Do Growth Factors Act the Same Way in Cartilage, Tendon, and Synovium? — Dr. Moriwaki Sorts Out Tissue-Specific Responses to TGF-β, IGF-1, and FGF in Joint Injection of Stem Cell Conditioned Media2026.07.10

Even when we speak simply of “joint injection with stem cell conditioned media,” the growth factors contained in the injectate do not necessarily act the same way in the different tissues that make up a joint — the cartilage of the knee, the tendons of the shoulder and elbow, and the synovium that lines the joint. The growth factors often discussed in orthopedics — TGF-β, IGF-1, and FGF — reach cells with different receptor profiles, blood-flow environments, and repair speeds, so it is not accurate to think that “a single joint injection works equally well for every joint pain.” In this article, Dr. Moriwaki, supervising physician at AVAN TOKYO Ginza Regenerative Medicine, honestly sorts out how the growth factors used in a joint injection may respond tissue by tissue, and what can and cannot be expected — with a fair view of indications and limitations.

Key Points of This Article

・The growth factors in a joint injection — TGF-β, IGF-1, FGF and others — interact with different receiving cells and elicit different responses in cartilage, tendon, and synovium.

・Because cartilage has poor blood supply and slow metabolism, it is hard to expect an injection alone to “newly rebuild the cartilage itself.”

・Tendons and enthesis heal slowly due to scarce blood flow, but growth factors can serve as signals for tenocyte migration and collagen remodeling.

・In the synovium, signals that suppress inflammatory cytokines tend to be active, so joint injection carries meaningful weight as an anti-inflammatory approach to the pain source.

・Rather than “a joint injection cures everything,” it is essential to sort out which tissue is being targeted and what is being expected — as part of the diagnostic order.

Why Growth Factors Do Not “Work the Same Way” in Every Tissue

Growth factors are, so to speak, proteins that act like “messengers” to cells. TGF-β (transforming growth factor beta), IGF-1 (insulin-like growth factor 1), and FGF (fibroblast growth factor) are representative factors that drive cell proliferation, differentiation, and production of the extracellular matrix (ECM). However, because the distribution and density of receptors on the receiving cells differ across tissues, even the same growth factor changes both “who receives the message” and “how the tissue reacts after receiving it.” When stem cell conditioned media is delivered by joint injection, what actually happens depends greatly on which tissues make up the joint being targeted.

Different Receptor Distribution Means Different Responses

Chondrocytes, tenocytes, and synoviocytes all derive from mesenchymal cells, yet once mature they express different receptor profiles on their surfaces. IGF-1 receptors are strongly expressed on chondrocytes and act as signals that support the synthesis of type II collagen and aggrecan; in tenocytes, however, the response can lean toward the synthesis of type I collagen. TGF-β can also contribute to synovial fibrosis, so it is not simply a matter of “the more, the better” — dose and timing must be designed carefully.

Blood Flow and Metabolic Speed Determine Response Time

Another important point is the difference in blood supply and metabolic speed among tissues. The synovium is the most vascular tissue in the joint and its cells turn over quickly. Cartilage, on the other hand, has almost no blood vessels and relies on diffusion through the joint fluid. Tendons and their attachments (enthesis) look sturdy at a glance but actually have limited blood flow and slow cellular metabolism, and it is well known that it takes time for the effects of growth factors to appear there. Even for the same joint injection, “clearly better next week” and “gradually improving over several months” belong to different worlds when viewed from tissue physiology.

What Happens Tissue by Tissue — Cartilage, Tendon, Synovium

Response in Cartilage — “Supporting Metabolism” More Than “Rebuilding”

In knee osteoarthritis, the problem is worn articular cartilage together with the chain of inflammation around it. When growth factors are delivered here by joint injection, basic research reports that IGF-1 can support matrix synthesis by the remaining chondrocytes and that TGF-β may act chondro-protectively in some settings. However, this does not mean that “the cartilage itself newly regrows to its original thickness.” In a place with poor blood flow and a limited number of chondrocytes to begin with, what an injection can primarily do is support the metabolic environment of the remaining cells. Saying “cartilage will regenerate with an injection” when articular cartilage has been broadly lost and the joint has progressed to KL grade IV is, honestly, an inaccurate framing of expectation.

Response in Tendon and Enthesis — Can Serve as a Repair Signal

What is at stake in the lateral epicondyle of the tennis elbow and at the Achilles enthesis is tendon degeneration under chronic loading (tendinosis). Here, FGF and IGF-1 are reported to act as signals that stimulate tenocyte migration and type I collagen synthesis, potentially setting a micro-scale wound-healing cascade in motion. However, tendon tissue originally has limited blood flow, healing takes time, and joint injection alone is not a treatment that completes itself. It is realistic to position stem cell conditioned media as a “boost to the repair environment” combined with a foundation of rest and staged rehabilitation.

Response in Synovium — Approaching the Pain Source

What tends to produce the most clinically visible response after a knee joint injection is the anti-inflammatory action in the synovium. Synovitis is a major source of pain in knee osteoarthritis, and signals such as TGF-β and IL-10 act in a direction that suppresses the production of inflammatory cytokines. Because blood flow is rich and cell metabolism is fast, it is clinically observed that pain and swelling can ease within a relatively short time after injection. That said, this is a phenomenon of “calming inflammation”; it does not mean that structural deformity has reverted. Sharing a clear understanding of the effect and its expected duration is essential.

What Joint Injection Can and Cannot Be Expected to Do

Taking the above into account, what can realistically be expected from joint injection with stem cell conditioned media revolves around three axes: (1) easing pain and swelling from synovitis, (2) supporting the metabolic environment of the remaining chondrocytes, and (3) boosting repair signals in tendon and enthesis. On the other hand, claims that lost cartilage recovers to its original thickness, that a complete tendon tear heals without suturing, or that structural joint deformity disappears are overreach that ignores both the biological limits of growth factors and tissue-specific response differences. Please see our detailed page on joint injection with stem cell conditioned media for an overview of the treatment, and refer also to the site of the Japanese Orthopaedic Association for information on joint disease itself, so that decisions are made from multiple sources.

What Dr. Moriwaki especially prioritizes is narrowing down “which tissue the pain source is.” Even for the same complaint of “knee pain,” the expected response differs depending on whether the pain is primarily driven by synovitis, an extra-articular enthesis such as pes anserine tendinitis, or subchondral bone stress. Taking each step of the diagnostic order carefully is a prerequisite for making the most of the joint injection option.

Frequently Asked Questions

Q. Do growth factors reach “the tissue that responds” with a single joint injection?

Whether the growth factors actually reach the target cells sufficiently depends on distribution within the joint cavity, diffusion through the synovial fluid, and receptor density on the tissue side. Delivery into a space filled with joint fluid, such as the knee joint cavity, differs from delivery into a narrow local area, such as an enthesis. Choosing the injection site to match the target tissue is a prerequisite.

Q. I heard TGF-β is “good for cartilage.” Is more always better?

It is not that simple. TGF-β can act chondro-protectively in some settings, yet depending on dose and environment it has also been reported to contribute to synovial fibrosis. Stem cell conditioned media used in joint injection is a complex of many growth factors and cytokines, and the overall balance, more than the amount of any single factor, is what carries clinical meaning.

Q. Can tendon pain also be treated by joint injection?

Pain from tendon and enthesis conditions such as tennis elbow or Achilles tendinopathy is, strictly speaking, local administration into the enthesis rather than “injection into the joint cavity.” Tendon tissue has limited blood flow and takes time to respond, but growth factors have room to act as repair signals, so it can be one option combined with rest and exercise therapy. It does not, however, mean that a complete tear can be healed without surgical repair.

Q. How long should we wait before judging the effect?

Because response times differ by tissue, no single number applies. For pain driven by synovitis, changes are often felt within days to a few weeks; for tendon and enthesis, we look on a scale of weeks to months. Rather than drawing conclusions from a single response, continuously recording changes in pain scores, range of motion, and activities of daily living is essential.

Q. What should be done if there is no effect?

When the response is poor, we revisit the following: (1) whether the diagnosis truly matched pain in a tissue reachable by injection, (2) whether the number and location of injections were designed appropriately, and (3) whether combined therapies such as exercise therapy were in place. Rather than forcibly repeating injections, considering the next step — including orthopedic re-evaluation — is the honest path forward.

──────────────

Supervising Physician: Shin Moriwaki, M.D.

Member, Japan Society of Aesthetic Surgery (JSAS) / Member, American Academy of Aesthetic Medicine

ECFMG Certificate (U.S. Medical Licensing Qualification)

──────────────

📍AVAN TOKYO Ginza Regenerative Medicine

English / 中文 / Tiếng Việt Available

Inquiries welcome via DM / LINE / Website / Phone.