abnormal embryonic epiblast morphology MP:0003886 MPheno.ontology The epiblast cells (appearing on day 8 of human embryonic development) make up a columnar epithelium with dense microvilli on the apical surface. During gastrulation, the epiblast cells undergo epithelial-to-mesenchymal transition and delaminate to become the loose mesenchyme of the primitive streak. The epiblast is present in postimplantation mouse embryos between E5.5-E7.5. abnormal EPI abnormal ectoblast morphology abnormal embryonic ectoderm morphology abnormal primary ectoderm morphology abnormal primitive ectoderm morphology any structural anomaly of the transient structure derived from the inner cell mass which lies above the hypoblast; the epiblast tissue gives rise to the three primary germ layers (ectoderm, definitive endoderm, and mesoderm) and to the extraembryonic mesoderm of the visceral yolk sac, the allantois, and the amnion abnormal bilaminar embryonic disc