Bone Structure and Function

• These notes cover bone structure and functions, classification of bones, and cartilage structures and classification.

Skeleton

• A structural compound of bones and cartilages connected by ligaments and joints.
• The human body has approximately 206 distinct bones in adults.
• Bones are rigid organs and weigh about 1/6th of body weight.

Bone

• Bone is connective tissue consisting of cells (osteocytes), collagen fibers, and a mineralized amorphous matrix.
• It's covered by the periosteum, a fibrous connective tissue membrane, and lined by the endosteum.

Periosteum

• The periosteum covers the external surface of the bone, except for articular surfaces.
• Functions of the periosteum include protecting the bone and supplying attachment points for muscles.
• It supports the growth of long bones in width.
• Further, the periosteum carries blood vessels and nerves to the bone.

Classification of Bones

• Bones can be classified according to several criteria: position, shape, structure, and development (ossification).
• By position, bones are classified as axial and appendicular.
• By shape, bones can be long, short, flat, or irregular.
• By structure, bones are either compact or spongy.
• By development, bones are membranous or cartilaginous.
• The axial skeleton includes 80 bones: skull (22), vertebral column (26), hyoid bone (1), auditory ossicles (6), and ribs and sternum (25).
• The appendicular skeleton includes 126 bones: upper extremities (64) and lower extremities (62).
• Examples for bone shape include: femur and humerus (long), carpal and tarsal (short), skull, ribs, sternum, and scapula (flat), vertebrae and hip bone (irregular).
• Pneumatic bones contain air spaces lined by respiratory epithelium, e.g., maxilla, sphenoid, ethmoid.
• Sesamoid bones are found embedded within certain tendons, e.g., patella.

Bone Structure

• Bones can be classified based on their structure: compact or spongy.
• Compact bone is dense and forms the hard, external portion of bones.
• Spongy bone is porous and found inside the compact bone.

Bone Ossification

• Bones can be classified according to their ossification process.
• Cartilaginous ossification (cartilaginous bone) occurs in most long bones, where mesenchyme becomes cartilage which then becomes bone.
• Membranous ossification (membranous bone) occurs in flat bones (skull cap, scapula) and involves rapid ossification for protection, with mesenchymal tissue transforming directly into bone.

Gross Structure of a Long Bone

• A long bone consists of a shaft (diaphysis) and two expanded ends (epiphyses).
• The metaphysis is the junction between the diaphysis and epiphysis.

Diaphysis

• The diaphysis is composed of compact bone covered by periosteum and contains a marrow cavity lined by endosteum.
• The medullary cavity is filled with bone marrow.

Epiphyses

• Epiphyses are composed of spongy bone covered by a thin layer of compact bone.

Metaphysis

• The metaphysis contains the epiphyseal plate made of a a thin plate of cartilage responsible for the length growth of long bones.
• The epiphyseal plate (growth plate) is a hyaline cartilage plate located at the metaphysis between the epiphysis and diaphysis.
• Proliferation of cells in the cartilaginous plate before puberty causes bone growth in length, nourished by epiphyseal and metaphyseal arteries.

Blood supply of bones

• Nutrient artery and vein: a single pair of large blood vessels enters the diaphysis through the nutrient foramen
• Periosteal vessels: provide blood to superficial osteons
• Epiphyseal vessels: usually arise from articular branches supplying the joint
• Metaphyseal vessels: supply the epiphyseal cartilage where bone growth occurs

Bone Functions

• Bones protect vital organs such as the heart, brain, lungs, and spinal cord.
• They transmit and support body weight.
• Bones serve as attachment points for muscles and thereby form joints.
• Bones produce blood cells (hematopoiesis).
• Bones store minerals, including calcium, phosphorus, potassium, and sodium.

Cartilage

• Cartilage is connective tissue composed of cells (chondrocytes) and fibers (collagen or elastic).
• Major cartilage feautures: support soft tissue, provide a smooth, gliding surface for bone articulations, enable the development and growth pf long bones, cushion and absorb shock.
• Cartilage lacks blood vessels and lymphatics; cell nutrition is achieved by diffusion through the matrix.
• Cartilage lacks nerves and is thus insensitive.
• Except for articular cartilage, cartilage is surrounded by a fibrous membrane called the perichondrium.
• Cartilage calcifies, the chondrocytes die, and the cartilage is replaced by bone-like tissue.

Types of Cartilage

• There are three types of cartilage: hyaline, fibrocartilage, and elastic cartilage.
• Hyaline cartilage, the most abundant type, is found in articular cartilages of synovial joints.
• Fibrocartilage, rich in collagen fibers, is present in intervertebral discs and the pubic symphysis.
• Elastic cartilage, rich in elastic fibers, is located in respiratory tubes (trachea and bronchi), the pinna of the ear, the external auditory meatus, the Eustachian tube, and the epiglottis.

Differences between bone and cartilage

• Bone is hard, has a rich nerve and vascular supply, and contains bone marrow.
• Cartilage is firm, lacks nerves and a vascular supply, and does not contain marrow.