Expression of a novel antigen, EEM-1, in the mouse embryo and embryonic stem cell-derived embryoid bodies

A novel monoclonal antibody designated EEM-1 is described. EEM-1 recognizes an intracellular protein with an apparent molecular weight of >250 kDa. Expression of the EEM-1 antigen is largely confined to extra-embryonic membranes, but some expression does occur in the embryo. In the embryonic day 6 (E6) and E7 embryo it is expressed in visceral and parietal endoderm; later derivatives of these structures in the yolk sac are negative. The outer layer of the amnion also stains. Within the embryo proper, antigen is expressed in limited regions of the gut, kidney, and pancreas. EEM-1 is also expressed in embryonic stem (ES) cells differentiating in vitro. Undifferentiated ES cells do not express the antigen. Embryoid bodies (EBs) derived from ES cells have patches of EEM-1-positive cells on their surface at 2 days in culture. Older EBs have increasing numbers of positive cells which are confined to the surface. A special class of EBs, termed 'cystic EBs,' are covered by a cell layer which strongly express EEM-1 antigen. The EEM-1 antibody will be useful for investigating the development of extraembryonic membranes and their counterparts in the ES cell in vitro model. (C) 2000 Wiley-Liss, Inc.