Spontaneous in vitro angiogenesis in a trout pronephric stromal cell line (TPS), and in TPS-haemopoietic co-cultures

This study describes angiogenic processes taking place in the in vitro microenvironment of a trout pronephric stroma cell line (TPS) under specific culture conditions, in which fetal calf serum, horse serum and hydrocortisone-sodium-21-hemisuccinate were used as supplements to the culture medium. When TPS cultures were kept in the same flask, i.e. without passages, for longer than 7 months, epithelioid cells differentiated into endothelial cells. Early stages of such differentiation were characterised by the presence of intracellular tubular vacuoles in clusters of neighbouring epithelioid cells. Subsequently, the endothelial cells reorganised and gave rise to microvascular structures, which branched over and into the TPS multilayers. The lining cells of the microvasculature showed typical characteristics of endothelial cells, such as ovoid or cubical shape, bundles of microfilaments and microtubules, and particularly numerous small vesicles at the apical pole, some of them fused to the plasma membrane. Similar angiogenic processes were also observed in long-term haemopoietic co-cultures formed by the TPS cell line and trout pronephric cell suspensions. Developing haemopoietic cells were observed at the basal pole of the vessels, and in the vascular lumen, where some immature cells appeared in close contact with the endothelium. These results indicate that the TPS cell line contains endothelial cell precursors, which are able to differentiate under certain culture conditions.