Classification of lymphoid organs
Depending on their role in the maturation of immune cells, all lymphoid organs are divided into two large groups: primary and secondary.
1. Primary lymphoid organs (organa lymphoidea primaria): This group includes the basic structures where immune cells originate:
- Bone marrow (medulla ossium).
- Thymus (thymus).
2. Secondary lymphoid organs (organa lymphoidea secundaria): Lymphocytes migrate here for further functional transformation:
- Regional lymph nodes (nodi lymphoidei regionales).
- Spleen (splen).
- Tonsils: palatine tonsils (tonsillae palatini), pharyngeal tonsil (tonsilla pharyngea), tubal tonsils (tonsillae tubarii), and lingual tonsil (tonsilla lingualis).
- Lymphoid nodules. They can be solitary (noduli lymphoidei solitarii), located in the walls of hollow organs of the respiratory and digestive systems, and aggregated (noduli lymphoidei aggregati), which are found in the wall of the ileum and appendix.
Functions of primary lymphoid organs
Primary organs play a key role in the formation of the body's defense system.
Bone marrow acts as the most important organ of hemocytopoiesis (hematopoiesis). Its structures harbor stem cells that give rise to absolutely all varieties of blood cells (hemocytes) and lymph (lymphocytes).
The thymus is the central organ of immunogenesis. In addition, the gland performs an endocrine function by producing a hormone-like substance — thymosin. This secretion helps increase the number of lymphocytes, globally enhances immunity, and also actively participates in the processes of growth and skeletal formation.
The process of lymphocyte differentiation is strictly divided:
- T-lymphocytes (thymus-dependent) are formed in the thymus through a complex transformation of some of the stem cells originating from the bone marrow.
- B-lymphocytes are presumably differentiated from cells of the bone marrow itself, as well as from the lymphoid nodules of the appendix and ileum.
After primary differentiation, lymphocytes migrate via the bloodstream to secondary lymphoid structures to complete their development.
Macroscopic structure of the thymus
The thymus (thymus) is an unpaired lymphoid organ with an elongated, asymmetrical shape.
In most cases, it consists of two closely adjoining lobes: the right lobe (lobus dexter) and the left lobe (lobus sinister). In rare cases, the organ may consist of three or four lobes. These structures are connected by a layer of loose connective tissue. The surface of the thymus is uneven and nodular.
Anatomically, the organ has a lower part, which is the widest and forms the base, as well as tapering upper sections that form the apex of the gland.
Age anatomy and topography of the thymus
The topography and dimensions of the thymus change significantly during development.
At the seventh month of prenatal development (in the fetus): The thymus is located in the superior mediastinum. The organ has a cone-like shape tapering upward. The lower margin of the thymus adjoins the base of the heart (basis cordis) and large vessels. Laterally, it borders the medial surfaces of the lungs (facies medialis pulmonis), and the upper pole extends high into the neck region. During this period, the organ grows actively.
In the newborn: The thymus shifts, occupying the anterior part of the superior mediastinum and extending into the anterior mediastinum. The organ increases significantly in size, while its lobular structure (two asymmetrical, tightly adhering lobes) is clearly visualized.
- Syntopy: anteriorly, the thymus adjoins the posterior surface of the sternum; posteriorly, with its broad base, it covers the anterior surface of the pericardium and the great vessels (aorta, pulmonary trunk, superior vena cava); laterally, it significantly overlaps the anterior borders of the lungs.
By birth, the thymus is a large immune organ occupying a substantial volume of the retrosternal space. It is in newborns that the thymus has the maximum mass relative to body weight.
Briefly: answers to topical questions
What is the microscopic structure of a thymic lobule?
A thymic lobule (lobuli thymi) consists of a peripheral cortical substance and a central medullary substance, formed by a network of epitheliocytes and filled with lymphocytes.
| Substance | Localization | Features |
|---|---|---|
| Cortical substance (cortex thymi) | Peripheral part of the lobule | Darker, densely populated with T-lymphocytes |
| Medullary substance (medulla thymi) | Middle part (center) of the lobule | Lighter, contains fewer lymphocytes, features concentric corpuscles |
Both substances contain maturing or mature T-cells, epithelial stroma, macrophage lineage cells, as well as blood and lymphatic vessels.
From which arteries is the blood supply to the thymus carried out?
The blood supply to the thymus is provided by thymic branches (rr. thymici) originating from multiple arterial sources.
These include:
- Internal thoracic artery (a. thoracica interna)
- Inferior thyroid artery (a. thyroidea inferior)
- Pericardiacophrenic artery (a. pericardiacophrenica)
- Superior phrenic artery (a. phrenica superior)
Childhood is characterized by an abundance of blood supply sources for this organ.
What functions does the spleen perform as a secondary lymphoid organ?
The main function of the spleen as a secondary lymphoid organ is the formation of the immune response. It implements cellular mechanisms of immunity, including antigen recognition and the functioning of immunocompetent cells. The spleen contains one of the largest lymphocyte populations in the body, accounting for 20–25% of the total number of cells in the immune system.
What happens to the thymus during age-related involution in adults?
During age-related involution, which begins after the age of 20, there is a gradual decrease in thymic mass and the replacement of its parenchyma with connective and adipose tissue.
The main changes include:
- Loss of lymphoid and epithelial cells with the formation of cysts at the sites of stromal cell death.
- Destruction of the corticomedullary structure and weakening of the function of the corticomedullary barrier.
- Migration of non-characteristic cells (predominantly B-lymphocytes) into the thymic cortex.
In elderly individuals, the organ is filled with adipose tissue interspersed with islands of normal parenchyma. At the same time, the thymus does not completely lose its functions, maintaining hormone secretion and cell emigration at a low level.
What hormones does the thymus produce?
The thymus produces polypeptide hormones that are secreted by stromal epithelial cells and influence the maturation of T-lymphocytes.
The main hormones and factors of the thymus include:
- Thymosins (including alpha-1 thymosin) are a group of polypeptide factors.
- Thymopoietins are produced by the same cells as thymosins.
- Thymulin is a nonapeptide that exhibits activity only in a complex with zinc ions.
- Parathymosin belongs to one of the main groups of gland hormones.
- Thymic humoral factor is a complex of peptides whose active fraction is gamma-2.
T-lymphocytes are named so because they are thymus-dependent (they mature in the thymus).
Class and exam questions
Which organs are primary lymphoid organs?
Primary lymphoid organs (organa lymphoidea primaria) include the bone marrow and thymus.
Where and how are T-lymphocytes formed?
T-lymphocytes are formed in the thymus. They differentiate by the transformation of some stem cells coming from the bone marrow.
What endocrine function does the thymus perform?
The thymus produces a hormone-like substance, thymosin, which increases the number of lymphocytes, enhances the immune response, and promotes skeletal growth processes.
What is the characteristic feature of the thymus in newborns?
In newborns, the thymus has the maximum relative body mass compared to other periods of life, a distinct lobular structure, and occupies a significant volume of the retrosternal space.
What else to study on this topic
- Regional lymph nodes: classification and topography
- Microscopic structure and functions of the spleen (splen)
- Solitary and aggregated lymphoid nodules of the digestive tract
- Anatomy of the tonsils: palatine, pharyngeal, lingual, and tubal
- Thymus involution in the postpubertal period
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