Intussusceptive angiogenesis (IA) is a complementary method to sprouting angiogenesis (SA). The hallmark of IA is formation of trans-capillary tissue pillars, their fusion and remodeling of the vascular plexus. In this study, we investigate the formation of the zebrafish caudal vein plexus (CVP) in Tg(fli1a:eGFP)y7 and the synergistic interaction of IA and SA in crafting the archetypical angio-architecture of the CVP. Dynamic in vivo observations and quantitative analyses revealed that the primitive CVP during development was initiated through SA. Further vascular growth and remodeling occurred by IA. Intussusception contributed to the expansion of the CVP by formation of new pillars. Those pillars arose in front of the already existing ones; and in a subsequent step the serried pillars elongated and fused together. This resulted in segregation of larger vascular segments and remodelling of the disorganized vascular meshwork into hierarchical tree-like arrangement. Blood flow was the main driving force for IA, particularly shear stress geometry at the site of pillar formation and fusion. Computational simulations based on hemodynamics showed drop in shear stress levels at locations of new pillar formation, pillar elongation and fusion. Correlative 3D serial block face scanning electron microscopy confirmed the morphological substrate of the phenomena of the pillar formation observed in vivo. The data obtained demonstrates that after the sprouting phase and formation of the primitive capillary meshwork, the hemodynamic conditions enhance intussusceptive segregation of hierarchical vascular tree i.e. intussusceptive arborization resulting in complex vascular structures with specific angio-architecture.

Essential hypertension is associated with decreased lumen area and a tendency for increased basement membrane thickening in capillaries of skeletal muscle. Exercise training may improve the diffusion conditions in essential hypertension by altering capillary structure and capillary number.

The ultrastructure of capillaries in skeletal muscle was morphometrically assessed in vastus lateralis muscle (VL) biopsies taken before and after exercise from 22 participants of two training studies. In study 1 (8 wk of ergometer training), light microscopy revealed capillary-fiber (C/F) ratio (+27%) and capillary density (+16%) to be higher (P ≤ 0.05) in postexercise biopsies than in preexercise biopsies from all 10 participants. In study 2 (6 mo of moderate running), C/F ratio and capillary density were increased (+23% and +20%; respectively, P ≤ 0.05) in VL biopsies from 6 angiogenesis responders (AR) after training, whereas 6 nonangiogenesis responders (NR) showed nonsignificant changes in these structural indicators (-4%/-4%, respectively). Forty capillary profiles per participant were evaluated by point and intersection counting on cross sections after transmission electron microscopy. In study 1, volume density (Vv) and mean arithmetic thickness (T) of endothelial cells (ECs; +19%/+17%, respectively) and pericytes (PCs; +20%/+21%, respectively) were higher (P ≤ 0.05), whereas Vv and T of the pericapillary basement membrane (BM) were -23%/-22% lower (P ≤ 0.05), respectively, in posttraining biopsies. In study 2, exercise-related differences between AR and NR-groups were found for Vv and T of PCs (AR, +26%/+22%, respectively, both P ≤ 0.05; NR, +1%/-3%, respectively, both P > 0.05) and BM (AR, -14%/-13%, respectively, both P ≤ 0.05; NR, -9%/-11%, respectively, P = 0.07/0.10). Vv and T of ECs were higher (AR, +16%/+18%, respectively; NR, +6%/+6%, respectively; all P ≤ 0.05) in both groups. The PC coverage was higher (+13%, P ≤ 0.05) in VL biopsies of individuals in the AR group but nonsignificantly altered (+3%, P > 0.05) in those of the NR group after training. Our study suggests that intensified PC mobilization and BM thinning are related to exercise-induced angiogenesis in human skeletal muscle, whereas training per se induces EC-thickening.

Our investigation demonstrates advancing senescence to be associated with increase in BM thickness and loss of EC and PC filopodia length in skeletal muscle capillaries.

(PAD). Impaired limb blood flow is a major casual factor of lower exercise tolerance in PAD but cannot entirely explain it. We hypothesized that IC is associated with structural changes of the capillary-mitochondria interface that could contribute to the reduction of exercise tolerance in IC patients. Capillary and mitochondrial morphometry were performed after light and transmission electron microscopy using vastus lateralis muscle biopsies of 14 IC patients and 10 age-matched controls, and peak power output (PPO) was determined for all participants using an incremental single-leg kneeextension protocol. Capillary density was lower (411 Ϯ 90 mm Ϫ2 vs. 506 Ϯ 95 mm Ϫ2 ; P Յ 0.05) in the biopsies of the IC patients than in those of the controls. The basement membrane (BM) around capillaries was thicker (543 Ϯ 82 nm vs. 423 Ϯ 97 nm; P Յ 0.01) and the volume density of mitochondria was lower (3.51 Ϯ 0.56% vs. 4.60 Ϯ 0.74%; P Յ 0.01) in the IC patients than the controls. In the IC patients, a higher proportion of capillaries appeared with collapsed slit-like lumen and/or swollen endothelium. PPO was lower (18.5 Ϯ 9.9 W vs. 33.5 Ϯ 9.4 W; P Յ 0.01) in the IC patients than the controls. We suggest that several structural alterations in skeletal muscle, either collectively or separately, contribute to the reduction of exercise tolerance in IC patients. capillary; morphometry; peripheral arterial disease; skeletal muscle; transmission electron microscopy PERIPHERAL ARTERIAL DISEASE (PAD) is an atherosclerotic disease characterized by stenosis or occlusion of the conduit arteries of the lower limbs (20,36). Risk factors associated with PAD include advancing age, cigarette smoking, diabetes mellitus, arterial hypertension, and disturbances in lipid metabolism (12). While many PAD patients are asymptomatic, the arterial narrowing leads to ischemia and hypoxia of the supplied tissues and in severe cases, this manifests as critical limb ischemia (CLI), usually requiring surgical interventions. Intermittent claudication (IC), which is typically described as a cramp-like muscle pain during exercise, is the most commonly reported symptom of PAD and is usually the first noted symptom of the disease.Patients with IC have limited muscular strength and endurance capacities (35), reduced cardiorespiratory fitness (V O 2 peak ), and demonstrate reduced oxygen uptake kinetics at the onset of exercise (6). They also exhibit a significantly reduced tolerance for physical activities such as walking and cycling (2, 13). This reduction in exercise tolerance leads to functional impairments that are similar to those observed in patients with heart failure (31) and contributes to their impaired mobility, reduced quality of life, and elevated risk for the development of cardiovascular complications (17).Impaired limb blood flow is commonly assumed to be the main casual factor of the reduction in exercise tolerance in PAD but cannot entirely explain it. For example, restoration of blood flow capacity with percutaneous endovascular revascularizat...