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Osteoblasts

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The osteoblasts are bone – forming cells. They synthesize the substances necessary for the growth, development and remodeling of bone tissue. They also regulate the work of bone destruction carried out by osteoclasts. Osteoblastic precursors produce substances that favor the stimulation of the remodeling cell.

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What are osteoblasts?

Osteoblasts are building blocks of bone . Its main function is to process –extract- the collagen, proteoglycans, glycoproteins and organic salts that form the bone matrix. These cells also participate in the synthesis of cytokines and other protein substances responsible for intercellular communication , induce changes in neighboring cells, altering their behavior and differentiation. They regulate the process of bone resorption.

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  • Location
  • Characteristics of osteoblasts
  • Origin and formation
  • Function
  • Diseases associated with osteoblasts
  • Importance

Location

Osteoblasts are aligned and attached to each other, forming the epithelial layer that covers the surface of the bone. Cells secrete an unmineralized organic substance called osteoid and other proteins necessary for the formation of bone mass. This matrix separates the osteoblasts from the mineralized bone matrix and later becomes the new bone tissue , after the maturation of the osteoid.

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Characteristics of osteoblasts

Osteoblasts have the capacity for differentiation , which suggests modifications in their shape and function during the process of formation, operability and interrelation with other organisms. They are prolific cells, but not divisible, they lack mitotic capacity.

Are recipients of calcitonin and parathyroid hormone, secreted by the glands thyroid and parathyroid. Furthermore, these cells have a secretory nature .

They have a rounded shape and once they come into operation they become cubic. They are found in compact groups constituting a layer – one cell thick – that maintains direct contact with the bone surface . This layer becomes thinner as the synthesis rate decreases. It occurs when the bones have reached their maximum length, but continue to feed their volume.

They measure between 20 and 30 microns. Its cytoplasm is acidic, basophilic. They are polarized cells , they contain a single nucleus, located far from the area that touches the bone. They are separated by the Golgi apparatus with its dilated secretory vesicles and a prominent folded endoplasmic reticulum. In addition, they are provided with multiple mitochondria that allow active synthesis.

Origin and formation

The name of osteoblasts comes from the Greek words: osteon, which means bone and blasts, which is equal to germ, genesis of bone .

Osteoblasts are derived from the osteoprogenitor cells that make up the bone marrow, endosteum, periosteum, and perivascular pericytes. Pluripotent mesenchymal cells , hence their ability to differentiate and specialize in the tissue that supports the body.

Osteoprogenitor cells become osteoblasts after stimulation of parathyroid hormone, which metabolizes calcium and phosphorus. Also involved in the process are bone morphogenetic proteins and other trophic substances or factors, including the transcription factor Cbfa1. The Cbfa1 factor or RUNX2 protein is responsible for the differentiation process.

As osteoformers they act in the development, growth and recomposition of bones, originated by endochondral or intramembranous ossification . Endochondral ossification develops from the inside out, forming bone from cartilaginous tissue . The intramembranous begins in the internal part of the periosteum, the layer that surrounds the bone.

After synthesizing the bone matrix, the osteoblasts that remain on the surface of the bone flatten into lining cells. While those that remain immersed in the osteoid, they change shape (acquire a starry appearance) and become osteocytes. Osteocytes make up 95% of the bone structure .

Another part of the cells is destined to undergo programmed death, this according to the genetic load and the production of local factors. Likewise, once the osteoblasts decrease in the process of development and regeneration of the bone matrix , the osteoprogenitor cells that expand and differentiate, will produce new osteoblasts.

Function

The main function of osteoblasts is to simplify the organic components of the bone matrix. They synthesize type I, type V collagen proteins , and to a lesser extent, those from Sharpey fibers. Also collagen produced under mechanical stress.

These cells summarize proteoglycans , glycoproteins that participate in morphogenesis and bone formation. At least four proteoglycans make up the osteoid matrix, modify cell adherence and intervene in the calcification of the organic matrix.

Stimulated by vitamins D, bone forming cells become synthesizers of osteocalcin, a biochemical marker of osteogenesis . And of the glycoprotein known as osteonectin, acting in the mineralization of the matrix. It also receives the enzyme alkaline phosphatase, a participant in the process.

Osteoblasts gather in their mitochondria calcium phosphate granules , which are digested and transported by vesicles to the extracellular substance. Vitamin D and parathyroid hormone regulate the concentration of calcium and phosphate in the matrix. In the middle, these elements accumulate and release crystalline particles, which together with those of collagen, make up the so-called hydroxyapatites .

The hydroxyapatites give the osteid substance the characteristic of a mineral, they consolidate the mineralization of the bone .

Osteoblast cells, together with osteclasts, participate in the synthesis of the glycoproteins necessary for the bone remodeling process . Event in which the fibroblast and other growth factors produced by blood cells also mediate.

Osteoblasts prepare the medium for the differentiation of osteoclast precursors. They directly or indirectly regulate the resorption process carried out by the osteoclasts .

Diseases associated with osteoblasts

One of the diseases most associated with osteoblast function is osteoporosis . It is characterized by decreased bone mass – tissue density – and bone fragility. It is controllable but has no cure. The bone wears out and the new bone tissue provided by the osteoblasts is insufficient to fill the cavity.

The imbalance in the bone remodeling unit may be due to interference in the differentiation process of osteogenic cells. The osteoblasts –differentiated cells- are responsible for filling the cavity made by the osteoclasts. Although there are other associated factors, which may be accelerating the action of the osteoclasts or the death of the cells that make up the bone.

The interaction between osteoblasts, osteoclasts, and osteocytes is also altered by inflammation. The remodeling process is deficient in inflammatory diseases such as rheumatoid arthritis or ankylosing spondyloarthritis . Given the difficulty of the cells to act freely on the surface of the bone, bone mass decreases and makes it prone to abnormality.

The rheumatoid arthritis or osteoarthritis is associated with osteoblasts and their participation in the remodeling process of the bone. It is the most common rheumatological disease, it generally occurs in elderly people, its progress is slow. It consists of the degradation of the hyaline articular cartilage, but affects the entire articular structure.

It is produced by mechanical, biochemical and endogenous factors. Factors that reduce the quality of the extracellular matrix, cause apoptosis and disintegration of cartilage. However, there is evidence of systemic and lipid disorders that affect stromal cell differentiation and maintenance of bone mass.

The Paget ‘s disease is also known as osteitis deformans, it is characterized by the deformation and bone weakening. It usually affects bones of the leg, pelvis, spine, or brain. There is no cure. It is related to genetic factors or viral infections, but what causes it is certainly unknown. The bone wears down and the socket is filled by a larger but fragile piece of bone. Something interferes with the interplay of osteoblasts and osteoclasts during the remodeling process.

Importance

Osteoblasts participate in the growth, development, continuous remodeling, and recomposition of bone. Bone, made up of living tissue , plays a fundamental role as a musculoskeletal structure and as a protective shield for organs. It remains in constant remodeling, allowing it to adapt to the physical demands of a prevailing activity. Also recover from fractures and other injuries.

Osteoblasts synthesize the bone matrix, which once mineralized constitutes the bone tissue, favoring its growth in length and thickness. It does the same with the remodeling process developed by osteoclasts. Osteoclasts resorb a small portion of the bone and osteoblasts secrete osteoid and protein substances capable of restoring the missing bone tissue . Osteoclasts and osteoblasts constitute the bone remodeling unit.

Its direct action, and interaction with other cells and organisms of the body, guarantee the normal functioning of the skeleton. It also intervenes in the regulation of calcium, due to the availability of calcium resulting from remodeling. It is also related to the homeostasis of the acid-base balance.

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