6.1 Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

Functional Properties of Cartilage Types

Cartilage is a specialized connective tissue that provides support, flexibility, and resilience to various parts of the skeleton. There are three main types of cartilage, each with distinct properties and functions:

• Hyaline Cartilage: Provides support with flexibility and resilience. It is the most abundant type and forms articular surfaces, costal cartilages, and parts of the respiratory tract.

• Elastic Cartilage: Maintains the shape of a structure while allowing great flexibility. Found in the external ear and epiglottis.

• Fibrocartilage: Resists compression and provides great tensile strength. Located in intervertebral discs and menisci.

Major Cartilages of the Adult Skeleton

Cartilage is distributed throughout the adult skeleton, serving various structural and functional roles.

How Cartilage Grows

Cartilage growth occurs by two mechanisms:

• Appositional Growth: New cartilage tissue is added at the periphery of existing cartilage tissue.

• Interstitial Growth: New cartilage grows from within existing cartilage tissue.

The hard extracellular matrix of cartilage tissue allows for mitosis.

6.2 Bones Perform Several Important Functions

Functions of the Skeleton and Bone Tissue

Bones perform a variety of essential functions for the body:

6.3 Bones are Classified by Their Location and Shape

Major Regions of the Skeleton

The skeleton is divided into two major regions:

• Axial Skeleton: Includes bones of the skull, vertebral column, and rib cage. Functions mainly in protection and support.

• Appendicular Skeleton: Consists of bones of the upper and lower limbs and the girdles that attach them to the trunk. Functions mainly in movement.

Bone Classes by Shape

Bones are classified into four classes based on their shape:

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Most limb bones are long bones except for the kneecap (patella).

6.4 All Bones Consist of Outer Compact Bone and Inner Spongy Bone

Gross Anatomy of a Typical Flat Bone and a Long Bone

Bones have a complex structure with distinct regions and tissues:

• Compact Bone: Dense outer layer that provides strength.

• Spongy Bone: Internal network of trabeculae that provides support and flexibility.

• Red Bone Marrow: Produces red blood cells; found in flat bones and some irregular bones.

• Yellow Bone Marrow: Stores fat; found in the medullary cavity of long bones.

• Periosteum: Outer fibrous layer covering bone.

• Endosteum: Membrane lining the internal bone surfaces.

• Articular Cartilage: Covers joint surfaces.

• Diaphysis: Shaft of a long bone.

• Epiphysis: Ends of a long bone.

Functional Importance of Bone Markings

Bone markings are features on bones that serve as sites for muscle, ligament, and tendon attachment, help form joints, or serve as passages for nerves and blood vessels.

Histology of Compact and Spongy Bone

Bones are composed of different microscopic structures:

• Osteons: Found only in compact bone; structural units.

• Trabeculae: Found only in spongy bone; supporting framework.

• Osteocytes: Found in both compact and spongy bone; mature bone cells.

• Canaliculi: Found in both; tiny canals connecting osteocytes.

Structures and Cell Types in Bone

• D: Small needle-like or flat pieces of bone that form a honeycomb structure (trabeculae).

• I: Cells that resorb (break down) bone (osteoclasts).

• B: Tiny canals filled with tissue fluid and containing extensions of bone cells (canaliculi).

• E: Layers of bone matrix in compact bone (lamellae).

• F: Canals that lie at right angles to the long axis of a long bone (perforating canals).

• G: Mature bone cells that monitor and maintain bone matrix (osteocytes).

• A: Canals that run through the core of each osteon (central canals).

• H: Bone-forming cells that secrete the bone matrix (osteoblasts).

Chemical Composition of Bone

Bones are composed of organic and inorganic components:

• Organic Components: Include osteogenic cells, osteoblasts, osteocytes, bone lining cells, and osteoclasts. These provide flexibility and tensile strength (like rebar in concrete).

• Inorganic Components: Mainly hydroxyapatite (mineral salts), which provide hardness and resistance to compression (like concrete in a highway).

6.5 Bones Develop Either by Intramembranous or Endochondral Ossification

Ossification Processes

Bone formation occurs by two main processes:

• Intramembranous Ossification: Bone develops from a fibrous membrane. This process forms flat bones such as those of the cranium.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage. This process forms most bones of the skeleton, including long bones.

Endochondral ossification is more complex than intramembranous ossification.

Events of Endochondral Ossification

1. A bone collar forms around the diaphysis of the hyaline cartilage model.

2. Cartilage calcifies in the center of the diaphysis and then develops cavities.

3. The periosteal bud invades the internal cavities and spongy bone forms.

4. Secondary ossification centers appear in the epiphyses; a medullary cavity forms in the diaphysis.

5. The epiphyses ossify.

Examples of bones formed by intramembranous ossification include the flat bones of the cranium.

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