Pluripotent stem cells have the unique ability to differentiate into cells of the three germ layers: ectoderm, mesoderm, and endoderm. Cells of the endoderm are initially arranged as flattened cells but later develop into a more columnar formation as gastrulation proceeds. The primary function of the endoderm is to provide the epithelial lining of the bodys two major tubes, the digestive tract and the respiratory tract. Endocrine glands which open into the digestive tract also originate from the endoderm, including cells for the liver, pancreas, and gall bladder. Endodermal lineage markers are available for the study of the earliest stages of endodermal lineage specification, as well as pancreatic and hepatic progenitor cells.