MYELOBLASTS: CHARACTERISTICS AND GRANULOPOIESIS - BIOLOGY - 2025

The myeloblasts or granuloblastos are cells that are in a state of primary development in the bone marrow. It is the first cell to be recognized in the granulocytic series. They finally differentiate into neutrophils, eosinophils, and basophils.

Structurally, a myeloblast has a large oval nucleus, which occupies a large volume; about four fifths of the whole cell. They have about two five nucleoli.

Formation of the different cell lines. The myeloblast is the first cell to be recognized in the granulocytic series.

Source: Unknown Author: Illu_blood_cell_lineage.jpgNunoAgostinho: Derivative work

characteristics

Myeloblasts are cells 15-20 µm in diameter. The nucleus is spheroidal or ovoid in appearance, quite large and generally reddish in color. Inside the nucleus, several nucleoli can be differentiated, from three to five on average. The outline of the cells is smooth.

The chromatin - a substance that is located inside the nucleus, made up of genetic material and proteins - of myeloblasts is lax.

Nucleoli are compartments that are located inside the nucleus but are not delimited by a system of membranes.

No granules are detected inside the cell and the cytoplasm is basophilic. Although some authors classify them as an agranular cell, others consider that myeloblasts have a fine and nonspecific granulation.

The term "basophilic" refers to the tendency of cells to stain with the application of basic dyes, such as hematoxylin.

However, when the term is used without further clarification, it refers to leukocytes belonging to the granulocyte family, as we will see later.

Granulopoiesis

Myeloblasts are immature cells from the bone marrow, and they are the precursors of granulopoiesis.

Granulopoiesis is the process of cell formation and differentiation that ends in the formation of granulocytes. Of all marrow cells, this type represents about 60% of the total, while the remaining 30% correspond to cells of the erythropoietic type.

During this process, the granulopoietic progenitor cell undergoes the following modifications:

-Size reduction: during maturation, progenitor cells progressively decrease their cell size. Furthermore, the nucleus / cytoplasm ratio is reduced. That is, the nucleus decreases and the cytoplasm increases.

- Chromatin condensation: chromatin is modified as the mature cell goes from a lax state to being increasingly dense. Maturation supposes the disappearance of the nucleoli.

-Loss of basophilic cytoplasm: the basophilic cytoplasm typical of the first cells in the series gradually lose its bluish color.

-Increased granulation: with the maturation of granulopoietic cells, granulation appears. The first step is the appearance of a fine granulation, called primary granulation. Subsequently, a typical specific granulation of each granulocyte appears, called secondary granulation.

Cells of the maturative sequence

In granulopoiesis, the first cells are the myeloblasts already described. These are successively transformed into other cell forms that receive the following names:

Promyelocyte

Myeloblasts undergo mitotic cell division and give rise to larger cells, called promyelocytes.

These cells represent 5% of the cells in the bone marrow. Compared to the myeloblast, it is a slightly larger cell, it is a range of 16 to 25 um. In all granulopoiesis, they are the largest cells. The nucleus is eccentric and may retain some nucleolus.

In this state, primary granulation begins to appear. The cytoplasm is still basophilic (basophilia is moderate).

Myelocyte

These cells represent 10% to 20% of the cells in the bone marrow. They are rounded structures, and their size decreases a little, reaching 12 to 18 um.

The nucleus continues to be eccentric and the chromatin has condensed. The nucleoli disappear. The cytoplasm is no longer basophilic and the granulation pattern is more pronounced.

Metamyelocyte

These cells represent 15% to 20% of the cells in the bone marrow. The size continues to reduce, on average they measure from 10 to 15 um. They are cellular structures quite similar to myelocytes.

At this stage, the nucleus takes on a reniform appearance. The capacity for cell division no longer exists. Of the entire series, it is the first cell that we can find in peripheral blood under normal conditions.

Band

The bada or cayado are cells that represent about 30% of all cells in the bone marrow. They are smaller than metamyelocytes, but retain the same basic structural features. The nucleus does undergo certain modifications, and acquires a shape similar to the letters S, C or L.

Segmented

The cayados or bands give rise to the segmented ones by means of nuclear segmentation; hence the name. These correspond to the most mature elements of the entire series. According to the type of granulation, they are classified into three types:

Neutrophil

These cells have a size in the order of 12 to 15 um. The nucleus takes on a dark purple color and is segmented into multiple lobes that are held together thanks to the presence of special bridges formed from chromatin.

The cytoplasm has a typical pink hue with a significant number of granules that, under the application of traditional dyes used in the laboratory, turn brown. Of all the leukocytes present in the peripheral blood, neutrophils constitute about 40-75%.

Basophil

This second cell type is slightly smaller in size than neutrophils, on the order of 12 to 14 um. The basophilic granules that distinguish this lineage of cells are found surrounding the nucleus. They are quite scarce elements of the peripheral blood, being in a proportion of less than 1%.

Eosinophil

These cells are the largest, with sizes around 12 to 17 um. One of its most prominent features are two lobes in the nucleus. This structure is reminiscent of glasses.

In the cytoplasm we find large orange or almost brown granules, which never overlap with the nucleus. In peripheral blood they constitute 1 to 7% of the leukocytes present.

These three types of cells stay in the peripheral blood for a few hours, 7 to 8 on average. They can circulate freely, or attached to a series of glasses. Upon reaching the white tissue, they perform their functions for about 5 days.

References

1. Abbas, AK, Lichtman, AH, & Pillai, S. (2014). Cellular and molecular immunology E-book. Elsevier Health Sciences.
2. Alexander, JW (1984). Principles of clinical immunology. I reversed.
3. Dox, I., Melloni, BJ, Eisner, GM, Ramos, RE, Pita, M. Á. R., Otero, JAD, & Gorina, AB (1982). Melloni's Illustrated Medical Dictionary. I reversed.
4. Espinosa, BG, Campal, FR, & González, MRC (2015). Hematological analysis techniques. Ediciones Paraninfo, SA.
5. Miale, JB (1985). Hematology: laboratory medicine. I reversed.
6. Ross, MH, & Pawlina, W. (2006). Histology. Lippincott Williams & Wilkins.