General Principles of Classification
The process of blood cell maturation is under strict control. The main mechanism is humoral regulation. Regulatory molecules are divided into two large groups depending on their physiological effect:
- Stimulators — trigger proliferation and cell maturation.
- Inhibitors — suppress these processes to maintain homeostasis.
Depending on their spectrum of action, these factors can be general, affecting the entire hematopoietic process, or specific, targeting the development of only one specific blood cell lineage (e.g., erythroid or thrombocytic).
Hematopoietic Growth Factors: Stimulators
Humoral stimulators are collectively termed hematopoietins. Traditionally, they are divided into two categories, although this division is largely conventional in practice.
- Colony-Stimulating Factors (CSFs). This group includes Multi-CSF, erythropoietin (EPO), thrombopoietin, granulocyte-CSF (G-CSF), and thymopoietins. Their primary target is cells at early stages of development. The main task of CSFs is to set the vector for further differentiation of immature cells.
- Interleukins (ILs). This category includes substances from IL-1 to IL-7. They act on relatively differentiated cell elements, finely regulating their functional activity and proliferative capacity.
It is important to note that the boundaries between these groups are blurred, and their biological action spectra often overlap. For example, Multi-CSF can affect cells of completely different degrees of maturity—from the earliest stem cells to almost fully formed cells. Another striking example is interleukin-3 (IL-3), whose presence is critically necessary at all stages and directions of hematopoiesis without exception.
To achieve the required physiological effect, a combination of factors is almost always needed rather than a single isolated factor. Each hematopoietic lineage at a specific stage requires its own unique set of stimulators, while the same factor may participate in different combinations.
Sources of Hematopoietin Production
The synthesis of stimulating factors can occur either directly within the hematopoietic zone or at a considerable distance from it, operating on a hormonal principle.
- Local Production (Microenvironment). Most factors are produced directly within hematopoietic organs. Sources include stromal cells, tissue macrophages, vascular endothelium, and even mature blood cells themselves (providing autocrine and paracrine regulation).
- Distant Production. A number of substances are synthesized in other organs and delivered to the bone marrow systemically via the bloodstream. For instance, thrombopoietin is produced in the liver, and erythropoietin is produced predominantly in the kidneys, liver, and lungs (with only a small fraction formed locally in the bone marrow).
- Interorgan Regulation. Hematopoietic factors from one organ can distantly control another. For example, thymopoietins (endocrine products of the thymus) enter the red bone marrow, where they direct lymphopoiesis precursors strictly down the pathway of T-cell formation.
- Classical Hormones. Hematopoiesis is also influenced by thyroid hormones (thyroxine) and growth hormone (somatotropin) from the pituitary gland.
The staged mechanism of action of thymus hormones (thymosin) deserves special attention. It works in stages across different locations:
- In the red bone marrow — stimulates the migration of pro-T cells to the thymus.
- In the thymus — triggers active proliferation of T-lymphoblasts.
- In the peripheral lymphoid tissue — stimulates the proliferation of T-immunoblasts (occurring after antigen encounter).
Inhibitors of Hematopoiesis
To maintain strict homeostasis, the circulatory system requires not only constant stimulation but also timely inhibition. Inhibitors perform this function.
- Specific Inhibitors (Chalones). These substances are produced by mature blood cells. Their mechanism of action is based on the principle of negative feedback: the more mature formed elements accumulate in the bloodstream, the stronger the chalones they release suppress the production of new cells in the bone marrow.
- Relatively Nonspecific Inhibitors. This group includes tissue hormones (such as prostaglandins and interferons) and true hormones, notably glucocorticoids (produced by the adrenal cortex). Glucocorticoids play a key role in the body's stress responses: they can potently suppress lymphopoiesis, which morphologically manifests as a significant reduction in the total volume of lymphoid tissue.