Monoblast
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Structure
A typical monoblast is about 12 to 20 μm in diameter, has a nuclear to cytoplasm ratio of 4:1 to 3:1, and, like most myeloid blasts, has a round to oval nucleus with fine chromatin structure. Compared to other myeloid blasts, monoblasts have more cytoplasm. The nucleoli it contains is usually distinct. One to four nucleoli are usually visible. The nucleus can be central or eccentric and it can show evidence of indentation or folding. The cytoplasm stains moderately to lightly basophilic and may contain small azurophilic granules. These granules contain enzymes that can damage or digest pathogens and also release inflammatory signals in the periphery. Auer rods are there, but rarely seen. Easily observed in the round monoblast nucleus is lacy, clear chromatin and distinct nucleoli.
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Development
The monoblast is the first stage of monocyte-macrophage maturation. The developmental stages of the monoblast are: CFU-GM (pluripotential hemopoietic stem cell or hemocytoblast) -> monoblast -> promonocyte -> monocyte-> macrophage/dendritic cell. During their development, monocytes are present in large packs in all of the lymph nodes in the body.
Hematopoietic stem cells mature into monoblasts by being in a concentrated environment of certain cytokines that induce activation of certain transcription factors. The development of monoblasts occurs when certain transcription factors are activated, crucial ones being PU.1 and GATA1. Development continues by, yet again, the activation of certain transcription factors. A monoblast matures into a monocyte if the transcription factors PU.1 and C/EBPa are expressed.
Monocytes will then develop into macrophages or dendritic cells upon tissue damage and recruitment of monocytes into the infected area. During recruitment monocytes are distinct from macrophages and dendritic cells, but upon entering the infected area, monocytes will acquire inflammatory effector functions and then differentiate into inflammatory cells such as macrophages or dendritic cells. These inflammatory cells are now better equipped to combat a foreign invader quickly (in the case of macrophages) and specifically (in the case of a dendritic cell) through activating different parts of the Immune response.
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