II. Capsular vaso-mimicry formed by transgenic mammary tumor spheroids implanted ectopically into mouse dorsal skin fold: implications for cellular mechanisms of metastasis

Abstract

Most cancer patients die of metastatic disease, not primary tumors, while biological mechanisms leading to metastases remain unclear and effective therapies are missing. Using a mouse dorsal skin chamber model we had observed that tumor growth and vasculature formation could be influenced by the way in vitro cultured (avascular) spheroids of N202 breast tumor cells were implanted; co-implantation of lactating breast tissue created stimulating microenvironment, whereas the absence of the graft resulted in temporary tumor dormancy. This report addressed the issue of cellular mechanisms of the vasculogenic switch that ended the dormancy. In situ ultrastructural analysis revealed that the tumors survived in ectopic microenvironment until some of host and tumor stem cells evolved independently into cells initiating the vasculogenic switch. The tumor cells that survived and proliferated under hypoxic conditions for three weeks were supported by erythrogenic autophagy of others. However, the host microenvironment first responded as it would to non-immunogenic foreign bodies, i.e., by encapsulating the tumor spheroids with collagen-producing fibroblasts. That led to a form of vaso-mimicry consisting of tumor cells amid tumor-derived erythrosomes (synonym of erythrocytes), megakaryocytes and platelets, and encapsulating them all, the host fibroblasts. Such capsular vaso-mimicry could potentially facilitate metastasis by fusing with morphologically similar lymphatic vessels or veins. Once incorporated into the host circulatory system, tumor cells could be carried away passively by blood flow, regardless of their genetic heterogeneity. The fake vascular segment would have permeability properties different from genuine vascular endothelium. The capsular vaso-mimicry was different from vasculogenic mimicry earlier observed in metastases-associated malignant tumors where channels formed by tumor cells were said to contain circulating blood. Structures similar to the vasculogenic mimicry were seen here as well but contained non-circulating erythrosomes formed between tumor nodules. The host’s response to the implantation included coordinated formation of new vessels and peripheral nerves.

I. Embryonal vasculature formation recapitulated in transgenic mammary tumor spheroids implanted pseudo-orthotopicly into mouse dorsal skin fold: the organoblasts concept

“…At the outset, the components of the tumor niche (tumor spheroid, graft and local host tissues) did not have an ongoing vasculature formation. In both variants of the model, with and without homologous tissue graft, the new qualities emerged after the implantation (this report and the accompanying article 43 , respectively). New tissues formed locally and integrated into the host animal.…”

“…Thus, tumors were subjected to selective pressure because those incapable of either inducing local nonmalignant SCs to form tumor-supporting vasculature or of generating their own vasculature 43 could not grow. The ability of the tumor to generate its own vasculature in the ectopic environment implied a conditional existence of CSCs, i.e.…”

“…(2) Heterogeneity with respect to cell cycle activity was detected even in such an iconic cell lineage as the HSCs of adults 42 . In case of the CSCs, their role in tumorigenesis appeared to depend on microenvironment as discussed below and in one of our accompanying articles 43 .…”

“…We carried out the ultrastructural in situ analysis of initially avascular breast tumor spheroids co-implanted with homologous tissue graft, into the mouse dorsal skin folds 44, 45 . A complementary study on such spheroids implanted without the graft is the subject of a separate report 43 . In contrast to the reductionism that dominated the ultrastructural studies of the second half of the last century, we analyzed tissue sections instead of isolated tissue components.…”

The Human Mesenchymal Stem Cells and the Chick Embryo Chorioallantoic Membrane: The Key and the Lock in Revealing Vasculogenesis

“…Tumor vasculature, for instance, has been thought to emerge by expansion of the host vasculature through postnatal sprouting of endothelial cells (angiogenesis). Therefore, it was proposed that inhibiting angiogenesis would stop tumor growth, but when this conceptually simple objective turned out to be impossible to reach, a notion of neo-vasculogenesis was born, suggesting that tumor vessels develop from circulating bone marrowderived endothelial precursors rather than from mature endothelial cells (5).…”