Topic 5

EXS 207 Topic 5: Bones

Functions of Bone

  • Support:

    • Provide a framework for the body.

    • Cradles soft organs, enabling structural integrity.

  • Protect:

    • Surrounds and encases important organs such as:

    • Brain

    • Spinal cord

    • Heart

    • Lungs

  • Facilitate movement:

    • Bones act as levers upon which skeletal muscles act, enabling movement.

  • Produce blood cells:

    • Hematopoiesis: The process of blood cell formation occurs in red bone marrow.

  • Mineral storage:

    • Serve as a reservoir for essential minerals, primarily calcium and phosphate.

  • Fat storage:

    • Triglycerides stored in yellow marrow serve as an energy reserve.

Note: The aforementioned functions contrast the common perception of bones as static structures.

Are Bones Organs?

  • Definition of an organ: Structures composed of two or more tissue types that have specific functions or functions.

    • Yes, bones are classified as organs due to their composition:

    • Bone (osseous tissue)

    • Marrow (red and yellow)

    • Nervous tissue

    • Cartilage

    • Fibrous connective tissue

    • Muscle cells

    • Epithelial cells

    • Blood vessels

Bone Structure

Types of Bones

  1. Long Bones

    • Characterized as being longer than they are wide.

    • Examples: Most limb bones, including small bones of fingers and toes.

  2. Short Bones

    • Approximately equal in length and width, roughly cube-shaped.

    • Examples: Carpals of the hand and tarsals of the feet.

  3. Flat Bones

    • Typically thin, broad, and often curved.

    • Examples: Skull bones, sternum, ribs.

  4. Sesamoid Bones

    • Small, flat, oval-shaped bones found within tendons.

    • Function: Increase muscle leverage and extend the longevity of tendons.

    • Example: Patella (kneecap).

  5. Irregular Bones

    • Bones that do not fit easily into previous classifications.

    • Example: Vertebrae.

Macroscopic Anatomy of Bone

  • Periosteum:

    • Vascularized dense irregular connective tissue that covers bone, excluding joint surfaces.

  • Compact bone:

    • Hard outer layer of bone made up primarily of concentric lamellar osteons and interstitial lamellae.

    • Strong and resistant to compression, twisting, and shearing stress.

  • Spongy bone:

    • Located beneath compact bone, featuring a honeycomb-like network of trabeculae.

    • Trabeculae is the bone around the marrow in the spongy.

  • Endosteum:

    • Thin, vascularized connective tissue lining the inner surfaces of bone.

Strucure of Long Bones

  • Diaphysis:

    • Long axis or shaft of a long bone, comprised primarily of a thick layer of compact bone with minimal spongy bone.

  • Epiphyses:

    • Rounded ends of bones comprising spongy bone encased in a compact bone shell.

    • Covered by hyaline cartilage to minimize friction from adjoining bones.

  • Metaphyses:

    • The narrow portion between the epiphysis and diaphysis; a major site of bone growth.

  • Epiphyseal lines:

    • Reside within the metaphyses, remnants of the growth plates (epiphyseal plates).

Structure of Short, Flat, Irregular, and Sesamoid Bones

  • Arrangement:

    • Simple arrangement consists of spongy bone encased in a compact bone shell. With periosteum on the outer surface of the compact bone.

Bone Marrow

  • Red marrow:

    • Contains hematopoietic stem cells that produce blood cells.

  • Yellow marrow:

    • Contains adipocytes.

    • Infants and children typically possess primarily red marrow for rapid growth and increasing blood cell requirements.

    • From age five, yellow marrow starts to replace red marrow, leading to adulthood typically exhibiting more yellow than red marrow.

Bone Cells

  • Osteogenic cell (stem cell):

    • Develop and give rise to osteoblasts.

  • Osteoblasts:

    • Develop into mature osteoblasts that maintain bone health.

  • Osteocyte:

    • Mature osteoblasts that maintain bone health.

  • Osteoclasts:

    • Break down bones and reasorbs bone.

Bone Matrix

  • Composition: Bone matrix is unique because it has both inorganic and organic components.

    • Inorganic Matrix:

    • Comprises 65% of bone weight; provides strength.

    • Organic Matrix (Osteoid):

    • Comprises 35% of bone weight; provides flexibility.

Inorganic Matrix of Bone

  • Mostly made up of hydroxyapatite:

    • Chemical composition: Calcium phosphate crystals - Ca5(PO4)3OH

    • Contributes to hardness, strength, and resistance to compression.

    • Also contains bicarbonate, potassium, magnesium, and sodium.

Organic Matrix of Bone (Osteoid)

  • Composed of collagen fibers which align with hydroxyapatite crystals to enhance hardness.

  • Contains various proteins (such as proteoglycans and bone-specific proteins) that help maintain hydration (some of these trap water) in the bone and resist compression.

Bone ECM (Extracellular Matrix) Composition

  1. Ground substance

  2. Collagen fibers

    • 1 & 2 Organic Matrix

  3. Calcium phosphate

    • 3 Inorganic matrix

Compact Bone - Microscopic Anatomy

  • Compact bone consists of tightly packed units called osteons (Haversian Systems).

    • A collection of osteons.

Osteons

  • Cylindrical structures: Comprising multiple concentric lamellae (4-20) surrounding a central canal.

    • Each osteon is a cylinder.

  • Lamellae:

    • Thin rings of bone tissue with collagen fibers oriented in opposite directions to resist twisting and bending forces in several directions.

Central Canal

  • Passageway for nerves and blood vessels that innervate and supply the cells of the osteon.

Perforating Canals (Volkmann’s Canals)

  • Perpendicular to central canals, allowing blood vessels and nerves to penetrate and connect central canals to each other.

Canaliculi

  • Tiny canals connecting lacunae to the central canal, supplying nutrients to osteocytes.

Spongy Bone - Microscopic Anatomy

  • Made up of trabeculae, forming a loose meshwork of extensively branched bone tissue.

  • Lacks osteons, central canals, or perforating canals; however, trabeculae contain lamellae, lacunae housing osteocytes, and canaliculi for nutrient access from blood vessels supplying the marrow.

Bone Formation, Growth & Remodeling

Ossification

  • Ossification = The process of bone formation.

  • Transforms the precursor skeleton composed of hyaline cartilage and fibrous membranes into bone during fetal development.

Types of Ossification

  1. Endochondral Ossification:

    • Bone develops from a hyaline cartilage model.

    • A complex process requiring the breakdown of hyaline cartilage as ossification occurs.

    • Forms almost all bones below the skull, excluding clavicles.

  2. Intramembranous Ossification:

    • Bone develops from within a fibrous membrane, forming facial and cranial bones, as well as clavicles.

Bone Growth After Birth

  • Two types occur:

  1. Longitudinal Growth:

    • Increase in bone length.

  2. Appositional Growth:

    • Increase in diameter.

  • Longitudinal Growth: Takes place at epiphyseal plates through several zones:

    • Proliferation Zone: Chondrocytes divide, pushing older cells toward the diaphysis.

    • Hypertrophic Zone: Chondrocytes increase in size and die.

      • As cells move toward diaphysis, they begin to hypertrophy and die.

    • Calcification Zone: Matrix becomes calcified by surrounding tissues.

      • When chondrocytes die, matrix becomes calcified by the surrounding tissue.

    • Ossification Zone: Invaded by blood vessels and bone cells such as osteoclasts and osteoblasts.

      • Osteoclats: break down the existing cartilaginous matrix.

      • Osteoblasts: can begin to deposit bone matrix.

Appositional Growth

Bones are getting wider and there is more build up occurring than breaking down.

  • Results from:

    1. Osteoblasts creating new bone at the surface.

    2. Osteoclasts resorbing old bone lining the medullary cavity.

Topic Understanding

  • Which of the following might explain why breakdown of bone lining the medullary cavity must occur at all during interstitial growth?

    • It allows bone diameter to increase without becoming too heavy.

Hormonal Regulation of Bone Growth

  • Several hormones influence bone growth:

  1. Growth Hormone (GH):

    • Increases length of bones by stimulating chondrocytes in the epiphyseal plate, and increases bone density via enhanced Ca2+ retention and osteoblast activity.

  2. Thyroxine:

    • Works alongside GH to promote osteoblast activity.

  3. Sex Hormones:

    • Testosterone and estrogen encourage osteoblast activity, promote adolescent growth spurts, and cause closure of the epiphyseal plate (converting it to bone - epiphyseal line.)

Bone Remodeling

  • Even after bones are fully grown, they continuously undergo remodeling:

    • Bone Creation: By osteoblasts.

    • Bone Resorption: By osteoclasts.

    • Influenced by hormonal regulation and physical forces (Wolff’s law).

      • Wolfs law is the demand being placed on bone.

    • Approximately 5-10% of the skeleton is entirely renewed each year during adulthood.

Calcium Homeostasis

  • Importance: Many physiological processes rely on Ca2+. Examples include muscle contraction, nerve impulse transmission, blood coagulation, gland secretions, and cell division.

  • Hormonal Regulation:

    • Parathyroid Hormone (PTH): (Calcium level too low)

      • Increases blood Ca2+ levels.

      • Calcium is being reabsorbed from urine by kidneys.

      • Calcium absorption in the small intestine increase via vitamin D synthesis.

      • Increases activity of osteoclasts which will lower bone density. You will destroy more than create.

    • Calcitonin: (Calcium level to high)

      • Decreases blood Ca 2+ levels.

      • Inhibits osteoclasts.

Wolff’s Law

  • Bones adapt to the loads they encounter; their growth and remodeling are influenced by how they are used.

Observations Supporting Wolff's Law

  • Bones in the dominant limb tend to be thicker and stronger.

  • Curved bones are thicker where stress is most likely to buckle.

  • Trabeculae align in the direction of forces experienced.

  • Large bony projections arise where large muscles attach.

  • Healed bone typically resembles the original structure when activity patterns return to normal.

Topic Understanding

  • Would growth hormone injections make a short, but otherwise healthy 35 year-old taller? Why or why not?

    • No, because the epiphyseal plates will have closed by this age.

Homeostatic Imbalances of Bone

Fracture Repair

  1. Fracture Hematoma Formation:

    • Occurs due to torn blood vessels bleeding and clot formation within 6-8 hours post-fracture.

  2. Calli Formation:

    • Internal Calli: Fibrocartilaginous.

    • External Calli: Hyaline cartilage and bone, providing stability. This occurs within approximately 48 hours.

  3. Callus Replacement:

    • Calli are replaced by trabecular bone through endochondral ossification after about 6-8 weeks.

    • Osteoclasts resorb the dead bone while osteogenic cells differentiate into osteoblasts.

  4. Remodeling:

    • The final stage where healing is complete; compact bone replaces spongy bone at outer margins.

    • The bone remodels according to usage to restore standard structure (over several months).

Osteoporosis

  • Description: The most commonly encountered bone disease characterized by an imbalance between osteoblast and osteoclast activity, resulting in decreased bone mass.

  • Osteoclasts have gained n advantage over time. With oteoporosis, balance tips toward osteoclasts and bone mass decline.

Symptoms of Osteoporosis

  • Weakened bones leading to inadequate weight support and increased fracture risk, presenting as:

    • Stooped posture and loss of vertical height.

    • Increased susceptibility to fractures in hips, forearms, wrists, and vertebrae.

Risk Factors for Osteoporosis

  •    Age is the primary determinant of Osteoporosis.

  • Age, chronic alcoholism, dietary imbalances (especially in calcium), removal of ovaries (leading to estrogen deficiency), endocrine disorders, kidney disorders, gastrointestinal tract disorders, heavy tobacco use, and a sedentary lifestyle.

Diagnosis of Osteoporosis

  • Measured bone density in the hip region using dual-energy X-ray absorptiometry (DXA).

  • Compare readings to the average bone density of a young adult female (30-40 years old) to obtain a T score.

  • T score Values:

    • Normal bone density: +1 to -0.99

    • Osteopenia (bone thinning): -1.0 to -2.49

    • Osteoporosis: -2.5 and below.

Treatment and Prevention of Osteoporosis

  • Encouragement of a balanced diet, potentially inclusive of calcium and vitamin D supplementation.

  • Advocating a physically active lifestyle with regular weight-bearing exercises.

  • Recommendations against smoking and limiting alcohol consumption.

  • Consideration of hormone (estrogen) replacement therapy (HRT) for postmenopausal women.

  • Potential use of calcitonin treatment.

Paget’s Disease

  • Overview: Characterized by problems with bone remodeling. Involves excessive bone resorption followed by increased bone formation leading to enlarged and deformed bones.

  • Symptoms can include bone pain, arthritis, deformities, and fractures. Etiology is idiopathic, potentially viral or inherited.

Final Thoughts: Dynamics of Bones

  • Ultrasound exams provide precise determination of fetal age due to predictable ossification timelines of the embryonic skeleton.

  • By birth, long bones are primarily ossified (except for epiphyseal plates).

  • By the age of 25, nearly all bones are fully ossified with closed epiphyseal plates.

  • Bone mass begins to decrease from the fourth decade onward, and the rate of loss is influenced by genetic and environmental factors, notably where bone resorption predominates in old age.