Essential Thrombocythemia

Slide 1 · Essential Thrombocythemia

Molecular pathogenesis · morphology · clinical consequences

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Explain how JAK2, MPL and mutant CALR converge on growth factor signaling.

Recognize marrow and peripheral-blood morphological features of ET.

Relate neoplastic platelet dysfunction to thrombosis and hemorrhage.

Distinguish ET conceptually from reactive thrombocytosis, PV and PMF.

Slide 3 · ET belongs to the myeloproliferative neoplasms

The essential lesion is autonomous myeloid proliferation

Myeloproliferative neoplasms (MPNs) are clonal disorders with excessive mature-cell production.

Acquired signaling changes reduce dependence on hematopoietic growth factors.

Shared biologic features create clinical and morphological overlap.

Slide 4 · Essential thrombocythemia: Working definition

A clonal MPN characterized predominantly by increased platelet production.

The key abnormality is dysregulated megakaryocytic proliferation.

Elevated platelets may coexist with dysfunction and clinical vascular events.

Unlike PV and PMF, erythrocytosis and overt marrow fibrosis are not defining.

Slide 5 · The disease is not simply a high platelet count

Observed feature

Elevated peripheral platelet count

Sometimes mild leukocytosis

Underlying disease

Clonal progenitor growth

Abnormal platelet function

Slide 6 · One clone, two opposing vascular consequences

Clonal megakaryocytic expansion

Platelet excess + abnormal function

Thrombosis AND hemorrhage

Clinical complications arise from both quantity and quality of platelets.

Slide 7 · ET in the MPN differential

Entity

Dominant phenotype

Key differentiator

ET

Megakaryocytic / platelet excess

No defining erythrocytosis or overt fibrosis

Polycythemia vera

Red-cell mass increase with panmyelosis

Erythrocytosis dominates symptoms

Primary myelofibrosis

Obliterative marrow fibrosis

Marrow failure and extensive extramedullary hematopoiesis

Chronic myeloid leukemia

Marked granulocytic proliferation

BCR::ABL fusion

Slide 8 · Normal thrombopoietin signaling: A reference

Thrombopoietin ligand

MPL receptor

JAK2 / STAT signaling

Progenitor survival and proliferation

ET driver mutations converge on this normally ligand-controlled axis.

Slide 9 · Three recurrent molecular drivers

Alteration

Approximate fraction of ET

Signaling consequence

JAK2 mutation

50–60%

Constitutive JAK2 kinase signaling

CALR mutation

25–35%

Mutant CALR acts as an alternative MPL ligand

MPL mutation

5–10%

Constitutive MPL pathway signaling

Slide 10 · Most ET cases harbor one of these drivers

JAK2 · CALR · MPL

JAK2, CALR or MPL alterations in over 90% of cases.

These lesions are distinct molecular routes to increased JAK–STAT signaling.

The absence of an identified driver does not by itself establish a reactive cause.

Slide 11 · JAK2: Constitutive kinase activation

Activating JAK2 alteration

Persistent signaling

Increased progenitor proliferation

JAK2 participates downstream of hematopoietic growth-factor receptors.

A driver alteration bypasses part of normal ligand dependence.

The consequence is excess production of mature blood elements.

Slide 12 · The JAK2 mutation is not ET-specific

A shared mutation does not define the complete phenotype

The same JAK2 mutation is found in nearly all PV cases.

Some individuals with a JAK2 mutation develop ET; Others develop PV.

The basis for this phenotypic divergence remains uncertain.

Slide 13 · MPL connects thrombopoietin to JAK2

Thrombopoietin

MPL receptor

JAK2 activation

MPL is the receptor activated by thrombopoietin.

Its downstream signal is transmitted through JAK2.

Activating MPL mutations produce inappropriate pathway activity.

Slide 14 · Mutant MPL creates ligand-independent signaling

Activating MPL mutation

Receptor pathway activation

Thrombopoietin independence

Excess megakaryopoiesis

The biological endpoint resembles JAK2-driven ET despite a different molecular starting point.

Slide 15 · Mutant CALR supplies an alternative MPL stimulus

CALR encodes calreticulin; Mutation provides a mechanism distinct from direct kinase activation.

Mutant CALR as secreted and able to bind the MPL receptor.

This abnormal interaction activates thrombopoietin-receptor signaling.

Slide 16 · Mutant CALR: Oncogenic autocrine feedback

Mutated CALR produced

Abnormal CALR binds MPL

MPL / JAK2 activation

Progenitor expansion

The ligand–receptor interaction as an oncogenic autocrine feedback loop.

Slide 17 · Separate mutations converge on one signaling axis

Driver

Direct level of action

Common outcome

JAK2

Intracellular kinase

Increased JAK–STAT pathway activity

MPL

Thrombopoietin receptor

Increased JAK–STAT pathway activity

CALR

Alternative ligand for MPL

Increased JAK–STAT pathway activity

Slide 18 · Thrombopoietin independence drives expansion

Abnormal signaling

Reduced dependence on TPO

Megakaryocyte proliferation

Constitutive JAK2 or MPL signaling supports progenitor growth without normal input.

Maturation is retained, so excess production appears as mature platelets.

The marrow response underlies the elevated platelet count.

Slide 19 · MPN proliferation generally preserves differentiation

Increased production without a primary differentiation block

MPN signaling lesions with defects that block maturation.

The major peripheral consequence is an increase in mature blood elements.

ET particularly emphasizes the megakaryocytic lineage.

Slide 20 · Clone behavior extends beyond the count

A clonal megakaryocytic population gives rise to functionally abnormal platelets.

Qualitative abnormalities are found in platelet-function testing.

The count alone therefore cannot predict an individual’s bleeding or thrombotic phenotype.

Slide 21 · Interpreting a JAK2-positive thrombocytosis

JAK2 positivity supports an MPN but cannot separate ET from PV on its own.

Marrow morphology and red-cell findings must be interpreted together.

Iron deficiency may complicate phenotypic recognition.

Slide 22 · Iron deficiency may disguise polycythemia vera

PV-driven erythropoiesis

Concurrent iron deficiency

Red-cell elevation obscured

A small proportion of apparent ET may represent masked PV.

Iron restriction can suppress the apparent red-cell phenotype.

A thrombocytosis evaluation should not ignore the iron-status context.

Slide 23 · Why identical drivers yield different MPNs

Genotype is informative; Phenotype still requires assessment

JAK2-mutated ET and JAK2-mutated PV show different predominant cell-lineage outputs.

Robbins does not establish a definitive mechanistic explanation.

Avoid assigning a single molecular test a phenotype it cannot resolve.

Slide 24 · Bone marrow cellularity is often only mildly elevated

Total cellularity may be near normal or mildly increased.

Megakaryocytes are disproportionally expanded relative to this modest change.

A striking platelet phenotype does not require uniform panmarrow hypercellularity.

Slide 25 · Megakaryocytes are markedly increased

Schematic — not a histologic specimen

Marrow interpretation

Megakaryocytes disproportionately numerous

Large cellular forms present

Overall marrow cellularity may rise only mildly

Slide 26 · Megakaryocyte size is diagnostically informative

Marrow megakaryocytes in ET include abnormally large forms.

The expansion is both quantitative and morphologic.

Recognition requires marrow assessment; The peripheral count is insufficient.

Slide 27 · Reticulin is not equivalent to overt myelofibrosis

Delicate reticulin fibrils may be seen in ET marrow.

Overt replacement-type fibrosis characterizes primary myelofibrosis.

The distinction is qualitative and architectural, not merely the presence of any fiber.

Slide 28 · Overt fibrotic marrow suggests a different process

Schematic — not a histologic specimen

Marrow interpretation

Dense replacement by fibrous tissue

Suppression of normal marrow activity

Contrasts with uncomplicated ET

Slide 29 · The peripheral smear may show giant platelets

Platelets may be abnormally large, even approaching erythrocyte dimensions.

The textbook figure caption explicitly describes giant platelets in ET.

Platelet size heterogeneity supports morphologic assessment but is not pathognomonic.

Slide 30 · Platelet–erythrocyte scale: A schematic

Schematic (not a blood film)

Peripheral blood

Increased platelet number

Platelet size heterogeneity

Some platelets approach erythrocyte size

Slide 31 · Mild leukocytosis may accompany thrombocytosis

The peripheral smear can show a modestly elevated leukocyte count.

This is compatible with a myeloproliferative process involving mature lineages.

The dominant feature remains the platelet / megakaryocytic abnormality.

Slide 32 · Marrow and blood offer different evidence

Bone marrow

Megakaryocyte number and size

Mild total-cellularity increase

Reticulin versus overt fibrosis

Peripheral blood

Elevated platelet count

Large or giant platelets

Sometimes mild leukocytosis

Slide 33 · Extramedullary hematopoiesis can occur

MPN clone in marrow

Homing to secondary organs

Extramedullary hematopoiesis

Possible organomegaly

In ET the extent is usually modest compared with overt primary myelofibrosis.

Slide 34 · Mild organomegaly has a pathological basis

Modest extramedullary hematopoiesis in ET.

Mild organ enlargement is reported in approximately half of patients.

Marked organ enlargement with marrow failure should prompt attention to the differential.

Slide 35 · ET incidence and age distribution

An annual U.S. Incidence of 1–3 per 100,000 people.

Presentation is most common after the age of 60 years.

Younger adults can also develop ET.

Slide 36 · The disease may remain clinically silent

Long asymptomatic intervals are characteristic.

Diagnosis may precede any symptomatic vascular event.

Clinical crises can interrupt an otherwise indolent course.

Slide 37 · Platelet excess does not guarantee effective hemostasis

Quantitative excess

High circulating platelet burden

Greater opportunity for abnormal platelet interactions

Qualitative dysfunction

Abnormal platelet-function tests

Impaired hemostatic performance

Potential bleeding despite high count

Slide 38 · ET has a dual complication phenotype

Neoplastic platelet clone

Abnormal number AND function

Thrombotic events / bleeding

Neither elevated count nor a single molecular variant replaces clinical assessment.

Slide 39 · Thrombosis may involve deep veins

Deep venous thrombosis is among the described manifestations.

The abnormal clotting tendency overlaps that seen in polycythemia vera.

A venous thrombotic presentation may bring previously silent ET to attention.

Slide 40 · Portal and hepatic veins are important sites

Portal and hepatic venous thrombosis.

These events extend ET beyond ordinary peripheral-vein thrombosis.

Unusual-site thrombosis should broaden consideration to myeloproliferative disease.

Slide 41 · Arterial events include myocardial infarction

Myocardial infarction among ET thrombotic manifestations.

Both arterial and venous vascular beds may be involved.

The clinical spectrum cannot be captured by platelet count alone.

Slide 42 · Erythromelalgia is a characteristic symptom

Patients may experience painful, throbbing burning of hands or feet.

The symptom to small-arteriole occlusion by platelet aggregates.

It is also recognized in polycythemia vera.

Slide 43 · Erythromelalgia: Platelet aggregation in arterioles

Blood flow

Schematic platelet aggregation in a small arteriole

Small-vessel obstruction

Burning, throbbing pain of the hands or feet

Shared manifestation with polycythemia vera

Slide 44 · Hemorrhage is not paradoxical once function is considered

Neoplastic platelets frequently display qualitative defects.

Bleeding is a major ET manifestation despite thrombocytosis.

Interpret hemostasis as the interaction of platelet number, function and other factors.

Slide 45 · Acquired von Willebrand disease may coexist

Acquired von Willebrand disease in up to one-third of ET patients.

It may contribute to the hemorrhagic tendency.

The excerpt characterizes its etiology as uncertain; Avoid inventing a mechanism here.

Slide 46 · The thrombosis–bleeding paradox

Finding

Consequence

Reasoning

Elevated platelet count

Propensity to abnormal clotting

Increased and dysregulated platelet activity

Platelet qualitative abnormalities

Potential hemorrhage

Excess number does not ensure effective function

Acquired von Willebrand disease

Additional bleeding contribution

Reported association; mechanism uncertain in source

Slide 47 · Very high platelet count modifies concern

Thrombotic complications to very high platelet counts.

The text also associates thrombotic events with homozygous JAK2 mutations.

These are textbook associations, not a standalone contemporary risk-scoring scheme.

Slide 48 · Bleeding history matters before antiplatelet treatment

The source qualifies aspirin use to patients without evidence of bleeding.

This reflects a central ET problem: Simultaneous hemorrhagic and thrombotic risks.

Management reasoning starts with the actual clinical phenotype.

Slide 49 · ET usually follows an indolent course

Long asymptomatic interval

Occasional vascular crisis

Continued clinical observation

The course is punctuated by thrombosis or bleeding rather than uniform rapid progression.

Slide 50 · A minority progress to marrow fibrosis

An uncommon spent phase with marrow fibrosis.

Fibrotic progression alters the primary production pattern.

The evolution connects ET to the broader MPN biology of spent-phase disease.

Slide 51 · Fibrotic progression changes hematopoiesis

Previous megakaryocytic proliferation

Spent-phase marrow fibrosis

Disrupted marrow production

MPN fibrosis can accompany peripheral-blood cytopenias.

Extramedullary hematopoiesis can become more important.

Uncomplicated ET and advanced fibrotic evolution must not be conflated.

Slide 52 · Transformation to acute myeloid leukemia is uncommon

The Robbins ET section notes rare progression to AML.

This represents a qualitative change in disease biology and prognosis.

No ET-specific transformation rate is supplied in the excerpt; None is assumed.

Slide 53 · Survival perspective

Median survival of approximately 12–15 years for ET.

The statement is a source-era summary, not an individualized survival prediction.

Heterogeneous complications and evolution prevent a single course for every patient.

Slide 54 · Diagnosis requires more than detecting thrombocytosis

Platelet elevation

Clonal evidence and morphology

Exclude mimics

Integrate clinical phenotype

This is a Robbins-grounded reasoning schematic, not a reproduction of formal WHO/ICC criteria.

Slide 55 · Inflammatory thrombocytosis is a major mimic

Reactive increases in platelets can occur in inflammatory disorders.

A high platelet count therefore does not uniquely signal ET.

Excluding reactive causes in driver-negative evaluations.

Slide 56 · Iron deficiency produces two diagnostic pitfalls

Reactive alternative

Iron deficiency may account for elevated platelets

A nonclonal explanation must be assessed

Masked PV context

Iron deficiency may obscure a PV red-cell phenotype

Avoid labeling every apparent isolated thrombocytosis as ET

Slide 57 · Marrow fibrosis is a high-value discriminator

ET may have delicate reticulin without overt fibrotic replacement.

Primary myelofibrosis is characterized by obliterative marrow fibrosis.

The distinction is essential when platelet counts and driver mutations overlap.

Slide 58 · ET versus polycythemia vera

Feature

Essential thrombocythemia

Polycythemia vera

Dominant lineage

Platelets / megakaryocytes

Red cells; panmyelosis

JAK2 status

Common, but not universal

Nearly all cases

Morphology

Many large megakaryocytes

Increased erythroid, granulocytic and platelet output

Key pitfall

Mistaking masked PV for ET

Iron deficiency can obscure hematocrit

Slide 59 · ET versus primary myelofibrosis

Feature

Essential thrombocythemia

Primary myelofibrosis

Typical marrow

Increased large megakaryocytes

Obliterative fibrosis

Fibers

Delicate reticulin may occur

Extensive collagen deposition

Extramedullary hematopoiesis

Usually modest

Often extensive

Clinical direction

Platelet excess / vascular events

Marrow failure, cytopenias

Slide 60 · ET versus chronic myeloid leukemia

Feature

Essential thrombocythemia

Chronic myeloid leukemia

Characteristic output

Megakaryocytes / platelets

Maturing granulocytes

Key genetic lesion

JAK2 / CALR / MPL

BCR::ABL fusion

Blood clues

Thrombocytosis, sometimes mild leukocytosis

Marked leukocytosis with immature forms

Diagnostic logic

Integrate clonality and phenotype

Define BCR::ABL status

Slide 61 · A coherent diagnostic assessment

Confirm phenotype

Evaluate plausible reactive causes

Assess marrow and molecular clues

Differentiate other MPNs

Formal numerical diagnostic criteria were not reproduced from the provided source.

Slide 62 · Therapy aims to reduce platelet-driven complications

Mild cytoreductive treatment to suppress thrombopoiesis.

The rationale is to reduce the neoplastic platelet burden.

Treatment choice is beyond the excerpt’s detailed scope; Drug regimens are not invented.

Slide 63 · Aspirin use is qualified by bleeding status

Low-dose aspirin in patients without evidence of bleeding.

Antiplatelet therapy addresses the thrombotic side of ET biology.

The possibility of hemorrhage and acquired von Willebrand disease requires clinical judgment.

Slide 64 · Case: Isolated thrombocytosis with iron deficiency

An adult is referred for persistently elevated platelets and evidence of iron deficiency. No marrow study or driver test is yet available.

What diagnostic assumptions would be premature?

What non-neoplastic explanation needs consideration?

Why does iron deficiency also complicate the ET–PV distinction?

Slide 65 · Case analysis: Test the phenotype, not a label

Iron deficiency is a recognized cause of reactive thrombocytosis.

In a separate scenario, iron deficiency can suppress an otherwise apparent PV erythrocytosis.

A single elevated platelet result cannot resolve these alternatives.

Slide 66 · Case: Thrombocytosis associated with bleeding

A patient with a known clonal MPN has very high platelets but develops mucosal bleeding.

How can bleeding coexist with a high platelet count?

Consider neoplastic platelet qualitative dysfunction.

Acquired von Willebrand disease is a reported ET association.

The case illustrates why the count alone cannot justify an aspirin decision.

Slide 67 · Case: Burning pain in the fingers and toes

A patient with thrombocytosis reports repeated episodes of throbbing, burning pain in the distal extremities.

What clinical manifestation links the symptom to platelet behavior?

Identify erythromelalgia as the characteristic symptom.

Explain its proposed mechanism: Platelet aggregates in small arterioles.

Recognize that a similar manifestation can occur in PV.

Slide 68 · Integrative reasoning: Four diagnostic questions

Is thrombocytosis persistent and clinically consequential?

Is there evidence of an MPN-associated driver or clonal megakaryocytic morphology?

Could inflammation or iron deficiency explain the platelet increase?

Are features of PV, PMF or CML more compelling?

Slide 69 · Essential thrombocythemia: Five propositions

JAK2, CALR and MPL lesions converge on excessive signaling.

Marrow megakaryocytes increase; Large forms and giant platelets are characteristic.

Reticulin may be delicate, but overt primary fibrotic disease is different.

Thrombosis and bleeding both arise in the setting of neoplastic platelets.

Molecular, morphologic and clinical context prevents diagnostic oversimplification.

Slide 70 · Source references

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th edition (2026). Chapter 13: Myeloproliferative Neoplasms and Essential Thrombocythemia.

Adjacent Robbins sections: Chronic Myeloid Leukemia, Polycythemia Vera, and Primary Myelofibrosis.

Mechanistic and morphologic schematics in this lecture are original editable teaching diagrams, not patient specimens.

Formal WHO/ICC diagnostic thresholds and therapeutic algorithms were not added because they are not present in the supplied excerpt.