What Is Reticulin Fibrosis and Its Health Impact?

Reticulin fibrosis is the abnormal buildup of reticulin fibers in an organ’s tissue, most commonly the bone marrow, where it disrupts normal blood cell production and can signal serious underlying disease. In the bone marrow, reticulin forms a fine scaffold that supports blood-forming cells, and when that scaffold thickens or multiplies beyond its normal density, doctors call it reticulin fibrosis. The condition ranges from a mild, incidental finding with little clinical consequence to a hallmark feature of life-threatening blood cancers. Its health impact depends heavily on what is driving the excess fiber deposition and how far it has progressed.

What Reticulin Actually Is

Reticulin is not a single molecule. Research dating back decades has shown that what pathologists call “reticulin” under the microscope is a composite structure made up of type III collagen, fibronectin, and at least one additional non-collagenous glycoprotein.¹ These fibers are much thinner and more delicate than the dense collagen bundles (types I and III in thick bundles) that form scar tissue in later-stage fibrosis. In healthy bone marrow, reticulin creates a loose, web-like framework that holds blood-forming cells in place without crowding them. In a healthy liver, a similar reticulin network supports the organ’s basic architecture.²

The definition of reticulin has historically been circular: for much of the twentieth century, “reticulin” was defined as whatever stained with silver impregnation techniques, a point researchers have flagged as problematic since the methods are not perfectly specific.³ Even so, silver staining remains the standard way to visualize reticulin in tissue samples today, with newer imaging approaches still building on the same basic chemistry.⁴

How Reticulin Fibrosis Is Graded

When a pathologist examines a bone marrow biopsy for fibrosis, the tissue is treated with a silver stain that turns reticulin fibers black against a lighter background. This process, while effective, can introduce artifacts such as silver droplet deposits or tissue damage during preparation, which is one reason grading requires a trained eye.⁵ The World Health Organization uses a four-tier scale:

MF-0: No increase in reticulin fibers; normal marrow architecture.
MF-1: A loose network of reticulin with some intersections, but no dense fiber bundles.
MF-2: Diffuse, dense reticulin with extensive intersections, sometimes with focal collagen or bone thickening.
MF-3: Dense reticulin with coarse collagen bundles, often accompanied by significant bone remodeling.

The distinction between reticulin-only fibrosis and collagen fibrosis matters for prognosis. Increased reticulin staining alone shows up in many conditions, both benign and malignant, and often does not correlate well with how severe the disease is. In contrast, the presence of dense collagen, detected by a different stain called trichrome, tends to appear in later-stage disease and is more strongly linked to worse outcomes.⁶ This is a nuance that can get lost when people hear the word “fibrosis” and assume the worst. A grade MF-1 finding on its own may be clinically insignificant, while MF-3 with collagen and bone changes is an entirely different situation.

To capture this difference more precisely, researchers have developed a combined scoring system called the RCO (reticulin, collagen, osteosclerosis) score. Each of the three components is graded from 0 to 3, producing a total between 0 and 9. In one study of 122 patients with primary myelofibrosis, those with high-grade RCO scores (5 to 9) had worse outcomes than those with low-grade scores (0 to 4).⁷ A larger validation study confirmed the pattern: pre-fibrotic disease showed the mildest stromal changes, and higher reticulin fibrosis grades tracked with more collagen and bone thickening.⁸

What Causes Reticulin Fibrosis in the Bone Marrow

The single most recognized cause is myelofibrosis, a type of blood cancer classified among the myeloproliferative neoplasms. In myelofibrosis, abnormal blood stem cells drive the process: they produce signals that stimulate the marrow’s structural cells to lay down excess fiber. Bone marrow fibrosis is so central to this disease that the World Health Organization lists it as a major diagnostic criterion.⁹ The abnormal stem cells create a marrow environment that increasingly favors the malignant cells over healthy blood-forming cells, which is why patients progressively lose the ability to make normal blood.

Megakaryocytes, the large bone marrow cells responsible for producing platelets, play a particularly outsized role. In myelofibrosis, these cells are both abnormally numerous and abnormally shaped. Their dysfunction leads to the release of inflammatory signaling molecules that, in turn, tell stromal cells to ramp up fiber production.¹⁰ This is why treatments that normalize megakaryocyte behavior can sometimes slow or reverse the fibrosis.

Myelofibrosis can also develop as a secondary event in people who initially had a different blood disorder. Essential thrombocythemia, a condition of chronically elevated platelet counts, can transform into overt myelofibrosis over time. The risk of this transformation is roughly 4 to 11 percent at 10 years and climbs to between 14 and 50 percent after 15 to 20 years.¹¹ When it happens, the marrow shows progressive reticulin and collagen fibrosis along with an enlarging spleen and falling blood counts.

Non-Cancer Causes

Reticulin fibrosis is not exclusive to blood cancers. The fibrotic stimulus can come from immune cells, infections, or even medications, and understanding these causes matters because the treatment and prognosis differ dramatically.¹²

Autoimmune diseases are an underappreciated cause. Autoimmune myelofibrosis is a rare but distinct condition in which the body’s immune attack on its own tissues leads to increased reticulin in the marrow. It shows up most often in young women with systemic lupus erythematosus, who develop low blood counts and sometimes an enlarged spleen. The key feature that separates autoimmune myelofibrosis from cancer-related myelofibrosis is that it typically responds to steroids.¹³ Even a short course of steroids combined with intravenous immunoglobulins has been reported to improve the fibrosis.¹⁴ Getting the distinction right is critical: you do not want to start cancer-directed therapy for a condition that responds to immune suppression.

Infections can trigger it too. Disseminated tuberculosis, for example, has been documented to cause marked reticulin fibrosis around the granulomas it forms in the bone marrow, sometimes producing a “dry tap” where the marrow cannot even be aspirated with a needle because of how fibrotic it has become.¹⁵ Other infections, including HIV and certain fungal diseases, can provoke similar changes. Treating the underlying infection is the path to resolving the fibrosis in these cases.

Medications are another overlooked contributor. Thrombopoietin-receptor agonists, drugs used to boost platelet counts in immune thrombocytopenia, can induce bone marrow fibrosis with prolonged use. One long-term follow-up study found that roughly one in five patients treated with these drugs developed grade 2 or 3 myelofibrosis, with the risk increasing after more than two years of treatment.¹⁶ Earlier observations noted that even mild reticulin increases (to MF-1) occurred during therapy, though collagen deposition was not seen.¹⁷ Stopping the drug may halt or reverse the progression, which is why monitoring marrow fibrosis is part of the clinical follow-up for patients on these medications long-term.

How Reticulin Fibrosis Affects Your Health

The health consequences depend on the organ involved and the severity. In bone marrow, the basic problem is straightforward: as fiber accumulates, there is less room and less functional support for normal blood cell production. The marrow becomes increasingly unable to make red blood cells, white blood cells, and platelets in adequate numbers, leading to anemia, infection risk, and bleeding. In myelofibrosis specifically, the disease produces cytopenias (low blood counts), abnormal-looking blood cells, and extramedullary hematopoiesis, where the body attempts to produce blood cells in organs not normally responsible for the job, particularly the spleen.⁹

This spleen-based blood production causes progressive splenomegaly, which itself brings a host of problems: abdominal discomfort, early satiety from the enlarged organ pressing on the stomach, and sometimes painful splenic infarcts. The spleen also develops its own fibrosis in these patients, with reticulin and collagen fibers accumulating particularly in areas of active extramedullary blood production.¹⁸ Constitutional symptoms are common and debilitating: fatigue, night sweats, weight loss, and bone pain. These symptoms collectively compromise quality of life in ways that go well beyond what blood test numbers alone would suggest.

When reticulin fibrosis is caused by something other than cancer, the health impact tends to be milder and more reversible. Autoimmune myelofibrosis, for instance, may cause low blood counts and spleen enlargement, but because the underlying process is inflammatory rather than malignant, treating the inflammation can restore near-normal marrow function. Infection-related fibrosis similarly resolves once the infection is controlled.

Can Reticulin Fibrosis Be Reversed

One of the more encouraging findings in this area is that reticulin fibrosis is not always permanent, even in cancer-related cases. The fiber network is maintained by ongoing signaling from diseased cells. Remove or suppress those signals, and the marrow’s normal remodeling processes can clear the excess fiber over time.

The clearest evidence comes from stem cell transplantation. In patients with primary myelofibrosis who underwent transplants using reduced-intensity conditioning, complete or near-complete regression of bone marrow fibrosis was seen in about 59 percent of patients by day 100 after transplant, 90 percent by day 180, and 100 percent by one year.¹⁹ Even dense reticulin fibrosis, collagen fibrosis, and osteosclerosis have been shown to gradually resolve after transplant, with serial biopsies documenting the restoration of normal marrow architecture.²⁰ Transplantation, however, carries significant risks and is not an option for every patient, particularly older or frailer individuals.

For patients who cannot undergo transplant, JAK inhibitor drugs, particularly ruxolitinib, have shown the ability to slow, stabilize, or partially reverse fibrosis. In clinical comparisons, ruxolitinib was associated with greater odds of fibrosis improvement or stabilization compared to best available conventional therapy.²¹ In individual cases, prolonged ruxolitinib treatment has achieved complete normalization of the reticulin pattern, with biopsies after roughly three years of therapy showing a return to a WHO fibrosis score of zero and only normal perivascular collagen remaining.²² These results suggest that sustained inhibition of the JAK signaling pathway, which is overactive in myelofibrosis, may not just manage symptoms but actually modify the disease at the tissue level.

It is worth being clear-eyed about the limits, though. Not every patient on ruxolitinib sees fibrosis reversal. Some see stabilization, some see slowing, and some continue to progress. The drug’s effect on fibrosis is an argument for its disease-modifying potential, but it is not a guaranteed cure. And for the non-malignant causes of reticulin fibrosis, the reversal approach is different: steroids for autoimmune causes, antimicrobial therapy for infections, and drug discontinuation for medication-induced cases.

Reticulin in the Liver

The bone marrow gets most of the attention, but the liver has its own reticulin story. In a healthy liver, reticulin forms a delicate web that supports the basic cell architecture. When liver injury occurs, whether from alcohol, viruses, metabolic disease, or toxins, this normal reticulin framework is disrupted. The fibers may collapse, compress, or be replaced by denser scar-like collagen. Pathologists use reticulin stains on liver biopsies to assess whether the normal architecture is intact, to identify areas of cell loss or collapse, and to detect nodule formation that signals cirrhosis.²

In liver disease, the pattern is the reverse of what happens in the marrow. Instead of fibers accumulating where they should not be, the normal reticulin framework is lost and replaced by dense scar tissue. A reticulin stain on a liver biopsy showing disrupted or absent reticulin in zones where it should be present points toward hepatocyte loss or regenerative nodule formation. Blood markers can also hint at type III collagen turnover: the N-terminal propeptide of procollagen type III (PIIINP) is significantly elevated in patients with alcoholic cirrhosis, non-alcoholic cirrhosis, and toxic liver injury compared to healthy controls.²³ This marker is sometimes used as a non-invasive way to track fibrosis progression without repeated biopsies, though it is not specific to the liver alone.

Why the Grading System Matters More Than the Word “Fibrosis”

If you or someone you know has been told there is reticulin fibrosis on a biopsy report, the natural reaction is alarm. Fibrosis sounds like scarring, and scarring sounds irreversible and dangerous. But the word covers an enormous range. A bone marrow biopsy showing MF-1 reticulin in an otherwise healthy person may have no clinical significance whatsoever. Mild reticulin increases can be seen in reactive conditions, infections, and even as a normal variant in some contexts. The same finding in a patient with known myelofibrosis carries a completely different weight.

What matters clinically is not just the reticulin grade but the full picture: whether collagen is also present, whether there are bone changes, what the blood counts look like, and what the underlying disease is. The RCO scoring system reflects this reality by combining reticulin, collagen, and osteosclerosis into a single composite, acknowledging that reticulin alone is a blunt instrument.⁷ Reticulin fibrosis without collagen or bone changes is generally less ominous than the same reticulin grade accompanied by dense collagen deposition.⁶

For patients navigating a new diagnosis, the most useful question to ask is not simply “how much fibrosis do I have” but “what is causing it and what else is happening in the marrow alongside the fibrosis.” A mild reticulin increase in autoimmune disease is a fundamentally different clinical situation from the same reticulin increase in a JAK2-mutated blood cancer, even if the biopsy photographs look identical under the microscope. The grading number is the beginning of the conversation, not the end of it.