Osteoblasts are mononucleate cell

Cell (biology)

The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

s that are responsible for bone formation; in essence, osteoblasts are specialized fibroblast

Fibroblast

A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing...

s that in addition to fibroblastic products, express bone sialoprotein

Bone sialoprotein

Bone sialoprotein is a component of mineralized tissues such as bone, dentin, cementum and calcified cartilage. BSP is a significant component of the bone extracellular matrix and has been suggested to constitute approximately 8% of all non-collagenous proteins found in bone and cementum...

 and osteocalcin

Osteocalcin

Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein , is a noncollagenous protein found in bone and dentin. In humans, the osteocalcin is encoded by the BGLAP gene.- Function :...

.

Osteoblasts produce a matrix

Extracellular matrix

In biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...

 of osteoid

Osteoid

In histology, osteoid is the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue. Osteoblasts begin the process of forming bone tissue by secreting the osteoid as several specific proteins...

, which is composed mainly of Type I collagen

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

. Osteoblasts are also responsible for mineralization of this matrix. Zinc, copper and sodium are some of the minerals required in this process. Bone is a dynamic tissue that is constantly being reshaped by osteoblasts, in charge of production of matrix and mineral, and osteoclasts, which remodel the tissue. Osteoblast cells tend to decrease with age, affecting the balance of formation and resorption in the bone tissue.

Osteogenesis

Osteoblasts arise from osteoprogenitor cells located in the deeper layer of periosteum

Periosteum

Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....

 and the bone marrow

Bone marrow

Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...

. Osteoprogenitors are immature progenitor cell

Progenitor cell

A progenitor cell is a biological cell that, like a stem cell, has a tendency to differentiate into a specific type of cell, but is already more specific than a stem cell and is pushed to differentiate into its "target" cell...

s that express the master regulatory transcription factor

Transcription factor

In molecular biology and genetics, a transcription factor is a protein that binds to specific DNA sequences, thereby controlling the flow of genetic information from DNA to mRNA...

 Cbfa1/Runx2.

Osteoprogenitors are induced to differentiate under the influence of growth factor

Growth factor

A growth factor is a naturally occurring substance capable of stimulating cellular growth, proliferation and cellular differentiation. Usually it is a protein or a steroid hormone. Growth factors are important for regulating a variety of cellular processes....

s, in particular the bone morphogenetic protein

Bone morphogenetic protein

Bone morphogenetic proteins are a group of growth factors also known as cytokines and as metabologens . Originally discovered by their ability to induce the formation of bone and cartilage, BMPs are now considered to constitute a group of pivotal morphogenetic signals, orchestrating tissue...

s (BMPs). Aside from BMPs, other growth factors including fibroblast growth factor (FGF), platelet-derived growth factor

Platelet-derived growth factor

In molecular biology, platelet-derived growth factor is one of the numerous growth factors, or proteins that regulate cell growth and division. In particular, it plays a significant role in blood vessel formation , the growth of blood vessels from already-existing blood vessel tissue. Uncontrolled...

 (PDGF) and transforming growth factor beta

Transforming growth factor

Transforming growth factor is used to describe two classes of polypeptide growth factors, TGFα and TGFβ....

 (TGF-β) may promote the division of osteoprogenitors and potentially increase osteogenesis.

Once osteoprogenitors start to differentiate into osteoblasts, they begin to express a range of genetic markers including Osterix, Col1

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

, BSP

Bone sialoprotein

Bone sialoprotein is a component of mineralized tissues such as bone, dentin, cementum and calcified cartilage. BSP is a significant component of the bone extracellular matrix and has been suggested to constitute approximately 8% of all non-collagenous proteins found in bone and cementum...

, M-CSF

Macrophage colony-stimulating factor

Macrophage colony-stimulating factor, or M-CSF, is a secreted cytokine which influences hematopoietic stem cells to differentiate into macrophages or other related cell types. Eukaryotic cells also produce M-CSF in order to combat intercellular viral infection. M-CSF binds to the Colony...

, ALP

Alkaline phosphatase

Alkaline phosphatase is a hydrolase enzyme responsible for removing phosphate groups from many types of molecules, including nucleotides, proteins, and alkaloids. The process of removing the phosphate group is called dephosphorylation...

, osteocalcin

Osteocalcin

Osteocalcin, also known as bone gamma-carboxyglutamic acid-containing protein , is a noncollagenous protein found in bone and dentin. In humans, the osteocalcin is encoded by the BGLAP gene.- Function :...

, osteopontin

Osteopontin

Osteopontin , also known as bone sialoprotein I , early T-lymphocyte activation , secreted phosphoprotein 1 , 2ar and Rickettsia resistance , is a human gene product, which is also conserved in other species...

, and osteonectin

Osteonectin

Osteonectin also known as secreted protein acidic and rich in cysteine or basement-membrane protein 40 is a protein that in humans is encoded by the SPARC gene....

. Although the term osteoblast implies an immature cell type, osteoblasts are in fact the mature bone cells entirely responsible for generating bone tissue in animals and humans.

Morphology and histological staining

Hematoxylin and eosin staining

H&E stain

H&E stain, HE stain or hematoxylin and eosin stain is a popular staining method in histology. It is the most widely used stain in medical diagnosis; for example when a pathologist looks at a biopsy of a suspected cancer, the histological section is likely to be stained with H&E and termed H&E...

 reveals that the cytoplasm of osteoblasts is basophilic

Basophilic

Basophilic is a technical term used by histologists. It describes the microscopic appearance of cells and tissues, as seen down the microscope, after a histological section has been stained with a basic dye. The most common such dye is haematoxylin....

 due to the presence of a large amount of rough endoplasmic reticulum. A large Golgi apparatus

Golgi apparatus

The Golgi apparatus is an organelle found in most eukaryotic cells. It was identified in 1898 by the Italian physician Camillo Golgi, after whom the Golgi apparatus is named....

 is also present in the centre. The nucleus is spherical and large. Active osteoblasts synthesize

Chemical synthesis

In chemistry, chemical synthesis is purposeful execution of chemical reactions to get a product, or several products. This happens by physical and chemical manipulations usually involving one or more reactions...

, and stain positively for, Type-I collagen

Type-I collagen

Type-I collagen is the most abundant collagen of the human body.It is present in scar tissue, the end product when tissue heals by repair.It is found in tendons, the endomysium of myofibrils and the organic part of bone.- See also :* Collagen...

 and alkaline phosphatase

Alkaline phosphatase

Alkaline phosphatase is a hydrolase enzyme responsible for removing phosphate groups from many types of molecules, including nucleotides, proteins, and alkaloids. The process of removing the phosphate group is called dephosphorylation...

.

Osteoblasts and osteocytes

Osteoblasts that become trapped in the bone matrix and remain isolated in lacunae become osteocyte

Osteocyte

An osteocyte, a star-shaped cell, is the most abundant cell found in compact bone. Cells contain a nucleus and a thin ring of cytoplasm. When osteoblasts become trapped in the matrix they secrete, they become osteocytes...

s. They cease to generate osteoid and mineralized matrix, and instead act in a paracrine manner on active osteoblasts. They are believed to respond to mechanosensory stimuli.

See also

• Bonesetter
  Bonesetter
  A bonesetter is a practitioner of joint manipulation. Before the advent of chiropractors, osteopaths, and physical therapists, bonesetters were the main providers of this type of treatment. Bonesetters would also reduce joint dislocations and 're-set' bone fractures.The original spinal adjustment...
  
  
• Chondrocyte
  Chondrocyte
  Chondrocytes are the only cells found in cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans...
  
  
• Endochondral ossification
  Endochondral ossification
  Endochondral ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike intramembranous ossification, which is the other process by which bone tissue is created, cartilage is present during endochondral...
  
  
• Intramembranous ossification
  Intramembranous ossification
  Intramembranous ossification is one of the two essential processes during fetal development of the mammalian skeletal system by which bone tissue is created. Unlike endochondral ossification, which is the other process by which bone tissue is created, cartilage is not present during intramembranous...
  
  

Further reading

• Netter, Frank H.
  Frank H. Netter
  Frank H. Netter was an artist, physician, and most notably, a leading medical illustrator. He was also a Fellow of The New York Academy of Medicine.-Early life, training, and medical career:...
  
  (1987). Musculoskeletal system: anatomy, physiology, and metabolic disorders. Summit, New Jersey: Ciba-Geigy Corporation ISBN 0914168886.