Bones and Bone Tissue: Structure, Function, and Microscopic Anatomy

Ch. 6 Bones and Bone Tissue

Introduction to Bone and Skeletal Tissue

The skeletal system is composed of bones and associated connective tissues. Bone is a dynamic, living tissue that responds to its environment and plays several critical roles in the human body.

• Support: Bones provide a rigid framework that supports the body and maintains its shape.

• Protection: Bones protect soft internal organs, such as the brain (protected by the skull) and the heart and lungs (protected by the rib cage).

• Movement: Bones act as levers for muscles, enabling movement.

• Blood Cell Production: Bones produce blood cells in a process called hematopoiesis, which occurs in the red marrow.

• Storage: Bones store minerals (especially calcium and phosphate) and triglycerides (fat) in yellow marrow.

Types of Bones

Bones are classified based on their shapes and structures, each adapted for specific functions.

• Long Bones: Longer than they are wide, with expanded ends (e.g., femur, humerus). Adapted for movement and leverage.

• Short Bones: Roughly cube-shaped (e.g., wrist and ankle bones). Provide stability and support with limited motion.

• Flat Bones: Thin, flattened, and often curved (e.g., sternum, ribs, cranial bones). Provide protection and broad surfaces for muscle attachment.

• Irregular Bones: Complex shapes that do not fit other categories (e.g., vertebrae, pelvis, facial bones).

• Sesamoid Bones: Develop within tendons (e.g., patella). Vary in number and protect tendons from stress and wear.

Gross Anatomy of Bone: Compact and Spongy Bone

Bones have two main structural arrangements: compact bone and spongy bone. These arrangements provide both strength and lightness to the skeleton.

• Compact Bone: Dense and solid, with no visible spaces. Optimized for strength and hardness. Found on the outer edges of all bones and in the shaft (diaphysis) of long bones.

• Spongy Bone: Porous, with a lattice-like structure called trabeculae. Reduces bone weight and contains spaces filled with marrow. Found mainly in the ends (epiphyses) of long bones and inside flat, short, and irregular bones.

Gross Anatomy of Bone: Periosteum and Endosteum

Bones are covered and lined by specialized connective tissue membranes that play roles in nourishment, growth, and repair.

• Periosteum: A double-layered membrane covering the external surface of bone (except at joints). The outer fibrous layer is dense irregular connective tissue, while the inner osteogenic layer contains bone stem cells. Perforating fibers anchor the periosteum to the bone matrix, providing strong attachment points for tendons and ligaments.

• Endosteum: A thin membrane lining the internal surfaces of bone, including the medullary cavity and trabeculae of spongy bone. Contains osteogenic cells similar to the periosteum's inner layer.

Gross Anatomy of Bone: Bone Marrow

The spaces within bones are filled with bone marrow, which exists in two forms:

• Red Marrow: Site of hematopoiesis (formation of blood cells). Found mainly in spongy bone of adults and in most bones of infants and children.

• Yellow Marrow: Stores triglycerides (fat). Found in the medullary cavity of long bones in adults. Can revert to red marrow if necessary (e.g., in cases of severe blood loss).

Gross Anatomy of Bones: Structure of a Long Bone

Long bones have a characteristic structure with distinct regions:

• Epiphysis: The expanded ends of the bone, covered with articular cartilage for joint movement.

• Diaphysis: The shaft of the bone, containing the medullary cavity filled with yellow marrow in adults.

• Metaphysis: The region where the diaphysis and epiphysis meet. Contains the epiphyseal plate (growth plate) in children and adolescents, which becomes the epiphyseal line (bone) in adults after growth ceases.

Gross Anatomy of Bones: Nerves and Blood Supply

Bones are highly vascularized and innervated. Blood vessels and nerves enter the bone through small openings called foramina.

• Nutrient Foramen: A small hole in the diaphysis for the passage of the nutrient artery, vein, and nerves.

• Blood Supply: The nutrient artery supplies the medullary cavity, while veins carry blood out. Smaller foramina are found in the metaphysis and epiphysis.

Microscopic Anatomy of Bones: Bone Matrix

The extracellular matrix of bone is composed of two main components, each contributing to the bone's unique properties:

• Inorganic Matrix: Consists mainly of hydroxyapatite crystals (calcium and phosphate), making up about 65% of bone mass. Provides hardness and the ability to resist compression.

• Organic Matrix (Osteoid): Composed of collagen fibers and ground substance, making up about 35% of bone mass. Provides tensile strength and flexibility.

Key Equation:

$\text{Hydroxyapatite:} \ \text{Ca}_{10}(\text{PO}_4)_6(\text{OH})_2$

Microscopic Anatomy of Bones: Bone Cells

Bone tissue contains several types of cells, each with specialized functions:

• Osteoprogenitor (Osteogenic) Cells: Stem cells found in the periosteum and endosteum that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells that secrete the organic matrix (osteoid). Some become trapped in the matrix and differentiate into osteocytes.

• Osteocytes: Mature bone cells that maintain the bone matrix. Located in lacunae and connected by canaliculi for communication and nutrient exchange.

• Osteoclasts: Large, multinucleated cells derived from white blood cells. Responsible for bone resorption (osteolysis) by secreting acids and enzymes to dissolve bone matrix.

Microscopic Anatomy of Bones: The Osteon

The osteon (Haversian system) is the structural unit of compact bone, providing strength and support.

• Central Canal: Runs parallel to the long axis of the bone, containing blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix around the central canal.

• Lacunae: Small spaces between lamellae that house osteocytes.

• Canaliculi: Tiny channels connecting lacunae, allowing communication and nutrient exchange between osteocytes.

• Perforating (Volkmann's) Canals: Run perpendicular to central canals, connecting blood and nerve supply of the periosteum to those in the central canals and medullary cavity.

Microscopic Anatomy of Bones: Structure of Lamellae

The arrangement of collagen fibers within and between lamellae provides bones with strength in multiple directions, allowing them to resist twisting and bending forces.

• Within Lamella: Collagen fibers run in the same direction, providing strength along that axis.

• Between Adjacent Lamellae: Collagen fibers run in alternating directions, increasing resistance to torsion and fracture.

Microscopic Anatomy of Bones: Trabeculae

Spongy bone is composed of trabeculae, which are small, needle-like pieces of bone aligned along lines of stress. Trabeculae contain lamellae, osteocytes, and canaliculi, but lack well-organized osteons and central canals. This structure allows spongy bone to be lightweight yet strong.

• Trabeculae: Provide structural support and house bone marrow.

• Canaliculi: Allow osteocytes in spongy bone to receive nutrients and communicate, despite the absence of central canals.

Additional info: The notes above include expanded academic context, definitions, and examples to ensure completeness and clarity for college-level anatomy and physiology students.