Thymus

Thymus is an annoying little organ that is in the mediastinum.

General

• Involutes after childhood.
• Location: anterior mediastinum.
• Important for development of the immune system.
• May contain within it parathyroid.

Histology

Features:

• No germinal centres.
• Hassall's corpusle (thymic corpusle).
  • Round eosinophilic thingy.
  • Thought to arise from medullary epithelial cells (see cell types).

Cell types

Cells of the thymus (short version):

1. Cortical epithelial cells.
   • Epithelioid.
   • Abundant cytoplasm.
   • Pale nuclei with small nucleoli.
2. Medullary epithelial cells.
   • Spindle morphology.
   • Scant cytoplasm.
   • Oval dark nuclei.
3. T lymphocytes.

Other cells:

• Macrophages.
• Dendritic cells.
• Other WBCs: B lymphocytes, neutrophils, eosinophils.
• Myoid cells.

Images:

• Thymic corpusle (wikimedia.org).

Di George syndrome

• Things go wrong with the thymus... very wrong.

Thymic follicular hyperplasia

• AKA thymic follicular hyperplasia.

Features:

• Follicular centres in the thymus.

Associations:

• Myasthenia gravis.
• Graves' diseases.
• SLE.
• Rheumatoid arthritis.
• Other autoimmune diseases.

Tumours of the thymus (overview)

Thymic tumours are derived from the epithelial component of the thymus, i.e. the cortical epithelial cells and medullary epithelial cells.

The WHO published a widely used system - WHO classification:

Type A

• AKA Spindle cell or medullary.
• Arise from medullary epithelial cells.
• Good prognosis.

IHC:

• Usu. keratin+.

Type AB

• Like Type A... but with foci of lymphocytes.

Type B1

• Near normal, expanded cortex.

Lesion consists of:

• >2/3 lymphocytes, <1/3 cortical epithelial cells.

Type B2

• Neoplastic cells with some resemblance to cortical epithelial cells.
  • Epithelioid cells with distinct nucleoli.
  • May be perivascular.
• Large population of lymphocytes.

Lesion consists of:

• <2/3 but >1/3 lymphocytes, >1/3 but <2/3 cortical epithelial cells.

Notes:

• Most common B type.

Type B3

• Neoplastic cells with some resemblance to cortical epithelial cells.
  • Polygonal/round shape.
  • Form sheets (of cells) - key feature.
• Lymphocytes - less than in Type B2.
• AKA well-differentiated thymic carcinoma.

Lesion consists of:

• <1/3 lymphocytes, >2/3 cortical epithelial cells.

Type C

• AKA thymic carcinoma.
• Neoplastic cells with some resemblance to cortical epithelial cells - with cytologic features of malignancy.
  • Any nuclear atypia of epithelial thymocytes... puts a tumour into this group.

Images:

• Type B1:
  • Thymoma Type B1 (WC).
  • Thymoma Type B1 (WC).
  • Thymoma Type B1 - CAM5.2 (WC).
• Type C:
  • Thymic carcinoma - Type C (pathconsultddx.com).

Thymic carcinoma

• AKA Thymic tumour type C.

IHC:

• CD5 +ve.

Staging

There is a system by Masaoka et al..

IHC and thymus

Types A, AB, B:

• CK7-, CK20-, CAM5.2+, CK5/6+, p63+, CD5-.

Type C:

• CD5+.

All types:

• CD1a (immature T cells), Lnagerhans cell, dendritic cells, CEA+ (focal), vimentin-.

Anterior mediastinum mass DDx

4 Ts (mnemonic):

• Thymoma.
• Terrible lymphoma.
• Teratoma.
• Thyroid (ectopic).

See also

• Thyroid gland.
• Basics.
• Lung.