abnormal embryonic epiblast morphology small embryonic epiblast 2011-06-22T01:26:36Z MP:0011189 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. csmith reduced epiblast size reduced size 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 small EPI small ectoblast small embryonic ectoderm small primary ectoderm small primitive ectoderm