Ultrastructural localization of type IV collagen and laminin in the seven-day-old mouse embryo

The localization of the basement membrane components type IV collagen and laminin was investigated in seven-day-old mouse embryos (NMRI) fixed with formaldehyde, using an immunoperoxidase technique. Posttreatment of the embryos with TBS (trishydroxymethylaminomethane buffered saline) buffer was prerequisite for restoration of the antigenicity after fixation. The localization of the peroxidase (PO) positive reaction after treatment with anti-type IV collagen and anti-laminin antibodies in the embryos has been compared with results obtained after fixating embryos with the addition of tannic acid. Tannic acid stained the basement membrane of the ectodermal cell layer, in particular the lamina densa. After immunostaining for type IV collagen and laminin, a strong PO-positive reaction in the lamina densa of the ectodermal basement membrane was observed. A basement membrane of the endodermal cell layer had not yet been formed at this developmental stage. In this region, which is where a basement membrane was to develop in later stages, a tannic acid positive material consisting of granules with a diameter of about 25 nm was found near the surface of the endoderm. Moreover, PO-positive patches were seen in this part of the embryo after staining for laminin as well as after staining for type IV collagen. These PO-positive patches were mainly localized in areas where mesodermal cells lay adjacent to the surface of the endodermal cell layer. No positive staining for type IV collagen and laminin was found in the cytoplasm of either ectodermal or endodermal cells.