Early developmental stages: promyelocytes and myelocytes
The process of granulocyte maturation originates from class IV cells — myeloblasts, which transform into maturing forms (class V cells).
The first morphologically recognizable cell along this pathway is the promyelocyte. It is characterized by a round nucleus and a basic, blue-staining (basophilic) cytoplasm. The key event of this stage is the appearance of non-specific azurophilic granulation (primary granules) in the cytoplasm. Specificity is absent at this stage, making it visually impossible to determine whether the cell will become a neutrophil, eosinophil, or basophil. Promyelocytes divide actively.
The next stage is the myelocyte stage. The nucleus still remains round, but secondary (specific) granules begin to form in the cytoplasm. It is at this point that a clear separation of cells into three parallel differentiation lineages occurs:
- Neutrophilic (synthesize and accumulate antibacterial substances).
- Eosinophilic (contain major basic protein, histaminase, and oxidases).
- Basophilic (accumulate histamine and heparin).
Myelocytes represent the predominant pool of cells of the granulocytic series in the red bone marrow. It is important to remember that this is the final stage capable of mitotic division.
Postmitotic maturation of granulocytes
Upon completion of the myelocyte stage, cells irreversibly lose their ability to divide and transition to postmitotic maturation. This group includes a strict sequence of cell forms: metamyelocytes, band cells, and finally, segmented granulocytes.
A common property of this entire group is the absence of mitotic activity and the gradual acquisition of an irregular nuclear shape. Mature forms and their immediate precursors (metamyelocytes) are capable of entering the peripheral blood.
Dynamics of nuclear shape changes
As the cell progresses along the maturation pathway from a metamyelocyte to a fully mature form, its nucleus undergoes a series of regular transformations:
- Metamyelocyte: the nucleus begins to be compressed from one side, adopting a characteristic kidney-shaped (indented) form.
- Band granulocyte: the nucleus elongates, turning into a thick curved band (sometimes resembling a horseshoe), although constrictions are still absent at this stage.
- Segmented granulocyte: the nucleus finally divides into several lobes (segments) connected by thin strands. An exception is basophils — their nucleus practically does not segment, remaining only weakly lobulated.
Monocyte development (Monocytopoiesis)
Parallel to granulocytopoiesis is the process of agranulocyte — monocyte formation. Their early precursor is the monoblast (class IV), which differentiates into the promonocyte (class V).
Unlike granulocytes, promonocytes are represented by only a single cell type without subdivision into subtypes. Morphologically, they are distinguished by a large round nucleus and the absence of granules. Upon maturation of a promonocyte into a mature monocyte, its nucleus becomes kidney-shaped, and small granulation, representing lysosomes, appears in the cytoplasm.
Other hematopoietic lineages
In addition to leukocytes, elements of other hematopoietic lineages mature from class IV cells:
- Erythroid series: the basophilic erythroblast sequentially transforms into a polychromatophilic, and then an oxyphilic one. After this, a reticulocyte and finally a mature erythrocyte (class VI) are formed.
- Thrombocytic series: the promegakaryocyte matures into a giant megakaryocyte, from which platelets are subsequently pinched off.
Briefly: answers to topical questions
What specific enzymes and marker substances are contained in primary azurophilic granules?
Primary azurophilic granules contain the following specific enzymes and marker substances:
- Myeloperoxidase—a marker enzyme of azurophilic granules.
- Membrane molecule CD63 — is also a marker of these granules.
- Enzymes — α-fucosidase, 5'-nucleotidase, β-galactosidase, arylsulfatase, α-mannosidase, N-acetylglucosaminidase, β-glucuronidase, acid glycerophosphatase, lysozyme (muramidase), acid hydrolases.
- Neutral proteases (serprocidins) — cathepsin G, elastase, collagenase, azurocidin.
- Antimicrobial peptides and other substances — defensins, cathelicidins, lactoferrin, granulophysin, acidic glycosaminoglycans.
Which hematopoietic growth factors and cytokines stimulate granulocytopoiesis?
Granulocytopoiesis is stimulated by the following colony-stimulating factors and cytokines:
- G-CSF — plays a major role in regulating neutrophil development and is used to stimulate granulocytopoiesis.
- GM-CSF — performs an auxiliary role in neutrophil development and is also used to stimulate granulocytopoiesis.
- IL-3 and IL-6 — play an auxiliary role in regulating neutrophil development.
- IL-17 and IL-23 — regulate the increase in neutrophil counts during inflammation.
- Leukopoietin — a specific growth factor affecting the differentiation of myelopoiesis progenitors.
Which hematopoietic cell classes (classes I-III) precede the myeloblast in the hematopoiesis scheme?
The myeloblast, belonging to class IV (Blasts), is preceded by the following cell classes:
- Class I (Hematopoietic stem cell, HSC) — the pluripotent foundation of hematopoiesis.
- Class II (Myelopoiesis progenitor cell) — a hemistem, multipotent cell (CFU-GEMM).
- Class III (Uni- and bipotent progenitor cells, CFU) — committed cells that restrict their potency to specific lineages. For the granulocytic lineage, this is CFU-GM, which gives rise to CFU-G (neutrophilic), as well as CFU-GEM, which gives rise to CFU-B (basophilic) and CFU-Eo (eosinophilic).
Through what membrane structures of the megakaryocyte cytoplasm does the detachment of platelets occur?
The detachment of platelets from the megakaryocyte cytoplasm occurs through the formation of the demarcation membrane system. This system forms cytoplasmic processes (protrusions) of the megakaryocyte that are pushed into the lumen of red bone marrow capillaries. The function of the demarcation membrane system is to divide the cytoplasm into separate fragments, which are then detached ("pinched off") and become independent platelets (thrombocytes). The remnants of this system form the open canalicular system.
To memorize the stages of granulocyte development, use the mnemonic: PROmyelocyte → MYELocyte → METAMYELocyte → BAND → SEGMENTED.
Class and exam questions
At what stage do specific granules appear in granulocytes?
Secondary (specific) granules first appear at the myelocyte stage. It is from this moment that cells can be divided into neutrophilic, eosinophilic, and basophilic lineages.
Which cells of the granulocytic series are capable of division?
The ability for mitosis is retained only by promyelocytes and myelocytes. Starting from the metamyelocyte stage, cells cease dividing and only undergo maturation.
What are "band" forms in a blood test?
The term "juvenile" is used in clinical practice to denote metamyelocytes that occasionally enter the peripheral blood.
What else to study on this topic
- Composition of specific granules of neutrophils, eosinophils, and basophils
- Structural features of megakaryocytes and the process of platelet formation
- Similarities of predominant cell pools (myelocytes and polychromatophilic erythroblasts)
- Morphological characteristics of erythroid lineage cells
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