Merriam-Webster Collegiate Dictionary - interstitial

 adjective  Date: 1646  1. relating to or situated in the interstices  2.  a. situated within but not restricted to or characteristic of a particular organ or tissue — used especially of fibrous tissue  b. affecting the ~ tissues of an organ or part  3. being or relating to a crystalline compound in which usually small atoms or ions of a nonmetal occupy holes between the larger metal atoms or ions in the crystal lattice  • ~ly adverb