Monoblasts are the committed progenitor cells that differentiated from a committed

macrophage Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...

dendritic cell A dendritic cell (DC) is an antigen-presenting cell (also known as an ''accessory cell'') of the mammalian immune system. A DC's main function is to process antigen material and present it on the cell surface to the T cells of the immune system ...

precursor (MDP) in the process of

hematopoiesis Haematopoiesis (; ; also hematopoiesis in American English, sometimes h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. In a healthy adult human, roughly ten ...

. They are the first developmental stage in the monocyte series leading to a macrophage. Their myeloid cell fate is induced by the concentration of

cytokine Cytokines () are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are produced by a broad range of cells, including immune cells like macrophages, B cell, B lymphocytes, T cell, T lymphocytes ...

s they are surrounded by during development. These cytokines induce the activation of

transcription factor In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription (genetics), transcription of genetics, genetic information from DNA to messenger RNA, by binding t ...

s which push completion of the monoblast's myeloid cell fate. Monoblasts are normally found in bone marrow and do not appear in the normal peripheral

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...

. They mature into

monocyte Monocytes are a type of leukocyte or white blood cell. They are the largest type of leukocyte in blood and can differentiate into macrophages and monocyte-derived dendritic cells. As a part of the vertebrate innate immune system monocytes also ...

s which, in turn, develop into

macrophages Macrophages (; abbreviated MPhi, φ, MΦ or MP) are a type of white blood cell of the innate immune system that engulf and digest pathogens, such as cancer cells, microbes, cellular debris and foreign substances, which do not have proteins that ...

. They then are seen as macrophages in the normal peripheral blood and many different tissues of the body. Macrophages can produce a variety of effector molecules that initiate local, systemic inflammatory responses. These monoblast differentiated cells are equipped to fight off foreign invaders using pattern recognition receptors to detect antigen as part of the innate immune response.

Structure

A typical monoblast is about 12 to 20 μm in diameter, has a nuclear to

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

ratio of 4:1 to 3:1, and, like most myeloid blasts, has a round to oval nucleus with fine

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

structure. Compared to other myeloid blasts, monoblasts have more cytoplasm. The nucleoli it contains is usually distinct.6 CAP TODAY JANUARY 2018 Monocyte, Immature (Promonocyte ... 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

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

s that can damage or digest

pathogen In biology, a pathogen (, "suffering", "passion" and , "producer of"), in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a Germ theory of d ...

s 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.

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 ->

. 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. Additional images File:Illu blood cell lineage.jpg, Blood cell lineage File:Hematopoiesis (human) diagram en.svg, Hematopoiesis File:Immune Cells Development.pdf, This image represents the development from a HSC to a monocyte through various transcription factors activation which are in italics. The lymphoid cell development junctures to the right while the myeloid cell development junctures to the left side and below. Each cell that does not contain much in their cytoplasm represents a type of precursor cell. Each cell name is above each cell.

References

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Leukocytes

Structure

Development

See also

References