What is the bone marrow responsible for?

Bone marrow is a spongy soft tissue found inside most bones in the human skeleton. Sometimes it is confused with the dorsal, but these tissues have nothing in common with each other. The spinal cord is located in the spine and performs conductive and reflex functions. The first is the transmission of nerve impulses to the brain and back, and the second, as you might guess from its name, is the organization of reflexes. But what features are inherent in the bone marrow, what it is responsible for and why it is so important for a person, now we will figure it out.

What is bone marrow

The bone marrow is the most important organ for the hematopoietic system, because its main function is the implementation of hematopoiesis, or hematopoiesis. He is directly involved in the creation of new blood cells to replace those that died, died. In addition, the only adult tissue that contains immature cells, also known as stem cells, is the bone marrow.

Bone marrow is of two types: yellow, which is represented mainly by fat, and red, which is the main organ of hematopoiesis. Unlike red, yellow bone marrow is not involved in hematopoiesis.

During hematopoiesis, blood cells are produced. Hematopoiesis starts in the early embryonic period. Accordingly, there are both embryonic hematopoietic organs and those that function after birth. Organs that are responsible for hematopoiesis during the embryonic period include the yolk sac, fetal liver, spleen, and bone marrow. The first hematopoietic stem cells appear in the yolk sac. This happens at the 3rd week of embryogenesis. Shortly after, from the 3rd month until birth, the liver becomes the main hematopoietic organ of the fetus, as some of the stem cells move there. From the 4th month of embryogenesis, the formation of blood cells and in the bone marrow begins. In addition, the thymus, lymph nodes and spleen are involved in hematopoiesis in the fetus. In the liver and spleen, hematopoietic stem cells are stored in a “dormant” state, which often explains the fact that hematopoietic foci appear outside the bone marrow. Such hematopoiesis is called extramedullary. It occurs with oncological diseases of the blood and as a result of excessive stimulation of hematopoiesis.

The volume of the medullary cavities in a newly born child is about 1,6 liters, of which the red bone marrow occupies almost 100% of the space. When a person grows up, hematopoiesis is centralized, while hematopoietically active tissue is preserved in the bones of the central part of the skeleton. The total volume of bone marrow in adults reaches approximately 4 liters.

The location of hematopoietic tissue in an adult is as follows: in the pelvic bones it is the most – 40%, in the vertebral bodies it is much less – 28%, in the bones of the skull it is 13%, in the epiphyses of tubular bones and ribs – 8%, in the sternum the least – only 2%. The remaining part of the bone marrow cavities is occupied by yellow bone marrow, which, as you remember, is adipose tissue. At the same time, red and yellow bone marrow are in an equal ratio: 1:1.

Structurally, red bone marrow is divided into: extravascular (actually, hematopoietic tissue) and vascular, which consists of wide venous vessels called sinuses. In the network of reticulin fibers inside the bone trabeculae there is a jelly-like dispersed material, which is the hematopoietic tissue.

The blood supply to the bone marrow is called perfusion. It is carried out by the main feeding artery and its small terminal arterioles. The outflow of blood occurs in this way: blood is collected through the venous capillaries into the central venous sinus through the venous vessels. The walls of venous vessels are composed of the following three layers of cells: adventitia, basement membrane, and endothelium. It is in the reticulum – a thin network of connective tissue fibers formed by processes of adventitial cells, that hematopoietic cells are located. The volume of the hematopoietic space is affected by changes in the adventitial cells: the number of hematopoietic cells decreases when the adventitial cells increase due to their increased fat content. When viewed under a microscope, this picture looks like трансформация red bone marrow to yellow.

At the moment when the requirements for hematopoiesis increase, adventitial cells decrease, thereby contributing to an increase in the hematopoietic component of the bone marrow.

What is red bone marrow responsible for?

The main function that the red bone marrow performs is the function of hematopoiesis or hematopoiesis. It is carried out constantly and extremely intensively – more than 300 million blood cells per minute are formed in the hematopoietic organs. The function of hematopoiesis is unique in that at the right time and in the right place, a huge, but at the same time, the optimal amount of blood cells of the required type is produced. The bone marrow can speed up the production of any type of blood cell by 5-6 times if the body needs more of them. All blood cells develop from a single parent cell. It has the morphology of a small lymphocyte and is called a multipotent hematopoietic stem cell (HSC). Its descendants are all peripheral blood cells. In the process of division and differentiation of multipotent HSC, the entire hematopoietic tissue is formed. It combines both progenitor cells and maturing and mature blood cells: erythrocytes, platelets and leukocytes, which make up human peripheral blood.

Hematopoiesis combines two large divisions of hematopoiesis: lymphopoiesis and myelopoiesis.

1. Myelopoiesis (or myeloid hematopoiesis). Normally, the only place where this process takes place after birth is the red bone marrow. All blood cells are formed there, except for lymphocytes (platelets, erythrocytes, monocytes and granulocytes), in order to then enter the peripheral blood.
2. Lymphopoiesis (formation of T-lymphocytes and B-lymphocytes). After birth, it is realized in the central and peripheral lymphoid organs. The former include the red bone marrow, as well as the thymus (which retains the functions of the lymphoid organ only until the puberty of the organism); to the second – lymph nodes, spleen and Peyer’s patches of the gastrointestinal tract.

Erythrocytes, also called “red blood cells,” are cells that do not have a nucleus and have a biconcave disc shape. It is maintained in erythrocytes by spectrin (a stabilizing membrane protein). The size of an erythrocyte normally ranges between 7,5 microns and 8,3 microns, and life expectancy is 90-120 days. The well-known main blood groups (I, II, III, IV) are isolated on the basis of the antigenic properties of erythrocytes. The function of erythrocytes is extremely important – they transport respiratory gases. The erythrocyte cytoplasm is 96% filled with hemoglobin. It is a chromoprotein consisting of two parts: globin and heme. The first is protein, and the second is non-protein and is a complex of protoporphyrin IX and iron. Oxygen from the alveoli of the lungs is transported to the cells of the whole organism precisely thanks to hemoglobin, and vice versa, from the cells to the alveoli – with the help of carbon dioxide. Normally, each hemoglobin molecule contains two pairs of identical protein chains. They are denoted by the letters α and β from the Greek alphabet. Depending on the composition of these chains, there are three types of hemoglobin: embryonic, fetal and adult hemoglobin.

In peripheral blood, in addition to mature erythrocytes, young erythrocytes, reticulocytes, can be found. These are cells without a nucleus, but contain a large amount of RNA and ribosomes that have membrane receptors for transferrin. Reticulocyte RNA continues to produce hemoglobin. At this stage, the production of hemoglobin up to 30% of the total amount in the erythrocyte is possible. Most of them are synthesized at the prereticulocyte stages of cell differentiation – 70-80% of hemoglobin. When a reticulocyte becomes a mature red blood cell, it can no longer produce hemoglobin as it loses RNA. In the bone marrow, an erythrocyte remains at the reticulocyte stage for one day, and then another day in the peripheral blood.

Leukocytes, in turn called white blood cells, are a heterogeneous (heterogeneous) group of peripheral blood cells that contain a nucleus. They perform the function of immunity and differ in several ways. According to the shape of the nucleus – segmented or rounded, according to the color and nature of the cytoplasm, as well as according to the granularity – its presence or absence.

If specific granularity is absent, leukocytes are called agranulocytes, and if present, granulocytes. The former include lymphocytes and monocytes.

Granulocytes, on the other hand, differ in the nature of the specific granularity and are of three types:

1. Neutrophils (contain copious fine, pale purple (“neutrophil”) granules).
2. Basophils (with large and not abundant dark purple (“basophilic”) granularity).
3. Eosinophils (differ in granularity, occupying the entire cytoplasm of the cell).

Thank you lleukocytes in the body realize a protective function – immunity, which can be specific and nonspecific.

One of the manifestations of nonspecific immunity involves neutrophils, monocytes and tissue macrophages (monocytes turn into them after they leave the bloodstream). They phagocytize (capture) with subsequent lysis (dissolution) of microbes, toxins and cellular detritus (in other words, garbage). Eosinophils provide protection against parasites and are involved in allergic reactions (as are basophils).

Lymphocytes carry out reactions of specific immunity, whether innate or acquired. Specific immunity, in turn, is humoral and cellular. Humoral immunity is realized due to the synthesis of immunoglobulins of classes A, M, G, E, D by B-lymphocytes; and cellular – with the help of the diverse functions of T-lymphocytes. Acquired immunity can be formed naturally, due to various infectious diseases, or as a result of immunization of the body.

The size of leukocytes ranges from 6 microns (small lymphocytes) to 14 microns (monocytes).

Leukocytes differ among themselves not only in appearance and function, but also in life expectancy. For example, the life span of lymphocytes varies between a few hours and tens of years. Monocytes circulate in the blood for 72 hours, and then go to the tissues, where they turn into fixed or migrating macrophages. Neutrophils are in the blood for 4-10 hours, followed by release into the tissues.

Platelets are the third formed element of blood. They are equated with true cells, although they are not. In fact, these are particles of the detached cytoplasm of bone marrow megakaryocytes, the so-called platelets. Platelets are characterized by the properties of aggregation (gluing) and adhesion (sticking). Their participation in the mechanisms of blood coagulation and fibrinolysis is determined by the presence of special biologically active substances. They also help maintain normal microvascular resistance and function (angiotrophic function). The size of platelets is 1-2 microns, and life expectancy is 8 days.

What is yellow bone marrow responsible for?

Yellow bone marrow is usually located in the diaphysis of tubular bones. It consists of reticular tissue and adipocyte cells, which contain a special pigment-lipochrome in the center of the cavity of long bones, and outside it is surrounded by a layer of red bone marrow. Fat from adipocytes, in case of emergency, for example, after a long fast, the body can use as an energy source. Under normal conditions, yellow bone marrow does not participate in hematopoiesis, but in exceptional cases, such as after severe blood loss or acute anemia, part of the yellow bone marrow may turn red to speed up blood recovery.

Main functions of the bone marrow

The first and main task of the bone marrow is the production of blood elements, or hematopoiesis. Therefore, disturbances in the process of hematopoiesis are directly related to problems in the functioning of the bone marrow. If it does not work properly, a person may feel worse for no apparent reason.

Insufficient bone marrow activity can cause conditions such as:

• weakness and fatigue (due to a lack of hemoglobin, which is responsible for transporting oxygen);
• fever (due to insufficient white blood cells);
• susceptibility to infectious diseases (due to a decrease in white blood cells, which are necessary to fight infections);
• uneven breathing (due to a lack of red blood cells and the oxygen starvation caused by this);
• bruising or bleeding (due to a lack of platelet mass).

In addition, if we remember that it is thanks to the blood flow that all tissues and organs receive oxygen and nutrients, it becomes clear that absolutely every cell of the human body depends on the bone marrow.

Also, the bone marrow is the core element of the lymphatic system. All lymphocytes originate in this tissue. And if we take into account that the immune system directly depends on the efficiency of the lymphatic system, then it turns out that without the bone marrow there would be no immunity. Most blood antibodies that protect the body from pathogens are synthesized in the bone marrow.

Diseases of the bone marrow

Regardless of what caused tissue damage, especially its red part – cancer or other factors – it always poses a threat to human health and life.

Myeloproliferative Disorders

Disorders with such a complex name arise if stem cells multiply incorrectly. There are several types of such diseases:

1. Primary myelofibrosis. It is characterized by the formation of predominantly megakaryocytes and granulocytes in the bone marrow. At the end of the disease, the connective tissue grows, and hematopoiesis develops outside the bone marrow.
2. Polycythemia. This is a disease characterized by an absolute increase in the number of red blood cells in the peripheral blood, an increase in the total volume of circulating blood, often leukocytosis, hyperthrombocytosis, an enlarged spleen, and frequent thrombosis of the cerebral and coronary vessels.
3. Essential thrombocythemia. Chronic myeloproliferative disease, manifested by hyperthrombocytosis (platelets above 450 × 109 / l) in combination with megakaryocytic hyperplasia of the bone marrow, in the absence of erythrocytosis, neutrophilic leukocytosis.

Aplastic anemia

Aplastic anemia is a disease due to which the hematopoietic function of the bone marrow is inhibited. In other words, it loses the ability to produce the number of blood cells necessary for the body. This happens due to damage to stem cells, which lose their ability to grow and develop into new blood cells.

Aplastic anemia can be acquired or congenital. In the first case, stem cells can lose their ability to turn into blood cells due to exposure to toxins, radiation, or after severe infectious diseases. In addition, this disease can sometimes appear as a complication of certain autoimmune disorders, such as lupus or rheumatoid arthritis.

Leukemia

Leukemia, or “leukemia” is a type of cancer in which a large number of white blood cells – leukocytes – appear in the peripheral blood. These diseases are united under the general name – hemoblastoses.

There are such principles for the division of hemoblastoses:

1. Depending on belonging to the department of hematopoiesis: myeloproliferative neoplasms are classified as myelopoiesis, and lymphoproliferative neoplasms are classified as lymphopoiesis;
2. Depending on the place of primary localization of tumor growth: leukemia (bone marrow) and hematosarcoma (outside the bone marrow);
3. Depending on the aggressiveness of the course, which correlates with the nature of the morphological substrate of the tumor: from immature cells (precursor cells, blasts) or from maturing and mature cells.

It’s hard for scientists to say exactly what causes leukemia. But it is generally accepted that radiation, exposure to certain chemicals, and a number of genetic diseases increase the risk of oncological processes in the bone marrow.

Bone marrow transplantation: how, to whom and why

Bone marrow damage is life-threatening. Fortunately, red bone marrow can be restored through a transplant. Bone marrow transplantation (BMT) is a treatment method in which bone marrow prepared in advance is injected into the patient. This procedure is practically the only salvation for such complex, deadly and previously incurable diseases as lymphoma, leukemia, a complex form of anemia, malignant tumors of various origins, autoimmune pathologies.

A bone marrow transplant is, in fact, the introduction of the necessary stem cells into the patient’s body. Stem cells are found in the embryo, bone marrow, human peripheral blood, and umbilical cord blood. All of the above options, except for the first one, can serve as a source for transplantation. During the transplant procedure, the harvested stem cells are injected intravenously into the patient. After entering the bloodstream, the donor hematopoietic stem particles move to the patient’s bone marrow, where, in the case of a successful procedure, they begin to produce erythrocytes, platelets and leukocytes.

It usually takes about 2-4 weeks for the transplanted material to take root. A transplant helps the diseased body restore its ability to independently produce the necessary blood elements. This procedure is used to treat both oncological and other types of diseases.

Consider the types of bone marrow transplantation. Exists:

1. Autologous – when the patient is injected with his own stem cells. Usually, they are removed earlier in the course of treatment, with intact bone marrow, from cord or peripheral blood, and stored frozen until the time of transplantation, when they are thawed and injected into the patient. This type of transplant is used after some types of cancer have been treated with high doses of chemotherapy, after which the bone marrow is destroyed.
2. Syngeneic – when the donor of stem cells is a person with the same set of genes – the identical twin of the patient.
3. Allogeneic – when stem cells are obtained from the patient’s brothers or sisters, or a person with whom the patient is not related can act as a donor, but the transplanted material is genetically as close as possible to the patient’s own cells. This type of transplantation is used in case of leukemia, severe aplastic anemia, severe complex immunodeficiency.
4. Haploidentical – when stem cells are transplanted from an incompletely compatible donor. As a rule, it becomes the mother or father of the patient.
5. Cord – when stem cells obtained from cord blood are transplanted. In this case, the material is frozen and stored until needed. At the moment, there are many cord blood banks in developed countries. The advantage of this type of transplant is that the cells from this source are always very immature, which means that there is no need to select compatibility. In other words, they suit everyone.

Possible Risks of a Bone Marrow Transplant

Despite the seemingly simple procedure, in fact, bone marrow transplantation is not without serious risks. After the introduction of donor material, the patient may experience a reaction, which is called “graft-versus-host” (GVHD). This is considered one of the most dangerous and most frequent complications after an allogeneic transplant. The essence of the reaction is that the donor’s bone marrow perceives the recipient’s body as an enemy and begins to work against it. Graft-versus-host disease occurs in almost 40% of stem cell transplants. This confrontation can even lead to the death of the recipient. It is believed that the risk of a THD reaction increases if the patient is older than 30 years. For a long time, doctors did not undertake to transplant bone marrow to people over the age of 50, the risk of death after the procedure is extremely high. Nowadays, doctors treat the age limit with less apprehension.

In addition to the THD reaction, ophthalmic, endocrine, pulmonary, neurological, musculoskeletal, immune, infectious diseases, heart failure, progression of oncological disease can develop as a complication for transplantation.

Red blood cells, white blood cells, platelets, and components of the lymphatic system are all produced by the bone marrow. He is rarely remembered and, as a rule, only in cases of serious illness. The bone marrow cannot be seen or touched, and if something is wrong with it, it does not hurt. However, it is one of the most important tissues in the body and malfunctions in its work in many cases end in death.

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