The 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 that of an adult. Its decrease over time is called the involution of the thymus. More on 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 (respiratory tube). It consists of two lobes connected by an isthmus. The body reaches a maximum mass of 30-40 grams when puberty occurs, 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 the normal immune response) and in the production of thymosin and thymopoietin, which, in turn, stimulate the formation of antibodies.
The role of thymus in the body of a child
The thymus performs its main function during the intrauterine development of the baby and after its birth at the age of 3 years. It was at this time that he actively synthesized T-lymphocytes. This is necessary to protect the baby from infections, since the children's body is most susceptible to the influence of pathogenic microorganisms.
Thymus produces the hormone thymosin, which is necessary for the normal formation of lymphocytes. With a decrease in the function of the thymus gland, the body's resistance to infection decreases. The child is prone to frequent respiratory illnesses that can easily become chronic.
With prolonged dysfunction of the thymus, 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 the normal state of immunity do not lead to the development of the disease. They are also called opportunists.
The main types of involution
Thymus size reduction can be of two types:
In both cases, the process of involution of the thymus consists in the gradual replacement of its tissue with fat structures. This process is characteristic only of the thymus gland. Neither in the bone marrow, nor in the spleen, such changes occur.
Age changes
Age-related involution of the thymus is considered the norm. It begins after puberty. Its main manifestations are presented below:
- organ mass reduction;
- decreased function, i.e. inhibition of T-lymphocyte production;
- replacing normal organ tissue with fat.
On micropreparations of pathological anatomy, it is seen that the tissue of the thymus during involution loses clear boundaries between the cortical and medulla. There is a gradual thickening of the partitions that separate the lobules from each other. Gassalle's corpuscles (epithelial cells in the thymus medulla) acquire large sizes, their number increases.
After puberty, almost the entire mass of the thymus is replaced by adipose tissue. Only certain islets 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.
Features of accinental changes
As already noted earlier in the article, age-related and incidental involution of the thymus are two main varieties of reducing the size of this organ. This section will discuss in more detail the second type of change.
The main difference between age-related changes in the thymus gland 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 "chance." And indeed, in essence, incidental involution is a random response of the thymus gland to a harmful factor that has affected it.
Causes of pathology
The reasons why the involution of the thymus begins is not fully understood. However, doctors identify a number of risk factors that increase the likelihood of these changes. These include:
- radiation exposure;
- taking anticancer drugs;
- hormonal medication;
- oncological diseases, primarily hemoblastoses (malignant neoplasms of the bone marrow);
- infectious inflammatory diseases.
There is also research on the importance of conditions such as hypothermia and hypoxia (a decrease in the concentration of oxygen in the tissues of the body) in the development of the pathology of the thymus. However, their meaning is not precisely clarified.
Milestones: first, second and third
When studying the pathanatomy of the incidental involution of the thymus, certain stages in the changes in the gland should be distinguished. Conventionally, five such stages or phases are distinguished.
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 healthy child.
In the second phase, a partial decrease in lymphocytes occurs, which are localized in the cortical (outer) layer of the gland. Moreover, they are destroyed randomly or "nestly". Macrophages adhere to these lymphocytes and “swallow” them. In the medical literature, this process is called phagocytosis. Part of the lymphocytes decreases due to their leakage into the general bloodstream.
In the third phase, the process progresses, and the collapse of the reticular thymus gland develops. There are more lymphocytes in the cerebral layer than in the cortical. As a result of this, when examining a micropreparation of the incidental involution of the thymus under a microscope, the brain substance looks darker, although normally it should be vice versa.
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 incidental involution, the cortical part also begins to colonize.
Milestones: fourth and fifth
In the fourth phase, the condition worsens even more. There is a decrease in lymphocytes from the brain substance, so it becomes extremely difficult to distinguish the cortical region from the brain. Timic bodies are combined with each other, which on a micropreparation looks like large cystic formations. These structures are filled with protein secretion with inclusions in the form of flakes. Over time, this content leaves cystic formations through the lymphatic capillaries.
In the fifth (or terminal) phase, atrophy and sclerosis of the organ develops. This means that the thymus is significantly reduced in size, the septum from the connective tissue is thickened. There are very few lymphocytes, over time, almost the entire organ is replaced by connective tissue. Calcium salts are deposited in thymic bodies, which is called calcification or petrification.
Thus, with incidental involution in the thymus, the following processes occur:
- a sharp decrease in organ size;
- a significant drop in the functional activity of the thymus;
- a decrease in the number of lymphocytes up to their complete absence;
- replacement of the thymus gland 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 decrease in its functional activity. With age-related changes, no symptoms develop, since this, in fact, is the norm for a person. And with incidental involution, when the decrease in the function of the thymus gland arises sharply and manifests itself to a large extent, a certain clinical symptomatology develops.
The general symptoms that develop regardless of the causes of the pathology include the following:
- general tiredness, 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;
- the heaviness of the eyelids, the feeling that someone is pressing on them.
Also, a person is characterized by the presence of clinical manifestations that correspond to a specific cause of thymic involution. For example, oncological diseases are characterized by the development of anemic syndrome, pallor or jaundice of the skin, decreased appetite, weight loss. With inflammatory diseases of the patient, an increase in body temperature, chills, and a deterioration in the general condition are disturbing.
Diagnosis of the disease
The diagnosis begins with a detailed survey of the patient about his complaints, anamnesis of life and disease. Thymus involution is not a definitive diagnosis. This is only one of the clinical manifestations of many pathological conditions. Therefore, the main task in the diagnosis of this process is to find its cause.
The involution itself can be seen using ultrasound (ultrasound), a panoramic 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 ratio of the organ to the structures surrounding it.
They also make 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.
Conclusion
Thymus involution is a rather complex anatomical process that requires special attention. After all, the thymus has a very important function - it protects a person from foreign microorganisms. Fortunately, with the timely elimination of the cause, this condition is reversible. Thyroid function may recover. 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.