Difference between Bone and Cartilage

Bones are hard, mineralized tissues that form the skeleton of vertebrates. They provide both structure and protection to the body. Cartilage is a stiff but resilient connective tissue found in many areas of the body in both vertebrate and invertebrate animals. It is composed mostly of cells called chondrocytes that produce large amounts of extracellular matrix composed of collagen fibers impregnated with calcium phosphate.

Difference in Composition

Bones are an organ that is made up of several tissues, including bone cells, fibrous connective tissue, nerves and blood vessels. They are composed mainly of three materials:

* Bone Cells called Osteocytes  – These cells make up 90% of the bones volume and are responsible for the mineralization of bones. The other 10% is taken up by blood vessels and nerve fibers. Osteocytes stay near the center of bodies to conserve calcium while osteoblasts are found on the surface producing new bone material through a process called ossification. Young adults have about 206 bones however this number can change over the course of ones life.

Cartilage is composed of 70 percent water and 30 percent extracellular matrix which makes up most of its volume. The extracellular matrix consists mainly of collagen fibers that are secreted by chondrocytes, with small amounts of elastin, glycoproteins and proteoglycans  (which give cartilage much of its elasticity). Cartilage does not contain any blood vessels; the only cells it contains are those near its surface. Unlike bone, cartilage cannot repair itself once damaged. Cartilage also lacks osteocytes, bone forming cells found inside the bony matrix (or bone marrow) . However undifferentiated stem cells calledondrocytes reside in the cartilage. There are two types of cartilage in the body, hyaline cartilage and elastic cartilage.

Differences in Composition Chondrocytes are cells found in cartilage. They secrete a well-organized extracellular matrix which makes up most of the volume in cartilage tissue. Cartilaginous tissues consists mainly of collagen fibers that are secreted by chondrocytes, with small amounts of elastin, glycoproteins and proteoglycans  (which give cartilage much of its elasticity). Cartilages lack blood vessels; the only cells it contains are those near its surfaces. As opposed to bones, cartilages cannot repair themselves once damaged because they lack osteocytes, bone forming cells found inside the bony matrix (or marrow).

Cartilage is mostly composed of extracellular protein fibers and water. The protein fibers are mainly collagen and elastin, while proteoglycans are also present in cartilages. Cartilage lacks blood vessels but has many nerve endings which makes it very sensitive to touch.


Differences Between Bone Cells vs Cartilage Cells

Many people have a hard time distinguishing between cells of bone and cells of cartilage, but understanding the difference is key to devising proper treatment methods for bone and joint injuries. Bone cells are called osteocytes, while cartilage cells are chondrocytes. Both cell types grow through mitosis (cell division) and help replace damaged tissue in living organisms.

Cell Types

Osteocytes, which are made up of spindle-shaped or stellate cells, make up 80 percent of the volume in compact bones . The other 20 percent belongs to other types of bone cells, including lining cells , blood vessels , nerves and osteoclasts . Chondrocytes form 70 to 80 percent of the volume within articular cartilage , the smooth, firm tissue covering the ends of bones. The other 20 to 30 percent is occupied by chondroblasts , which synthesize the matrix proteins that give cartilage its elasticity.

Cells of Bone vs. Cells of Cartilage

Osteocytes are roughly 10 to 25 micrometers in diameter and can be as long as 100 micrometers when stretched out fully . Chondrocytes are smaller during their initial growth phase but become larger than osteocytes once they begin secreting collagen molecules for use in cartilaginous structures. Articular chondrocytes also contain large vesicles called extrachromosomal Golgi bodies within their cytoplasm, which make enzymes used to hydroxylate procollagen molecules. As a cell ages, it also begins to produce lipofuscin granules , which are brownish-yellow in color and contain byproducts of incompletely metabolized proteins or other potentially harmful materials.

Osteocytes maintain the bones they inhabit through the secretion of osteoid , an organic matrix containing collagen fibers that become mineralized with calcium phosphate salts during development . This process occurs faster near growing ends of bone, where osteoblasts operate. Chondrocytes are responsible for generating cartilage matrix that eventually becomes calcified , rendering it stronger than noncalcified cartilage but making it more brittle as well. New chondrocytes divide close to the surface of articular cartilage, where they are exposed to nutrients in the blood. Once generated, these cells move deeper into the tissue toward bone , where they lose their ability to divide.

Bone cells are responsible for bone remodeling , which occurs when osteoblasts stimulate resorption of old or damaged areas on bones by secreting collagenase and other enzymes . This process also happens at growing ends of bones where active chondrocytes reside. When cells stop dividing during puberty , this function shifts from osteoblasts to osteoclasts , which are large multinucleated cells filled with digestive vacuoles that assist them in breaking down minerals within bones.

Cell Signaling

Chondrocytes receive signals via growth factors secreted by osteoblasts and other cells. These factors move toward the center of cartilage, where they bind to a receptor on surface cells that sends an influx of calcium into these cells . Calcium ions then stimulate chondrocytes to maintain biosynthesis within their matrix as long as possible before dying.

Osteocytes receive signals from both chondrocytes and osteoblasts by secreting cytokines , which are chemicals designed to communicate with immune cells . This process participates in bone development but also plays a role in disease onset and metastasis (the spread of cancerous tumors).

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Bone cells are larger than cartilage cells. Bone cells usually have multiple nuclei while cartilage is made up of one single nucleus. Both bone and cartilage cells can be affected by metastasis.

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