simple squamous epithelium periderm EVL In some mammals, Eyelid Fusion is thought to be driven by a population of cells which are derived from the periderm, the outermost layer of the developing epidermis relationship type change: OBO_REL:part_of ectoderm (TAO:0000016) CHANGED TO: develops_from ectoderm (UBERON:0000924)[TAO] 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 EHDAA2:0001846 Originally the epidermis is one layer thick, in most vertebrates it soon becomes a two-layered structure. The outer layer gives rise to the periderm. The periderm goes through distinct developmental phases and is ultimately sloughed into the amniotic fluid when differentiation of the underlying epidermal layers is complete. The function of the periderm is not known, but is thought to be related to transport/exchange between the fetus and the amniotic fluid (http://courses.washington.edu/hubio567/devbio/periderm.html) The outermost epidermal layer covering the fish at embryonic stages; derived from the EVL and thought to eventually be replaced by the superficial stratum of the epidermis. Sometimes used synonymously with EVL. Le Guellec et al, 2004.[TAO] In mice, the first non-basal layer formed at ~E9.5; it is a temporary structure composed of simple squamous epithelium that serves as the first barrier to the embryo's physical environment, exists throughout the entire keratinocyte stratification process, and sheds off at ~E17, when it is replaced by corneocytes[MP] epitrichium TAO:0001185 In humans, the periderm is a monolayer and is shed prenatally. BTO:0003377 XAO:0000029 In mice, the periderm is bilayered. It consists of the outer layer, which possesses numerous microvilli, and the inner layer that develops specific periderm granules. Both layers can be well recognized as early as at E14.5 and they are shed at different time points: the outer layer is shed in utero, in conjunction with barrier acquisition (E16-18), while the inner layer is retained postnatally. enveloping layer UBERON:0003055 UMLS:C1518973 uberon VHOG:0001680 outer epithelium external integument structure