Bones and Skeletal Tissue

Why This Matters

Understanding bone anatomy and the process of bone remodeling is essential for working with patients with bone diseases such as osteoporosis. The skeletal system provides structure, protection, and support for the body, and is crucial for movement and mineral storage.

Skeletal Cartilages

Basic Structure, Types, and Locations

• Skeletal cartilage: Highly resilient, molded tissue consisting primarily of water. It does not contain blood vessels or nerves.

• Perichondrium: Dense connective tissue surrounding cartilage, acting as a girdle. Contains blood vessels for nutrient delivery.

• Cartilage is made up of chondrocytes, cells encased in small cavities (lacunae) within a jelly-like extracellular matrix.

Types of Cartilage

• Hyaline cartilage: Most abundant type; provides support, flexibility, and resilience. Contains collagen fibers only. Found in articular (joint), costal (rib), respiratory (larynx), and nasal cartilage.

• Elastic cartilage: Similar to hyaline but contains elastic fibers. Found in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, providing great tensile strength. Found in menisci of knee and intervertebral discs.

Growth of Cartilage

• Appositional growth: New matrix is laid down on the surface of cartilage.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

• Calcification of cartilage occurs during normal bone growth in youth, but hardened cartilage is not the same as bone.

Functions of Bones

Major Functions

• Support: Provides framework for body and soft organs.

• Protection: Protects brain, spinal cord, and vital organs.

• Movement: Acts as levers for muscle action.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities.

• Triglyceride (fat) storage: Fat is stored in bone cavities.

• Hormone production: Osteocalcin helps regulate insulin secretion, glucose levels, and metabolism.

Classification of Bones

By Location

• Axial skeleton: Long axis of body (skull, vertebral column, rib cage).

• Appendicular skeleton: Bones of upper and lower limbs, girdles attaching limbs to axial skeleton.

By Shape

• Long bones: Longer than wide (limb bones).

• Short bones: Cube-shaped (wrist, ankle); sesamoid bones form within tendons (e.g., patella).

• Flat bones: Thin, flat, slightly curved (sternum, scapulae, ribs, skull bones).

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

Bone Structure

Tissue Types in Bone

• Bones are organs containing bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels.

• Three levels of structure: Gross, Microscopic, Chemical.

Gross Anatomy

• Compact bone: Dense outer layer; appears smooth and solid.

• Spongy bone: Honeycomb of small, needle-like or flat pieces called trabeculae. Spaces between trabeculae are filled with red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

• Consist of thin plates of spongy bone (diploë) covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes: periosteum (outer) and endosteum (inner).

• No shaft or epiphyses; bone marrow is scattered throughout spongy bone.

• Hyaline cartilage covers area of bone that is part of a movable joint.

Structure of Typical Long Bone

• Diaphysis: Tubular shaft forming the axis of bone; contains compact bone surrounding medullary cavity filled with yellow marrow in adults.

• Epiphyses: Ends of long bones; consist of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of childhood epiphyseal plate where bone growth occurs.

Membranes

• Periosteum: Double-layered membrane covering external bone surfaces except joints.

  • Fibrous layer: Dense irregular connective tissue; contains Sharpey's fibers anchoring to bone matrix.

  • Osteogenic layer: Inner layer containing osteogenic stem cells.

• Endosteum: Delicate connective tissue membrane covering internal bone surface; contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red marrow found within trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum).

• In adults, red marrow is located in heads of femur and humerus, and most active areas are flat bone diploë and some irregular bones (e.g., hip bone).

• Yellow marrow can convert to red if a person becomes anemic.

Bone Markings

• Sites of muscle, ligament, and tendon attachment; areas involved in joint formation or conduits for blood vessels and nerves.

• Three types:

  • Projection: Outward bulge of bone (due to increased stress from muscle pull or joint modification).

  • Depression: Bowl or groove-like cut that can serve as passageways for vessels and nerves or places for joints.

  • Opening: Hole or canal in bone serving as passageways for blood vessels and nerves.

Microscopic Anatomy of Bone

Cells of Bone Tissue

• Osteogenic cells: Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells secreting unmineralized bone matrix (osteoid), made up of collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress or strain sensors.

• Bone-lining cells: Flat cells on bone surfaces; help maintain matrix. Called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Giant, multinucleate cells for bone resorption (breakdown); derived from hematopoietic stem cells. Located in depressions called resorption bays.

Compact Bone Structure

• Osteon (Haversian system): Structural unit of compact bone; elongated cylinder parallel to long axis of bone. Consists of rings of bone matrix called lamellae.

• Lamellae contain collagen fibers running in different directions; withstand stress and resist twisting.

• Bone salts are found between collagen fibers.

Canals and Canaliculi

• Central (Haversian) canal: Runs through core of osteon; contains blood vessels and nerve fibers.

• Perforating (Volkmann's) canals: Canals lined with endosteum at right angles to central canal; connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

• Lacunae: Cavities containing osteocytes.

• Canaliculi: Hairlike canals connecting lacunae to each other and to central canal; allow communication and nutrient transfer between osteocytes.

Interstitial and Circumferential Lamellae

• Interstitial lamellae: Lamellae not part of osteon; fill gaps between forming osteons or remnants of osteons cut by bone remodeling.

• Circumferential lamellae: Extend around entire surface of diaphysis; help long bone resist twisting.

Spongy Bone

• Trabeculae are organized along lines of stress to help resist stress.

• No osteons present; trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid.

• Osteoid: Makes up one-third of organic bone matrix; secreted by osteoblasts. Consists of ground substance and collagen fibers, contributing to strength and flexibility.

• Resilience due to sacrificial bonds in or between collagen molecules that stretch and break to dissipate energy and prevent fractures.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up 65% of bone by mass; mainly calcium phosphate crystals in and around collagen fibers.

• Responsible for hardness and resistance to compression.

• Bones are half as strong as steel in resisting compression and as strong as steel in resisting tension.

• Mineral composition lasts long after death and can reveal information about ancient people.

Summary Table: Types of Cartilage

Key Equations

• Bone matrix composition:

Additional info: These notes expand on the original points with definitions, examples, and academic context for clarity and completeness.