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

Abstract

Inadequate understanding of cancer biology is a problem. This work focused on cellular mechanisms of tumor vascularization. According to earlier studies, the tumor vasculature derives from host endothelial cells (angiogenesis) or their precursors of bone marrow origin circulating in the blood (neo-vasculogenesis) unlike in embryos. In this study, we observed the neo-vasculature form in multiple ways from local precursor cells. Recapitulation of primitive as well as advanced embryonal stages of vasculature formation followed co-implantation of avascular (in vitro cultured) N202 breast tumor spheroids and homologous tissue grafts into mouse dorsal skin chambers. Ultrastructural and immunocytochemical analysis of tissue sections exposed the interactions between the tumor and the graft tissue stem cells. It revealed details of vasculature morphogenesis not seen before in either tumors or embryos. A gradual increase in complexity of the vascular morphogenesis at the tumor site reflected a range of steps in ontogenic evolution of the differentiating cells. Malignant- and surgical injury repair-related tissue growth prompted local cells to initiate extramedullar erythropoiesis and vascular patterning. The new findings included: interdependence between the extramedullar hematopoiesis and assembly of new vessels (both from the locally differentiating precursors); nucleo-cytoplasmic conversion (karyolysis) as the mechanism of erythroblast enucleation; the role of megakaryocytes and platelets in vascular pattern formation before emergence of endothelial cells; lineage relationships between hematopoietic and endothelial cells; the role of extracellular calmyrin in tissue morphogenesis; and calmyrite, a new ultrastructural entity associated with anaerobic energy metabolism. The central role of the extramedullar erythropoiesis in the formation of new vasculature (blood and vessels) emerged here as part of the tissue building process including the lymphatic system and nerves, and suggests a cellular mechanism for instigating variable properties of endothelial surfaces in different organs. Those findings are consistent with the organoblasts concept, previously discussed in a study on childhood tumors, and have implications for tissue definition.