Assertion: skin's surface | /r/HasProperty | exposed to outside world
Facets:
Source sentences:
text: Cells that lie on surfaces exposed to the outside world or gastrointestinal tract (epithelia) or internal cavities (endothelium) come in numerous shapes and forms – from single layers of flat cells, to cells with small beating hair-like cilia in the lungs, to column-like cells that line the stomach.