Angiogenesis describes the formation of new blood vessels from pre-existing vascular structures. While the most studied mode of angiogenesis is vascular sprouting, specific conditions or organs favor intussusception, i.e., the division or splitting of an existing vessel, as preferential mode of new vessel formation. In the present study, sustained (33-h) intravital microscopy of the vasculature in the chick chorioallantoic membrane (CAM) led to the hypothesis of a novel non-sprouting mode for vessel generation, which we termed “coalescent angiogenesis.” In this process, preferential flow pathways evolve from isotropic capillary meshes enclosing tissue islands. These preferential flow pathways progressively enlarge by coalescence of capillaries and elimination of internal tissue pillars, in a process that is the reverse of intussusception. Concomitantly, less perfused segments regress. In this way, an initially mesh-like capillary network is remodeled into a tree structure, while conserving vascular wall components and maintaining blood flow. Coalescent angiogenesis, thus, describes the remodeling of an initial, hemodynamically inefficient mesh structure, into a hierarchical tree structure that provides efficient convective transport, allowing for the rapid expansion of the vasculature with maintained blood supply and function during development.

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The chick chorioallantoic membrane (CAM) is extensively used as an in vivo model. Here, structure and hemodynamics of CAM vessel trees were analyzed and compared with predictions of Murray's law. CAM microvascular networks of Hamburger-Hamilton stage 40 chick embryos were scanned by videomicroscopy. Three networks with ∼3,800, 580, and 480 segments were digitally reconstructed, neglecting the capillary mesh. Vessel diameters ( D) and segment lengths were measured, and generation numbers and junctional exponents at bifurcations were derived. In selected vessels, flow velocities ( v) and hematocrit were measured. Hemodynamic simulations, incorporating the branching of capillaries from preterminal vessels, were used to estimate v, volume flow, shear stress (τ), and pressure for all segments of the largest network. For individual arteriovenous flow pathways, terminal arterial and venous generation numbers are negatively correlated, leading to low variability of total topological and morphological pathway lengths. Arteriolar velocity is proportional to diameter ( v∝ D^1.03measured, v∝ D^0.93modeling), giving nearly uniform τ levels (τ∝ D^0.05). Venular trees exhibit slightly higher exponents ( v∝ D^1.3, τ∝ D^0.38). Junctional exponents at divergent and convergent bifurcations were 2.05 ± 1.13 and 1.97 ± 0.95 (mean ± SD) in contrast to the value 3 predicted by Murray's law. In accordance with Murray's law, τ levels are (nearly) maintained in CAM arterial (venular) trees, suggesting vascular adaptation to shear stress. Arterial and venous trees show an interdigitating arrangement providing homogeneous flow pathway properties and have preterminal capillary branches. These properties may facilitate efficient oxygen exchange in the CAM during rapid embryonic growth.

The palliative treatment options for advanced hepatocellular carcinoma (HCC) are currently not satisfying. The use of photodynamic therapy (PDT) has gained much attention in the treatment of several cancers and has been approved as an alternative approach in treating different forms of cancers. We investigated for the first time the PDT effects of tetra-triethyleneoxysulfonyl zinc phthalocyanine (ZnPc) on HCC cells. Photoactivation of ZnPc loaded HCC cells resulted in a dose- and time- dependent growth inhibitory effect, the production of reactive oxygen species (ROS), induced cytotoxic effects and the induction of apoptosis in the investigated HCC cells (HepG2 and Huh-7). ZnPc-PDT inhibited the proliferation of HCC cells by up to 90% accompanied by a down-regulation of the activity and expression of the proliferation relevant mitogen-activated protein kinase (MAPK)-protein extracellular signal-regulated (ERK ½). Moreover, an up-regulation of proapoptotic BAX and a down-regulation of antiapoptotic B-cell lymphoma 2 (Bcl-2) expressions were observed with both proteins implicated in mitochondria-driven apoptosis. The investigation of the anti-tumor effect of ZnPc-PDT in vivo using the chicken chorioallantoic membrane assays (CAM) revealed a strong reduction in the size of HCC tumor plagues >80% after 4 days of PDT-treatment without affecting the survival of the developing embryo. The pronounced anti-proliferative and anti-tumor effects of ZnPc-PDT both in vitro and in vivo render ZnPc-PDT as a promising palliative treatment option for hepatocellular carcinoma.