Thymic Hyperplasia

Slide 1 · Thymic Hyperplasia

Architecture • Germinal centers • Autoimmunity • Differential diagnosis

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish follicular hyperplasia from an enlarged but architecturally normal thymus.

Recognize normal thymic compartments and the significance of ectopic germinal centers.

Explain the clinicopathologic link with myasthenia gravis and central tolerance.

Differentiate benign thymic changes from key mediastinal neoplastic lesions.

Slide 3 · “Hyperplasia” describes two different findings

Follicular hyperplasia

Increased intrathymic B-cell follicles.

Germinal centers are the defining tissue clue.

The whole gland need not be larger.

Age-relative enlargement

Thymus is large for the patient’s age.

Histologic architecture may remain normal.

May imitate a thymic mass on imaging.

Slide 4 · Why terminology matters

The same clinical label may describe different histologic processes.

Organ enlargement is not required to diagnose follicular hyperplasia.

An enlarged thymus does not automatically imply follicular hyperplasia.

The distinction determines what pathologists and radiologists are describing.

Slide 5 · The thymus is a primary T-cell organ

Marrow-derived progenitors

Thymic maturation and selection

Mature T-cell export to periphery

Slide 6 · Embryologic origin

The thymus derives mainly from the third pair of pharyngeal pouches.

A contribution from the fourth pouch pair is described as inconsistent.

Developmental abnormalities can affect both anatomy and immune function.

Slide 7 · Thymus size changes with age

Birth: 10–35 g

Growth to puberty: 20–50 g

Involution in older adults: 5–15 g

Slide 8 · Size must be interpreted in context

Thymic size varies widely between individuals.

The organ grows before puberty and progressively involutes thereafter.

Severe illness or HIV infection may accelerate involution in younger persons.

A seemingly prominent thymus is not sufficient evidence of neoplasia.

Slide 9 · Normal thymic lobules: Cortex and medulla

Original schematic — not a photomicrograph

Cortex

Medulla

Central whorl: Hassall corpuscle

How to read the diagram

Fibrous septa divide the gland into lobules.

Each lobule has an outer cortex and central medulla.

Lobulation and compartment relationships organize interpretation.

Slide 10 · The thymic cortex

Cortical thymic epithelial cells are polygonal with abundant cytoplasm.

Dendritic epithelial extensions connect with adjacent cells.

Immature thymocytes are a dominant cellular component.

Cortex and medulla are histologically distinct compartments.

Slide 11 · The thymic medulla

Medullary epithelial cells are densely packed and often spindle-shaped.

They have relatively scant cytoplasm and lack prominent interconnecting processes.

Degenerated epithelial whorls create Hassall corpuscles.

Slide 12 · Hassall corpuscles identify medullary differentiation

Original schematic — not a photomicrograph

Cortex

Medulla

Central whorl: Hassall corpuscle

How to read the diagram

Hassall corpuscles are whorls of epithelial cells.

Their central material is characteristically keratinized.

They help orient the observer within the medullary compartment.

Slide 13 · Thymic residents beyond epithelial cells

Immature T cells are the principal lymphoid population.

Macrophages and dendritic cells also populate the tissue.

B lymphocytes are normally a minor population.

Myoid cells with muscle-like features are scattered in the gland.

Slide 14 · From thymocyte arrival to immune competence

Progenitors enter thymus

TCR-bearing cells mature

Self-reactive clones are removed or regulated

T cells enter peripheral pool

Slide 15 · Central tolerance operates in the thymus

Immature T cells develop receptors through somatic gene rearrangement.

Some receptors happen to recognize self-antigens strongly.

Selection mechanisms prevent many potentially harmful clones from entering the mature pool.

Slide 16 · Negative selection removes dangerous T cells

Self-reactive thymocyte

Recognition of thymic self-antigen

Apoptosis / clonal deletion

Slide 17 · AIRE and tissue-restricted self-antigens

Thymic antigen-presenting cells display many self-proteins.

AIRE facilitates expression of selected peripheral tissue–restricted antigens.

This supports deletion of developing T cells reactive to those antigens.

Failure of this pathway illustrates why thymic tolerance is clinically important.

Slide 18 · Central tolerance is not absolute

Not every self-antigen is represented in central lymphoid organs.

Some autoreactive lymphocytes escape thymic deletion.

Peripheral tolerance is then necessary to prevent tissue injury.

Autoimmunity need not imply a visible structural thymic lesion.

Slide 19 · A thymic route to regulatory T cells

A subset of developing CD4-lineage T cells becomes regulatory T cells.

The balance between deletion and regulatory differentiation is incompletely understood.

Central tolerance therefore includes more than simple destruction of lymphocytes.

Slide 20 · B cells are normally sparse in the thymus

The thymus is primarily a site of T-cell differentiation.

B-cell follicles are usually present in only small numbers.

Conspicuous germinal centers therefore change the normal architectural expectation.

Slide 21 · Follicular hyperplasia: The microscopic concept

Original schematic — not a photomicrograph

Cortex

Medulla

Central whorl: Hassall corpuscle

Large circles: B-cell follicles

How to read the diagram

The defining change is an increase in B-cell follicles.

Some follicles develop germinal centers.

Background lobular thymic tissue may still be recognizable.

Slide 22 · Why the name can be misleading

“Hyperplasia” suggests growth of an organ or one of its structural cell populations.

Here the term usually emphasizes germinal-center formation.

The tissue finding and total thymic size should be reported as separate observations.

Slide 23 · Architecture: Normal versus follicular change

Expected normal thymus

Cortex and medulla remain identifiable.

B cells are a minor component.

Few small follicles may be present.

Follicular hyperplasia

B-cell follicles become conspicuous.

Germinal-center formation is evident.

The appearance reflects a prominent immune response.

Slide 24 · What a germinal center signifies

A germinal center is a specialized structure within a B-cell follicle.

Its recognition locates an organized B-cell immune response inside the thymus.

The key diagnostic observation is the follicular pattern, not merely scattered B cells.

Slide 25 · Thymic follicular hyperplasia: Spatial map

Original schematic — not a photomicrograph

Cortex

Medulla

Central whorl: Hassall corpuscle

Large circles: B-cell follicles

How to read the diagram

Schematic dark outer cortex and pale medulla orient the viewer.

Round B-cell follicles with paler centers represent germinal centers.

Real diagnosis requires genuine microscopy and architectural context.

Slide 26 · Describe follicles without overinterpretation

Record the presence and prominence of intrathymic lymphoid follicles.

Describe germinal centers when they are actually visible.

Relate the lymphoid change to preserved or altered thymic compartments.

Do not label an epithelial tumor solely because lymphoid tissue is dense.

Slide 27 · Organ weight and follicle burden answer different questions

Anatomic size

How large is the gland for this age?

May fluctuate with physiologic or clinical context.

Main differential issue can be an apparent mediastinal mass.

Follicular architecture

Are B-cell follicles conspicuous microscopically?

Requires tissue-based assessment.

Links most strongly to autoimmune clinicopathologic settings.

Slide 28 · Age-relative thymic enlargement

Some morphologically normal thymuses are large for the patient’s age.

True hyperplasia versus normal variation is not always clear.

The principal clinical hazard is radiologic confusion with thymoma.

Slide 29 · Two benign patterns should not be conflated

Follicular phenotype

Defined microscopically by germinal-center proliferation.

Linked to myasthenia gravis and other autoimmune conditions.

Large organ volume is not essential.

Morphologically normal enlargement

Maintains ordinary histology.

May represent age-related variation rather than a distinct lesion.

Potentially triggers unnecessary surgery if misidentified as tumor.

Slide 30 · Immunologic diseases linked to follicular change

Myasthenia gravis is the emphasized association.

Similar thymic changes also appear with several other immune disorders.

The histologic pattern does not, by itself, identify the specific autoimmune disease.

Slide 31 · Autoimmune associations beyond myasthenia gravis

Graves disease.

Systemic lupus erythematosus.

Systemic sclerosis (scleroderma).

Rheumatoid arthritis and other autoimmune disorders.

Slide 32 · A cautious clinicopathologic interpretation

Intrathymic B-cell follicles

Suggest immune activation

Correlate with autoimmune history

Do not infer a unique diagnosis

Slide 33 · Myasthenia gravis: The central association

Myasthenia gravis is usually an autoantibody-mediated disorder of the neuromuscular junction.

Thymic follicular hyperplasia is particularly encountered in the associated thymic pathology.

A thymoma is a separate thymic abnormality associated with some patients.

Slide 34 · Patient age modifies thymic findings

The myasthenia gravis chapter emphasizes hyperplasia in younger patients.

Older-onset myasthenia gravis often lacks a detectable thymic abnormality.

Age and autoimmune phenotype therefore matter when interpreting a thymic specimen.

Slide 35 · The proposed thymus–autoimmunity link

Thymic self-antigen presentation

Altered tolerance in susceptible patients

Anti–ACh-receptor autoimmunity

Neuromuscular dysfunction

Slide 36 · Myoid cells provide relevant tissue context

The thymus contains scattered muscle-like myoid cells.

Their possible importance in myasthenia gravis.

The proposed link is biologically plausible but does not establish causation in an individual patient.

Slide 37 · What remains mechanistically uncertain

The chapter frames disruption of thymic self-antigen expression as a hypothesis.

Follicular hyperplasia does not prove that a thymic lesion initiated autoimmunity.

Anti–ACh-receptor autoimmunity can occur without visible thymic abnormalities.

Slide 38 · Neuromuscular transmission under normal conditions

Presynaptic acetylcholine release

Binding to postsynaptic ACh receptors

Muscle membrane activation

Contraction

Slide 39 · Anti–ACh-receptor antibodies impair transmission

ACh-receptor autoantibodies

Receptor aggregation and loss

Complement-associated membrane injury

Reduced postsynaptic response

Slide 40 · Complement damage is mechanistically important

Anti–ACh-receptor antibodies can activate complement.

The postsynaptic membrane becomes structurally and functionally impaired.

The reduction in effective ACh-receptor function produces fatigable weakness.

Slide 41 · Not all myasthenia gravis antibodies act alike

Anti–ACh-receptor MG

Autoantibodies target postsynaptic receptors.

Receptor depletion and complement-mediated injury are important.

Strongly linked to characteristic thymic changes.

Anti-MuSK MG

Targets muscle-specific receptor tyrosine kinase.

Interferes with ACh-receptor trafficking and clustering.

Does not rely on the same complement-fixing mechanism.

Slide 42 · Why weakness fluctuates in myasthenia gravis

Impaired transmission reduces the functional safety margin of the neuromuscular junction.

Clinical weakness commonly worsens with repeated use.

Symptoms may worsen later in the day and improve with rest.

Slide 43 · Ocular findings offer a clinical clue

Diplopia and ptosis are common presentations.

Extraocular muscle involvement is less typical in many primary myopathies.

An ocular presentation can precede or accompany generalized weakness.

Slide 44 · Generalized disease has clinical consequences

Patients may develop bulbar, limb, or respiratory muscle weakness.

Severe involvement may require mechanical ventilatory support.

Thymic histology should be interpreted in the context of the full clinical phenotype.

Slide 45 · Assessing suspected myasthenia gravis

Characteristic fatigable weakness

Autoantibody testing

Electrophysiologic evaluation

Integration with thymic assessment

Slide 46 · Electrophysiology tests neuromuscular function

Repeated stimulation can produce a decremental muscle response.

This result supports a disorder of neuromuscular transmission in the appropriate setting.

It diagnoses physiological dysfunction, not the microscopic type of thymic change.

Slide 47 · The thymic finding does not replace MG diagnosis

Follicular hyperplasia is a tissue association, not an independent MG diagnostic test.

Clinical features, autoantibodies, and electrophysiology remain central.

Likewise, normal-appearing thymic tissue does not exclude myasthenia gravis.

Slide 48 · Thymic histology does not determine MG therapy

A thymoma and non-neoplastic hyperplasia are different pathologic entities.

Microscopic follicular hyperplasia is not, by itself, a treatment algorithm.

Clinical subtype, severity, imaging, and contemporary evidence are needed for management.

Robbins provides a pathology framework rather than a current treatment guideline.

Slide 49 · Thymoma is a thymic epithelial neoplasm

The word thymoma is restricted to tumors of thymic epithelial cells.

Benign immature thymocytes can be present within thymomas.

A lymphocyte-rich field is therefore not sufficient to classify a lesion.

Slide 50 · Reactive follicles versus epithelial neoplasia

Follicular hyperplasia

Prominent B-cell follicles with germinal centers.

Non-neoplastic reactive thymic pattern.

Autoimmune association is common.

Thymoma

Neoplastic proliferation of thymic epithelial cells.

Often accompanied by non-neoplastic immature T cells.

Invasion and epithelial cytology matter clinically.

Slide 51 · The thymoma differential has an anatomic setting

Thymomas most often arise in the anterior superior mediastinum.

Other thymic-region neoplasms include lymphomas and germ cell tumors.

An apparently enlarged thymus requires consideration of non-neoplastic and neoplastic causes.

Slide 52 · Invasion is not a feature of ordinary hyperplasia

The chapter distinguishes thymomas by epithelial cytology and invasive behavior.

Invasive thymoma extends beyond the capsule to adjacent tissues.

A benign-appearing thymic gland must not be conflated with locally invasive epithelial tumor.

Slide 53 · Three pathologic categories encountered at the thymus

Finding

Primary feature

Clinical implication

Follicular hyperplasia

Intrathymic B-cell germinal centers

Seek compatible autoimmune setting

Large normal-appearing thymus

Age-relative enlargement, preserved architecture

Risk of radiologic tumor mimicry

Thymoma

Neoplastic thymic epithelial population

Assess epithelial tumor and invasion

Slide 54 · Cystic findings require a different question

Isolated thymic cysts are uncommon and usually clinically insignificant.

Neoplastic masses may compress normal thymus and cause cyst formation.

A symptomatic cystic thymic lesion warrants evaluation for an underlying neoplasm.

Slide 55 · Hypoplasia and hyperplasia are opposite thymic problems

Developmental hypoplasia/aplasia

Associated with DiGeorge syndrome.

Severe deficiency of cell-mediated immunity.

Often part of 22q11 deletion syndrome.

Hyperplastic patterns

Preserved thymus with B-cell follicles or relative enlargement.

May be associated with autoimmune disease.

Does not imply a primary absence of T-cell development.

Slide 56 · A structured approach to a prominent thymus

Consider patient age and history

Assess imaging for a discrete mass

Ask whether tissue shows follicles or neoplasia

Integrate clinical + morphologic evidence

Slide 57 · Diagnostic pitfalls in small or partial samples

Sampled tissue may fail to represent the overall thymic architecture.

Scattered lymphocytes must not be mistaken for a thymoma without epithelial evidence.

The presence of follicles alone does not diagnose a particular autoimmune disorder.

Morphology, age and clinical data should be considered together.

Slide 58 · Case 1 — fatigable ocular weakness

A young adult reports fluctuating ptosis and diplopia. A resected thymus shows preserved thymic compartments with numerous B-cell follicles and germinal centers.

Interpretation question

What is the best morphologic designation?

Slide 59 · Case 1 — interpretation

Thymic follicular hyperplasia

Germinal-center-rich B-cell follicles are the central tissue finding.

Myasthenia gravis is the classic clinical association.

The finding does not require a grossly enlarged thymus.

Slide 60 · Case 2 — a prominent anterior mediastinal gland

An adolescent has a prominent thymic contour on imaging. Histology, if obtained, shows ordinary lobular cortex–medulla organization without excess follicles or an epithelial tumor.

Interpretation question

Is the term follicular hyperplasia justified?

Slide 61 · Case 2 — interpretation

No: Prominence of size is not follicular hyperplasia

Age-relative normal size is variable.

Normal architecture argues against a follicular lesion.

Robbins warns that this situation may be mistaken radiologically for thymoma.

Slide 62 · Case 3 — a symptomatic cystic lesion

A symptomatic patient has an anterior mediastinal cystic lesion. Adjacent thymic tissue appears distorted and compressed.

Interpretation question

What pathologic possibility must be actively considered?

Slide 63 · Case 3 — interpretation

Search for an associated neoplasm

Cystic change can be secondary to a thymic-region neoplasm.

Thymoma and lymphoma are specific considerations.

Cystic appearance alone does not establish a harmless thymic cyst.

Slide 64 · Synthesis: Tissue observation versus diagnosis

Observation in the thymus

Germinal centers → follicular process.

Normal architecture with prominence → age-relative size question.

Epithelial proliferation/invasion → consider thymoma.

Required interpretation

Correlate follicles with autoimmune context.

Correlate size with age and radiology.

Classify tumors through epithelial morphology and behavior.

Slide 65 · What students should retain

Follicular hyperplasia means prominent B-cell germinal centers—not necessarily a large gland.

Myasthenia gravis is a key association, but causality should not be overstated.

Age-relative thymic enlargement is a recognized mimic of thymoma.

When a mass is suspected, separate reactive thymus from epithelial neoplasia.

Slide 66 · Selected source material

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th ed. (2026).

Chapter 13: Thymus — normal architecture, developmental disorders, thymic hyperplasia, and thymoma.

Chapter 6: Central tolerance, clonal deletion, AIRE, and regulatory T-cell differentiation.

Chapter 27: Myasthenia gravis — autoantibodies, neuromuscular pathogenesis, and thymic associations.