MONOBLASTS: CHARACTERISTICS, MORPHOLOGY, FUNCTIONS - BIOLOGY - 2025

The monoblast is an immature cell belonging to the monocytopoietic lineage. It is the first cell of this lineage that can be identified in the bone marrow thanks to its morphological characteristics. However, its recognition is not easy, as it requires the experience of qualified personnel.

The monoblast is the precursor to an extremely important mononuclear blood cell, which may be called a monocyte or macrophage, depending on whether it is in a resting state in the circulation or activated in the tissues respectively.

Summary scheme of the differentiation, maturation, and activation of the monocytic lineage. From the Stem Cell to the macrophage stage. Source: No machine-readable author provided. G.tuttobene assumed (based on copyright claims). Image edited by the author (translated into Spanish)

Likewise, the monoblast after going through a process of differentiation and maturation, becomes a monocyte. The monoblast together with the promonocyte, the monocyte and the different types of macrophages are part of the large mononuclear phagocytic system.

This cell is located in the bone marrow, since it is an immature cell; In other words, it is not yet ready to go out into the peripheral circulation. The appearance of this cell in peripheral blood is a sign of disease.

Diseases in which monoblast can be seen in peripheral blood smears along with other immature precursors are chronic and juvenile myelomonocytic leukemia and acute monoblastic leukemia.

Monoblast characteristics

Size

Immature cells are usually large, but of all the blasts present in the bone marrow, the monoblast is the largest. In this case the cell measures between 14-25 µm.

The monoblast also has a prominent nucleus, which covers almost the entire cell. Therefore there is a high nucleus-cytoplasm ratio.

It is often confused with myeloblasts, due to their great resemblance; but the monoblast is a bit bigger.

Location

It is characterized by being only in the bone marrow and its presence is not very abundant in the absence of disease. Peripheral blood should not contain immature cells under normal conditions. If these were to be present in circulation, this would be a cause for alarm.

Diseases with the presence of immature cells in the blood generally have a poor prognosis.

Origin

The monoblast phase is just a small step in the great process called monocytopoiesis.

The monoblast comes from the differentiation of the pluripotential cell called CFU-monocytic.

This cell, upon receiving stimuli from interleukin 3 (IL 3), granulocytic and macrophage colony stimulating factor (GM-CSF) and macrophage colony stimulating factor (M-CSF), becomes a monoblast.

Morphology

The shape of the monoblast is rounded, and like any precursor cell it has characteristics of its immaturity state.

The shape of the nucleus can appear in several forms: round, and oval with or without a small notch or slight invagination.

The chromatin is quite lax, and the nucleoli can be clearly and forcefully observed. These can vary in quantity, and they are observed from 2 to 6 nucleoli approximately.

The cytoplasmic space is reduced and is stained blue-gray with the usual stains (Wright and May-Grunwald Giemsa). Therefore, it is said to be moderately basophilic. In this phase, no granulations are evident in the cytoplasm.

Function

The function of the monoblast is to continue in its maturation process until it becomes the mature cell, monocyte and / or macrophage. Mature cells fulfill specific functions in innate and acquired immunity.

The monocyte is the circulating cell, it is normally at rest and when it passes into the tissues it becomes a macrophage. This is where it is activated.

Macrophages are distributed in many organs and tissues. These receive a specific name depending on the site where they are. For example, in the lungs they are called alveolar macrophages, in the liver Kupffer cells, in the CNS microglia cells, and in the skin Langerhans cells, among others.

Macrophages are strategically located in the wall of the splenic sinusoids and in the medullary sinuses of the lymph nodes, in these sites they are able to filter and eliminate foreign agents that enter the body.

The most important functions of the macrophage are to engulf infectious agents, act as antigen-presenting cells, participate in inflammation processes, and participate in tissue healing and repair.

On the other hand, it also participates in coagulation homeostasis through the release or synthesis of substances, such as plasminogen, factor VII, factor XIII, thrombomodulin, among others.

ID

As already mentioned, monoblast and myeloblast can be confused because of their close resemblance, but there are some differences. By carefully comparing both cells, it can be seen that the monoblast is larger and has more cytoplasm than the myeloblast.

Another important molecular detail is that the monoblast has a marker called CD14 in its membrane.

The special test to differentiate monoblast from myeloblast is cytochemical staining for nonspecific esterases. For this, alpha-naphthylacetate is used as a reagent. The monoblast gives a strongly positive reaction, while the myeloblast gives a negative reaction.

With this staining the monoblast is stained red. There is also another variant of the technique called combined esterase staining in which the monoblast is stained brown.

Diseases that occur with monoblasts in peripheral blood

Myelomonocytic leukemia (AML-M4)

There are two variants of this disease, chronic and juvenile. The first is common in elderly patients and the second in children under 6 years of age.

It is characterized by the presence of up to 20% immature cells in peripheral blood. Immature cells that are usually present are monoblasts, myeloblasts, and promonocytes.

Chronic myelomonocytic leukemia is characterized by symptoms and signs, such as bleeding and bruising, weakness, recurrent infections, hepatosplenomegaly (an enlarged liver and spleen), and fatigue.

Whereas in juvenile myeloid monocytic leukemia the most prominent symptoms are the appearance of paleness, rash and hepatosplenomegaly.

Acute monoblastic leukemia (AML M5a and M5b)

There are 2 variants: a little differentiated called myeloblastic (M5a) and a differentiated called monocytic (M5b). In the first, there is an 80% predominance of monoblasts in bone marrow and it occurs especially in children, with a frequency of 5 to 8%.

In the second, monoblasts represent 10-15% and the highest predominance is of promonocytes and monocytes. It occurs mainly in adults with a frequency of 3 to 6%.

References

1. Medical dictionary. University of Navarra Clinic. Available at: cun.es
2. Information on Chronic Myelomonocytic Leukemia and Juvenile Myelomonocytic Leukemia. 2016. Leukemia, Lymphoma Society. Available at: lls.org
3. Rodak B. (2004). Hematology and fundamentals and clinical applications. 2nd edition, Editorial Médica Panamericana, Buenos Aires, Argentina.
4. Fernández J, Armario J, Conde T, Pujol R, Rodríguez J. (2007). Cutaneous lymphomas. 1st Edition, Publications Service of the University of Cadiz. Spain.
5. Manascero A. (2003). Hematology, tool for diagnosis. Atlas of cell morphology, alterations and related diseases. 1st ed. Javeriano publishing center. Bogota Colombia.
6. Macrophage. Wikipedia, The Free Encyclopedia. 13 Feb 2019, 00:48 UTC. 12 Jun 2019, 04:37 wikipedia.org