Vascular morphogenesis and remodeling in a human tumor xenograft: blood vessel formation and growth after ovariectomy and tumor implantation

To determine mechanisms of blood vessel formation and growth in solid tumors, we used a model in which LS174T human colon adenocarcinomas are grown in the isolated ovarian pedicle of nude mice. Reconstruction of 3500 histological serial sections demonstrated that a new vascular network composed of venous-venous loops of varying sizes grows inside the tumor from the wall of the adjacent main vein. Loops elongate and remodel to establish complex loop systems. The mechanisms of loop formation and remodeling correspond to intussusceptive microvascular growth (IMG). In the tissue surrounding the tumor segmentation, another mechanism of IMG is prevalent in venous vessels. Comparison to vascular morphogenesis in the ovariectomized pedicle not only confirms the existence of corresponding mechanisms in both systems, but also reveals numerous sprouts that are superimposed onto loop systems and pathological deviations of loop formation, remodeling, and segmentation in the tumor. These pathological mechanisms interfere with vessel patency that likely cause heterogenous perfusion and hypoxia thus perpetuating angiogenesis. Blood vessel formation based on IMG was also detected in a large thrombus that completely occluded a part of an ovarian artery branch.

Vascular morphogenesis and remodeling in a model of tissue repair: blood vessel formation and growth in the ovarian pedicle after ovariectomy

To investigate mechanisms of vascular morphogenesis in tissue repair, we performed ovariectomy with resection of the corresponding branches of the ovarian vessels in nude mice. This induces a vascular network remodeling response in the healing ovarian pedicle. Reconstruction of 2000 histological serial sections demonstrated that a new vascular network composed of venous-venous loops forms in the wall of the dilated ovarian vein. Preexisting veins of all sizes, including a branch of the main artery, are subjected to segmentation. Loop formation and segmentation are based on intussusceptive microvascular growth. Loop formation is followed by elongation. Loop remodeling occurs also by intussusception and results in the formation of compound loop systems. All loop systems observed were completely patent. Blind-ending sprouts were extremely rare. Anastomoses between the preexisting vessels subjected to segmentation and the loop systems were established to include the newly formed vessels into the preexisting vascular network. The formation of an increasing number of patent loop systems likely decreases hypoxia and subsequently arrests angiogenesis with transformation of the granulation tissue into a scar. Loop formation also occurred inside a large thrombus that occluded a part of the lumen of the main vein.

Vasculogenesis and angiogenesis as mechanisms of vascular network formation, growth and remodeling

Two distinct mechanisms, vasculogenesis and angiogenesis implement the formation of the vascular network in the embryo. Vasculogenesis gives rise to the heart and the first primitive vascular plexus inside the embryo and in its surrounding membranes, as the yolk sac circulation. Angiogenesis is responsible for the remodeling and expansion of this network. While vasculogenesis refers to in situ differentiation and growth of blood vessels from mesodermal derived hemangioblasts, angiogenesis comprises two different mechanisms: endothelial sprouting and intussusceptive microvascular growth (IMG). The sprouting process is based on endothelial cell migration, proliferation and tube formation. IMG divides existing vessel lumens by formation and insertion of tissue folds and columns of interstitial tissue into the vessel lumen. The latter are termed interstitial or inter-vascular tissue structures (ITSs) and tissue pillars or posts. Intussusception also includes the establishment of new vessels by in situ loop formation in the wall of large veins. The molecular regulation of these distinct mechanisms is discussed in respect to the most important positive regulators, vascular endothelial growth factor (VEGF) and its receptors flk-1 (KDR) and flt-1, the Angiopoietin/tie system and the ephrin-B/EpH-B system. The cellular mechanisms and the molecular regulation of angiogenesis in the pathological state are summarized and the differences of physiological and pathological angiogenesis elaborated.

TIE1 and TIE2 receptor tyrosine kinases inversely regulate embryonic angiogenesis by the mechanism of intussusceptive microvascular growth

As shown previously, TIE1 and TIE2 receptor tyrosine kinases are specifically expressed in endothelial cells during embryonic angiogenesis. A detailed analysis of the vascular malformations of homozygous mice for a targeting mutation of both receptors was performed at the histological and cellular level. The data demonstrate that the TIE1 and TIE2 receptor inversely and concomitantly mediate interactions between endothelial cells with their extracellular matrix and with surrounding mesenchymal cells. These interactions are obviously crucial for normal endothelial cell motility and/or attachment and also for recruitment of periendothelial cells. The analysis of the TIE2-deficient embryos demonstrates how these cell/cell- and cell/matrix interactions subsequently influence the formation of normally structured tissue folds that divide the vessel lumen. They are also essential for the formation of vessel loops that compose a new vascular network and for the development of the ventricle in the heart. Fold and loop formation follow the principles of intussusceptive microvascular growth. The localization of the cardiovascular malformations corresponds to the temporal and spatial expression pattern of the TIE2 receptor. Angiopoietin-1, a ligand that activates the TIE2 receptor, is expressed in mesenchymal cells surrounding the endothelium. This local relationship is indicative of a paracrine regulation.

Implementation of intussusceptive microvascular growth in the chicken chorioallantoic membrane (CAM)

Intussusceptive microvascular growth (IMG) is a new mechanism of capillary growth: The vascular network expands by insertion of newly formed columns of interstitial tissue (interstitial tissue structures) into the vascular lumen called tissue pillars or posts (diameter: 0.5-2.5 microm). IMG has so far been described during organ development and growth and in tumor angiogenesis. Different modes of its implementation could be demonstrated in the rat lung and the chicken chorioallantoic membrane (CAM). In the present investigation a further mechanism of IMG is reported in the chicken CAM: tissue pillars form by splitting of larger interstitial tissue structures and intercapillary walls located between neighboring capillary segments which will consecutively fuse. Splitting is dependent on the existence of a pillar's core composed of a bundle of collagen fibrils ensheathed by extensions of endothelial-like cells inside these structures. Pillar cores thus represent the smallest unit of interstitial tissue around which the vascular lumen might expand. This mode of IMG is obviously connected to physiological remodeling of the capillary network and appears to be dominant during later stages of CAM development.

Requisite role of angiopoietin-1, a ligand for the TIE2 receptor, during embryonic angiogenesis

Vascular endothelial growth factor (VEGF), which acts via members of a family of endothelial-specific receptor tyrosine kinases, is the only factor that has been shown definitively to play a role in the formation of the embryonic vasculature. Only one other family of receptor tyrosine kinases, comprising TIE1 and TIE2, is largely endothelial cell specific. We have recently cloned a ligand for TIE2, termed Angiopoietin-1. Here we show that mice engineered to lack Angiopoietin-1 display angiogenic deficits reminiscent of those previously seen in mice lacking TIE2, demonstrating that Angiopoietin-1 is a primary physiologic ligand for TIE2 and that it has critical in vivo angiogenic actions that are distinct from VEGF and that are not reflected in the classic in vitro assays used to characterize VEGF. Angiopoietin-1 seems to play a crucial role in mediating reciprocal interactions between the endothelium and surrounding matrix and mesenchyme.

Intussusceptive microvascular growth in a human colon adenocarcinoma xenograft: a novel mechanism of tumor angiogenesis

Intussusceptive microvascular growth refers to vascular network formation by insertion of interstitial tissue columns, called tissue pillars or posts, into the vascular lumen and subsequent growth of these columns, resulting in partitioning of the vessel lumen. While intussusception has been reported in normal developing organs, its existence in solid tumors has not been previously documented. By observing the growth of the human colon adenocarcinoma (LS174T) in vivo for a period of 6 weeks, we demonstrate that intussusception is an important mechanism of tumor angiogenesis. At the leading edge of the tumor, vascular growth was found to occur by both intussusception and endothelial sprouting. In the stabilized regions, intussusception led to network remodeling and occlusion of vascular segments. The formation of some tissue pillars appears to depend on intravascular blood-flow patterns or changes in intravascular shear stress. The rapid vascular remodeling by intussusception could possibly contribute to intermittent blood flow in tumors.

Implementation of intussusceptive microvascular growth in the chicken chorioallantoic membrane (CAM): 1. pillar formation by folding of the capillary wall

Intussusceptive microvascular growth is a new mode of capillary network growth originally described in the lungs of rabbits and rats. It constitutes an alternative to endothelial sprouting. The capillary network grows by insertion of new intercapillary meshes with dimensions around 1.5 microns called tissue pillars or posts. In a recent investigation, growth by intussusception was demonstrated in the chicken chorioallantoic membrane (CAM). In the present study the first of several modes of its implementation can now be presented in the CAM by in vivo video microscopy and analyses of light and electron microscopic serial sections: Cores of tissue pillars containing collagen fibrils ensheathed by extensions of endothelial-like cells will form within the tips of vertically running tissue folds that project into the capillary lumen. Due to retraction of tissue toward the intercapillary space the fold is thinning. Finally, the pillar's core is connected to its fold by a very slender extension of a single endothelial cell. Cell membrane fusion within that slender membrane-like structure causes subsequent separation of the pillar from its fold throughout an increasing vertical distance. This mechanism allows for expansion of the capillary network into the surrounding tissue, leaving behind tissue pillars as remnants of folds.

Evidence for intussusceptive capillary growth in the chicken chorio-allantoic membrane (CAM)

The aim of our investigations was to test whether the chicken chorio-allantoic membrane (CAM) could be an adequate in vivo model for a new mode of capillary growth, originally described in the rat lung and termed intussusceptive microvascular growth. According to that concept the capillary system does not grow by sprouting of vessels, but expands by insertion of transcapillary tissue pillars or posts which form new intercapillary meshes. In the present study, we observed slender transcapillary tissue pillars with diameters around 1 microns in the CAM by in vivo microscopy, and analyzed their ultrastructure by transmission electron microscopic investigation of serial sections. The pillars corresponded in size to those previously described in rat lung microvasculature. On day 7, the pillar core contained endothelial-, endothelial-like cells and collagen fibers, and on day 12 additionally chorionic epithelial cells. As a hypothesis we propose that slender cytoplasmic extensions of endothelial cells, heavily interdigitated in the post area and often projecting into the vascular lumen, could initiate the first step of pillar formation, i.e., interconnect opposite capillary walls. During both stages of development endothelial-like cells were observed in close relationship with the pillars. These cells seem to be relevant for tissue post completion and growth, as they were found to invade the core of the pillars. From the localization of the interendothelial junctions in the post region, a certain similarity to the concept proposed for the lung can be found. The observations confirm that the CAM is a very suitable material for the in vivo investigation of intussusceptive capillary growth.

Intussusceptive microvascular growth: a common alternative to capillary sprouting

Intussusceptive capillary growth represents a new principle for microvascular growth as described in the lungs of growing rats. According to this concept, the capillary network expands by the formation of slender transcapillary tissue pillars, which give rise to new vascular meshes. The process was first observed in Mercox casts of the lung microvasculature, which revealed the existence of multiple tiny holes with diameters around 1.5 microns. Consecutive transmission electron microscopic investigation of serial sections demonstrated that the holes corresponded to slender tissue pillars (Burri and Tarek, 1990). The corrosion cast technique thus appears to be an adequate screening method for intussusceptive growth. In the present investigation, Mercox casts of various vascular systems, namely, those of the eye, submandibular gland, heart, liver, stomach, small and large intestine, trachea, kidney, uterus and ovary were prepared from rats aged between 4 and 9 weeks in order to screen them for the existence of the typical tiny holes representing tissue pillars. In all organs investigated, these structures were observed in various locations to a variable degree. They were mainly encountered within dilated vascular segments or at triple or quadruple branching points of the circulation. Even in capillary networks with a three-dimensional arrangement could these pillars be detected. Intussusception thus appears to be a principle of growth appertaining to many vascular systems.