Thymus involution: definition, norms and meaning

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Thymus involution: definition, norms and meaning
Thymus involution: definition, norms and meaning

Thymus or thymus gland is one of the most important organs of the immune system. It plays a special role in the normal development of the child. That is why the size of this endocrine organ in children is much larger than in an adult. Its decrease over time is called thymus involution. More about this phenomenon later in the article.

Basic information

The thymus is located in the upper part of the chest cavity, in front of the trachea (breathing tube). It consists of two lobes connected by an isthmus. The organ reaches its maximum mass of 30-40 grams at the onset of puberty, after which its size gradually decreases.

Thymus belongs to both the group of immune organs and endocrine organs. That is, it performs a dual function: it participates in the synthesis of T-lymphocytes (white blood cells responsible for a normal immune response) and in the production of thymosin and thymopoietin, which in turn stimulate the formation of antibodies.

human thymus

The role of the thymus in the child's body

Basicthe thymus performs its function during the intrauterine development of the baby and after his birth at the age of 3 years. It was at this time that he actively synthesizes T-lymphocytes. This is necessary to protect the baby from infections, since the child's body is most susceptible to the influence of pathogenic microorganisms.

The thymus produces the hormone thymosin, which is necessary for the normal formation of lymphocytes. With a decrease in the function of the thymus, the body's resistance to infection decreases. The child is prone to frequent respiratory problems that can easily become chronic.

When the thymus function is impaired for a long time, an immunodeficiency state occurs. It is manifested not only by a decrease in resistance to pathogenic viruses and bacteria, but also to microorganisms that live inside each person, but in a normal state of immunity do not lead to the development of the disease. They are also called opportunists.

normal thymus

Main varieties of involution

Thymus size reduction can be of two types:

  • age;
  • accidental.

In both cases, the process of thymus involution consists in the gradual replacement of its tissue with fatty structures. This process is typical only for the thymus gland. Neither in the bone marrow nor in the spleen such changes occur.

Age changes

The age-related involution of the thymus is considered the norm. It begins after puberty. Its main manifestations are presented below:

  • organ mass reduction;
  • decrease in function, that is, inhibition of T-lymphocyte production;
  • replacing normal organ tissue with fat.

Pathological anatomy micropreparations show that the thymus tissue loses clear boundaries between the cortical and medulla during involution. There is a gradual thickening of the partitions that separate the lobules from each other. Hassall's corpuscles (epithelial cells in the thymus medulla) become larger and more numerous.

After puberty, almost the entire mass of the thymus is replaced by adipose tissue. Only separate islands of epithelial and reticular cells are noted. However, even in this form, the thymus continues to participate in the body's immune response, producing T-lymphocytes.

thymus involution

Features of accidental changes

As noted earlier in the article, age-related and accidental involution of the thymus are the two main types of reduction in the size of this organ. This section will discuss the second type of change in more detail.

The main difference between accidental changes in the thymus gland and age-related changes is that in the first case, there is a decrease in the size of the lobules of this organ and a decrease in the number of lymphocytes. At the same time, with age-related involution, the gland tissue is replaced by fat cells.

The term "accidental" was proposed back in 1969, but still has not lost its relevance. Literally, it means "accident". Indeed, in essence, accidental involution israndom response of the thymus gland to a harmful factor that acted on it.

Causes of pathology

The reasons why the involution of the thymus begins are not fully understood. However, doctors identify a number of risk factors that increase the likelihood of developing these changes. These include:

  • radiation exposure;
  • taking anticancer drugs;
  • taking hormonal drugs;
  • oncological diseases, primarily hemoblastoses (malignant neoplasms of the bone marrow);
  • infectious inflammatory diseases.

There are also studies on the importance in the development of thymus pathology of such conditions as hypothermia and hypoxia (decrease in oxygen concentration in body tissues). However, their meaning is not exactly clear.

thymus micropreparation

Main stages: first, second and third

When studying the pathology of the accidental involution of the thymus, certain stages in the changes in the gland should be distinguished. Conventionally, there are five such stages, or phases.

The first phase is characterized by the absence of changes in the thyroid gland. The volume and structure of the thymus correspond to those of a he althy child.

In the second phase, there is a partial loss of lymphocytes, which are localized in the cortical (outer) layer of the gland. Moreover, they are destroyed chaotically or "nested". Macrophages stick to these lymphocytes and "swallow" them. In the medical literature, this process is called phagocytosis. Part of the lymphocytes decreases due to their leakage intototal blood flow.

In the third phase, the process progresses, the collapse of the reticular mesh of the thymus develops. There are more lymphocytes in the medulla than in the cortex. As a result, when examining a micropreparation of an accidental involution of the thymus under a microscope, the medulla looks darker, although it should normally be the other way around.

Also at this stage, there is an increased synthesis of small thymic bodies. Normally, they are observed only in the medulla, and at the third stage of accidental involution, they begin to populate the cortical part as well.

Main stages: fourth and fifth

In the fourth phase, the condition worsens even more. There is a decrease in lymphocytes from the medulla, so it becomes extremely problematic to distinguish the cortical region from the brain. Thymic bodies are combined with each other, which looks like large cystic formations on a microslide. These structures are filled with protein secretion with inclusions in the form of scales. Over time, this content leaves the cystic formations through the lymphatic capillaries.

In the fifth (or terminal) phase, atrophy and sclerosis of the organ develop. This means that the thymus is significantly reduced in size, the connective tissue septa are thickened. There are very few lymphocytes; over time, almost the entire organ is replaced by connective tissue. Calcium s alts are deposited in thymic bodies, which is called calcification or petrification.

Thus, during accidental involution in the thymus, the following processes occur:

  • dramatic reduction in sizeorgan;
  • a significant drop in the functional activity of the thymus;
  • decrease in the number of lymphocytes up to their complete absence;
  • replacement of the thymus with connective tissue;
  • deposition of petrificates in thymic bodies.

Main symptoms

The main outcome of both complete and incomplete involution of the thymus is a drop in its functional activity. With age-related changes, no symptoms develop, since this, in fact, is the norm for a person. And with accidental involution, when the fall in thymus function occurs abruptly and manifests itself to a large extent, certain clinical symptoms develop.

General symptoms that develop regardless of the causes of the pathology include the following:

  • general fatigue, weakness;
  • an increase in the size of almost all groups of lymph nodes;
  • shortness of breath - shortness of breath;
  • frequent colds, infectious diseases due to a decrease in immune resistance;
  • heaviness of the eyelids, feeling as if someone is pressing on them.

It is also common for a person to have clinical manifestations that correspond to a specific cause of thymus involution. For example, oncological diseases are characterized by the development of anemic syndrome, pallor or yellowness of the skin, loss of appetite, and weight loss. In inflammatory diseases, the patient is concerned about fever, chills, deterioration of the general condition.

thymus ultrasound

Diagnosis of disease

The diagnosis begins with a detailed questioning of the patient about his complaints, anamnesis of life and disease. Thymus involution is not yet a definitive diagnosis. This is just one of the clinical manifestations of many pathological conditions. Therefore, the main task in diagnosing this process is to find its cause.

The involution itself can be seen with the help of ultrasound (ultrasound), plain radiography of the chest cavity. But ultrasound is a more reliable diagnostic method. It allows you to see the structure, size, shape of the thymus, the presence of pathological inclusions in it, the relationship of the organ to the structures surrounding it.

Also do an immunogram. Using this examination method, you can see the number of different fractions of lymphocytes and thus evaluate the function of the thymus gland.

thymus anatomy


Thymus involution is a rather complex anatomical process that requires special attention. After all, the thymus performs a very important function - it protects a person from foreign microorganisms. Fortunately, with timely elimination of the cause, this condition is reversible. Thyroid function can be restored. The main thing is to recognize the problem as early as possible in order to contact a specialist in time, who will prescribe an effective treatment.

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