Capillarization

Capillarization is the process by which capillaries form or expand within a tissue, resulting in increased capillary density and enhanced microvascular perfusion. It encompasses angiogenesis, the growth of new capillaries from preexisting vessels, and in some contexts vasculogenesis, the formation of vessels from endothelial progenitor cells. Capillarization is commonly described in terms of metrics such as capillary density (capillaries per square millimeter) and the capillary-to-fiber ratio in muscle, as well as the diffusion distance from capillaries to surrounding cells.

Physiological significance and mechanisms: Increased capillarization improves the delivery of oxygen and nutrients and the removal of metabolic byproducts, supporting tissue metabolism and function. It occurs during development, wound healing, and in response to endurance training, where hypoxic stimuli trigger signaling pathways involving vascular endothelial growth factor (VEGF), hypoxia-inducible factors (HIF-1α), and angiopoietins that promote capillary sprouting and remodeling. Capillarization also shapes perfusion in organs such as the brain and heart, contributing to tissue resilience and function.

Measurement and interpretation: Capillarization is typically assessed histologically by staining endothelial markers (e.g., CD31/PECAM-1) to quantify metrics like capillary density and capillary-to-fiber ratio, as well as diffusion distance between capillaries and cells. Abnormal capillarization appears in various conditions: capillary rarefaction in diabetes and some vascular diseases, or excessive, disorganized capillary growth in tumor angiogenesis.

Limitations and context: Capillarization varies with age, physical activity, metabolic demand, and disease state. Interpretations depend on tissue type and methodology, as increased capillary density does not automatically translate to improved function if the microvascular network is dysfunctional.