has part has part has_part characteristic of has modifier abnormal embryonic epiblast morphology thick embryonic epiblast increased thickness 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 MP:0011190 csmith thick embryonic ectoderm thick EPI thick primary ectoderm MPheno.ontology 2011-06-22T01:26:36Z thick primitive ectoderm 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. thick ectoblast abnormal increased thickness inner cell mass derived epiblast