3-DIMENSIONAL ANALYSIS OF THE WHOLE MESANGIUM IN THE RAT

The three-dimensional structure of the mesangium was analyzed by means of reconstruction from serial semithin and ultrathin sections of the rat glomerulus. The mesangial domains traced on light micrographs of s emithin sections were transferred to styrene models, which were stacke d up to reconstruct the whole mesangium. The reconstructed mesangium w as tree-like in shape nd was divided into three lobes that were connec ted to the vascular pole by a slender neck. The glomerulus contained n o islets of mesangium which were not connected to the vascular pole. T he mesangium contained mesangial loops that were penetrated by capilla ries. Reliability of the findings on the mesangial loops was ascertain ed by various methods including reconstruction of part of the mesangiu m from ultrathin sections. Electron microscopic observations revealed that the arms of the mesangial loops were frequently very slender and consisted of mesangial cell processes containing prominent bundles of actin filaments. The mesangial loops were distributed evenly within th e mesangium. Considering previous reports showing about 400 capillary the rat glomerulus as well as the present findings, we the mesangial l oops may change the distribution of intraglomerular blood flow bq dyna mic contraction of the mesangial cells, in or serve a's an additional safety device to prevent the expansion of glomerular capillaries.