Bone Cells definition
Bone cells are the cells that makeup bone tissue. Bone is a specialized connective tissue with three main functions: to protect internal organs, create a rigid frame for muscular movement, and store minerals such as calcium and phosphorous.
There are four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and bone lining cells. Osteoblasts are responsible for building new bone tissue, while osteoclasts break down old or damaged bone tissue.
Osteocytes are mature bone cells that help maintain the structure of existing bone tissue. Bone lining cells regulate the movement of calcium in and out of the bones.
What Are Bone Cells?
Bone cells are the cells that make up bone tissue, which is a highly specialized connective tissue with three main functions: to protect internal organs, to create a rigid frame for muscular movement, and to store minerals such as calcium and phosphorous.
Bone tissue is comprised of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. Osteoblasts are cuboidal cells arranged in a densely packed layer along the bone surface.
They account for 4-6% of all bone cells and their main function is to form new bone tissue. Osteoclasts constantly break down and reabsorb old bone tissue while osteoblasts form new bone tissue. Together, these two cells control the gradual reshaping of bones.
Osteocytes are post-proliferative cells lying within the bone itself and are ‘entrapped’ osteoblasts. They represent the most mature differentiation state of the osteoblast lineage. There are about 25,000 osteocytes per mm3 of bone.
The osteocytes occupy lacunae which are regularly distributed between the cell membrane and the surrounding bone and contain an unmineralized matrix and a few collagen fibrils. Osteocytes are also in communication with osteoblasts at the surface.
Osteoprogenitor cells originate from pluripotent mesenchymal stem cells of the bone marrow. The evidence of mesenchymal stem cells as precursors for osteoblasts is based on the capacity of bones to regenerate by regulating the level of bone matrix deposited.
Types of Bone Tissue
Bone is living tissue that makes up the body’s skeleton. There are three types of bone tissue: compact tissue, cancellous tissue, and subchondral tissue. Compact tissue is the harder, outer tissue of bones, while cancellous tissue is the sponge-like tissue inside bones.
Subchondral tissue is the smooth tissue at the ends of bones, which is covered with another type of tissue called cartilage. Cartilage is a specialized connective tissue that is present in adults and from which most bones develop in children.
Bones are classified by their shape as long, short, flat, and irregular. There are 206 bones in the human skeleton, not including teeth and sesamoid bones (small bones found within cartilage). The different types of bone cells include osteoblasts and osteoclasts.
Osteoblasts are found within the bone and form new bone tissue while osteoclasts absorb and remove unwanted bone tissues.
Bone tissues consist mainly of a collagen matrix that is mineralized with calcium and phosphorus crystals. The combination of flexible collagen and hard mineral crystals makes bone tissues hard without making them brittle. Bone tissues are composed of four different types of bone cells: osteoblasts, osteocytes, osteoclasts, and osteogenic cells.
Compact Bone Tissue
Compact bone tissue is one of the two types of bone tissue found in the human skeleton, the other being spongy bone tissue. Compact bone forms the hard external layer of all bones and surrounds the medullary cavity or bone marrow. It provides protection and strength to bones. Compact bone tissue consists of units called osteons or Haversian systems.
The osteon consists of a central canal called the osteonic (Haversian) canal, which is parallel to the long axis of the bone. These blood vessels interconnect, by way of perforating canals, with vessels on the surface of the bone.
Mature compact bone is lamellar or layered in structure. It is permeated by an elaborate system of interconnecting vascular canals, forming structural units called Haversian systems that contain blood vessels and nerve fibers.
Compact bone makes up 80 percent of the human skeleton; it is found in long bones such as the femur and humerus where its greater strength and rigidity are needed. The diaphysis or central shaft contains compact bone tissue in a marrow cavity while epiphyses are covered with articular cartilage and filled with red bone marrow that produces blood cells.
Spongy Bone Tissue
Spongy bone tissue, also known as cancellous bone or trabecular bone, is a porous type of bone found in animals and humans. It is lighter and less dense than compact bone.
Spongy bone consists of plates (trabeculae) and bars of bone adjacent to small, irregular cavities that contain red bone marrow.
The canaliculi connect to the adjacent cavities instead of a central haversian canal to receive their yellow bone marrow, which is composed primarily of fat, in its medullary cavity.
Spongy bone contains red bone marrow that is used in erythropoiesis – the production of red blood cells. Inside spongy bone, red blood cells are produced in the red bone marrow at a rate of about 2 million per second.
This rapid production is made possible by the highly vascularized nature of spongy bones, which can deliver adequate amounts of glucose, lipids, amino acids, and trace elements.
Spongy bones form the porous interior while compact bones form the hard outer layer of bones. Trabeculae form a mesh-like network of bony spicules aligned along regions of biomechanical stress. They project into the medullary cavity from the internal surface of spongy bones.
Types of Bone Cells
Bone tissue is made up of four types of cells: osteoblasts, osteoclasts, osteocytes, and osteoprogenitor cells. Osteoblasts are bone-forming cells that synthesize and secrete collagen fibers and other organic components needed to build new bone tissue.
Osteoclasts are large cells that break down and reabsorb old bone tissue. Osteocytes are mature bone cells that maintain the daily cellular activities of the bone tissue.
Finally, osteoprogenitor cells (also known as osteogenic cells) are stem cells found in the bone marrow that can differentiate into either osteoblasts or bone lining cells.
Osteoblasts and osteoclasts work together to maintain homeostasis of the bone tissue via a process called bone remodeling. Bone remodeling is the replacement of old bone tissue with new bone tissue.
Osteoclasts constantly break down and reabsorb old bone tissue, while osteoblasts form new bone tissue. Together, these two cell types control the gradual reshaping of bones.
Osteocytes are responsible for maintaining daily cellular activities in the bones. They communicate with each other through long cytoplasmic processes that extend through channels within the bone matrix called canaliculi. Although they cannot undergo mitosis, they can be replenished by undifferentiated stem cells called osteogenic cells.
Osteoprogenitor cells are stem cells found in the bone marrow that can differentiate into either osteoblasts or bone lining cells. They have high mitotic activity and are responsible for replenishing both cell types when they die.
Functions of Bone Cells
There are four types of bone cells: osteoblasts, osteoclasts, osteocytes, and bone lining cells. Osteoblasts are responsible for bone formation by producing collagenous and non-collagenous proteins of the bone.
Osteocytes help maintain bone structure by regulating the mineral concentration of the bone. Osteoclasts have the function of absorbing bone through a process called resorption.
They are giant bone cells with specialized membrane structures, ruffled borders, and clear zones that help them function in bone resorption.
Bone lining cells regulate the movement of calcium into and out of bones. They also play a role in repairing damaged bones by differentiating into osteoblasts or osteoclasts as needed.
Osteogenic cells are undifferentiated with high mitotic activity. They are capable of differentiating into osteoblasts or other types of bone cells as needed to replenish old or damaged cells.
Together, these four types of bone cells work to grow, shape, maintain, and repair bones.
Osteoblasts
Osteoblasts are specialized mesenchymal cells that play an important role in bone development and remodeling. Their primary function is to lay down new bone during skeletal development and remodeling by depositing and mineralizing new bone.
Osteoblasts directly interact with other cell types within the bone, including osteocytes and hematopoietic stem cells. They also regulate osteoclastogenesis, which is the process of breaking down old bone tissue.
Osteoblasts have been found to play a role in energy homeostasis as well. Studies have shown that osteocalcin, a protein produced by osteoblasts, regulates glucose metabolism and insulin secretion. Osteoblasts also produce matrix Gla protein, which has been linked to vascular calcification.
Osteocytes
Osteocytes are cells that lie within the substance of fully formed bone. They occupy a small chamber called a lacuna, which is contained in the calcified matrix of bone. Osteocytes derive from osteoblasts, or bone-forming cells, and are essentially osteoblasts surrounded by the products they secreted.
Cytoplasmic processes of the osteocyte extend away from the cell toward other osteocytes in small channels called canaliculi. By means of these canaliculi, nutrients and waste products are exchanged to maintain the viability of the osteocyte.
Osteocytes are involved in bone remodeling by transmitting signals to other osteocytes in response to even slight deformations of bone caused by muscular activity.
Osteocytes regulate local mineral deposition and chemistry at the bone matrix level, and they also function as endocrine cells producing factors that target distant organs such as the kidney to regulate phosphate transport.
They are actively involved in bone turnover and play a role in mineral homeostasis versus mechanoreception. Osteocytes regulate the formation of new bone and resorption of old bone, dictating bone remodeling and repair.
In summary, osteocytes are cells found within fully formed bones that play an essential role in regulating local mineral deposition and chemistry at the bone matrix level.
They also function as endocrine cells producing factors that target distant organs such as kidneys to regulate phosphate transport. Osteocytes play an active role in bone turnover, and mineral homeostasis versus mechanoreception, and dictate bone remodeling and repair.
Osteoclasts
Osteoclasts are cells that play a crucial role in bone remodeling and mediate bone loss in pathological conditions by increasing their resorptive activity. They are derived from precursors in the myeloid/monocyte lineage that circulate in the blood after their formation in the bone marrow.
Osteoclasts are giant cells containing between 10 and 20 nuclei, and they closely attach to the bone matrix by binding their surface integrins to a bone protein.
Osteoclasts are unique polykaryons whose activity, in the context of osteoblasts, dictates skeletal mass. They differentiate from hematopoietic stem cells under the influence of two cytokines: macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor kappa-B ligand (RANKL).
M-CSF is necessary for osteoclast differentiation, while RANKL is essential for osteoclast activation.
In summary, osteoclasts are specialized cells that play a vital role in maintaining skeletal health. They degrade bone to initiate normal bone remodeling and mediate bone loss in pathological conditions. Osteoclast differentiation and activation require two cytokines: M-CSF and RANKL.
Osteoprogenitor cells
Osteoprogenitor cells, also known as osteogenic cells, are stem cells in the bone marrow that play a crucial role in bone repair and growth. They are the precursors to more specialized bone cells such as osteocytes and osteoblasts.
Osteoprogenitor cells originate from mesenchymal stem cells (MSCs) and have the potential to differentiate into osteoblasts, fibroblasts, and adipocytes. The differentiation of osteochondroprogenitor cells into the osteoblast cell lineage requires Runx2 and Osx transcription factors.
Osteoprogenitor cells are responsible for mediating bone regeneration through endochondral or intramembranous ossification. Recent evidence shows that major osteoprogenitor cells in the cranial region are primarily located within the periosteum or suture.
A novel lineage of osteoprogenitor cells with dual epithelial-mesenchymal properties has been discovered in mice that can contribute to craniofacial development and repair.
