For spintronic device applications, large and in particular tunable tunnel magnetoresistance (TMR) ratios are inevitable. Fully crystalline and epitaxially grown Fe/MgO/Fe magnetic tunnel junctions (MTJs) are well suited for this purpose and, thus, are being intensively studied [1]. However, due to imperfect interfaces it is difficult to obtain sufficiently large TMR ratios that fulfill industrial demands (e.g. [2]). A new means to increase TMR ratios is the insertion of ultra-thin tetallic buffer layers at one or at both of the Fe/MgO interfaces. With regard to their magnetic and electronic properties as well as their small lattice mismatch to Fe(001), Co and Cr spacer are being preferably investigated. We report on a systematic first-principles study of the effect of Co and Cr buffers (with thicknesses up to 6 ML) in Fe/MgO/Fe magnetic tunnel junctions (MTJs) on the spin-dependent conductance. The reults of the transport calculations reveal options to specifically tune the TMR ratio. Sysmmetric junctions, i.e. with Co buffers at both interfaces, exibit for some thickness much larger TMR ratios in comparison to those obtained for Fe-only electrodes. Further, antiferromatgnetic Cr films at a single interface introdce TMR oscillations with a period of 2 ML, a feature which provdes another degree of freedom in device applications. The comparison of our results with experimental findings shows agreement and highlights the importance of interfaces for the TMR effect.