Bone and Cartilage

A human skeleton is made of bones and cartilage. 

Bone: Synonymous terms for bone are, Osteon (G), Os (L). There are 206 bones in an adult human body. The bone forms the structural framework of the skeleton and one-third fraction of the connective tissue in the body. The regenerative power of bone is much higher as compared to other tissues, excluding blood. 

Cartilage: Synonymous terms for cartilage are Chondros (G), Gristle. Cartilage is a connective tissue with its cells present in a gel-like matrix. It is surrounded by a perichondrium membrane. The primary cell that initiates cartilage formation is the chondrocyte.

Based on – The Composition of Elements

• The bone constitutes inorganic and organic substances/ matrices. Inorganic salts like calcium phosphate (chiefly) and some calcium carbonate give the bone its hardness and resistance. The organic element is called the collagen fibres (predominantly collagen type I), giving bone flexibility and resilience
• Cartilage is embedded in a mucopolysaccharide rich matrix. It is composed of chondrocytes and collagen fibres

Based on – Growth and Formation

• Bone: it has a foundation laid by mesodermal connective tissues. Surface accretion/ appositional growth occurs at the end of bones, resulting in bone growth. The bone follows a bidirectional growth pattern i.e. displays both longitudinal and radial growth

Membranous type of bone formation occurs in flat bones, whereas endochondral and membranous bone formation is a characteristic feature of the formation of long bones.

• Cartilage: the process of cartilage formation is called chondrogenesis. Differentiation of mesenchyme into chondroblasts cells takes place. The chondroblasts secrete aggrecan and type II collagen, forming a vital part of cartilage formation

Cell division in cartilage is not much significant, therefore minimal change in size and mass takes place.

Based on – Shape and Type

Bones can be classified based on their structure.

• Long bones: examples of such types of bones found in the body are clavicle, humerus, radius, ulna, femur, tibia, fibula etc.
• Short bones: this category includes carpals, tarsals
• Flat bones: examples include the skull, scapulae, sternum, mandible and ribs
• Pneumatic bones: maxilla, ethmoid etc. As the name suggests, they are light in weight
• Sesamoid bones: examples of sesamoid bones are patella, pisiform etc.
• Irregular bones:  their examples include vertebrae, sacrum, coccyx, and hyoid bone

Cartilage is of three types:

• Hyaline cartilage:

• It is the most extensive cartilage present in the body
• Primary constituents of hyaline cartilage are proteoglycans and type II collagen
• The cartilage present in the trachea, nose, epiphyseal growth plate, sternum, etc is hyaline cartilage

• Fibrocartilage:

• The primary constituent is type-one collagen. Unlike hyaline cartilage, it contains small quantities of proteoglycans
• Tendons, ligaments, intervertebral (IV) disc, articular surfaces of some bones, and menisci are covered by fibrocartilage
• Perichondrium layering is absent

• Elastic cartilage: 

• Most commonly present in the larynx, epiglottis and eustachian tube
• The layer of perichondrium is present

Based on – Vascularity, Presence or Absence of Nerve Supply and Bone Marrow

• Bones have their blood supply, they are vascular structures. The source of blood supply to bones are the nutrient artery, periosteal artery, epiphyseal artery and metaphyseal artery. In contrast, cartilage is avascular. It receives its nutrition via matrix.
• Cartilage is insensitive because they lack nerve supply. On the other hand, Bones have their Nerve supply accompanying the blood vessels. Few of them are sensory which supply the articular ends and periosteum of the long bones, flat bones and vertebra.
• Bone marrow is absent in the cartilage whereas bone marrow is found in bones.

Based on – The Presence or Absence of Volkmann and Haversian Canals

Bones contain both Volkmann and Haversian canal, whereas the cartilage lacks both the canals.

Based on – Functional Variation

The function of Bone:

1. They protect vital organs of the body.
2. They act as a lever for the musculature.
3. The bone marrow is the site of the formation of blood cells like RBCs, WBCs etc.

The function of cartilage:

1. It can withstand compression forces.
2. It provides resilience to deforming forces.
3. The chondroitin in the matrix of cartilage, adds to the flexibility and elasticity component.

Conclusion

Bone and cartilage form the basic framework of the human body and they are a kind of connective tissue. The growth pattern is different for both components. They differ with each other based on structure and types, presence or absence of Haversian canal, Volkmann canal, nerve supply, vascularity etc.