MEGAKARYOBLAST: CHARACTERISTICS, FUNCTION, HISTOLOGY - BIOLOGY - 2025

The megakaryoblast is the most immature cell belonging to the megakaryocytic lineage, that is, it is the first recognizable precursor in the formation of platelets. Thanks to its characteristics, it can be identified in bone marrow samples. However, sometimes it is necessary to use cytochemical techniques for its identification. This cell reacts positively to alpha-naphthyl acetate esterase.

Another way to identify it is by using specific antibodies against the cell's own markers, such as platelet glycoproteins (IIb / IIIa or Ib) and the membrane receptors CD41a, CD42b, and CD61.

Differences between megakaryoblast and megakaryocyte. Source: Jmarchn / Original raster: User: A. Rad.SVG file drawn with FlashMX by birdy.Cropped by: User: Timothy Gu

On the other hand, from a histological point of view the megakaryoblast is a small cell compared to other blasts. As the cell differentiates, it increases considerably in size, until it reaches the mature megakaryocyte or metamegakaryocyte. This is the largest cell in the bone marrow.

The increase in size occurs because the cell changes the mitotic process for the endomitotic. This process makes it possible to significantly increase the cytoplasmic mass and the number of chromosomes. This is how the mature metamegakaryocyte can then fragment its cytoplasm to give rise to thousands of platelets.

Disorders or defects in the megakaryocytic lineage can lead to serious diseases such as acute megaloblastic leukemia.

There are also other disorders that can affect several cell lines at the same time where platelets are affected, such as myelodysplastic syndrome and myeloproliferative disorders.

characteristics

Location

The megakaryoblast is an immature cell, and therefore under normal conditions it is only observed in bone marrow.

Origin

The megakaryoblast comes from the megacatriocytic colony-forming unit (CFU-MK). This cell, when stimulated by thromocytopoietin, becomes a megakaryoblast.

Later, by the action of the same cytokine, the cell differentiates into its successor, the promegakaryocyte; and subsequently in megakaryocyte.

The maturation process continues until the metamegakaryocyte stage, which will fragment the cytoplasm to form platelets.

The process of differentiation and maturation of the megakaryoblast until the formation of platelets can take up to 5 days to carry out.

ID

Cytochemical tests are a good option to differentiate blasts.

In the case of megakaryoblast, it is negative against the following cytochemical stains: black sudan, myeloperoxidase or alpha-naphthyl butyrate esterase. While it reacts positively against alpha-naphthyl acetate esterase.

On the other hand, the megakaryoblast gives a positive reaction against platelet peroxidase using the ultrastructural cytochemical PPO technique. This technique is highly sensitive, but has the disadvantage of the need for an electron microscope, which is not common in routine laboratories.

Other viable options are the detection of platelet glycoproteins (IIb / IIIa or Ib), factor VIII and membrane receptors, such as CD41, CD42 or CD61, through the use of specific antibodies for each of these antigens.

Histology

Morphology

It is a cell with an irregular or rounded shape, which has a single nucleus that can be oval or bilobed. Its cytoplasm is scarce, agranular and strongly basophilic, therefore, with common stains it stains an intense blue. Chromatin is lax, it has several nucleoli.

Size

The megakaryoblast is small compared to other blasts present in the bone marrow. Its size and shape are similar to those of a lymphocyte (15-25 µm). Unlike other cell lines, the megakaryoblast is smaller than its successors, that is, the promegakaryocyte and the megakaryocyte.

In this cell series, the opposite occurs than in the rest: as the cell differentiates and matures, its size increases, until it reaches the metamegakaryocyte, which is the largest cell observable in the bone marrow.

The latter, curiously, will give rise to platelets, which are the smallest functional structures that blood contains (2-4 µm).

The increase in these cells is due to a process called endomitosis. This process is characterized by the replication of chromosomes without cell division. Therefore the cell grows and the nucleus transforms from diploid (2N) to polyploid (4N, 8N… 16N), being able to reach up to 64N in the megakaryocyte stage.

The endomitosis process is necessary for there to be a significant increase in functional genes to ensure greater protein synthesis.

This results in an increase in the cytoplasmic mass. The size of the cytoplasmic mass is proportional to the number of platelets, since these are formed from the fragmentation of the cytoplasm of the mature megakaryocyte.

Function

The function of the megakaryoblast is to continue its differentiation and maturation process until it reaches the mature megakaryocyte, which will give rise to the definitive cell, which are platelets.

Platelets fulfill fundamental functions for life. They are basically in charge of covering and repairing damaged tissues, avoiding exaggerated bleeding; that is, they act concomitantly with coagulation factors.

Diseases

Acute megakaryoblastic leukemia (M7)

It is a rare disease. It occurs mainly in childhood. It is characterized by producing coagulation disorders, the most common being disseminated intravascular coagulation.

It also presents ecchymosis or petechiae and gingival hyperplasia. In addition, there may or may not be the appearance of acute hemorrhages and neurological disorders.

Myelodysplastic syndrome

This pathology is characterized by a disorder in the bone marrow that does not allow any cell to mature; with the exception of lymphocytes, which mature extra-medullary.

The cells that are produced are abnormal. This disorder is also known as pre-leukemia, because it usually progresses to acute myeloid leukemia.

Myeloproliferative disorders

In this type of affectation there is an excessive proliferation of at least one cell type. When the increase is in the megakaryocytic line, an exaggerated increase in platelets occurs. This disorder is known as essential thrombocythemia.

If there is an increase in several cell lines then it is called polycythemia vera (increase in red blood cells, leukocytes and platelets).

References

1. Rivadeneyr L, Ivani P, Schattner M, Pozner R. Thus begins platelet life: a journey from medullary megakaryocytes to circulating platelets. Acta Bioquím Clín Latinoam 2016; 50 (2): 233-45. Available at: redalyc.org.
2. Röllig C, Beelen W, Braess J, Greil R, Niederwieser D, Passweg, J and et al. Akute Myeloische Leukämie (AML). Onkopedia. 2018.Available at: onkopedia.com/
3. "Myelodysplastic syndrome." Wikipedia, The Free Encyclopedia. 3 Apr 2018, 00:26 UTC. 13 Jun 2019, 02:57 en.wikipedia.org.
4. Tuset E, Toll T, Rives S, Alcota I, Pérez-Iribarne M and Estella J. Megakaryoblastic Congenital Leukemia. Hematology, 2004; 89 (4): 340-346 Available at: sehh.es/images
5. A. González-Villalva, P. Bizarro-Nevares, M. Rojas-Lemus et al. The megakaryocyte a very original cell. Journal of the Faculty of Medicine of the UNAM. 2019; 62 (1): 6-18. Available: medigraphic.com