling retained cells,dye exclusion,stem cell antigen-1 labeling,cell surface marker labeling,and lineage tracing.The isolation and identification of mammary stem cells by different methods is helpful to understand the heterogeneity.Mammary research is divided into six stages:Embryo,adolescence,sexual maturity,pregnancy,lactation and involuted phases.The types and characteristics of mammary gland stem/progenitor cells are different in different stages of mammary gland development.Studies have shown that the progenitor cells with basal and luminal lineages are produced in embryo phase.Quiescent mammary stem cells may also come from the embryo stage.After birth,quiescent mammary stem cells can be stimulated by ovarian hormones to re-enter the cell cycle,producing a mammary cell lineage and self-renewal.This paper introduced the existence,morphological characteristics and identification methods of mammary stem cells and progenitor cells,briefly described the characteristics of mammary stem cells in embryonic,adolescent and pregnancy,analyzed the current shortcomings and prospects for future application,in order to further study animal breast stem cells.

Key words: mammary gland development; mammary stem cell; identification; cell surface marker; lineage tracer