Intussusceptive Pillar Formation in Developing Porcine Glomeruli.

BACKGROUND/AIMS: Intussusceptive angiogenesis (IA) is a dynamic process which contributes to vascular expansion and remodeling. Intraluminal pillars have long been the distinctive structural indicator of IA. However, the mechanism of their formation has not been fully elucidated. METHODS: Using light and electron microscopy, we studied intussusceptive vascular growth in the developing porcine metanephric kidney. RESULTS: We observed intraluminal pillars formed by endothelial cells in the vasculature of developing glomeruli. Their diameter was < 2.5 µm, consistent with the diameter of nascent pillars. TEM revealed that the majority of these pillars consisted only of endothelium. However, a central core of extracellular matrix (ECM) covered by endothelium, reminiscent of a more mature intussusceptive pillar, was also found in the lumen of a glomerular capillary. Perivascular cells or pericytes were not involved in the pillar structure during these stages of formation. CONCLUSION: This study shows ECM presence in a mature intussusceptive pillar without any perivascular cell involvement in the structure. This leads to the hypothesis that ECM deposition precedes the participation of these cells in the formation of intraluminal pillars during IA in porcine metanephric glomerular capillaries.

Stereological and immunogold studies on TIE1 and TIE2 localization in glomeruli indicate angiopoietin signaling in podocytes.

Angiopoietins and their TIE receptors are important regulators of vascular stability and remodeling. These molecules are involved not only in the normal development of kidney glomeruli, but also in disease, thus making them promising targets for therapies. Although TIE receptors are mainly found in endothelial cells, some reports observed TIE2 expression in glomerular podocytes as well. This suggests a role of angiopoietins in the regulation of podocytes. In the present study, we aimed to map the subcellular localization of TIE receptors in metanephric glomeruli of fetal pigs using high-resolution immunogold electron microscopy and the relative labeling index stereological approach. TIE1 and TIE2 antibody labeling was detected on the abluminal side of endothelial cell membranes. In endothelial cells, 4.5% of TIE2 was observed close to cell-cell contacts and 11.9% of TIE2 was found in closely associated pairs, which suggests the presence of homodimers. Interestingly, both receptors were also expressed in podocyte foot processes indicating that TIE1 and TIE2 may play a similar role in podocytes as in endothelial cells.

Intussusceptive angiogenesis and expression of Tie receptors during porcine metanephric kidney development.

Intussusceptive angiogenesis (IA) is required for normal embryonic vascular development. The Tie family of receptors and their ligands, the angiopoietins, play an important role in the growth or regression of blood vessels which are important not only during development but also throughout an organism's life. The presence of IA was investigated in glomerular capillaries of the fetal porcine metanephros using Mercox II resin casts. The first signs of IA were observed in stage III glomeruli. Stage IV and V glomeruli showed numerous signs of aligned pillar formation and their successive merging to delineate the vascular entities. Furthermore, immunohistochemistry was used to determine the exact locations of the Tie receptors in the developing porcine metanephric kidneys. Tie1 and Tie2 were found in endothelial cells of all glomeruli. Strong expression of the receptors was found in podocytes of stage V glomeruli whereas a weaker expression was observed in the cuboidal epithelial cells of stage III and IV glomeruli. Remarkably, the receptors were also found in the parietal epithelium of Bowman's capsule. These findings indicate that there might be an association between the Tie receptors and the IA during porcine metanephric development and during glomerulogenesis in particular.