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# Essential thrombocythaemia and primary myelofibrosis

- Lesion

Two marrow neoplasms with the same genes: one makes too many platelets, the other scars the marrow so blood is made in the spleen.

Updated Nov 12, 2026

## Key points

- ET: platelets persistently high; reactive thrombocytosis (iron deficiency, inflammation, splenectomy) must be excluded.
- Erythromelalgia: burning, red hands and feet from platelet plugs in arterioles, in ET and PV.
- PMF: PDGF and TGF-β from megakaryocytes drive fibrosis; aspirate is a dry tap.
- Leukoerythroblastosis (nucleated red cells and immature granulocytes) plus teardrop red cells (dacrocytes).
- Massive splenomegaly with extramedullary haematopoiesis; late osteosclerosis.

On this page

- The basics Key points
- What it is
- Structure
- Step by step
- Markers
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Clinical notes
- In the lab

- Study Test yourself

- Connections Connections

## What it is

Essential thrombocythaemia (ET) and primary myelofibrosis (PMF) are JAK2-, CALR- or MPL-driven MPNs in which the megakaryocyte is central. In ET, large abnormal megakaryocytes produce persistently high platelet counts, with thrombosis and sometimes bleeding, in an otherwise indolent disease. In PMF, neoplastic megakaryocytes release fibrogenic cytokines that replace the marrow with collagen; haematopoiesis moves to the spleen and liver, the blood shows teardrop cells and leukoerythroblastosis, and the disease ends in marrow failure or AML.

## Structure

- ET marrow: mildly hypercellular, clusters of giant megakaryocytes with hyperlobated (stag-horn) nuclei; no marked fibrosis.
- PMF early: hypercellular marrow with clustered bizarre megakaryocytes with cloud-like nuclei; little fibrosis.
- PMF late: dense reticulin and collagen, haematopoietic islands in dilated sinusoids, new bone (osteosclerosis).
- Spleen and liver: trilineage haematopoiesis in sinusoids, later in cords; subcapsular infarcts.

## Step by step

- A stem cell acquires a JAK2, CALR or MPL mutation activating thrombopoietin receptor signalling.
- Megakaryocytes proliferate: in ET platelets rise; in PMF the megakaryocytes are atypical.
- PMF: megakaryocytes release PDGF and TGF-β; fibroblasts lay down collagen and new vessels form.
- Fibrosis expels stem cells, which settle in spleen and liver.
- Extramedullary haematopoiesis is disordered: anaemia, leukoerythroblastosis, teardrop cells.

## Markers

- JAK2 V617F
- CALR
- MPL
- Reticulin stain

## Clinical notes

- ET: often found on a routine count; headaches, visual symptoms, thrombosis or bleeding.
- PMF: older adults with fatigue, weight loss, a massive spleen, night sweats and gout.
- JAK2 inhibitors shrink the spleen and relieve symptoms in myelofibrosis; transplantation is the only cure.

## In the lab

- Reticulin and trichrome stains grade marrow fibrosis.
- Teardrop cells and leukoerythroblastosis also occur with other marrow-infiltrating diseases (metastases, granulomas).

## Memory aid

Teardrops fall from a scarred marrow: the cells are squeezed out of the fibrosis and cry on the smear.

## Test yourself

Think of the answer, then tap the question to see it.

## What causes marrow fibrosis in primary myelofibrosis?

Fibrogenic factors (PDGF, TGF-β) released by neoplastic megakaryocytes that activate non-neoplastic fibroblasts.

## What is leukoerythroblastosis?

Nucleated red cells and immature granulocytes in the blood, released from a distorted marrow or from extramedullary sites.

## What is erythromelalgia?

Throbbing, burning, red hands and feet caused by platelet aggregates occluding small arterioles, seen in ET and PV.

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## Elsewhere in DurgiPedia

- [Hematopoiesis](https://durgical.com/en/physiology/hematopoiesis/)
- [Hematopoiesis and bone marrow](https://durgical.com/en/histology/hematopoiesis-and-bone-marrow/)

## Connections

## Related 5

- [Myeloproliferative neoplasms: overview](https://durgical.com/en/general-pathology/myeloproliferative-neoplasms-overview/)
- [Polycythaemia and polycythaemia vera](https://durgical.com/en/general-pathology/polycythaemia-and-polycythaemia-vera/)
- [Fibrosis in organs: TGF-β and the myofibroblast](https://durgical.com/en/general-pathology/fibrosis-in-organs-tgf-and-the-myofibroblast/)
- [Other marrow failure: PRCA and myelophthisis](https://durgical.com/en/general-pathology/other-marrow-failure-prca-and-myelophthisis/)
- [Splenomegaly and hypersplenism](https://durgical.com/en/general-pathology/splenomegaly-and-hypersplenism/)

## Related conditions 3

- [Essential thrombocythaemia and primary myelofibrosis](https://durgical.com/en/conditions/essential-thrombocythaemia-and-primary-myelofibrosis/)
- [Polycythaemia vera](https://durgical.com/en/conditions/polycythaemia-vera/)
- [Portal vein thrombosis](https://durgical.com/en/conditions/portal-vein-thrombosis/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Essential thrombocythemia: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/essential-thrombocythemia/) (MedlinePlus, US National Library of Medicine, 2026); [Primary myelofibrosis: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/primary-myelofibrosis/) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/essential-thrombocythaemia-and-primary-myelofibrosis/