Anatomy & Physiology: Structure and Function of Bones and Cartilage

6.1 Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

Functional Properties of the Three Types of Cartilage

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:

• Elastic cartilage: Maintains the shape of a structure while allowing great flexibility. Example: External ear

• Fibrocartilage: Resists compression and provides great tensile strength. Example: Intervertebral discs

• Hyaline cartilage: Provides support with flexibility and resilience. Example: Articular surfaces of bones

Major Cartilages of the Adult Skeleton

Cartilage is distributed throughout the body, serving various structural and functional roles. The table below summarizes the location and type of major cartilages:

Type of cartilage	Location in the body
Hyaline cartilage	Covers the ends of most bones at movable joints
Elastic cartilage	Cartilage of the external ear
Hyaline cartilage	Cartilage that supports the nasal cavity
Fibrocartilage	Discs of cartilage that join the hip bones anteriorly
Hyaline cartilage	Coastal cartilage that connects ribs to the sternum (breastbone)
Fibrocartilage	Patella cartilages (menisci) of the knee joint
Elastic cartilage	Forms the epiglottis (the flap that bends to cover the larynx)
Hyaline cartilage	Cartilage that reinforces the walls of the trachea

Cartilage Growth and Flexibility

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

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

• Flexible: The extracellular matrix of cartilage tissue allows for mitosis and growth.

6.2 Bones Perform Several Important Functions

Functions of the Skeleton and Bone Tissue

Bones are dynamic organs that perform a variety of essential functions for the body. The table below summarizes these functions:

Function	Description
Support	Bones provide a framework that supports the body. For example, the bones of the lower limbs support the body when standing.
Protection	The brain and spinal cord are protected by the bones of the skull and vertebrae. The rib cage protects the heart and lungs.
Anchorage	Skeletal muscles attach to bones, which act as levers allowing us to move the body and its parts.
Mineral storage	Bones store minerals, most importantly calcium and phosphate.
Blood cell formation	Blood cell formation occurs in the red bone marrow of certain bones.
Triglyceride (fat) storage	Yellow bone marrow stored in the shaft of long bones is fat, a source of energy.
Hormone production	Bones produce osteocalcin, a hormone that helps to regulate insulin secretion, glucose homeostasis, and energy expenditure.

6.3a Name the Major Regions of the Skeleton and Describe Their Relative Functions

Axial and Appendicular Skeleton

• Appendicular skeleton: Consists of the bones of the upper and lower limbs and the girdles that attach the limbs to the trunk.

• Axial skeleton: Includes bones of the skull, vertebral column, and rib cage.

General Functions:

• The bones of the axial skeleton protect, support, or carry other body parts.

• The bones of the appendicular skeleton help us move from place to place (locomotion) and manipulate the environment.

Bone Classes and Examples

Bones are classified into four main types based on their shape:

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

• Short bones: Cube-shaped (e.g., wrist and ankle bones)

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

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

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

Gross Anatomy of a Typical Flat Bone and a Long Bone

• Compact bone: Dense outer layer that looks smooth and solid.

• Spongy bone: Internal layer with a honeycomb structure (trabeculae).

• Red bone marrow: Site of blood cell formation, found in spongy bone of flat bones and some long bones.

• Yellow bone marrow: Fat storage, found in the medullary cavity of long bones.

• Periosteum: Outer fibrous layer covering the bone.

• Endosteum: Thin membrane lining the internal bone surfaces.

Functional Importance of Bone Markings

Bones have various markings that serve as sites for muscle and ligament attachment, help form joints, or serve as passages for nerves and blood vessels.

Bone Marking	Projections (muscle/ligament attachment)	Surfaces (form joints)	Depressions/Openings
Meatus			X
Condyle		X
Tubercle	X
Foramen			X
Spine	X
Facet		X
Fossa			X
Epicondyle	X
Tuberosity	X
Head		X

Histology of Compact and Spongy Bone

• Osteons: Compact bone only

• Trabeculae: Spongy bone only

• Osteocytes: Both compact bone and spongy bone

• Canaliculi: Both compact bone and spongy bone

6.4d: Chemical Composition of Bone and Advantages of Components

Organic and Inorganic Components

• Organic components: Cells and osteoid (ground substance and collagen fibers). Provide flexibility and tensile strength.

• Inorganic components: Mineral salts, mainly hydroxyapatite (calcium phosphates). Provide hardness and resistance to compression.

Highway Analogy: Organic components are like reinforcing bars (rebar) in concrete, providing flexibility and tensile strength. Inorganic components are like the concrete, providing hardness and resistance to compression.

6.5: Bone Development by Intramembranous or Endochondral Ossification

Intramembranous vs. Endochondral Ossification

• Intramembranous ossification: Bone develops from a fibrous membrane. Most flat bones form this way.

• Endochondral ossification: Bone develops by replacing hyaline cartilage. Most bones of the skeleton are formed this way.

Steps of Endochondral Ossification:

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

2. The epiphyses ossify.

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

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

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

Key Points:

• Endochondral ossification is more complex than intramembranous ossification.

• Most bones that form by intramembranous ossification are flat bones.

Equations and Scientific Terms

• Osteoid: Unmineralized bone matrix composed of ground substance and collagen fibers.

• Hydroxyapatite: The chief inorganic component of bone, with the formula $Ca_{10}(PO_4)_6(OH)_2$.

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