Ch. 6: Bone Tissue and the Skeletal System

Support

Provides a rigid framework for the body, holding up soft tissues and providing attachment points for muscles.

Protection

Encases delicate internal organs (e.g., the skull protects the brain, the rib cage protects the heart and lungs).

Movement

Serves as levers for muscles to act upon, enabling a wide range of body movements at joints.

Mineral Storage

Acts as a reservoir for essential minerals, primarily calcium and phosphate, releasing them into the bloodstream as needed to maintain mineral homeostasis.

Hematopoiesis

Red bone marrow, found within certain bones, is the site of blood cell formation (red blood cells, white blood cells, and platelets).

Fat Storage

Yellow bone marrow, located in the medullary cavities of long bones, stores triglycerides (fats) as an energy reserve.

Hormone Production

Bone cells produce osteocalcin, a hormone that regulates bone formation and protects against glucose intolerance and obesity.

Axial Skeleton

This division forms the central axis of the body.

Components of Axial Skeleton

Includes the skull, vertebral column (spinal column), and thoracic cage (ribs and sternum).

Function of Axial Skeleton

Provides primary support for the body, protects the brain, spinal cord, and organs of the thoracic cavity, and provides attachment points for muscles that move the head, neck, and trunk.

Appendicular Skeleton

This division consists of the limbs and their girdles.

Components of Appendicular Skeleton

Includes the pectoral (shoulder) girdles (clavicle and scapula), upper limbs (humerus, radius, ulna, carpals, metacarpals, phalanges), pelvic (hip) girdle (coxal bones), and lower limbs (femur, patella, tibia, fibula, tarsals, metatarsals, phalanges).

Function of Appendicular Skeleton

Primarily involved in locomotion, manipulation of objects, and carrying the body's weight.

Long Bones

Characterized by being longer than they are wide, with a shaft and two expanded ends. Examples include the femur, humerus, tibia, fibula, radius, ulna, phalanges.

Short Bones

Roughly cube-shaped, with nearly equal length, width, and thickness. Examples include the carpals (wrist bones) and tarsals (ankle bones).

Flat Bones

Thin, flattened, and often slightly curved, providing protection and large surface areas for muscle attachment. Examples include the skull bones (parietal, frontal), sternum, scapulae, and ribs.

Irregular Bones

Have complex, irregular shapes that do not fit into the other categories. Examples include the vertebrae and hip bones.

Sesamoid Bones

Small, round, and flat bones embedded within tendons, often found at joints. Their primary function is to alter the direction of pull of a tendon and reduce friction. The patella (kneecap) is the most well-known example.

Diaphysis

The cylindrical, elongated shaft or central part of a long bone. It is composed of compact bone surrounding a central medullary cavity.

Epiphysis

The expanded ends of a long bone, typically more bulbous than the diaphysis.

Articular Cartilage

A layer of hyaline cartilage covering the joint surfaces of the epiphyses, reducing friction and absorbing shock during joint movement.

Metaphysis (Epiphyseal Plate/Line)

The region where the diaphysis and epiphysis meet, containing the epiphyseal plate responsible for longitudinal bone growth.

Epiphyseal Plate

A hyaline cartilage plate responsible for longitudinal bone growth; replaced by bone to form the epiphyseal line once growth ceases.

Periosteum

A tough, fibrous connective tissue membrane covering the outer surface of the bone, providing attachment for tendons and ligaments.

Endosteum

A delicate connective tissue membrane lining the internal bone surfaces, including the medullary cavity.

Medullary Cavity

The hollow central cavity within the diaphysis of long bones, typically containing yellow bone marrow in adults.

Bone Markings

Specific surface features on bones that serve as sites for muscle attachment, passages for nerves and blood vessels, or articulation points.

Projections/Outgrowths

Areas that protrude from the bone surface, indicating muscle or ligament attachment.

Depressions/Openings

Indentations, hollows, or holes in the bone serving as passageways for nerves and blood vessels.

Osteogenic (Osteoprogenitor) Cells

Mitotically active stem cells located in the periosteum and endosteum that differentiate into osteoblasts.

Osteoblasts

Bone-forming cells that synthesize and secrete the organic components of the bone matrix.

Osteocytes

Mature bone cells derived from osteoblasts that maintain bone tissue and act as stress or strain sensors.

Osteoclasts

Large, multinucleated cells responsible for bone resorption, breaking down the bone matrix.

Bone Matrix

The extracellular component of bone tissue responsible for its unique properties.

Organic Components (Osteoid)

Approximately one-third of the matrix, consisting primarily of collagen fibers and ground substance.

Inorganic Components (Mineral Salts)

Approximately two-thirds of the matrix, primarily calcium phosphate crystals providing bone hardness.

Compact Bone (Cortical Bone)

The dense, outer layer of bone providing strength, support, and protection.

Osteon (Haversian System)

The fundamental structural unit of compact bone.

Central (Haversian) Canal

A canal running longitudinally through the center of an osteon, containing blood vessels and nerves.

Concentric Lamellae

Rings of calcified matrix that surround the central canal in an osteon.

Lacunae

Small spaces or cavities within the lamellae where osteocytes reside.

Canaliculi

Tiny canals radiating from the lacunae, connecting them to each other and to the central canal.

Perforating (Volkmann's) Canals

Canals running perpendicular to the central canals, connecting the blood and nerve supply of the periosteum.

Spongy Bone (Cancellous Bone/Trabecular Bone)

The internal, porous layer of bone.

Trabeculae

Irregularly arranged lamellae that form a network of thin, interconnecting bony plates or rods.

Absence of Osteons in Spongy Bone

Spongy bone does not contain osteons.

Location of Spongy Bone

Found in the epiphyses of long bones, the interior of short, flat, and irregular bones.

Function of Spongy Bone

Provides strength without adding excessive weight, accommodates shifts in weight distribution, and houses red bone marrow for hematopoiesis.

Epiphyses

The ends of a long bone.

Metaphysis

The intervening section between the diaphysis and epiphyses.

Bone Tissue Components

Composed of osteogenic cells, osteoblasts, osteocytes, and osteoclasts.

Compact Bone

Structured into osteons, providing robust support.

Ossification (Osteogenesis)

The process of bone tissue formation.

Intramembranous Ossification

The process that forms flat bones of the skull and the clavicles directly from fibrous connective tissue membranes.

Endochondral Ossification

The process that forms most of the bones of the body by replacing a hyaline cartilage model with bone.

Longitudinal Growth

Increases the length of long bones.

Epiphyseal Plates

Growth plates where longitudinal growth occurs.

Zone of Resting Cartilage

Nearest the epiphysis, composed of small, inactive chondrocytes.

Zone of Proliferation

Chondrocytes rapidly divide and arrange into columns.

Zone of Hypertrophy

Chondrocytes enlarge significantly.

Zone of Calcification

Matrix around hypertrophied chondrocytes calcifies, causing chondrocyte death.

Zone of Ossification

Calcified cartilage is invaded by osteoclasts and osteoblasts, and bone is laid down.

Closure of Epiphyseal Plate

At the end of adolescence, epiphyseal cartilage cells stop dividing, and the entire plate is replaced by bone, forming the epiphyseal line, signifying the cessation of longitudinal growth.

Appositional Growth

Increases the thickness or diameter of bones.

Mechanism of Appositional Growth

Osteoblasts in the periosteum deposit new bone matrix on the outer surface of the bone, while osteoclasts in the endosteum simultaneously resorb bone from the inner surface of the medullary cavity, thus enlarging the cavity while maintaining bone strength.

Results of Appositional Growth

Wider and stronger bones.

Bone Formation

Occurs via two primary methods: intramembranous ossification and endochondral ossification.

Bone Remodeling

A continuous process throughout life where old bone tissue is removed and new bone tissue is formed.

Definition of Bone Remodeling

The ongoing process of bone resorption (breakdown) by osteoclasts and bone deposition (formation) by osteoblasts.

Purpose of Bone Remodeling

Maintains calcium and phosphate homeostasis in the blood, adapts bone to mechanical stress, repairs microscopic damage, and replaces old, brittle bone.

Hormonal Regulation in Bone Remodeling

Primarily maintains blood calcium homeostasis.

Parathyroid Hormone (PTH)

Released when blood calcium levels are low; stimulates osteoclasts to resorb bone, releasing calcium into the blood.

Calcitonin

Released when blood calcium levels are high; inhibits osteoclast activity and promotes calcium deposition into bone.

Mechanical Stress (Wolff's Law)

States that bone grows or remodels in response to the demands placed on it.

Examples of Wolff's Law

The robust bones of athletes compared to sedentary individuals; the curved, thick bones of the femur that best resist bending stresses.

Bone Repair (Fracture Healing)

The natural process of bone regeneration after a fracture.

Formation of Hematoma

Immediately after a fracture, blood vessels are torn, leading to bleeding and the formation of a hematoma at the fracture site.

Formation of Fibrocartilaginous Callus

Capillaries grow into the hematoma, and phagocytic cells clear debris; fibroblasts produce collagen fibers, and chondroblasts secrete cartilage matrix.

Formation of Bony Callus

Osteoblasts begin to convert the fibrocartilaginous callus into a bony callus of spongy bone.

Bone Remodeling after Fracture

Over months to years, the bony callus is continually remodeled, excess material is removed, and compact bone is laid down.

Wolff's Law

Describes how bone structure adapts to mechanical demands.

Mechanical Stress

Stimulates osteoblasts to increase bone deposition and density.

Weight-bearing Exercise

Activities like walking, jogging, running, weightlifting, dancing, and high-impact sports that benefit bone health.

Calcium

A critical structural component of hydroxyapatite; essential for bone hardness.

Vitamin D

Essential for calcium absorption from the intestine and reabsorption in the kidneys.

Calcitriol

The active form of Vitamin D that enhances calcium absorption.

Vitamin K

Important for the synthesis of bone proteins, particularly osteocalcin.

Magnesium

Plays a role in bone matrix formation and is a cofactor for enzymes involved in bone metabolism.

Fluoride

Can be incorporated into hydroxyapatite, making bone stronger and more resistant to demineralization.

Omega-3 Fatty Acids

May reduce inflammation contributing to bone loss and promote bone formation.

Growth Hormone (GH)

Stimulates insulin-like growth factors (IGFs) in the liver, promoting bone growth during childhood and adolescence.

Thyroid Hormones

Modulate the activity of growth hormone, ensuring proper proportions of skeletal growth.

Sex Hormones

Estrogen and testosterone maintain bone density by inhibiting osteoclast activity and promoting osteoblast activity.

Osteoporosis

A condition characterized by reduced bone mass and increased fragility, often associated with aging and estrogen deficiency.

Osteomalacia/Rickets

Softening of bones due to inadequate mineralization; in children, it's called rickets.

Paget's Disease

A chronic bone disorder characterized by excessive and haphazard bone remodeling.

Normal blood calcium levels

Approximately 10 mg/dL, which the body rigorously maintains to prevent severe health consequences.

Hypocalcemia

Abnormally low blood calcium levels that can negatively impact circulation, muscles, nerves, and bones.

Symptoms of Hypocalcemia

Poor blood coagulation, irregular heartbeat, difficulties with muscle contraction, nerve dysfunction, and brittle bones (long-term effect).

Hypercalcemia

Abnormally high blood calcium levels that result in an underactive nervous system.