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What is Thymus Gland?

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A gland in an animal is a collection of cells that produce compounds, such as hormones, for release into the bloodstream (endocrine gland), into cavities inside the body, or onto the animal’s surface (exocrine gland). Each gland is created by the ingrowth of epithelial tissue. This ingrowth may initially have a tubular form, but glands can also begin as a solid column of cells that later develops into a tubular structure.

The column of cells may divide or release offshoots as growth progress, in which case a compound gland is created. Many glands have a finite number of branches, whereas others (such as the pancreas and salivary glands) repeatedly expand and divide to eventually form a very massive structure. Normally, the branches don’t join together, but the liver is one exception since a reticulated compound gland develops there. The terminal portion of each branch in compound glands is made up of the more common or secretory epithelium, and the connecting segments, which ducts, are lined with a less altered type of epithelial cell.

Thymus Gland

One of the essential parts of the lymphatic system, which is located in the mediastinum beneath the sternum, is the thymus gland.

  • The thymus stretches from the fourth intercostal gap of the vertebrae to the lower margin of the thyroid gland.
  • Unlike other endocrine glands, the thymus gland is exclusively active prior to puberty. At puberty, the gland expands to its maximum size before being gradually replaced by adipose tissue.
  • A vital component of the immune system, the thymus gland acts as a barrier against various diseases, cancers, and antigens.
  • The thymus is particularly crucial for the adaptive or thymus-dependent arm of the immune system since it is necessary for the growth and activation of immune cells.
  • The thymus contains immune cells such as macrophages, neutrophils, and dendritic cells in addition to secretory cells.
  • Cytokines like tumour necrosis factor (TNF) and interferon sustain the growth of the secretory cells and the control of their secretions.
  • Autoimmune conditions like myasthenia gravis can affect the anatomy and operation of the gland.

Structure of Thymus Gland

  1. The thymus gland is a bilobed gland with two pyramid-shaped lobes, each of which has a surface that is lobulated and has two distinct regions: the outer cortex and the inner medulla.
  2. An interior connective tissue septum and a thick connective tissue capsule surround and separate each of the gland’s lobes.
  3. An intricate three-dimensional network of star-shaped reticular cells makes up the majority of the gland’s mass.
  4. Based on many characteristics, including antigenic expression, ultrastructure, and the ability to create thymic hormones, the epithelial cells of the thymus gland are categorised into four main subtypes.
  5. Subcapsular cortical, inner cortical, medullary, and Hassall’s corpuscles are the subtypes.
  6. Loosely packed lymphocytes can be found in the gland’s outer cortex, while reticulocytes with abundant cytoplasm can be found in the medulla.
  7. Small bodies made up of concentric arrays of squamous cells are also present in the structure and are known as Hassall’s concentric corpuscles, Hassall’s bodies, or thymic corpuscles.
  8. Infants and early children have big thymuses, which reach their maximum size during puberty. After puberty, the gland gradually starts to consolidate so that adipose tissue can take its place.
  9. The inferior thyroid, internal thyroid, and intercostal arteries all give blood to the thymus gland.
  10. Sternohyoid and sternothyroid muscles, both of which are bilateral, link the gland to the sternum.
Thymus Gland


Anatomy of Thymus Gland

  • Location: A soft, bilobed organ with encapsulation, the thymus gland. It is located near the pericardium in the superior mediastinum and the anterior region of the inferior mediastinum. The thymus is located deep in the sternum and anterior to the large heart arteries. It reaches the fourth costal cartilage from the level of the inferior poles thyroid gland above. Parallel to the gland on its left and right side are the phrenic nerves (which go on to supply the diaphragm) (which go on to supply the diaphragm). An isthmus connects the thymus’ two separate lobes in the middle.
  • Blood supply: The internal thoracic artery, as well as the superior and inferior thyroid arteries, supplies blood to the thymus. Drainage is to the superior, middle, and inferior thyroid veins, as well as the left innominate vein. There are many thymic arteries that can enter the organ’s content and follow the path of the interlobular septae. The blood thymus barrier is made up of the arteries, which organise themselves into a series of intricate arcades in the cortex of the thymus, along with the reticular endothelial cells and white blood cells (lymphocytes and macrophages). Proteins cannot pass through the non-fenestrated endothelium and thick basal lamina of thymic capillaries. After that, blood enters the medullary veins.
  • Lymphatics: There are no afferent lymphatics in the thymus. The thymic lymph nodes, which include the internal mammary-parasternal, tracheobronchial-hilar, and mediastinal-brachiocephalic, are close to the gland where the lymph drains.
  • Nerve supply: The sympathetic nervous system and vagus nerves, which reach deeply into the thymus via postganglionic noradrenergic fibres, provide the thymus with its meagre nerve supply.

Histology of Thymus Gland

The connective tissue capsule that protects the thymus has septa that pierce the tissue and divide it into unfinished lobules. Each lobule has a centre, lighter zone called the medulla, and a peripheral, darker zone called the cortex. The lymphocytes are located between the collagen and reticular fibre layers that make up the capsule’s inner and outer layers.

  1. Cortex: This is the thymus gland’s periphery, and it is filled with many tiny, tightly packed T lymphocyte progenitors (thymocytes). Additionally, macrophages and epithelial reticular cells are present. This network of epithelial reticular cells contains the thymus’ blood arteries as well. The very early stages of thymocyte growth and the rearranging of the genes for the receptors on the surface of T cells both occur in the brain.
  2. Corticomedullary junction: This has few connective tissues, abundant blood arteries, and mature T lymphocytes. Dendritic cells and B cells can also be found here. There are also type IV epithelial reticular cells in this region.
  3. Medulla: The network of reticular endothelial cells is denser and the number of lymphoid cells is lower in the centre region. Hassall’s corpuscles are a group of concentrically arranged bodies. They are concentrically organised, flattened epithelial reticular cells that are packed with keratin filaments. Also, There is a core mass of a few granular cells within these corpuscles. Later stages of thymocyte formation also take place in the medulla. The thymocytes in this region have gone through the cortex, and undergone positive selection, receptor gene rearrangement, and a little amount of negative selection. Therefore, the medulla is the location where the great majority of negative selection occurs.

Hormones of Thymus Gland

Thymosin, thymopoietin, and serum thymic factor are the three hormones that are produced by the thymus gland. The T cells are not all affected by thymic hormones in the same way. The hormones’ mode of action at the cellular level is based on their interaction with adenylyl cyclase and binding to the appropriate cell receptors.


  1. The main hormone secreted by the gland is thymosin, which is generated by the epithelial cells of the cortex and medulla.
  2. Thymosin’s ability to trigger T cell differentiation and improve various immune cells’ immunological capabilities is its most significant function.
  3. The phenotypic markers in lymphocytes have also been linked to an increase in thymosin levels.
  4. Thymosin is a protein that can withstand heat up to 80°C and may also contain a small quantity of sugar.


  1. A polypeptide hormone produced by the thymus called thymopoietin serves more neuromuscular purposes than immunological ones.
  2. Increased hormone levels are known to also cause T cell activation and differentiation, though.

Functions of Thymus Gland

  1. The thymus gland’s primary job is to stimulate the growth, activation, and differentiation of T cells so that they can act as the messengers of cellular immunity.
  2. Prothymocytes are stimulated to develop into thymocytes and T lymphocytes by the thymic hormones thymosin and thymopoietin.
  3. The gland also triggers the secretion of cytokines, which are crucial for regulating T cell growth at various stages.
  4. By the 12th week of pregnancy, the thymus is already contributing to foetal immunity, and it continues to function throughout foetal life and into childhood.
  5. The thymus is an endocrine gland that is known to create human growth hormone, which is crucial for the body’s expansion and development.

Disorders of Thymus Gland

Thymic cysts

  1. The thymus gland may include congenital thymic cysts, which would prevent the thymocytes from proliferating.
  2. These cysts are benign and may typically be removed. Because their immune systems are compromised, people with thymic cysts experience symptoms like coughing and upper respiratory tract infections.


  1. Infants that experience neglect, malnourishment, and abuse frequently experience thymic involution.
  2. The thymus is vulnerable to stress, therefore it can also happen after chemotherapy, radiation therapy, and steroid therapy.
  3. When it comes to newborns and young children, thymus involution is far more uncommon.
  4. Premature involution is a sign of societal issues like abuse.


  1. Hypoplasia, which typically occurs in newborns, is a decrease in the number of cells in an organ.
  2. The third and fourth pharyngeal pouches’ abnormal development leads to hypoplasia of the thymus.
  3. It has been hypothesised that substances like alcohol or organic acids affect the neural crest, which interferes with the pharyngeal pouch’s ability to differentiate.
  4. The degree of hypoplasia and the amount of genetic material that is defective determine how the illness will appear clinically.


Low serum levels of immunoglobulin or antibodies are the cause of the condition known as hypogammaglobulinemia. The primary elements of the humoral immune response, immunoglobulins are capable of recognising antigens to initiate a biological response and eliminate the infectious cause. The most prevalent primary immunodeficiency, hypogammaglobulinemia, affects the majority of patients with immune system deterioration. It must be swiftly detected and treated in order to reduce the substantial morbidity and death linked to this disorder. Laboratory studies, clinical observations, vaccine response, and genetic testing are all included in the evaluation. IVIG transfusions, antibiotics, and glucocorticoids are used as treatment. In order to examine, evaluate, manage, and treat patients with this condition appropriately, the practitioner or interprofessional team is highlighted in this activity.

Pure red cell aplasia

The illness known as pure red cell aplasia (PRCA) is characterised by severe reticulocytopenia, severe normocytic normochromic anaemia, and a substantial reduction or absence of erythroid precursors from the bone marrow. A congenital type of PRCA is Diamond-Blackfan anaemia. PRCA that has been acquired might occur as the main disorder or as a result of another disorder or agent. An autoimmune condition known as primary acquired PRCA is typically antibody-mediated. The morphologic features of PRCA can also be seen in myelodysplastic syndromes. Secondary acquired PRCA may be linked to autoimmune or collagen vascular diseases like systemic lupus erythematosus, lymphoproliferative diseases like chronic lymphocytic leukaemia or large granular lymphocyte leukaemia, infections, particularly B19 parvovirus, thymoma and other solid tumours, as well as a number of other diseases, medications, or toxic substances. Immunosuppression is commonly used as a treatment for PRCA, however particular pathogenic subtypes have particular therapeutic techniques connected with them. Whether used in conjunction with corticosteroids or not, cyclosporine A seems to be the most potent immunosuppressive medication.

Myasthenia gravis

  1. Antibodies in myasthenia gravis (MG) prevent neurons and muscles from communicating with one another, which causes the skeletal muscles to weaken.
  2. The voluntary muscles of the body, particularly those that govern the eyes, mouth, throat, and limbs, are impacted by myasthenia gravis.
  3. Although the illness can affect anyone at any age, young women (between the ages of 20 and 30) and older men are more likely to contract it.
  4. Breathing or swallowing issues can occur during a myasthenia gravis crisis.
  5. Myasthenia gravis has no known cause and no known treatment, although early diagnosis and immediate medical attention can help people live longer, more active lives.

Thymus Cancer

Under the breastbone in your chest, there is an organ called the thymus gland. It is a component of your body’s immune system’s lymphatic system. Lymphocytes, a type of white blood cell that aids in the body’s ability to fight infection, are produced by the thymus gland.

  • Thymoma and thymic carcinoma are the two main types of thymus cancer, both of which are extremely uncommon. When cancerous cells develop on the thymus’ outer surface, cancer develops.
  • Thymic carcinoma is more aggressive than thymoma and is also more challenging to treat. Type C thymoma is another name for thymic cancer.
  • Thymoma patients may also have autoimmune conditions including rheumatoid arthritis, myasthenia gravis, or acquired pure red cell aplasia.

Symptoms of Thymus Cancer

Near the airways and certain blood veins, the thymus is located in the centre of the chest. The following symptoms may result from tumours in the thymus pressing on surrounding structures:

  1. Shortness of breath
  2. Cough (which may bring up bloody sputum)
  3. Chest pain
  4. Trouble swallowing
  5. Loss of appetite
  6. Weight loss

The superior vena cava, the major blood channel that transports blood from the head and upper body to the heart, is close to the thymus. Symptoms of superior vena cava syndrome can be brought on by tumours that stress this blood artery and include:

  • Swelling in the face, neck, and upper chest, sometimes with a bluish colour
  • Swelling of the visible veins in this part of the body
  • Headaches
  • Feeling dizzy or light-headed

Difference between Thyroid and Thymus




Definition A gland of the endocrine system An organ of the lymphatic system
Anatomy Two lobes comprised of follicles surrounded by epithelia An outer capsule and inner medulla section, two lobules
Changes in size with age  Does not change in size as you age Becomes smaller as you age 
What it produces Two thyroid hormones called thyroxine (T4) and triiodothyronine (T3) T cells of the immune system
Functions Controls metabolic rate and activity in the body Cell-mediated immune response
Disorders Too few hormones or too many hormones may be produced The disorder, myasthenia gravis and the condition, hypogammaglobulinemia

FAQs on Thymus

Question 1: What hormone affects the thymus?


Three important thymic hormones, thymosin, thymopoietin, and thymrin, are believed to reside in the cytoplasm of thymic epithelial cells.

Question 2: What regulates the thymus?


The thymus is physiologically controlled by neuroendocrine systems. It is clear that a number of biological processes involving the organ’s lymphoid cells and microenvironment depend on the levels of certain peptide hormones in circulation.

Question 3: What is the role of the thymus in immunity?


The immune system acts as the body’s defensive mechanism, keeping an eye out for and protecting against a variety of pathogens, cancers, antigens, and mediators of tissue damage. The thymus is a vital organ in this process.

Question 4: Where does the thymus develop from?


The endodermal gut tube, which has a single layer of epithelial cells and gives rise to all gut-derived organs via the creation of evaginating buds, is where the thymus and parathyroid glands originate.

Question 5: Which cells mature in the thymus?


T-cells can develop in the lymph nodes or the thymus gland. The lymph nodes play a significant role in maturation since the adult thymus is only 10-15% functioning.

Question 6: What is the structure of the thymus?


A superior mediastinal retrosternal organ is the thymus. It is bilobed and composed of epithelial, dendritic, mesenchymal, and endothelial cells. The cortex and the medulla are its two subcomponents. One organ that matures in utero and involutes as people get older is the thymus.

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Last Updated : 21 Oct, 2022
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