Bones are hard connective tissues which form skeletal system while cartilages are soft connective tissues which are present in nose, ear, joints and on the coverings of bones.
What is Bone?
Bones are strong, flexible and semi rigid supporting connective tissues. Bone is made up of cells and extracellular matrix. Cells of bones are called osteoblasts and osteocytes. Two other types of cells are also present which are osteoclasts and osteoprogenitor cells. The extracellular matrix of bones is made up of an organic matrix (30%) containing proteoglycans, glycosaminoglycans, glycoproteins, osteonectin, and osteocalcin. Besides these, collagen fibers are present. 70% bone is made up of a bone mineral called hydroxyapatite, and 30% is made up of water. Bones are resistant to twisting, bending, compression, and stretch. It is hard because it is calcified and collagen fibers help it to resist tensile stresses. The primary bone which is formed at any site is called woven bone which is soon replaced by lamellar bone. In woven bone, the collagen fibers are random while in lamellar bone, these are parallel in layers. There are two types of mature bones; compact and spongy bones. Compact bones are found in the shaft of long bones in the diaphysis. These are 80% of all bones. Spongy bones are located at the ends of long bones in the epiphysis. These make 20% of all bones. Red bone marrow and a network of bony trabeculae are present in spongy bones. A dense fibrous layer where muscles insert, called periosteum is found on the outside of bones. Bone forming cells are also found in the periosteum. Tissues that cover the inner surface of bones are called endosteum. The inner cavity of bones contains blood vessels and bone marrow which nourish the bone and play a significant role in its growth. There are two ways of growth in bones; endochondral and intramembranous. Endochondral is the formation of bone onto temporary cartilage model while intramembranous is the formation of bone directly onto fibrous connective tissues. This type of bone formation occurs in a few specialized places such as the flat bones of skull, maxilla, mandible, and clavicles. Cells of mesenchyme differentiate into osteoprogenitor cells, then into osteoblasts, which secrete the bone matrix. Once the osteoblasts are embedded in the matrix, they are called as osteocytes.
What is Cartilage?
Cartilage is a flexible, strong and semi rigid supporting tissue. It can withstand compression but also show bending. It is made up of cells (chondroblasts and chondrocytes) and extracellular matrix. Besides these, 10% aggrecan, 75% water and a mix of collagen fibers are also present. Cartilages are used to form supporting framework of some organs, articulating surfaces of some organs and to constitute the template for the growth and development of long bones. Cartilages are of three types; hyaline cartilage, fibrocartilage, and elastic cartilage. Hyaline cartilage is most common which has a glassy appearance. Fibrocartilages are tendon insertions and invertebral discs which are reinforced with parallel bundles of collagen fibers. Elastic cartilages are external ear and epiglottis and resilient which has elastic fibers as well as collagen fibers. Cartilages are avascular so are nourished by long range diffusion from nearby capillaries in the perichondrium. So cartilages cannot become thick because diffusion would not be sufficient to supply the cartilage with nutrients and oxygen. There are two ways of cartilage growth; interstitial growth and appositional growth. In interstitial growth, chondrocytes grow and divide and lay down more matrix inside the existing cartilage. It happens during childhood and adolescence. In appositional growth, new surface layers of the matrix are added to the pre-existing matrix by new chondroblasts from the perichondrium.
Bone vs. Cartilage
- Cells of bones are known as osteocytes while cells of cartilage are called chondrocytes.
- The matrix of bones is vascular while the matrix of cartilage is avascular.
- The matrix of bones is organic and inorganic while cartilage’s matrix is entirely organic.
- The matrix of bones contains lamellae while the matrix of cartilage is homogenous without lamellae.
- The matrix of bones has a protein which is called ossein while protein of cartilage matrix is called chondrin.
- Growth pattern of bones is bidirectional while growth pattern of cartilage is unidirectional.
- Bones have a high potential of healing while cartilages have the poor healing ability.
- Bone stores high concentration of calcium while cartilage has no significance of calcium stores.