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. Ex pression of the EEM-1 antigen is largely confined to extra-embryonic membra nes, 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 lay er of the amnion also stains. Within the embryo proper, antigen is expresse d in limited regions of the gut, kidney, and pancreas, EEM-1 is also expres sed 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 c ulture. Older EBs have increasing numbers of positive cells which are confi ned to the surface. A special class of EBs, termed "cystic EBs," are covere d by a cell layer which strongly express EEM-1 antigen. The EEM-1 antibody will be useful for investigating the development of extra-embryonic membran es and their counterparts in the ES cell in vitro model. (C) 2000 Wiley-Lis s, Inc.