Thymus Gland – Structure, Functions

Common Diseases Of The Thymus

Thymus Normal Structure:

The thymus gland is a complex lymphoreticular organ lying buried within the mediastinum. At birth, the gland weighs 10-35 gm and grows in size upto puberty, following which there is progressive involution in the elderly. In the adult, thymus weighs 5-10 gm.

The gland consists of right and left encapsulated lobes, joined together by fibrous connective tissue. Connective tissue septa pass inwards from the capsule and subdivide the lobe into large number of lobules. The histologic structure of the lobule shows outer cortex and inner medulla.

Read And Learn More: General Pathology Notes

Both cortex and medulla contain two types of cells: epithelial cells and lymphocytes (thymocytes).

• Epithelial cells are similar throughout the thymus gland. These cells have elongated cytoplasmic processes forming network in which thymocytes and macrophages are found. Hassall’s corpuscles are distinctive structures within the medulla composed of onion skin-like concentrically arranged epithelial cells having central area of keratinisation.
• Thymocytes are predominantly present in the cortex. These cells include immature T lymphocytes in the cortex and mature T lymphocytes in the medulla. Well-developed B-cell lymphoid follicles with germinal centres are rare in thymus gland.

The main function of the thymus is in the cell-mediated immunity by T-cells and by secretion of thymic hormones such as thymopoietin and thymosin-α1.

Thymic lesions are associated with diverse conditions which may be immunologic, haematologic or neoplastic. These can be broadly categorised into structural and functional changes (thymic hypoplasia and agenesis, thymic hyperplasia) and thymic tumours (thymomas); thymic involvement in myasthenia gravis.

Thymus Gland Disorders

Structural And Functional Changes:

Thymic Hypoplasia And Agenesis:

Thymic hypoplasia and agenesis are acquired and congenital disorders respectively in which the gland is either unusually small or absent. These conditions are various types of hereditary (primary) immunodeficiencies such as DiGeorge’s syndrome, severecombined immunodeficiency and reticular dysgenesis.

Acquired hypoplasia occurs as an ageing phenomenon or may occur in the young due to severe stress, malnutrition, irradiation, therapy with cytotoxic drugs and glucocorticoids.

Thymic Hyperplasia:

Enlargement of the thymus or failure to involute produces thymic hyperplasia. Hyperplasia is usually associated with appearance of lymphoid follicles in the medulla of the thymus and is called thymic follicular hyperplasia. Most common cause of follicular hyperplasia of the thymus is myasthenia gravis.

Less common causes are:

Addison’s disease, Graves’ disease, rheumatoid arthritis, SLE, scleroderma and cirrhosis of liver.

Thymic Tumours (Thymomas):

Most common primary tumour present in the anterosuperior mediastinum is thymoma. Although thymus is a lymphoepithelial organ, the term thymoma is used for the tumour of epithelial origin.

Most of the patients are adults. In about half the cases, thymoma remains asymptomatic and is accidentally discovered in X-rays of the chest. Other patients have associated conditions like myasthenia gravis or local symptoms such as cough, dyspnoea and chest pain.

Thymus Gland Disorders

Morphologic Features:

Grossly, the tumour is spherical, measuring 5-10 cm in diameter with an average weight of 150 gm. Sectioned surface is soft, yellowish, lobulated and may be either homogeneous or contain cysts due to the presence of haemorrhage and necrosis.

Microscopically, the tumour has a thick fibrous capsule from which extend collagenous septa into the tumour dividing it into lobules. The histology of lobule shows various patterns.

The tumour consists of neoplastic epithelial cells and variable number of non-neoplastic lymphocytes:

1. The epithelial component consists of round, oval to spindle-shaped cells. However, cytologic features of epithelial cells are not reliable predictor of biologic behaviour of the tumour.
2. Lymphocytes are benign polyclonal cells.

All thymomas have malignant potential. The main prognostic feature is whether thyoma is invasive (35%) or noninvasive (65%). The WHO classification has graded thymomas into A, B and C based on histologic description such as medullary, cortical or mixed respectively. Genetic abnormalities are common in thymoma, most commonly affecting chromosome 6.

Thymomas are known for their association with paraneoplastic syndrome. These include:

Myasthenia gravis (most common), hypogammaglobulinaemia, erythroid hypoplasia (pure red cell aplasia), peripheral T cell leukaemia/lymphoma, multiple myeloma, other autoimmune disease associations and other systemic malignancies.

Thymus Gland Disorders

Common Diseases of the Thymus:

• Thymic hypoplasia may occur due to primary immunodeficiency.
• Thymic hyperplasia is most often due to myasthenia gravis.
• Thymoma is an epithelial tumour in the anterosuperior mediastinum. Benign lymphocytes are also present in it. It has always a malignant potential. Prognosis depends upon presence or absence of invasion.
• Thymoma may produce paraneoplastic syndrome of myasthenia gravis and hypogammaglobulinaemia.