HEMATOPOIETIC SYSTEM: FUNCTIONS, TISSUES, HISTOLOGY, ORGANS - BIOLOGY - 2025

The hematopoietic system is the set of organs and tissues in which the formed elements of the blood are formed, differentiated, recycled and destroyed. That is, it encompasses the sites where they originate, mature and exert their functional action.

The mononuclear phagocytic system is also considered part of the hematopoietic system, which is responsible for eliminating blood cells that are no longer functional, thus maintaining balance. In this sense, it can be said that the hematopoietic system is made up of blood, hematopoietic organs and tissues, and the reticulum endothelial system.

Blood circulation. Source: Pixabay.com

On the other hand, hematopoietic organs (formation and maturation of blood cells) are classified into primary and secondary organs. The primary organs are the bone marrow and the thymus, while the secondary organs are the lymph nodes and the spleen.

The formation of hematopoietic cells complies with a complex hierarchy system in which each cell type gives rise to a slightly more differentiated progeny, until reaching the mature cells that enter the bloodstream.

The failure of the hematopoietic system causes serious diseases that compromise the life of the patient.

Functions of the hematopoietic system

Hematopoietic tissue is the place where the formation and maturation of the formed elements of the blood takes place. This includes red blood cells and platelets as well as cells of the immune system. That is, it is in charge of carrying out erythropoiesis, granulopoiesis, lymphopoiesis, monocytopoiesis and megakarypoiesis.

Blood is one of the most dynamic tissues in the body. This tissue is constantly in motion and its cells need to be constantly renewed. The homeostasis of this blood system is in charge of the hematopoietic tissue.

It should be noted that each cell line fulfills various functions of great importance for life.

Erythrocytes or red blood cells

Human blood, erythrocytes or red blood cells and two white blood cells. Taken and edited from: Viascos.

Erythrocytes are the cells responsible for carrying oxygen to the various compartments of the human body. Erythrocytes measure 8 µ in diameter, but due to their great flexibility they can pass through the smallest capillaries.

White blood cells or leukocytes

White blood cells

White blood cells or leukocytes are the body's defense system; These are in permanent surveillance in the blood circulation and are increased in infectious processes to neutralize and eliminate the offending agent.

These cells secrete chemotactic substances to attract certain types of cells to a certain site according to need. This nonspecific cellular response is led by segmented neutrophils and monocytes.

They also secrete cytokines capable of activating nonspecific humoral defense elements such as the complement system, among others. Subsequently, the elements of the specific response are activated, such as T and B lymphocytes.

Platelets

Platelets for their part comply with the maintenance of the endothelium through the coagulation process, in which they actively participate. When there is an injury, platelets are attracted and aggregate in large numbers to form a plug and initiate the process of repairing the injured tissue.

At the end of the useful life of each cell, they are eliminated by the mononuclear phagocytic system, which is distributed throughout the body with specialized cells for this function.

Tissues of the hematopoietic system

Hematopoietic tissue has a complex structure organized in hierarchical levels, simulating a pyramid, in which mature cells of both the lymphoid and myeloid lineage participate, as well as some immature cells.

Hematopoietic tissues are divided into myeloid tissue and lymphoid tissue (generation, differentiation and maturation of cells), and the mononuclear phagocytic system (destruction or elimination of cells).

Myeloid tissue

It is made up of the bone marrow. This is distributed inside the bones, especially in the epiphysis of long bones and in short and flat bones. Specifically, it is located in the bones of the upper and lower extremities, bones of the skull, sternum, ribs and vertebrae.

Myeloid tissue is the place where the different types of cells that make up the blood form. That is, erythrocytes, monocytes, platelets, and granulocytic cells (neutrophils, eosinophils, and basophils).

Lymphoid tissue

It is divided into primary and secondary lymphoid tissue

The primary lymphoid tissue is made up of the bone marrow and the thymus: lymphopoiesis and maturation of B lymphocytes take place in the bone marrow, while T lymphocytes mature in the thymus.

The secondary lymphoid tissue is made up of the lymphoid nodes of the bone marrow, lymph nodes, spleen, and mucosa-associated lymphoid tissue (appendix, Peyer's patches, tonsils, adenoids).

In these places the lymphocytes come into contact with the antigens, being activated to carry out specific functions in the individual's immune system.

The monocuclear phagocytic system

The mononuclear phagocytic system, also called the reticulum endothelial system, helps in the homeostasis of the hematopoietic system, since it is in charge of eliminating cells that are no longer competent or that have reached their useful life.

It is made up of cells from the monocytic lineage, which includes the macrophages of tissues, which change their name according to the tissue in which they are found.

For example: histiocytes (macrophages of connective tissue), Kupffer cells (macrophages of the liver), Langerhans cells (macrophages of the skin), osteoclasts (macrophages of bone tissue), microglia cell (macrophage of the central nervous system), macrophages alveolar (lung), among others.

Histology of the hematopoietic system

The cells of the hematopoietic tissue comply with the following rule: the more immature the cell, the greater the capacity to renew themselves but the less power to differentiate. On the other hand, the more mature a cell is, the more it loses its ability to renew itself but its power to differentiate will increase.

Hematopoiectic stem cells (HCM)

They are multipotential cells that have the ability to self-renew over time, therefore guaranteeing their repopulation, thus remaining throughout life for the maintenance of blood homeostasis. They are found in a very small number (0.01%).

It is the most immature or undifferentiated cell found in the bone marrow. It is divided asymmetrically.

A small population divides to form 10 ¹¹ to 10 ¹² immature cells (multipotent hematopoietic progenitors) for the renewal of circulating cells and also for the maintenance of the population within the bone marrow. Another percentage remains undivided.

Multipotent hemtopoietic progenitors

These cells have a greater capacity for differentiation, but little power to self-renew. That is, they have lost some properties of their precursor (stem cell).

From this cell, myeloid or lymphoid progenitors will be formed, but not both. This means that once it is formed it will respond to growth factors to give rise to a progenitor of the myeloid lineage or a progenitor of the lymphoid lineage.

The progenitor cells of the myeloid lineage are the Megakaryocytic Erythroid Progenitor (PME) and the Granulocytic or Macrophage Colony Forming Unit (CFU-GM). While the progenitor cell of the lymphoid lineage is called the Common Lymphoid Progenitor (PCL).

But these multipotent hematopoietic cells that will give rise to the different lineages are morphologically indistinguishable cells from each other.

These cells, according to differentiation, will have the function of forming a specific lineage of cells, but they do not maintain their own population.

Myeloid progenitors

These cells have a high capacity for differentiation.

The Megakaryocytic-Erythroid Progenitor (PME) will give rise to the precursor cells of platelets and erythrocytes, and the Granulocytic or Macrophage Colony Forming Unit (CFU-GM) will give rise to the various precursor cells of the granulocytic series and of the monocytes.

The cells that come from the Megakaryocytic Erythroid Progenitor (PME) are given the following names: Megakaryocytic Colony Forming Unit (CFU-Meg) and Burst Erythroid Forming Unit (BFU-E).

Those that come from the Granulocytic or Macrophagic Colony Forming Unit (CFU-GM) are called: Granulocytic Colony Forming Unit (CFU-G) and Macrophagic Colony Forming Unit (CFU-M).

Lymphoid progenitors

The Common Lymphoid Progenitor (PCL) has a high capacity to differentiate and produce precursors of T lymphocytes, B lymphocytes and NK lymphocytes. These precursors are called Pro-T lymphocyte (Pro-T), Pro-B lymphocyte (Pro-B), and Pro natural cytotoxic lymphocyte (Pro-NK).

Mature cells

They are comprised of platelets, erythrocytes, the granulocytic series (segmented neutrophils, segmented eosinophils, and segmented basolyphs), monocytes, T lymphocytes, B lymphocytes, and cytotoxic lymphocytes.

These are the cells that pass into the bloodstream, which are easily recognized according to their morphological characteristics.

Hematopoietic organs

-Primary organs

Bone marrow

It consists of a red (hematopoietic) and a yellow (fatty tissue) compartment. The red compartment is larger in newborns and decreases with age, being replaced by fatty tissue. Usually in the epiphysis of long bones is the hematopoietic compartment and in the diaphysis is the fat compartment.

Scam

The thymus is an organ that is located in the anterior superior mediastinum. It is structurally made up of two lobes, in which two areas called the medulla and cortex are distinguished. The medulla is located towards the center of the lobe and the cortex towards the periphery.

Here the lymphocytes acquire a series of receptors that complete the process of differentiation and maturation.

-Secondary organs

Lymph nodes

Lymph nodes play a fundamental role at the level of the immune system, since they are in charge of filtering infectious agents that enter the body.

It is there that the antigens of the foreign agent will come into contact with the cells of the immune system, and then trigger an effective immune response. The lymph nodes are strategically distributed throughout the body near the large lymphatic capillaries.

Four well-defined zones are distinguished: capsule, para-cortex, cortex and central medullary area.

The capsule is made up of connective tissue, has several entrances of lymphatic afferent vessels and a cleft called the hilum. At this site blood vessels enter and exit and efferent lymphatic vessels exit.

The para-cortex zone is rich in certain cell types such as T lymphocytes, dendritic cells, and macrophages.

The cortex contains two main areas called primary and secondary lymphoid follicles. The primary ones are rich in naive and memory B cells, and the secondary ones contain a germ zone composed of activated B lymphocytes (plasma cells) surrounded by a zone of inactive lymphocytes.

Finally, the central medullary area contains the medullary cords and the medullary sinuses through which the lymphatic fluid circulates. Macrophages, plasma cells and mature lymphocytes are found in the medullary cords, which after passing through the lymph are going to be incorporated into the bloodstream.

Spleen

It is located near the diaphragm in the left upper quadrant. It has several compartments; Among them we can distinguish the capsule of connective tissue that is internalized through trabecular septa, the red pulp and the white pulp.

In the red pulp, the elimination of damaged or non-functional erythrocytes occurs. Erythrocytes pass through the splenic sinusoids and then pass into a filter system called Billroth's cords. Functional red cells can pass through these cords, but old ones are retained.

The white pulp is made up of nodules of lymphoid tissue. These nodules are distributed throughout the spleen, surrounding a central arteriole. Around the arteriole are T lymphocytes and more externally there is an area rich in B lymphocytes and plasma cells.

Microenvironment

The microenvironment is made up of hematopoietic cells and the hematopoietic stem cell from which all the cell series in the blood come.

In the hematopoietic microenvironment, a series of interactions take place between a variety of cells, including stromal, mesenchymal, endothelial cells, adipocytes, osteocytes and macrophages.

These cells also interact with the extracellular matrix. The various cell-to-cell interactions aid in the maintenance of hematopoiesis. Substances that regulate cell growth and differentiation are also secreted in the microenvironment.

Diseases

-Hematological cancer

There are 2 types: acute or chronic myeloid leukemias and acute or chronic lymphoid leukemias.

-Medullary aplasia

It is the inability of the bone marrow to produce the different cell lines. It can occur for several reasons, including: chemotherapy treatments for solid tumors, constant exposure to toxic agents, generally of the occupational type, and exposure to ionizing radiation.

This disorder causes severe pancytopenia (a significant decrease in the number of red blood cells, white blood cells, and platelets).

-Genetic diseases of the hematopoietic system

These include hereditary anemias and immunodeficiencies.

Anemias can be:

Fanconi anemia

Hematopoietic stem cells are compromised in this disease. It is a rare hereditary recessive disease and there is a variant linked to the X chromosome.

The disease brings congenital consequences such as polydactyly, brown spots on the skin, among other malformations. They present anemia manifested from the first years of life due to bone marrow failure.

These patients have a great genetic disposition to suffer from cancer, specifically acute myeloid leukemia and squamous carcinoma.

Severe combined immunodeficiencies

They are rare, congenital diseases that produce a severe primary immunodeficiency. Patients with this anomaly need to live in a sterile environment, since they are unable to interact with the most harmless microorganisms, which is a very difficult task; for this reason they are known as “bubble kids”.

One of these diseases is called a deficiency in DNA-PKcs.

Deficiency in DNA-dependent protein kinase (DNA-PKcs)

This disease is very rare and is characterized by the absence of T and B cells. It is only reported in 2 cases.

References

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