The molecular processes underlying the formation of the first desmosomes in the mouse early embryo have been examined by immunocytochemical and biochemical techniques using antibody probes recognising desmosomal proteins 1 and 2 (dp1 + 2, desmoplakins), dp3 (plakoglobin), desmosomal glycoprotein 1 (dg1, desmoglein) and dg2 + 3 (desmocollins). Immunofluorescence labelling of staged intact embryos and synchronised cell clusters indicates that dp1 + 2, dg1 and dg2 + 3 are first detectable on the lateral membrane contact sites between trophectoderm cells in early cavitating blastocysts, coincident with the onset of desmosome formation as seen in ultrastructural preparations. Membrane localisation of these antigens is predominantly punctate in appearance, occurs after division to the 32-cell stage and appears to be coincident with blastocoele formation since non-cavitated embryos/cell clusters of equivalent age/cell cycle are usually unlabelled. In contrast, dp3 is first detectable at the 32-cell stage at all internal membrane contact sites (including those with inner cell mass cells) in a continuous linear pattern, and appears in both cavitated and non-cavitated specimens. Subsequently during blastocyst expansion, dp3 localisation becomes punctate and restricted to trophectodermal membranes. Immunoprecipitation of desmosomal antigens following metabolic labelling indicates that synthesis of dp3 is underway from at least compaction in the 8-cell embryo, while dp1 + 2 synthesis is first evident in 16-cell morulae. Synthesis of dg1 and dg2 + 3 is not detectable until the early blastocyst stage. These results suggest that desmosome biogenesis in the preimplantation embryo might be regulated by transcription or translation of desmosomal glycoproteins and by maturational changes in the trophectoderm layer associated with blastocoele formation. The earlier expression and wider distribution of dp3 at cell contact areas may reflect non-desmosomal sites (eg, adherens junctions) for this protein and a possible role for dp3 in the development of intercellular junctions.

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