Osseous Tissue Lecture Review

Osseous Tissue Overview

• The adult human skeleton comprises 206 named bones.
• It is broadly categorized into two main groups:
  • Axial Skeleton: Forms the long axis of the body and includes the skull, vertebral column, and rib cage.
  • Appendicular Skeleton: Consists of the bones of the upper and lower limbs, as well as the girdles that attach these limbs to the axial skeleton.

Primary Functions of Bones

• Support: Bones provide a foundational framework that gives shape and support to the body's tissues and organs.
• Storage of Minerals and Lipids:
  • Serve as an essential reserve for calcium and phosphate ions, crucial for various physiological processes.
  • Store lipids (fats) within the yellow marrow.
• Blood Cell Production: Hematopoiesis, the process of blood cell formation, occurs within the red marrow found in certain bones.
• Protection: The axial skeleton and the pelvic girdle of the appendicular skeleton shield delicate internal organs from injury.
• Leverage (Force of Motion): Bones act as rigid levers upon which muscles exert force, facilitating movement.

Classification of Bones by Shape

• Sutural Bones (Wormian bones):
  • Small, irregular bones found within the sutures, or joints, between the flat bones of the skull.
  • They vary in size and number among individuals.
• Long Bones:
  • Characterized by being significantly longer than they are wide.
  • Examples include the bones of the limbs (e.g., humerus, femur).
• Short Bones:
  • Typically cube-shaped, with roughly equal length, width, and thickness.
  • Found in the wrist (carpals) and ankle (tarsals), such as the talus.
• Flat Bones:
  • Thin, flattened, and often slightly curved.
  • Examples include the sternum, scapulae, ribs, and most bones of the skull.
• Irregular Bones:
  • Have complex and varied shapes that do not fit into other categories.
  • Examples include the vertebrae (spinal column) and coxal bones (hip bones).
• Sesamoid Bones:
  • Small, rounded bones that develop within tendons and muscles.
  • The patella (kneecap) is a prominent example.

Bone Markings

• Bone markings are structural features on the external surfaces of bones, serving several purposes:
  • Sites for the attachment of muscles, ligaments, and tendons.
  • Forming joint surfaces for articulation with other bones.
  • Providing conduits for nerves and blood vessels to pass through.
• These markings are categorized into projections and depressions/openings:
  • Projections (Processes): Outgrowths that often indicate areas where muscles pull or joints articulate.
    • Processes formed where joints (articulations) occur between adjacent bones:
      • Head: An expanded articular end of an epiphysis, typically separated from the shaft by a narrower neck (e.g., head of humerus).
      • Neck: A narrow connection between the epiphysis and the diaphysis (e.g., neck of femur).
      • Facet: A small, flat articular surface (e.g., on vertebrae).
      • Condyle: A smooth, rounded articular process (e.g., femoral condyles).
      • Trochlea: A smooth, grooved articular process shaped like a pulley (e.g., trochlea of humerus).
    • Processes formed where tendons or ligaments attach:
      • Trochanter: A very large, blunt, irregularly shaped process (only on the femur).
      • Crest: A prominent, narrow ridge of bone (e.g., iliac crest).
      • Spine: A sharp, slender, often pointed projection (e.g., scapular spine).
      • Line: A less prominent ridge than a crest, often long and low (e.g., linea aspera of femur).
      • Tubercle: A small, rounded projection (e.g., greater tubercle of humerus).
      • Tuberosity: A large, rounded, roughened projection, often site of muscle attachment (e.g., deltoid tuberosity).
  • Depressions and Openings: Features that generally allow nerves and blood vessels to pass or provide articulation space.
    • Sulcus (Groove): A narrow groove (e.g., intertubercular sulcus).
    • Fossa: A shallow, basin-like depression (e.g., olecranon fossa).
    • Other openings include sinus, fissure, foramen, and ramus.

Gross Anatomy of Bones

• Bones exhibit two main types of texture:
  • Compact Bone: The dense, outer layer that is smooth and solid, forming the exterior of all bones.
  • Spongy Bone (Cancellous or Trabecular Bone): Found deep to the compact bone, it consists of a honeycomb-like network of flat, needle-like or flat pieces of bone called trabeculae.

Structure of a Typical Long Bone

• Diaphysis: The tubular shaft that forms the long axis of the bone. It consists of compact bone surrounding a central medullary cavity.
• Epiphyses: The enlarged ends of the bone. Each epiphysis is composed of an external layer of compact bone covering an internal core of spongy bone. Articular cartilage, typically hyaline cartilage, covers the joint surfaces of the epiphyses.
• Epiphyseal Line: In adult bones, this is a distinct line found between the diaphysis and epiphysis, representing the remnant of the childhood epiphyseal plate where bone growth in length occurred.
• Medullary Cavity: The central cavity within the diaphysis that contains bone marrow.
  • Red Bone Marrow: The primary site of blood cell formation.
  • Yellow Marrow: Primarily composed of adipose tissue and serves as a fat storage reserve.
• Metaphyses: The region in a growing bone where the diaphysis and epiphysis meet, containing the epiphyseal plate.
• Periosteum: A double-layered connective tissue sheath covering the outer surfaces of all bones, except at articular cartilages.
  • Outer Fibrous Layer: Composed of dense irregular connective tissue (DICT).
  • Inner Cellular Layer: Contains osteoblasts, osteoclast, and osteoprogenitor cells.
  • Perforating Fibers (Sharpey’s Fibers): Thick collagen fibers that anchor the periosteum firmly to the underlying bone.
  • Functions: Protects the bone, aids in fracture healing, provides nourishment, and serves as an attachment point for tendons and ligaments.
• Endosteum: A thin, delicate membrane that lines the internal surfaces of the medullary cavity and covers the trabeculae of spongy bone.
  • Composed of a single layer of bone-forming cells, including osteoblasts, osteoprogenitor cells, and osteoclasts.
  • It is active in bone growth and repair.

Structure of Short, Irregular, and Flat Bones

• These bones typically consist of thin plates of spongy bone sandwiched between two layers of compact bone.
• They lack a diaphysis and epiphyses, or a distinct medullary cavity.
• Bone marrow is found throughout the spongy bone areas.
• Articular surfaces are covered by hyaline cartilage.

Microscopic Anatomy of Bone Tissue

Compact Bone (Lamellar Bone)

• Osteon (Haversian System): The fundamental structural unit of compact bone.
  • It is an elongated cylinder that runs parallel to the long axis of the bone.
  • Composed of concentric rings of bone matrix called lamellae.
  • Collagen fibers within each lamella run in different directions, providing significant strength to resist twisting forces.
  • Osteocytes, mature bone cells, are arranged around the central canal.
• Canals within Compact Bone:
  • Central (Haversian) Canal: Runs through the core of each osteon, containing blood vessels and nerve fibers that supply the osteon.
  • Canaliculi: Tiny, hair-like canals that radiate from the lacunae (small cavities housing osteocytes) and connect them to each other and to the central canal. These allow for communication and the relay of nutrients and wastes between osteocytes throughout the osteon.
  • Perforating (Volkmann's) Canals: Canals lined with endosteum that run perpendicular (at right angles) to the central canals. They connect the blood vessels and nerves of the periosteum, medullary cavity, and central canals.
• Lamellae in Compact Bone:
  • Interstitial Lamellae: Incomplete lamellae located between osteons. They are remnants of older osteons that have been partially resorbed and remodeled.
  • Circumferential Lamellae: Located just deep to the periosteum and extending around the entire surface of the diaphysis. These resist twisting forces applied to the long bone.

Spongy Bone

• Appears less organized than compact bone.
• Trabeculae: The main structural elements of spongy bone. These needle-like or flat pieces of bone are arranged along lines of stress, providing resistance to mechanical forces.
• Unlike compact bone, spongy bone does not contain osteons.
• It consists of irregularly arranged lamellae and osteocytes, which are interconnected by canaliculi.
• Nutrients are supplied to the osteocytes by capillaries within the endosteum.
• Red marrow is typically found in the spaces between the trabeculae.

Cells of Bone Tissue

• Bone tissue is dynamic and contains five major cell types, all specialized forms derived from the same basic cell lineage:
  • Osteogenic Cells (Osteoprogenitor Cells):
    • Unspecialized bone stem cells derived from mesenchyme.
    • These are the only bone cells capable of undergoing mitosis.
    • They are mitotically active and found primarily in the endosteum and the inner cellular layer of the periosteum.
    • Differentiate into osteoblasts.
  • Osteoblasts:
    • Bone-forming cells responsible for synthesizing and secreting the unmineralized organic bone matrix, known as osteoid.
    • Osteoid primarily consists of collagen fibers and calcium-binding proteins.
    • When osteoblasts become completely surrounded and trapped within their own secreted matrix, they differentiate into osteocytes.
  • Osteocytes:
    • Mature bone cells that reside in lacunae within the bone matrix.
    • Their primary functions include monitoring and maintaining the mineralized bone matrix, including daily metabolic activities.
    • They act as stress or strain sensors, responding to mechanical stimuli and communicating these signals to osteoblasts and osteoclasts to initiate bone remodeling.
  • Bone Lining Cells:
    • Flat cells found on bone surfaces where bone remodeling is not actively occurring.
    • The exact functions are still being studied, but they are thought to regulate calcium and phosphate movement into and out of the bone matrix.
  • Osteoclasts:
    • Large, multinucleate cells derived from hematopoietic stem cells (which also give rise to macrophages).
    • Their primary role is bone resorption, the breakdown of bone matrix.
    • When active, they rest in depressions called resorption bays and possess a highly folded cell membrane called a ruffled border.
    • The ruffled border significantly increases the surface area for the enzymatic degradation of bone and creates a sealed-off compartment where bone breakdown occurs.
    • Bone resorption is a normal and essential process for bone development, maintenance, and growth.

Bone Matrix Composition

• The bone matrix is a composite material, providing both hardness and flexibility.
  • Minerals (2/3 of matrix weight):
    • Primarily composed of calcium phosphate (Ca3(PO4)_2).
    • This calcium phosphate reacts with calcium hydroxide (Ca(OH)2) to form crystals of hydroxyapatite (Ca{10}(PO4)6(OH)_2).
    • Hydroxyapatite crystals incorporate other calcium salts and ions, giving bone its hardness and ability to resist compression.
  • Matrix Proteins (1/3 of matrix weight):
    • Consist mainly of collagen fibers.
    • Collagen provides bone with its tensile strength and flexibility, allowing it to resist stretching and twisting forces.
• Normal bone growth and maintenance depend on both nutritional and hormonal factors:
  • A regular dietary source of calcium and phosphate salts is essential.
  • Small amounts of other minerals like magnesium, fluoride, iron, and manganese are also required.

Bone Remodeling

• The adult skeleton is under continuous remodelment, a dynamic process involving constant breakdown and renewal.
• This process allows the skeleton to:
  • Maintain itself structurally.
  • Replace mineral reserves.
  • Recycle and renew the bone matrix.
• All bone cell types (osteoblasts, osteocytes, osteoclasts, and their precursors) are involved in this continuous cycle.
• The rate of bone turnover varies depending on the specific bone and individual activity levels.
  • If bone deposition (by osteoblasts) is greater than bone removal (by osteoclasts), bones tend to become thicker and stronger.
  • Conversely, if bone removal is faster than replacement, bones become weaker and less dense.

Effects of Exercise on Bone

• Mineral recycling allows bones to adapt to the stresses they experience.
• Heavily stressed bones, such as those in athletes or individuals engaged in physical labor, tend to become thicker and stronger due to increased osteoblast activity.
• Conversely, bone degeneration occurs quickly with inactivity.
  • It is estimated that up to one-third of bone mass can be lost within just a few weeks of complete inactivity (e.g., bed rest, prolonged immobilization).
• Mechanism of adaptation to stress: The application of compressive force to a slightly bent bone generates weak electrical currents on the inside curvature. These electrical currents stimulate osteoblasts, leading to bone deposition on the inside curvature and removal from the outside curvature over time. This process results in bone that is optimally matched to the compressive forces it frequently encounters.

Growth Patterns

• Appositional Growth: Refers to the thickening of bones, occurring at the bone surface.
• Interstitial Growth: Refers to the lengthening of long bones, primarily at the epiphyseal plates (growth plates).

Bone Formation and Growth (Ossification)

• Human bones generally grow until approximately age 25.
• Osteogenesis: The fundamental process of bone formation.
• Ossification: The specific process of replacing existing tissues (either cartilage or mesenchymal connective tissue) with bone tissue.
• Calcification: The process of depositing calcium salts within a tissue, which occurs during bone ossification and can also happen in other tissues under pathological conditions.
• There are two primary forms of ossification:
  1. Endochondral Ossification
  2. Intramembranous Ossification

Endochondral Ossification (Seven Main Steps)

• This is the process by which most bones of the body are formed. It typically ossifies bones that originate as a hyaline cartilage model.
  1. Cartilage Enlargement and Cavity Formation: As the cartilage model enlarges, chondrocytes (cartilage cells) near the center of the shaft significantly increase in size. The surrounding matrix is reduced to small struts and soon begins to calcify. The enlarged chondrocytes then die and disintegrate, leaving cavities within the cartilage model.
  2. Bone Collar Formation: Blood vessels grow around the edges of the cartilage model. The perichondrium (connective tissue surrounding the cartilage) converts into an osteogenic periosteum, and its cells differentiate into osteoblasts. These osteoblasts then secrete bone matrix, forming a superficial bone collar around the diaphysis.
  3. Primary Ossification Center Formation: Blood vessels penetrate the cartilage and invade the central region. Fibroblasts migrating with these blood vessels differentiate into osteoblasts, which begin producing spongy bone at a region known as the primary ossification center within the diaphysis. Bone formation then spreads along the shaft towards both ends of the former cartilage model.
  4. Medullary Cavity Formation and Diaphyseal Growth: As growth continues, remodeling occurs, leading to the formation of a medullary cavity in the center of the diaphysis. The osseous tissue of the shaft thickens, and the cartilage near each epiphysis is progressively replaced by bone. Further growth involves both increases in bone length and diameter.
  5. Secondary Ossification Centers: Capillaries and osteoblasts migrate into the epiphyses, creating distinct secondary ossification centers in both the proximal and distal ends of the bone.
  6. Epiphyseal Plate Formation and Continued Growth: The epiphyses eventually become filled with spongy bone. A relatively narrow cartilaginous region, called the epiphyseal cartilage or epiphyseal plate, separates the epiphysis from the diaphysis. At the epiphyseal side of this plate, chondrocytes continue to divide and enlarge, producing new cartilage. On the diaphyseal side, chondrocytes degenerate, and osteoblasts migrate upward from the diaphysis to continuously invade the cartilage, replacing it with bone. This balanced process allows the bone to lengthen.
  7. Epiphyseal Closure and Line Formation: At puberty, the rate of new cartilage production at the epiphyseal plate slows down, while the rate of osteoblast activity and bone replacement accelerates. Consequently, the epiphyseal cartilage narrows and eventually disappears in an event called epiphyseal closure. The former location of the epiphyseal cartilage becomes a distinct epiphyseal line, which remains visible in adult bones on X-rays, marking where growth in length has ended. A thin cap of the original cartilage model persists as the articular cartilage, covering the bone ends within joint cavities to prevent bone-to-bone contact and damage.

Intramembranous Ossification (Dermal Ossification) (Five Main Steps)

• This process forms dermal bones, which develop within fibrous connective tissue membranes, without a cartilage model. Examples include the mandible, clavicle, and most of the flat bones of the skull (e.g., parietal, occipital, frontal bones). Intramembranous ossification begins around the eighth week of embryonic development.
  1. Ossification Center Formation: Mesenchymal cells cluster together within the embryonic connective tissue. They then differentiate into osteoblasts and begin to secrete the organic components of the bone matrix (osteoid). The osteoid then becomes mineralized with calcium salts, forming unorganized bone matrix at an ossification center.
  2. Spicule Formation: As ossification proceeds, some osteoblasts become trapped within the growing bony pockets. These trapped cells differentiate into osteocytes. The developing bone grows outward from the ossification center in small, needle-like struts called spicules.
  3. Blood Vessel Trapping: Blood vessels begin to branch and grow into the region, weaving between the spicules. The reliable supply of oxygen and nutrients carried by these vessels significantly accelerates the rate of bone growth. As spicules interconnect, they fuse and trap the blood vessels within the forming bone tissue.
  4. Spongy Bone Plate Formation: Continued deposition of bone by osteoblasts, particularly those located close to the blood vessels, results in the formation of a plate of spongy bone with blood vessels ramifying throughout.
  5. Compact Bone and Periosteum Formation: Subsequent remodeling around the entrapped blood vessels leads to the formation of osteons, characteristic of compact bone. Osteoblasts on the bone surface, along with the surrounding connective tissue, organize to form the fibrous and cellular layers of the periosteum. Areas of spongy bone within the flat bones of the skull are remodeled to form the diploë (internal spongy bone layer), which is covered by thin plates of compact (cortical) bone on both the inner and outer surfaces.

Blood Supply of Mature Bones

• Bones are highly vascularized to support their metabolic needs and growth.
  1. Nutrient Artery and Vein: Typically, a single pair of large blood vessels enters the diaphysis of a long bone through an opening called the nutrient foramen. The femur is an exception, often having more than one pair.
  2. Metaphyseal Vessels: These vessels supply blood to the epiphyseal cartilage, the site of bone growth in length.
  3. Periosteal Vessels: These arteries and veins supply blood to the superficial osteons of the compact bone and to the secondary ossification centers during bone development.

Nutritional and Hormonal Factors for Bone Growth and Maintenance

• Calcitriol:
  • Produced in the kidneys.
  • Plays a crucial role in the absorption of calcium and phosphorus from the digestive tract.
  • Its synthesis requires vitamin D_3 (cholecalciferol), often acquired from diet or synthesized in the skin upon exposure to UV light.
• Vitamin C (Ascorbic Acid):
  • Essential for proper collagen synthesis, which forms the organic framework of bone matrix.
  • Also stimulates the differentiation of osteoblasts.
• Vitamin A:
  • Stimulates osteoblast activity.
• Vitamins K and B_{12}:
  • Both are important for the synthesis of various bone proteins.
• Growth Hormone (GH):
  • Produced by the pituitary gland.
  • Stimulates bone growth, particularly at the epiphyseal plates, by promoting protein synthesis and overall tissue growth.
• Thyroxine:
  • Secreted by the thyroid gland.
  • Works synergistically with growth hormone to stimulate osteoblast activity and the synthesis of bone matrix, thus promoting skeletal growth.
• Sex Hormones (Estrogens and Androgens):
  • Estrogens (from ovaries) and Androgens (from testes) stimulate osteoblast activity and the synthesis of bone matrix, especially during puberty.
  • Estrogens are known to stimulate epiphyseal closure more rapidly than androgens, which explains why females typically stop growing taller earlier than males.
• Calcitonin and Parathyroid Hormone (PTH):
  • These hormones are critical regulators of calcium and phosphate levels in the blood, influencing bone remodeling and mineral homeostasis.

Calcium Homeostasis

The Skeleton as a Calcium Reserve

• Bones serve as the body's primary storage site for calcium and other minerals.
• Calcium is the most abundant mineral in the human body.
• Calcium ions (Ca^{2+}) are vital for numerous physiological functions:
  • Maintaining the integrity and function of cell membranes.
  • Neuronal signaling and nerve impulse transmission.
  • Muscle cell contraction, particularly in the cardiac muscle cells where precise calcium regulation is critical for heart function.

Calcium Regulation

• The concentration of calcium ions in body fluids must be tightly regulated to maintain homeostasis.
• This critical balance is primarily maintained by two hormones:
  • Calcitonin
  • Parathyroid Hormone (PTH)
• These hormones control Ca^{2+} levels by influencing its storage in bones, absorption from the digestive tract, and excretion by the kidneys.

Parathyroid Hormone (PTH)

• Source: Produced and secreted by the parathyroid glands, four small glands located in the neck, typically on the posterior surface of the thyroid gland.
• Effects on Calcium Ion Levels (Increases Ca^{2+}):
  • Bones: Stimulates osteoclasts to resorb bone matrix, releasing stored calcium ions from the bone into the bloodstream.
  • Digestive Tract: Indirectly increases intestinal absorption of calcium by stimulating the kidneys to produce calcitriol (vitamin D_3).
  • Kidneys: Decreases the excretion of calcium ions by promoting their reabsorption from urine back into the blood.
• Response to Low Blood Calcium: When blood calcium ion concentrations fall below a threshold (e.g., 8.5 ext{ mg/dL}), the parathyroid glands are stimulated to secrete PTH. This leads to increased osteoclast activity, enhanced intestinal calcium absorption, and reduced renal calcium excretion, all working to elevate Ca^{2+} levels back to normal.

Calcitonin

• Source: Secreted by the C cells (also known as parafollicular cells) located in the thyroid gland.
• Effects on Calcium Ion Levels (Decreases Ca^{2+}):
  • Bones: Inhibits osteoclast activity, thus reducing the rate of bone resorption and preventing the release of calcium from bone. Concurrently, osteoblast activity is relatively enhanced, leading to calcium being locked into the bone matrix.
  • Digestive Tract: Indirectly decreases intestinal absorption of calcium by reducing calcitriol production.
  • Kidneys: Increases the excretion of calcium ions by inhibiting their reabsorption, leading to greater calcium loss in the urine.
• Response to High Blood Calcium: When blood calcium ion concentrations rise above a threshold (e.g., 11 ext{ mg/dL}), the thyroid gland's C cells secrete calcitonin. This results in inhibited osteoclast activity, reduced intestinal calcium absorption, and increased renal calcium excretion, all contributing to lowering Ca^{2+} levels back to normal.

Hormones Involved in Bone Growth and Maintenance Summary (Table ext{ }6-2)

Fractures

• Fractures are cracks or breaks in bones, primarily caused by physical stress or trauma.
• Fractures undergo a predictable four-step repair process:
  1. Bleeding and Hematoma Formation:
     • Immediately following a fracture, extensive bleeding occurs, leading to the formation of a large blood clot called a fracture hematoma.
     • Bone cells in the vicinity of the break that are deprived of nutrients due to damaged blood vessels will die.
  2. Callus Formation (Internal and External):
     • Cells from the endosteum and periosteum near the fracture zone rapidly divide and migrate into the area.
     • They form two types of calluses to stabilize the break:
       • External Callus: Consists of cartilage and bone, forming a protective bony and cartilaginous sheath around the exterior of the fracture.
       • Internal Callus: Develops within the medullary cavity, consisting primarily of spongy bone, unifying the inner edges of the broken bone.
  3. Replacement of Cartilage with Spongy Bone:
     • Osteoblasts actively invade the external callus and replace the central cartilage with spongy bone.
  4. Bone Remodeling:
     • Over a period of up to a year, osteoblasts and osteocytes continuously remodel the fracture site.
     • This remodeling process reduces the size of the bone calluses, gradually restoring the bone to its original shape and strength, though a slight swelling may initially mark the location.

Major Types of Fractures

• Transverse Fractures: Break occurs straight across the bone's long axis.
• Displaced Fractures: Bone ends are out of normal alignment.
• Compression Fractures: Bone is crushed, common in porous bones (osteoporotic vertebrae).
• Spiral Fractures: Ragged break occurs when excessive twisting forces are applied to a bone, common sports injury.
• Epiphyseal Fractures: Break occurs across the epiphyseal plate, potentially causing issues with bone growth.
• Comminuted Fractures: Bone fragments into three or more pieces, common in the elderly with brittle bones.
• Greenstick Fracture: Bone breaks incompletely, much like a green twig breaks, common in children whose bones are more flexible.
• Colles' Fracture: A fracture of the distal radius, typically resulting from a fall onto an outstretched hand (FOOSH) and causing a dorsal displacement of the wrist.
• Pott's Fracture: A bimalleolar fracture of the ankle involving both the medial and lateral malleoli, often with dislocation.

Effects of Aging on the Skeletal System

• Age-Related Changes:
  • Bones generally become thinner and weaker with advancing age.
  • Osteopenia, the condition of reduced bone mass, typically begins between ages 30 and 40.
  • Women experience a more rapid loss of bone mass, approximately 8\% per decade, compared to men who lose about 3\% per decade.
  • The epiphyses of long bones, vertebrae, and jawbones are commonly the most affected areas.
  • These changes can lead to fragile limbs, a reduction in overall height (due to vertebral compression), and potential tooth loss.
• Osteoporosis:
  • A severe form of bone loss that significantly impairs normal bone function.
  • It affects a substantial portion of the population over age 45: approximately 29\% of women and 18\% of men.
• Hormones and Bone Loss:
  • Sex hormones, including estrogens and androgens, play a critical role in maintaining bone mass throughout adulthood.
  • Bone loss in women accelerates significantly after menopause due to a dramatic decrease in estrogen production, which normally helps to inhibit osteoclast activity and promote osteoblast function.
• Cancer and Bone Loss:
  • Certain cancerous tissues can release osteoclast-activating factor.
  • This substance directly stimulates osteoclasts, leading to excessive bone resorption and potentially severe osteoporosis, even in areas distant from the primary tumor.