Plasminogen activator secretion during mouse embryogenesis

Midgestation mouse embryos between the 7th day and the 10th day of development were dissected into their major component tissues and fragments of each were assayed for the production of plasminogen activator by plasminogen-dependent lysis of casein- or fibrin-containing agar overlays. Parietal endoderm was positive throughout this period. Portions of the visceral endoderm overlying the embryonic region of the egg cylinder and the underlying mesoderm and ectoderm became positive during the 8th day. The extraembryonic portion of the visceral endoderm was the last region of the endoderm layer to produce plasminogen activator. By the 10th day all of the tissues around the embryo—parietal endoderm, visceral yolk sac endoderm and mesoderm, and amnion—produced an activator. The assay of activator production by these tissue layers was differentially inhibited in the presence of 0.2 M NaCl. Although it might be possible to find diagnostic assay conditions to distinguish parietal endoderm from other plasminogen activator producing tissues of the early mouse embryo, plasminogen activator production is not a tissuespecific biochemical marker for parietal endoderm cells. The pattern of activator distribution in the rapidly growing midgestation embryo is consistent with this protease activity playing a role in tissue modeling.