Why Do Tumors Need New Blood Vessels? Angiogenesis Explained
Why Do Tumors Need New Blood Vessels? Angiogenesis Explained
A growing tumor needs a continuous supply of oxygen and nutrients to survive. In its earliest stages, a small cluster of cancer cells can obtain these essentials from nearby, pre-existing blood vessels by diffusion through the surrounding tissue. As the tumor expands, its inner cells end up too far from that blood supply. Many tumors then stimulate new vessel growth—a process called angiogenesis—though some tumors can also tap into existing blood vessels or use alternative vascular pathways.[1]
By Chris Morais, MSc, MPhil, PhD
- What Is Angiogenesis?
- Why Can a Small Tumor Grow Without Making New Blood Vessels?
- Why Does a Growing Tumor Eventually Need Its Own Blood Supply?
- How Does Low Oxygen Trigger Tumor Angiogenesis?
- What Is VEGF and How Does It Promote New Blood Vessel Growth?
- How Do New Blood Vessels Grow Toward a Tumor?
- Are Tumor Blood Vessels Normal?
- How Does Angiogenesis Help a Tumor Grow?
- Can Tumor Blood Vessels Help Cancer Cells Spread?
- Can Tumors Obtain a Blood Supply Without Ordinary Angiogenesis?
What Is Angiogenesis?
Angiogenesis is the biological process of forming new blood vessels from pre-existing ones. It is a normal physiological function essential for natural human growth, wound healing, and tissue repair. During angiogenesis, the specialized cells lining the interior of blood vessels—known as endothelial cells—multiply, migrate, and assemble into new vascular branches.
In healthy body tissues, signals that stimulate vessel growth are normally balanced by signals that restrain it. Tumors disrupt this natural equilibrium. Cancer cells and other cells within the surrounding tumor environment release molecular signals that prompt nearby blood vessels to branch toward the tumor mass.
This transition is widely termed the angiogenic switch. It does not occur as an instant on-off event. Instead, the pro-growth signals gradually overwhelm the inhibitory signals that normally keep blood vessel formation in check.[2]
Why Can a Small Tumor Grow Without Making New Blood Vessels?
A small tumor can grow without forming new vessels because its cells remain close enough to pre-existing capillaries to obtain oxygen and nutrients by diffusion.
Substances carried in the blood cross capillary walls and move through the surrounding extracellular space. Oxygen, glucose, amino acids, and ions can therefore reach cancer cells without those cells needing to touch a vessel directly.
However, passive diffusion operates effectively only over very short physical distances. As cancer cells divide and the tumor mass expands outward, the outer layers may remain closer to host vessels, while the deeper interior receives progressively less oxygen and nourishment.
Because of this physical limit, solid tumors generally need increased access to blood vessels to expand beyond a microscopic scale. According to the National Cancer Institute, solid tumors generally require their own blood supply to grow larger than a few millimeters.[1]
Importantly, going without new blood vessels does not mean a tumor operates without a blood supply. Microscopic tumors can rely on neighboring healthy vessels, and some cancers can grow along and exploit existing vascular tracks rather than building new ones right away.
Why Does a Growing Tumor Eventually Need Its Own Blood Supply?
A growing tumor typically needs increased access to a blood supply because an expanding mass increases metabolic demand while placing interior cells farther away from functioning circulation.
As total cell numbers multiply, the tumor consumes greater amounts of oxygen and glucose while generating higher volumes of metabolic waste like carbon dioxide. Without adequate perfusion, parts of the tumor can develop shortages of oxygen and nutrients as well as impaired waste removal:
- Cellular adaptation: Some tumor cells alter their metabolism to tolerate low-oxygen or low-nutrient conditions.
- Growth arrest and death: Cells unable to adapt stop dividing or undergo cell death.
- Tissue necrosis: Deep areas of poorly supplied tissue may die, forming zones of dead tissue called necrosis.
Developing new vessels—or tapping into existing networks—places more tumor cells within physical reach of circulating blood, allowing the tumor to survive and continue expanding.
How Far Can Oxygen and Nutrients Diffuse Through Tissue?
In many tissues, oxygen delivery becomes limited at distances of roughly 100 to 200 micrometers (0.1 to 0.2 millimeters) from a functioning capillary.
This biological range is an operational estimate rather than a fixed boundary[4]:
- Continuous oxygen use: Because living cells actively consume oxygen along the way, oxygen levels drop steeply as distance from the vessel wall increases.
- Nutrient variation: Molecules like glucose, amino acids, and ions diffuse and are consumed or transported at different rates depending on their properties and tissue demand.
- Tissue environment: Local blood flow rates, cell density, metabolic rates, and tissue architecture all influence how far nutrients travel.
The primary obstacle for a tumor is not that every nutrient stops at an identical physical line; it is that unassisted diffusion cannot sustain a solid tumor mass once it exceeds microscopic dimensions.
How Does Low Oxygen Trigger Tumor Angiogenesis?
Low oxygen promotes tumor angiogenesis through hypoxia-inducible factors (HIFs), proteins that control gene activity in response to oxygen availability.
When tumor cells outgrow their oxygen supply, they enter a state called hypoxia. Hypoxia simply means the cells are receiving less oxygen than their normal metabolism demands.
Cells respond to this deficit through HIF proteins, whose activity is regulated by oxygen-sensitive enzymes. When oxygen is low, HIF-α proteins become more stable, allowing HIF complexes to increase the expression of many adaptive genes[3]. This hypoxic response changes how cells produce and use energy, helps some cells tolerate low oxygen, and increases production of signals that encourage nearby endothelial cells to form new blood vessels.
How Does Hypoxia Activate Angiogenic Signals?
Hypoxia activates angiogenic signaling through the accumulation of HIF-1α, the oxygen-sensitive subunit of the HIF-1 protein complex.
Under well-oxygenated conditions, oxygen-dependent enzymes modify HIF-1α so that the cell can recognize it and target it for destruction. When cellular oxygen drops:
- Destruction slows: The oxygen-dependent marking process becomes less active, so HIF-1α degradation decreases.
- Protein accumulation: HIF-1α becomes more stable and accumulates inside the cell.
- Nuclear translocation: HIF-1α moves into the cell nucleus, joins with HIF-1β, and activates target genes.[3]
- Growth factor production: Increased activity of these target genes raises production of angiogenic signaling molecules, most notably vascular endothelial growth factor (VEGF).
HIF activation coordinates a broad cellular adaptation program. While VEGF is an important output, hypoxia also stimulates additional growth factors and tissue-remodeling enzymes that assist in vessel formation.
What Is VEGF and How Does It Promote New Blood Vessel Growth?
Vascular endothelial growth factor (VEGF) is one of the most important signaling proteins that stimulate the growth, migration, and survival of blood vessel cells.
Although the VEGF family includes several related proteins, VEGF-A is a major driver of sprouting angiogenesis. Cancer cells and their surrounding stromal cells produce and release VEGF-A into the surrounding space, particularly when triggered by hypoxia.
- Receptor binding: VEGF moves through tissue and binds to surface receptors—predominantly VEGFR-2—on nearby endothelial cells.[1], [2]
- Cellular activation: Receptor binding signals endothelial cells to survive, divide, and travel toward the chemical source.
- Vessel permeability: VEGF can increase vascular permeability by altering the barrier formed by endothelial cells. In tumors, persistent VEGF signaling can contribute to abnormal vessel leakiness.
While VEGF is a major angiogenic signal, tumor angiogenesis relies on additional cooperative pathways, including fibroblast growth factors, angiopoietins, platelet-derived growth factors, and Notch signaling, which together regulate how vessels mature and branch.
How Do New Blood Vessels Grow Toward a Tumor?
New vessels grow toward a tumor primarily through sprouting angiogenesis, a process where a new vascular branch buds outward from a pre-existing host vessel.
- Vascular activation: Angiogenic factors released by the tumor bind to receptors on nearby blood vessels.
- Matrix remodeling: The vascular basement membrane and surrounding extracellular matrix are locally remodeled, allowing activated endothelial cells to move outward.
- Sprout extension: Activated endothelial cells migrate outward, forming a growing vascular sprout that extends through the tissue in response to angiogenic signals, including VEGF gradients.
- Vascular loop formation: Advancing sprouts eventually fuse with neighboring sprouts or vessels, forming a continuous hollow channel through which blood can circulate.
How Do Endothelial Cells Form New Vessels?
Endothelial cells in a growing sprout can adopt different functional roles, including leader “tip cells” and follower “stalk cells.”
- Tip cells: Positioned at the front of the growing sprout, tip cells mainly migrate rather than proliferate. They extend slender cellular projections called filopodia that sense guidance signals, including VEGF gradients, and help steer the growing branch.
- Stalk cells: Located behind the tip cell, stalk cells generally proliferate more than tip cells, helping lengthen the sprout and form the internal channel, or lumen, through which blood will eventually flow.
- Path coordination: Communication via the DLL4–Notch pathway regulates this division of labor. When a tip cell activates, it signals its immediate neighbors through Notch to remain stalk cells, preventing every cell from trying to lead its own independent branch.
As normal vessels mature, supporting cells called pericytes wrap around the endothelial tube, and a stable basement membrane forms to reinforce the wall. In tumor tissue, however, these stabilizing steps are often incomplete or abnormal.
Are Tumor Blood Vessels Normal?
No. Many newly formed tumor blood vessels are structurally abnormal and functionally inefficient compared with normal healthy vessels.
Healthy blood vessels form neat, hierarchical branching networks that distribute blood evenly. In contrast, newly formed tumor vessels often develop under persistent and uneven angiogenic signaling. They are often abnormally twisted (tortuous), irregular in diameter, excessively branched, and unevenly spaced.
| Feature | Normal Blood Vessels | Typical Newly Formed Tumor Blood Vessels |
|---|---|---|
| Architecture | Orderly, branching hierarchical tree | Chaotic, twisted, irregular diameter, tangled |
| Endothelial Lining | Tightly joined, smooth cellular monolayer | Discontinuous, overlapping, loose junctions |
| Pericyte Support | Closely attached, complete perivascular coat | Loosely attached, sparse, or absent |
| Basement Membrane | Smooth, continuous, uniform thickness | Irregular, loosely associated, or uneven in thickness |
| Vascular Permeability | Controlled, selective barrier | Often abnormally permeable (“leaky”) |
| Blood Flow | Uniform, unidirectional, efficient | Sluggish, chaotic, intermittent, or stagnant |
Because of these structural flaws, having a dense supply of tumor blood vessels does not guarantee effective blood delivery. A tumor can build many vessels and still remain deeply hypoxic inside.
Why Are Tumor Blood Vessels Often Leaky and Disorganized?
Tumor blood vessels are often leaky and disorganized because persistent, imbalanced angiogenic signaling can produce vessels that do not undergo normal maturation.
- Defective junctions: Endothelial cells can form irregular or poorly organized connections with one another, weakening the normal vascular barrier.
- Abnormal structural support: The basement membrane may be irregular or loosely associated with the vessel wall, while stabilizing pericytes may be sparse or abnormally attached.
- Vessel compression: As tumor cells multiply within a confined space, solid mechanical pressure compresses the fragile, thin-walled vessels, causing erratic, intermittent flow.
- Elevated fluid pressure: Abnormally permeable vessels allow fluid and plasma proteins to leak into the surrounding tumor tissue. Because lymphatic drainage within tumors is often poor, interstitial fluid pressure can rise, which can further compress vessels and reduce blood flow.
How Does Angiogenesis Help a Tumor Grow?
Angiogenesis supports tumor growth by improving access to oxygen and nutrients while helping remove metabolic waste.[1]
By expanding circulation deeper into the tumor mass, angiogenesis allows a small cluster of cells to overcome diffusion limits and expand into a larger solid mass.
However, because tumor vessels are structurally compromised, vascularization often does not fully correct the tumor’s oxygen deficit. Areas of persistent hypoxia can continue to stabilize HIFs and sustain VEGF production, promoting further vascular remodeling.
Beyond nutrient delivery, the newly built vascular network alters the wider tumor microenvironment. Endothelial cells actively communicate with cancer cells, immune cells, and surrounding structural tissue, influencing overall tumor behavior, growth rate, and therapeutic response.
Can Tumor Blood Vessels Help Cancer Cells Spread?
Yes. Tumor blood vessels provide a physical entryway through which cancer cells can enter the bloodstream and spread to distant organs.
To metastasize via the circulatory system, a cancer cell must migrate through local tissue, reach a blood vessel, and cross through the vessel wall into the bloodstream—a step known as intravasation:
- Easier entry: Abnormal endothelial junctions, basement membrane defects, and irregular pericyte coverage can make some tumor vessels easier for cancer cells to penetrate.
- Increased surface area: A denser vascular bed expands the physical surface area where cancer cells can come into direct contact with blood vessels.
Entering circulation does not guarantee a secondary tumor will form. A circulating cancer cell must survive mechanical fluid shear stress, evade immune cells, exit the vessel wall at a distant organ (extravasation), and survive in an unfamiliar tissue bed.
Tumor blood vessels can therefore provide one pathway for cancer-cell dissemination, but metastasis requires surviving this difficult multistep journey.
Can Tumors Obtain a Blood Supply Without Ordinary Angiogenesis?
Yes. Sprouting angiogenesis is only one of several distinct ways a tumor can secure a functional blood supply.
Cancers can acquire access to blood through several alternative mechanisms:
- Vessel Co-option: Rather than growing new blood vessels, cancer cells migrate along and use pre-existing host vessels. This non-angiogenic strategy is well documented in cancers growing in vascular organs such as the brain, lungs, and liver.[5]
- Intussusceptive (Splitting) Angiogenesis: An existing capillary develops tiny internal tissue pillars that extend into the lumen, splitting a single vessel into two parallel channels from within.
- Vasculogenesis: New vascular structures may involve the recruitment of circulating endothelial precursor or progenitor cells. How much these cells directly become part of tumor-vessel endothelium varies among tumor types and experimental models, and their contribution remains debated.[5]
- Vasculogenic Mimicry: Aggressive cancer cells adapt to mimic endothelial behavior, creating their own fluid-conducting, vessel-like channels without using true endothelial cells.
A single tumor can utilize several of these vascularization strategies at the same time. While sprouting new branches through classic angiogenesis remains common, cancer cells can exploit multiple alternative routes to maintain the circulation they need to survive and spread.
Morais C. Why Do Tumors Need New Blood Vessels? Angiogenesis Explained [Internet]. BiologyWithin.com; 2026 Sep 13. Available from: https://www.biologywithin.com/2026/09/why-do-tumors-need-new-blood-vessels-angiogenesis-explained.html
- National Cancer Institute. Angiogenesis inhibitors [Internet]. Bethesda (MD): National Cancer Institute; reviewed 2018 Apr 2.
- Lugano R, Ramachandran M, Dimberg A. Tumor angiogenesis: causes, consequences, challenges and opportunities. Cell Mol Life Sci. 2020;77(9):1745–1770.
- Bae T, Hallis SP, Kwak MK. Hypoxia, oxidative stress, and the interplay of HIFs and NRF2 signaling in cancer. Exp Mol Med. 2024;56(3):501–514.
- Fakhrejahani E, Toi M. Tumor angiogenesis: pericytes and maturation are not to be ignored. J Oncol. 2012;2012:261750.
- Belotti D, Pinessi D, Taraboletti G. Alternative vascularization mechanisms in tumor resistance to therapy. Cancers (Basel). 2021;13(8):1912.
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Chris is a biologist dedicated to making complex biological and physiological sciences clear, rigorous, and accessible.
Disclaimer: This article is for general educational purposes only and does not constitute medical advice, diagnosis, or treatment. Consult a qualified healthcare professional about personal health concerns.
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