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