Monoblasts are the committed progenitor cells that differentiated from a committed

macrophage Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cel ...

dendritic cell Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their 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 (, from Greek , 'blood' and 'to make'; also hematopoiesis in American English; sometimes also h(a)emopoiesis) is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells. ...

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

cytokines Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to enter the cytoplasm. Cytokines have been shown to be involved in autocrin ...

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

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

which push completion of the monoblast's myeloid cell fate. Monoblasts are normally found in

bone marrow Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It is composed of hematopoietic ce ...

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 in the c ...

. 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 conventional dendritic cells. As a part of the vertebrate innate immune system monocytes also inf ...

s which, in turn, develop into

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer ce ...

. 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 Pattern recognition receptors (PRRs) play a crucial role in the proper function of the innate immune system. PRRs are germline-encoded host sensors, which detect molecules typical for the pathogens. They are proteins expressed, mainly, by cells of ...

to detect antigen as part of the

innate immune response The innate, or nonspecific, immune system is one of the two main immunity strategies (the other being the adaptive immune system) in vertebrates. The innate immune system is an older evolutionary defense strategy, relatively speaking, and is the ...

Structure

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

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

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

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucle ...

with fine

chromatin Chromatin is a complex of DNA and protein found in 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 roles in 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

enzymes Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

that can damage or digest

pathogens In biology, a pathogen ( el, πάθος, "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 ...

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 Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This process is called haematopoiesis. In vertebrates, the very first definitive HSCs arise from the ventral endothelial wall of the embryonic aorta within t ...

or hemocytoblast) -> monoblast ->

promonocyte A promonocyte (or premonocyte) is a cell arising from a monoblast and developing into a monocyte. See also * Pluripotential hemopoietic stem cell Hematopoietic stem cells (HSCs) are the stem cells that give rise to other blood cells. This proc ...

. During their development, monocytes are present in large pacts in all of the

lymph nodes A lymph node, or lymph gland, is a kidney-shaped organ of the lymphatic system and the adaptive immune system. A large number of lymph nodes are linked throughout the body by the lymphatic vessels. They are major sites of lymphocytes that includ ...

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 Transcription factor PU.1 is a protein that in humans is encoded by the ''SPI1'' gene. Function This gene encodes an ETS-domain transcription factor that activates gene expression during myeloid and B-lymphoid cell development. The nuclear pro ...

and

GATA1 GATA-binding factor 1 or GATA-1 (also termed Erythroid transcription factor) is the founding member of the GATA family of transcription factors. This protein is widely expressed throughout vertebrate species. In humans and mice, it is encoded by ...

. 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 An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...

. 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 doesn't contain much in their cytoplasm represents a type of precursor cell. Each cell name is above each cell.

References

Leukocytes {{Immunology-stub