Congratulations on a careful methodological study! Under in vivo conditions, there is presumably a vascular tone, and this is emphasised by the in vivo diameters which you measure: around 120 um, considerably less than I would expect of vessels in this region (first branches off the superior mesenteric artery?) under relaxed conditions. Your in situ fixation method causes a (small) reduction in this diameter. This reduced diameter is also seen in the in vitro system, which suggests that there is also some vascular tone there. You suggest that the measurements are being made in the absence of tone? Are you sure about this? This is of course vital, when making investigations of vascular structure.