Nidogen-1: Expression and ultrastructural localization during the onset ofmesoderm formation in the early mouse embryo

Nidogen-1, a key component of basement membranes, is considered to function as a link between laminin and collagen Type IV networks and is expressed b y mesenchymal cells during embryonic and fetal development. It is not clear which cells produce nidogen-1 in early developmental stages when no mesenc hyme is present. We therefore localized nidogen-1 and its corresponding mRN A at the light and electron microscopic level in Day 7 mouse embryos during the onset of mesoderm formation by in situ hybridization, light microscopi c immunostaining, and immunogold histochemistry. Nidogen-1 mRNA was found n ot only in the cells of the ectoderm-derived mesoderm but also in the cytop lasm of the endoderm and ectoderm, indicating that all three germ layers ex press it. Nidogen-1 was localized only in fully developed basement membrane s of the ectoderm and was not seen in the developing endodermal basement me mbrane or in membranes disrupted during mesoderm formation. In contrast, la minin-1 and collagen Type IV were present in all basement membrane types at this developmental stage. The results indicate that, in the early embryo, nidogen-1 may be expressed by epithelial and mesenchymal cells, that both c ell types contribute to embryonic basement membrane formation, and that nid ogen-1 might serve to stabilize basement membranes in vivo.