Successful engineering of a new endocrine pancreas to treat diabetes mellitus represents a major challenge in endocrinology. One strategy is to isolate or generate insulin-secreting cells in vitro and transplant them in sufficient numbers to normalize glucose tolerance in a diabetic host [31]. Transplantation of embryonic organ primordia to replace the function of diseased organs followed by growth and differentiation in vivo (organogenesis) offers theoretical advantages relative to transplantation of either pluripotent embryonic stem (ES) cells, or of fully differentiated (adult) organs (whole pancreas or islets) [15].