simple squamous epithelium periderm epitrichium TAO:0001185 XAO:0000029 FMA:295662 The outermost layer (or layers, depending on the species) of embryonic skin, essential for the development of the epidermis and establishment of the barrier functions. The periderm derives from the outer layer of the ectoderm. ZFA:0001185 NCIT:C34247 EHDAA:6538 UBERON:0003055 enveloping layer EHDAA2:0001846 VHOG:0001680 outer epithelium external integument structure