BIOL2401 Exam 2 Study Guide: Chapter 6 - Bones and Skeletal Tissues

Chapter 6: Bones and Skeletal Tissues

Skeletal Cartilage: Characteristics and Location

Skeletal cartilage is a specialized connective tissue that lacks nerves and blood vessels (it is avascular). It is primarily composed of water (typically $60-80\%$), which allows it to be resilient and compress without permanent deformation. Chondrocytes, the mature cartilage cells, are found within spaces called lacunae. Cartilage is surrounded by a perichondrium, a layer of dense irregular connective tissue that provides nutrients by diffusion and resists outward expansion.

There are three main types of skeletal cartilage:

• Hyaline Cartilage: This is the most abundant type. It provides strong, flexible support. It is found as:
  • Articular cartilage: Covers the ends of most bones at movable joints.
  • Costal cartilage: Connects the ribs to the sternum.
  • Respiratory cartilage: Forms the skeleton of the larynx, trachea, and bronchi.
  • Nasal cartilage: Supports the external nose.
• Elastic Cartilage: Similar to hyaline cartilage but contains more elastic fibers, making it more flexible. It is found in the external ear and the epiglottis.
• Fibrocartilage: This is highly compressible with great tensile strength. It contains thick collagen fibers. It is found in sites subjected to heavy pressure, such as the menisci of the knee, intervertebral discs, and the pubic symphysis.

Classifications of Bones

Bones are classified into five main groups based on their shape:

• Long Bones: These are longer than they are wide, with a shaft (diaphysis) and two ends (epiphyses). They are primarily compact bone. Examples include the femur, tibia, fibula, humerus, radius, ulna, phalanges (fingers and toes).
• Short Bones: These are roughly cube-shaped. They contain mostly spongy bone. Examples include the carpals (wrist bones) and tarsals (ankle bones).
• Flat Bones: These are thin, flattened, and often curved. They have two parallel layers of compact bone with a layer of spongy bone (diploe) in between. Examples include the sternum, scapulae, ribs, and most skull bones.
• Irregular Bones: These have complicated shapes that do not fit into the other categories. Examples include the vertebrae and hip bones.
• Sesamoid Bones: These are small, round bones that are embedded within tendons, acting to alter the angle of pull of a muscle. An example is the patella (kneecap).

Seven Functions of the Skeletal System

The skeletal system performs several vital functions:

1. Support: Provides a framework that supports the body and cradles its soft organs. For example, the vertebral column supports the torso.
2. Protection: Hard, bony coverings protect delicate organs. The skull protects the brain, the ribs and sternum protect the heart and lungs, and the vertebral column protects the spinal cord.
3. Movement: Bones act as levers for muscles. Skeletal muscles attach to bones by tendons, and when muscles contract, they pull on bones, causing movement at joints.
4. Mineral and Growth Factor Storage: Bone tissue is a reservoir for minerals, most importantly calcium ($Ca^{2+}$) and phosphate ($PO_{4}^{3-}$). These minerals can be released into the bloodstream as needed. Bone matrix also stores growth factors, which are important for cell growth and repair.
5. Blood Cell Formation (Hematopoiesis): Red marrow, found within certain bones, is the site of hematopoiesis, the production of all blood cellular components (red blood cells, white blood cells, and platelets).
6. Triglyceride Storage: Yellow marrow, found in the medullary cavity of long bones, stores triglycerides (fats), which serve as an energy reserve.
7. Hormone Production: Bones produce osteocalcin, a hormone that helps regulate insulin secretion, glucose homeostasis, and energy expenditure.

Compact Bone vs. Cancellous (Spongy) Bone and Osteon Structure

Bone tissue exists in two main forms: compact and cancellous (spongy).

• Compact Bone: This is the dense outer layer of bone that appears smooth and solid. It is found on the external surfaces of all bones and provides strength and protection.
  • The structural unit of compact bone is the osteon (also called the Haversian system). An osteon is an elongated cylinder oriented parallel to the long axis of the bone, acting as a tiny weight-bearing pillar.
  • Osteon Components:
    • Haversian (Central) Canal: Runs through the core of each osteon, containing blood vessels and nerve fibers that serve the osteon's cells.
    • Lamellae: Concentric rings of bone matrix within an osteon. They are collagen fibers that run in different directions in adjacent lamellae, which helps the bone resist twisting forces.
    • Lacunae: Small cavities or spaces within the lamellae that house mature bone cells called osteocytes.
    • Canaliculi: Hair-like canals that connect the lacunae to each other and to the central canal. These allow osteocytes to communicate and receive nutrients.
    • Volkmann's (Perforating) Canals: These canals lie at right angles to the long axis of the bone and connect the blood and nerve supply of the periosteum to the central canals and the medullary cavity.
• Cancellous (Spongy) Bone: This is found internal to compact bone. It is a honeycomb structure of small, needle-like or flat pieces of bone called trabeculae.
  • Trabeculae: These tiny bone spicules are organized along lines of stress, providing strength where needed while being much lighter than solid bone. They contain irregularly arranged lamellae, osteocytes, and canaliculi, but no osteons. The spaces between trabeculae are filled with red or yellow bone marrow.

Long Bone Anatomy

A typical long bone exhibits the following anatomical features:

• Diaphysis: The main shaft or body of the long bone. It is composed of a thick collar of compact bone surrounding a central medullary cavity.
• Epiphysis: The expanded ends of the long bone. Each epiphysis consists of an outer shell of compact bone enclosing an interior of spongy bone. Articular cartilage covers the joint surfaces.
• Articular Cartilage: A layer of hyaline cartilage that covers the external surface of the epiphyses where they articulate with other bones. Its function is to cushion the bone ends and reduce friction during joint movement.
• Medullary Cavity: The central cavity within the diaphysis. In adults, it contains yellow bone marrow, which is primarily adipose tissue for fat storage. In infants, it contains red bone marrow.
• Primary Ossification Center: The first area of a bone to start ossifying. In long bones, this forms in the center of the diaphysis during fetal development.
• Secondary Ossification Centers: These appear in the epiphyses usually around the time of birth or during childhood. They typically ossify the epiphyses, leaving hyaline cartilage only at the epiphyseal plates and articular surfaces.
• Epiphyseal Plate (Growth Plate/Disc): A plate of hyaline cartilage located between the diaphysis and epiphysis in growing bones. It is the site of longitudinal bone growth. Once growth ceases, the epiphyseal plate ossifies and becomes the epiphyseal line.

Ossification Processes: Intramembranous vs. Endochondral Bone Formation and Cell Roles

Ossification (or osteogenesis) is the process of bone tissue formation. It can occur via two different mechanisms:

1. Intramembranous Ossification: This process forms bone directly from fibrous connective tissue membranes. There is no cartilage model involved.

   • Steps:
     1. Ossification centers appear: Mesenchymal cells in the fibrous connective tissue membrane cluster and differentiate into osteoblasts, forming an ossification center.
     2. Osteoid is secreted and calcified: Osteoblasts secrete organic bone matrix (osteoid), which then calcifies. Trapped osteoblasts become osteocytes.
     3. Woven bone and periosteum form: Accumulating osteoid is laid down between embryonic blood vessels, forming a network of trabeculae (woven bone). The vascularized mesenchyme condenses on the external face of the woven bone to form the periosteum.
     4. Lamellar bone replaces woven bone: Trabeculae deep to the periosteum thicken and are replaced with lamellar bone. Spongy bone forms in the center, and red marrow appears.
   • Examples: Most flat bones of the skull (frontal, parietal, occipital, temporal) and the clavicles are formed by intramembranous ossification.

2. Endochondral Ossification (Cartilaginous Bones): This process forms bone by replacing a hyaline cartilage model. Most bones of the body form this way.

   • Steps:
     1. Bone collar forms: Mesenchymal cells in the cartilage matrix differentiate into osteoblasts and secrete osteoid, forming a bone collar around the diaphysis of the hyaline cartilage model.
     2. Cartilage calcifies and deteriorates: Chondrocytes within the primary ossification center enlarge, calcify the surrounding matrix (blocking nutrient diffusion), and then die, leaving behind cavities within the cartilage model.
     3. Periosteal bud invades cavities: The periosteal bud (containing a nutrient artery and vein, nerves, red marrow, osteogenic cells, and osteoclasts) invades the internal cavities. Osteoclasts erode calcified cartilage, and osteoblasts secrete osteoid, forming spongy bone.
     4. Diaphysis elongates and medullary cavity forms; secondary ossification centers appear: The primary ossification center enlarges. As the diaphysis elongates, osteoclasts break down newly formed spongy bone to open up a medullary cavity. Secondary ossification centers appear in the epiphyses around the time of birth, following a similar process.
     5. Epiphyses ossify: Spongy bone forms in the epiphyses. Hyaline cartilage remains only in the epiphyseal plates (for longitudinal growth) and on the articular surfaces.
   • Examples: All bones below the skull (except the clavicles) are formed by endochondral ossification, including long bones like the femur and humerus.

Roles of Bone Cells:

• Osteocytes: Mature bone cells that reside in lacunae within the bone matrix. They act as stress or strain sensors, responding to mechanical stimuli and communicating with other cells to initiate bone remodeling. They maintain the bone matrix.
• Osteoclasts: Large, multi-nucleated cells that resorb (break down) bone matrix. They are derived from hematopoietic stem cells (the same lineage as macrophages). They are crucial for bone remodeling and calcium homeostasis.
• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), which consists primarily of collagen and calcium-binding proteins. Once osteoblasts are completely surrounded by matrix, they differentiate into osteocytes.

Interstitial vs. Appositional Growth

These terms describe how cartilage and bone tissues grow:

• Interstitial Growth: This refers to growth from within. In cartilage, chondrocytes within the lacunae divide and secrete new matrix, pushing the cells apart and expanding the cartilage mass from the inside, thus increasing its length. In bone, while bone tissue itself doesn't grow interstitially, longitudinal bone growth at the epiphyseal plates is an interstitial process of cartilage, followed by its replacement with bone, effectively lengthening the bone.
• Appositional Growth: This refers to growth from the outside. In cartilage, chondroblasts in the perichondrium secrete new matrix onto the external surface of the existing cartilage, increasing its width or thickness. In bone, osteoblasts beneath the periosteum secrete new bone matrix onto the external surface of the bone, increasing its diameter and thickness. Simultaneously, osteoclasts on the endosteal surface resorb bone from the medullary cavity, maintaining appropriate bone proportions and hollowing out the cavity.

Interstitial Bone Growth and Development Details

Role of the Epiphyseal Plate

The epiphyseal plate, or growth plate, is a crucial structure for longitudinal bone growth in children and adolescents. It is a hyaline cartilage plate located at each end of a long bone between the diaphysis and epiphysis. Its activity allows bones to lengthen.

The epiphyseal plate consists of several functional zones:

• Proliferation Zone (Growth Zone): Chondrocytes at the epiphysis-facing side rapidly divide, forming tall columns of cells. This pushes the epiphysis away from the diaphysis, lengthening the bone.
• Hypertrophic Zone: Older chondrocytes closer to the diaphysis enlarge (hypertrophy), and their lacunae erode and enlarge, leaving large, interconnecting spaces.
• Calcification Zone: The surrounding cartilage matrix calcifies, and the chondrocytes die and deteriorate, leaving calcified cartilage spicules.
• Ossification Zone (Osteogenic Zone): The calcified spicules are invaded by marrow elements from the medullary cavity. Osteoclasts resorb the calcified cartilage, and osteoblasts deposit new bone matrix (spongy bone) onto the calcified cartilage spicules. This spongy bone is then replaced by compact bone.

This continuous process of cartilage growth on the epiphyseal side and cartilage replacement by bone on the diaphyseal side results in the lengthening of the diaphysis while the epiphyseal plate maintains a relatively constant thickness. Bone growth typically ends when the epiphyseal plate closes, usually in late adolescence or early adulthood, forming the epiphyseal line.

Most Important Inorganic and Organic Compounds

• Inorganic Compounds: The most important inorganic component of bone is hydroxyapatites, which are mineral salts, primarily calcium phosphates, represented by the formula $Ca<em>{10}(PO</em>4)<em>6(OH)</em>2$. These inorganic salts make up about $65\%$ of bone mass, providing its exceptional hardness and resistance to compression. They allow bone to resist pressure and protect vital structures.
• Organic Compounds: The primary organic components (collectively called osteoid) are collagen fibers (about $90\%$ of the organic matrix) and ground substance (proteoglycans and glycoproteins). These organic components, particularly collagen, contribute to bone's flexibility and tensile strength, allowing it to resist stretching and twisting.

Vitamin Functions and Associated Diseases Affecting Bone Development

Vitamins play critical roles in bone health and development:

• Vitamin A: Essential for osteoblast and osteoclast activity and normal bone remodeling. It is also important for cell differentiation and growth. Deficiency can impair bone growth and development, although severe deficiency is rare in developed countries.
• Vitamin D: Crucial for calcium absorption from the small intestine into the blood, and also for regulating phosphate levels. Without sufficient Vitamin D, calcium cannot be properly absorbed, leading to poor bone mineralization.
  • Diseases:
    • Rickets (children): Characterized by soft, weak bones that become deformed, such as bowed legs and pelvic deformities. Caused by insufficient calcium or vitamin D, or impaired metabolism of vitamin D.
    • Osteomalacia (adults): Similar to rickets but occurs in adults. Bones become demineralized, soft, and weak, leading to pain and increased fracture risk, often due to insufficient vitamin D or kidney issues.
• Vitamin C: Necessary for the synthesis of collagen, the main organic component of bone matrix. Collagen provides bone's tensile strength. It is also an antioxidant.
  • Diseases:
    • Scurvy: Caused by severe Vitamin C deficiency, leading to weak collagen, fragile bones, brittle blood vessels, poor wound healing, and bleeding gums. Affects bone matrix formation.
• Vitamin K: Involved in the synthesis of several proteins essential for bone health, including osteocalcin, a calcium-binding protein in the bone matrix. Deficiency can impair bone mineralization and increase fracture risk.

How Hormones Regulate Bone Growth/Development

Several hormones regulate bone growth and development, primarily during childhood and adolescence:

• Growth Hormone (GH): Produced by the anterior pituitary gland, GH is the single most important stimulus for epiphyseal plate activity, leading to longitudinal growth of bones. It acts directly on bone but also indirectly by stimulating the liver to produce insulin-like growth factors (IGFs).
• Thyroid Hormones: (e.g., Thyroxine) Modulate the activity of growth hormone, ensuring that the skeleton grows with proper proportions. Insufficient thyroid hormone can lead to underdeveloped bones.
• Sex Hormones (Testosterone in males, Estrogen in females): These hormones are critically important during puberty. They promote the adolescent growth spurt by stimulating a burst of GH secretion and promoting osteoblast activity. However, they also ultimately cause the eventual closure of the epiphyseal plates (estrogen has a slightly stronger and earlier effect on epiphyseal plate closure than testosterone), marking the end of longitudinal bone growth. High levels for prolonged periods lead to epiphyseal fusion.
• Parathyroid Hormone (PTH) and Calcitonin: While primarily involved in calcium homeostasis (see below), they indirectly influence bone remodeling and thus bone development and maintenance.

Bone Homeostasis, Appositional Growth, Calcium, Bone Remodeling, and Mechanical Stress

Bone Homeostasis

Bone homeostasis refers to the state of dynamic balance within bone tissue, maintained by continuous opposition of bone deposit by osteoblasts and bone resorption by osteoclasts. This constant process, known as bone remodeling, allows the skeleton to adapt to mechanical stresses, repair microfractures, and maintain appropriate blood calcium levels.

Involvement of Appositional Growth in Bone Homeostasis

While appositional growth is primarily responsible for increasing bone width, it is an integral part of bone remodeling and plays a role in maintaining bone homeostasis. Throughout life, osteoblasts beneath the periosteum deposit new bone matrix on the external surface of the bone, increasing its diameter. Simultaneously, osteoclasts on the endosteal surface (lining the medullary cavity) resorb bone. This coordinated activity ensures that bones become wider without becoming excessively heavy and that the medullary cavity maintains appropriate size. It helps to strengthen the bone against bending stresses.

Role of Calcium in Bone Homeostasis

Calcium is the most abundant mineral in the body and plays essential roles in numerous physiological processes beyond bone structure, including:

• Nerve impulse transmission
• Muscle contraction
• Blood coagulation
• Glandular and nerve cell secretions
• Cell division

Because of its widespread importance, blood calcium levels must be kept within a very narrow physiological range (normally about $9 ext{ to } 11 ext{ mg/dL}$). Bone acts as the body's main reservoir for calcium. Hormonal controls primarily regulate calcium homeostasis:

• Parathyroid Hormone (PTH): Released by the parathyroid glands when blood $Ca^{2+}$ levels fall below the homeostatic range. PTH stimulates osteoclasts to resorb bone, releasing $Ca^{2+}$ into the blood. It also enhances $Ca^{2+}$ reabsorption by the kidneys and promotes activation of vitamin D (which then increases $Ca^{2+}$ absorption from the intestine).
• Calcitonin: Produced by the parafollicular cells of the thyroid gland in response to high blood $Ca^{2+}$ levels. Calcitonin inhibits osteoclast activity and promotes calcium salt deposition into the bone matrix. Its role in adult human calcium homeostasis is less significant than PTH, but it may be more important in children.

Bone Remodeling

Bone remodeling is a continuous and tightly regulated process of bone deposit and resorption, occurring constantly throughout life. This process involves bone cells (osteocytes, osteoblasts, osteoclasts) and is regulated by various hormones and mechanical stresses.

• Bone deposit: Occurs where bone is injured or added strength is needed. Osteoblasts lay down new osteoid, which then calcifies.
• Bone resorption: Osteoclasts move along bone surfaces, breaking down existing bone matrix and releasing its components into the blood.

Remodeling allows the skeleton to:

• Maintain calcium homeostasis.
• Repair micro-damage that accumulates over time due to mechanical stress.
• Alter bone shape and density in response to mechanical demands.

How Bones Respond to Mechanical Stress (Wolff's Law)

Wolff's Law states that a bone grows or remodels in response to the demands placed on it. This means that bones subjected to greater stress become thicker and stronger, while bones not subjected to stress become weaker and thinner (atrophy).

Key observations consistent with Wolff's Law:

• Trabeculae orientation: Trabeculae in spongy bone align precisely along lines of stress, providing greater strength for specific forces.
• Shaft thickness: The walls of the diaphysis (shaft) of long bones are thickest where bending stresses are greatest, typically midway along the shaft.
• Bony projections: Large, bony projections (e.g., tuberosities, trochanters) occur at sites where heavy, active muscles attach, indicating increased stress and bone remodeling.
• Lack of stress: Inactivity, such as prolonged bed rest or immobilization in a cast, leads to significant bone atrophy and loss of bone mass, as there are no mechanical forces stimulating bone deposit.
• Handedness: The bones of one upper limb are generally thicker than those of the less-used limb, reflecting the greater stresses placed on them.

This adaptation to stress allows the skeletal system to be dynamic and optimally structured for its functions.

Four Steps in Bone Repair

When a bone fractures, it typically undergoes a four-step repair process:

1. Hematoma Formation: Immediately after the fracture, blood vessels in the bone and periosteum are torn, leading to hemorrhage. A hematoma (a mass of clotted blood) forms at the fracture site. Bone cells deprived of nutrients begin to die, and the area becomes swollen, painful, and inflamed.
2. Fibrocartilaginous Callus Formation: Within a few days, phagocytic cells invade the area to clean up debris. Fibroblasts, chondroblasts, and osteoblasts from the periosteum and endosteum invade the fracture site. Fibroblasts produce collagen fibers to connect the broken bone ends, while chondroblasts secrete a cartilaginous matrix. Osteoblasts begin forming spongy bone. These activities create a soft, gelatinous fibrocartilaginous callus over the fracture, splinting the broken bone ends.
3. Bony Callus Formation: Within a week, new bone trabeculae begin to appear in the fibrocartilaginous callus. Osteoblasts and osteoclasts continue to convert the fibrocartilaginous callus into a hard, bony callus of spongy bone. This process continues for about two months until a firm union forms.
4. Bone Remodeling: During the next several months, the bony callus is remodeled. Excess material on the outside of the bone shaft and within the medullary cavity is removed by osteoclasts. Compact bone replaces spongy bone where needed, and the bone gradually returns to its near-original shape and strength, mirroring the structure of the adjacent unaffected bone regions.

Bone Fractures: Classifications and Treatment

Bone fractures are breaks in the bone. They are classified based on several characteristics:

• Position of bone ends after fracture:
  • Nondisplaced: The bone ends retain their normal position.
  • Displaced: The bone ends are out of normal alignment.
• Completeness of the break:
  • Complete: The bone is broken all the way through.
  • Incomplete: The bone is not broken all the way through (e.g., a crack).
• Orientation of the break relative to the long axis of the bone:
  • Linear: The fracture is parallel to the long axis.
  • Transverse: The fracture is perpendicular to the long axis.
• Whether the skin is penetrated:
  • Closed (Simple): The bone does not break through the skin.
  • Open (Compound): The bone breaks through the skin, exposing the bone to the external environment, increasing infection risk.

Common Types of Fractures (based on nature of break):

• Comminuted: Bone fragments into three or more pieces. Common in older people whose bones are more brittle.
• Compression: Bone is crushed, common in porous bones (osteoporotic bones) subjected to extreme trauma (e.g., a fall).
• Spiral: Ragged break occurs when excessive twisting forces are applied to a bone. Common sports injury.
• Epiphyseal: Epiphysis separates from the diaphysis along the epiphyseal plate. Tends to occur where cartilage cells are dying and calcification is occurring. Common in children.
• Depressed: Broken bone portion is pressed inward, typical of skull fractures.
• Greenstick: Bone breaks incompletely, much in the way a green twig breaks, only one side of the shaft breaks, and the other bends. Common in children, whose bones have relatively more organic matrix and are more flexible.

Treatment of Fractures:

Treatment involves reduction, which is the realignment of the broken bone ends, followed by immobilization to allow healing.

• Reduction:
  • Closed Reduction: The physician manipulates the bone ends into their normal position externally by hand.
  • Open Reduction: The bone ends are surgically secured together with pins, wires, plates, or screws.
• Immobilization: After reduction, the bone must be immobilized by a cast, traction, or internally by plates and screws to prevent movement and allow the natural bone repair processes to occur. The time required for healing depends on the severity of the break, the bone broken, and the age and health of the patient (e.g., $6-8$ weeks for a simple fracture of a medium-sized bone in a young adult, much longer for older adults).

Osteoporosis: Cause, Risk Factors, Treatment, and Prevention

Osteoporosis is a bone disease characterized by bone resorption outpacing bone deposit, leading to porous, fragile bones with greatly reduced bone mass. The bone structure deteriorates, making bones highly susceptible to fractures, especially in the spine, wrist, and hip. Spongy bone is particularly vulnerable, as the trabeculae thin and become fewer, making the bone appear moth-eaten. Compact bone also thins.

Cause: The fundamental cause is an imbalance in bone remodeling, where the rate of bone breakdown (resorption) exceeds the rate of bone formation (deposit), resulting in a net loss of bone tissue.

Risk Factors:

• Age: Bone density naturally declines with age, increasing risk in older individuals.
• Sex: Women are significantly more susceptible than men. After menopause, estrogen levels drop sharply. Estrogen has a protective effect on bones by inhibiting osteoclast activity. This decline accelerates bone loss. Men also lose bone mass with age, but they start with a higher bone mass and don't experience the sudden drop in sex hormones.
• Petite Body Form: Individuals with a small body frame and low body weight have less bone mass to begin with.
• Insufficient Calcium and/or Vitamin D Intake: A lifelong diet lacking these essential nutrients leads to low bone density.
• Lifestyle Factors:
  • Sedentary Lifestyle: Lack of weight-bearing exercise fails to stimulate bone remodeling and deposit.
  • Smoking: Nicotine inhibits osteoblasts and interferes with estrogen's protective effects.
  • Excessive Alcohol Consumption: Impairs osteoblast activity and calcium absorption.
• Certain Medications: Glucocorticoids (e.g., prednisone) used long-term, some antiseizure medications, and certain cancer treatments can increase risk.
• Certain Medical Conditions: Hyperthyroidism, diabetes, hyperparathyroidism, and conditions affecting nutrient absorption increase risk.
• Ethnicity: Caucasians and Asians are at higher risk than African Americans.

Treatment:

• Calcium and Vitamin D Supplements: To ensure adequate building blocks for bone formation, although they cannot reverse severe bone loss alone.
• Weight-bearing Exercise: Activities like walking, jogging, and weightlifting help stimulate osteoblast activity and increase bone density.
• Bisphosphonates: (e.g., Fosamax, Actonel) These drugs decrease osteoclast activity, slowing down bone resorption and thus reducing bone loss.
• Selective Estrogen Receptor Modulators (SERMs): (e.g., Raloxifene) Mimic estrogen's beneficial effects on bone density without affecting other estrogen-sensitive tissues.
• Denosumab: A monoclonal antibody that inhibits the formation and survival of osteoclasts, leading to reduced bone resorption.
• Teriparatide: A form of parathyroid hormone (PTH) that, when given intermittently, stimulates new bone formation by osteoblasts, rather than inhibiting osteoclasts.

Prevention:

• Adequate Calcium Intake: Ensuring sufficient calcium consumption throughout life, especially during childhood and adolescence to achieve peak bone mass (usually by age $20-30$).
• Sufficient Vitamin D: Adequate intake through diet, supplements, or sun exposure is crucial for calcium absorption.
• Regular Weight-Bearing Exercise: Engaging in activities that put stress on bones helps maintain and increase bone density. Examples include walking, running, dancing, and strength training.
• Avoid Smoking and Excessive Alcohol: These habits have detrimental effects on bone health.
• Fall Prevention: For individuals at high risk, measures to prevent falls (e.g., improving balance, removing hazards) are important to reduce fracture risk.
• Bone Density Screenings: Regular screenings, especially for postmenopausal women and older men, can help detect bone loss early.