anatomical boundary A non-material anatomical entity of two dimensions. Anatomical boundaries are contiguous structures. AEO:0000192 CARO:0000010 Except in the case of abstracted fiat boundaries such as the midline plane of an organism, all 2D anatomical entities have a 3 dimensional projection. For example, the surface of the shell of a muscle has a distinct shape that projects into the third dimension. Note that boundaries are 2D structures. They have no thickness - and so can not be sites of gene expression or gene product localisation. For this, use boundary region terms. FMA:50705 UBERON:0000015 uberon immaterial anatomical entity