Chapter 6 - Bones/Skeletal System

Bones & Skeletal Tissue

Functions of the Skeletal System

• Support: Provides a robust structural framework for the body, supporting soft tissues and organs; the skeleton maintains the shape of the body and anchors muscles.

• Storage of minerals: Bones are crucial reservoirs for essential minerals such as calcium and phosphorus, which are released into the bloodstream as needed to maintain mineral balance and physiological functions.

• Storage of lipids: Yellow marrow serves as an important energy reserve by storing lipids (fats), which can be metabolized for energy during times of increased energy demand or nutritional deficiency.

• Hemopoiesis: The process of blood cell formation occurs primarily in the red marrow of bones, where red blood cells, white blood cells, and platelets are produced, playing vital roles in oxygen transport, immune response, and blood clotting.

• Protection: Bones play a critical protective role by encasing and safeguarding vital organs; for example, the skull protects the brain, the rib cage safeguards the heart and lungs, and the vertebrae shield the spinal cord.

• Movement: Bones work in concert with muscles, acting as levers that allow for a wide range of motion in the body. Joints serve as pivot points that enable movement in various directions.

Structures of the Skeletal System

• Total Count: The adult human skeleton typically comprises 206 bones that vary in size and shape, which collectively serve various functions.

• Components: The skeletal system also includes important supportive tissues such as:

  • Cartilage, which provides flexibility and cushioning at joints.

  • Ligaments that connect bones to other bones, supporting the stability of joints.

  • Tendons that attach muscles to bones, facilitating movement.

Types of Skeletons

• Axial Skeleton: Comprises 80 bones along the body's central axis, including the skull, vertebral column, and rib cage, playing a key role in protecting the central nervous system and the thoracic cavity.

• Appendicular Skeleton: Includes 126 bones of the limbs and girdles (shoulder and pelvic), responsible for facilitating movement and providing further leverage for muscle action.

Types of Cartilage

• Hyaline Cartilage: The most abundant type, found in joints, coastal cartilages, and respiratory structures; it provides smooth surfaces for joint movement and functions as a precursor to bone in the developing skeleton.

• Elastic Cartilage: Found in structures requiring flexibility such as the epiglottis and external ear, allowing these structures to maintain shape while still being pliable.

• Fibrocartilage: Provides strong support and can withstand pressure; it is found in intervertebral discs and the pubic symphysis, acting as a shock absorber in areas subject to stress.

Classification of Bones

• By Shape: Bones are categorized into:

  • Long Bones: Such as the humerus and femur, primarily function in leverage and movement.

  • Flat Bones: Such as the sternum and skull bones, serve protective functions and provide broad areas for muscle attachment.

  • Irregular Bones: Such as vertebrae, which have complicated shapes that fit specific function requirements.

  • Short Bones: Such as carpals in the wrist, which provide stability and support while allowing for some motion.

• By Location: Bones are further classified into:

  • Sesamoid Bones: Embedded within tendons (e.g., patella), enhancing leverage and protecting tendons from excessive wear.

  • Sutural Bones (Wormian Bones): Tiny bones located within the sutures of the skull, which can vary in number and help with the growth of the skull.

Bone Tissue Overview

• Characteristics of Bone Tissue:

  • Dense, supportive connective tissue that forms a hard skeletal structure.

  • Contains specialized bone cells that produce a solid matrix of calcium salts (mineralized) around collagen fibers, providing both strength and flexibility.

Compact Bone Tissue

• Composition: Characterized by a dense matrix comprised of calcium salts and bone cells, contributing to its strength.

• Structural Features:

  • Osteon/Haversian System: The basic structural unit of compact bone, consisting of concentric lamellae surrounding central canals.

  • Central Canal (Haversian Canal): Contains blood vessels and nerves essential for bone health and maintenance.

  • Canaliculi: Microscopic canals that connect lacunae, facilitating the exchange of nutrients and waste.

  • Perforating Canals (Volkmann’s Canals): Channels providing pathways for blood vessels and nerves to connect to the Haversian systems from the periosteum.

Spongy Bone Tissue

• Characteristics: Comprised of trabeculae (thin rods or plate-like structures) instead of osteons, creating an open network which reduces weight while maintaining strength.

• Avascular: Lacks blood vessels and relies on diffusion for nutrient supply; this structure is particularly beneficial in helping to reduce overall bone mass while maintaining crucial functions.

• Contains red bone marrow, a primary site of hematopoiesis, and can also contain yellow marrow that stores fat.

Anatomy of a Long Bone

• Diaphysis: The long, tubular shaft that acts as the weight-bearing and supporting element of the bone.

• Epiphysis: Ends of the bone enriched with spongy bone and red marrow, contributing to joint formation and housing hematopoietic tissue.

• Metaphysis: The transitional region where diaphysis meets epiphysis, which contains the growth plate in developing bones.

• Medullary Cavity: The hollow interior of the diaphysis containing yellow marrow, crucial for lipid storage and energy metabolism.

• Periosteum: A vital dense layer of vascular connective tissue that surrounds all bones, containing Sharpey's fibers anchoring it firmly; plays a role in bone growth and repair.

• Endosteum: A thin layer of connective tissue lining the medullary cavity and bony canals, involved in bone remodeling and maintenance.

Anatomy of a Flat Bone

• Structure: Flat bones resemble a sandwich, composed of layers of compact bone (the outer cortex) surrounding spongy bone (the diploë), providing strength with minimal weight.

Matrix Minerals and Proteins

• Bone Matrix Composition:

  • Minerals (2/3): Primarily consisting of calcium phosphate ($Ca<em>3(PO</em>4)<em>2$), which reacts with calcium hydroxide ($Ca(OH)</em>2$) to form hydroxyapatite ($Ca<em>{10}(PO</em>4)<em>6(OH)</em>2$), providing hardness and strength to the bone structure.

  • Proteins (1/3): Mainly composed of collagen fibers, which confer tensile strength and flexibility, preventing fractures during stress.

Bone Cells Types

• Osteocytes: The most abundant type, these mature bone cells maintain the bone matrix and can communicate with other bone cells to respond to mechanical stress.

• Osteoblasts: Immature bone cells that synthesize and secrete the bone matrix during the formation of new bone; when surrounded by matrix, they become osteocytes.

• Osteoclasts: Large multinucleated cells responsible for resorption of bone matrix, important for bone remodeling and calcium homeostasis.

• Osteoprogenitor Cells: Stem cells found in the endosteum and periosteum that can differentiate into osteoblasts, playing a crucial role in bone repair and maintenance.

Bone Development

• Terminology:

  • Osteogenesis: The intricate process by which bone tissue is created, both during embryonic development and throughout life.

  • Ossification: Involves the transformation of other types of tissue into bone, critical for skeletal development.

  • Calcification: The deposition of calcium salts during ossification, essential for the hardening of bone tissue.

Forms of Ossification

• Intramembranous Ossification: A process where bone develops directly from mesenchymal (connective) tissue, as seen in the formation of flat bones of the skull.

• Endochondral Ossification: This method involves the development of bone from a cartilage template, which is prominent in the formation of long bones, including the limb bones.

Endochondral Ossification Steps

1. Bone Collar Formation: Mesenchymal cells differentiate into osteoblasts, forming a bone collar around the diaphysis of the hyaline cartilage model.

2. Chondrocyte Hypertrophy and Matrix Calcification: Chondrocytes undergo enlargement and signal the calcification of the surrounding matrix; dying chondrocytes create spaces for blood vessel invasion.

3. Periosteal Bud Invasion: Blood vessels invade the cavities, leading to the formation of spongy bone from the primary ossification center.

4. Diaphysis Elongation: The diaphysis lengthens, a medullary cavity forms, and secondary ossification centers begin emerging in the epiphyses.

5. Epiphyseal Ossification: Final bone formation occurs in the epiphyses, with remaining cartilage limited to articular surfaces and epiphyseal plates regulating growth.

Effects of Hormones on Bone Growth

• Calcitriol: Increases absorption of calcium and phosphorus in the intestines; its synthesis is dependent on adequate Vitamin D levels in the body.

• Growth Hormone & Thyroxine: Both hormones stimulate bone growth, impacting general skeletal health.

• Estrogens and Androgens: Hormones that promote osteoblast activity, leading to increased bone density during the growth spurts of puberty.

• Calcitonin: A hormone that decreases osteoclast activity, thereby lowering blood calcium levels and promoting bone deposition.

• Parathyroid Hormone (PTH): Increases osteoclast activity, raising blood calcium levels; plays a critical role in calcium homeostasis and bone remodeling.

Calcium Homeostasis

• Importance of Calcium: Essential for various physiological processes including muscle contraction, nerve impulse transmission, and blood coagulation.

• Regulatory Mechanisms:

  • PTH: Amplifies blood calcium levels by stimulating osteoclasts to release calcium from bones and enhancing dietary absorption.

  • Calcitonin: Functions to lower blood calcium levels by inhibiting osteoclast activity and promoting calcium retention in bones.

Aging and Bones

• Osteopenia: Initiates typically between ages 30 and 40; characterized by gradual bone loss and reduced bone density, making bones more susceptible to fractures.

• Osteoporosis: A medical condition affecting about 29% of women and 18% of men over age 45, leading to a significant reduction in bone mass and an increased risk of fractures due to compromised structural integrity.

Osteomalacia and Rickets

• Osteomalacia: Softening of bones due to inadequate mineralization, leading to soft and weak structures; often caused by vitamin D deficiency in adults.

• Rickets: A pediatric equivalent of osteomalacia, resulting from vitamin D deficiency during periods of rapid growth, leading to bone deformities and growth issues.

Fractures

• Definition: Defined as cracks or breaks in bones resulting from physical stress; the body can initiate repair processes as long as blood supply and the periosteum/endosteum are intact.

Fracture Repair Steps

1. Formation of a hematoma: Initial formation of a blood clot that initiates the inflammatory response and sets the stage for healing.

2. Formation of a fibrocartilaginous callus: Stabilizes the fracture site through new blood vessel growth and cartilage formation around the injured area.

3. Formation of a bony callus: Osteoblasts replace the cartilage with spongy bone; firm union of the bone is typically established within two months post-injury.

4. Remodeling of bone structure: Ongoing remodeling reduces the size of calluses and restores the bone's original structure; this process may take up to a year to complete.

Types of Fractures

• Transverse Fractures: Breaks that occur straight across the bone’s long axis, often due to direct impact.

• Spiral Fractures: Result from twisting forces applied to the bone, typically indicative of a torque impact.

• Displaced Fractures: Bones are misaligned due to the fracture, whereas non-displaced fractures retain anatomical alignment.

• Colles' Fracture: A common distal radius fracture caused by falling on an outstretched hand, often seen in older adults.

• Greenstick Fracture: An incomplete fracture where one side of the bone bends; prevalent in children due to the pliability of their bones.

• Epiphyseal Fractures: Occur at the growth plates and can disrupt normal growth patterns; require careful management to prevent long-term complications.

• Compression Fractures: Frequently seen in vertebrae subjected to extreme stress, potentially leading to a decrease in height and significant pain.

Paget’s Disease

• Overview: A chronic disorder characterized by excessive bone formation resulting in weak bones; though bones may be enlarged, they become structurally disorganized, increasing their vulnerability to fractures.

• Progression: The disease typically begins with peak osteoclast activity, leading to increased spongy bone formation, which is then followed by heightened osteoblast activity, creating irregular, thickened bones that do not have normal strength.