A rapidly proliferating population of cells that differentiate from stem cells of the intestinal crypt of the small intestine. Stem cells located in the crypts of Lieberkühn give rise to proliferating progenitor or transit amplifying cells that differentiate into the four major epithelial cell types. These include columnar absorptive cells or enterocytes, mucous secreting goblet cells, enteroendocrine cells and paneth cells. [ http://orcid.org/0000-0003-4183-8865 http://www.ncbi.nlm.nih.gov/pubmed/20801415 ]