Skeletal System: Bone Tissue Notes

The Skeletal System: Bone Tissue

General Functions of Bone

• Support and Protection:

  • Provides structural support for the body.

  • Acts as a framework for the body.

  • Protects vital organs.

• Hemopoiesis:

  • The production of blood cells.

  • Occurs in the red bone marrow.

• Storage:

  • Stores calcium mineral.

Anatomy of a Bone

• Diaphysis:

  • The elongated, usually cylindrical shaft of a long bone.

• Epiphysis:

  • The knobby regions at the ends of a long bone.

  • Proximal epiphysis: The end of the bone closest to the trunk.

  • Distal epiphysis: The end of the bone farthest from the trunk.

• Articular Cartilage:

  • A thin layer of hyaline cartilage covering the joint surface.

  • Reduces friction and absorbs shock during movement.

• Metaphysis:

  • The region of mature bone located between the diaphysis and the epiphysis.

• Medullary Cavity:

  • A hollow, cylindrical space located within the diaphysis.

• Epiphyseal Plate (Growth Plate):

  • Located in the metaphysis.

  • Responsible for continued lengthwise bone growth during development.

  • In adults, its remnant is termed the epiphyseal line, appearing as a thin line of compact bone.

• Endosteum:

  • A membrane that covers all internal surfaces of bone within the medullary cavity.

• Periosteum:

  • A tough sheath that covers the outer surface of bone.

  • Consists of an outer fibrous layer and an inner osteogenic layer.

  • Attached to the bone by perforating (Sharpey's) fibers.

Bone Marrow

• Red Bone Marrow:

  • Hemopoietic, meaning it is responsible for the formation of red blood cells.

• Yellow Bone Marrow:

  • A product of red bone marrow degeneration that occurs in adults.

  • Primarily composed of fatty substances.

Cells of Bone

• Osteoprogenitor Cells:

  • Primordial, undifferentiated cells.

  • Found in the endosteum of bones.

  • Develop into osteoblasts.

  • Belong to the bone cell lineage.

• Osteoblasts:

  • Cells that build bone by secreting bone matrix, called osteoid.

  • Responsible for bone deposition.

• Osteocytes:

  • Mature osteoblasts that have become entrapped in the bone matrix.

  • Function in the maintenance of bone tissue.

  • Reside in small spaces called lacunae and extend processes through canaliculi.

• Osteoclasts:

  • Large, multinucleated cells that break down bone tissue.

  • Function in resorption, the breakdown of the extracellular matrix of bone.

  • Release calcium from bone.

  • Characterized by a 'ruffled border' where resorption occurs.

  • Belong to the white blood cell lineage.

Histology of Bone

• Compact Bone:

  • Strong and dense tissue, excellent for providing protection and support.

• Spongy Bone:

  • Lightweight tissue that provides internal tissue support.

Blood Supply of Bone

• Bones are highly vascularized, receiving blood from several arteries and veins:

  • Periosteal arteries and veins.

  • Nutrient artery and nutrient veins (enter through the nutrient foramen).

  • Epiphyseal artery and epiphyseal vein.

  • Metaphyseal artery and metaphyseal vein.

Compact Bone Structure

• Composed of osteons (also known as Haversian systems).

  • Osteon: The basic functional and structural unit of mature compact bone.

  • Appears like concentric rings, similar to a tree trunk.

  • Central Canal (Haversian Canal): A cylindrical channel located at the center of each osteon, containing blood vessels and nerves.

  • Concentric Lamellae: Rings of bone matrix that surround the central canal.

  • Osteocytes: Mature bone cells found in small spaces called lacunae, located between the concentric lamellae.

  • Canaliculi: Tiny canals that extend from each lacuna, connecting osteocytes to each other and to the central canal, allowing for nutrient and waste exchange.

• Other structures within compact bone:

  • Interstitial lamellae: Remnants of older osteons between intact osteons.

  • External circumferential lamellae: Layers of bone immediately deep to the periosteum.

  • Internal circumferential lamellae: Layers of bone surrounding the medullary cavity.

  • Interosteonic (Volkmann's or Perforating) Canals: Canals that run perpendicular to the central canals, connecting blood vessels and nerves of adjacent osteons and the periosteum.

Spongy Bone Structure

• Components of spongy bone include:

  • Trabeculae: A meshwork of crisscrossing bars and plates of bone.

  • These trabeculae provide great resistance to stresses and help to distribute weight.

  • The spaces between trabeculae are often filled with red bone marrow.

Bone Formation (Ossification or Osteogenesis)

• Ossification is the process of bone formation.

• Bones form in four main situations:

  1. During embryological and fetal development.

  2. When bones grow before adulthood.

  3. When bones remodel throughout life.

  4. When fractures heal.

• Ossification primarily takes place in two forms:

  • Intramembranous ossification.

  • Endochondral ossification.

Intramembranous Ossification

• Produces most of the flat bones of the skull and the mandible (facial bones).

• Steps of intramembranous ossification:

  1. Development of Ossification Center: Mesenchymal cells cluster in thickened regions and differentiate into osteoblasts, which then begin to secrete the organic extracellular matrix (osteoid).

  2. Calcification: Calcium and other mineral salts are deposited into the osteoid, causing the extracellular matrix to calcify (harden). Osteoblasts trapped within the matrix become osteocytes.

  3. Formation of Trabeculae: The extracellular matrix develops into trabeculae, which fuse together to form spongy bone. Blood vessels grow into the spaces within the trabeculae.

  4. Development of the Periosteum: Mesenchyme at the periphery of the developing bone condenses to form the periosteum. A thin layer of compact bone eventually forms superficial to the spongy bone.

Endochondral Ossification

• Begins with a hyaline cartilage model.

• Produces most of the bones of the skeleton, including long bones.

• Process:

  1. Development of Cartilage Model: Mesenchymal cells develop into chondroblasts, which secrete extracellular matrix to form the hyaline cartilage model of the bone.

  2. Growth of Cartilage Model: The cartilage model grows in length (interstitial growth) and width (appositional growth) due to the division and matrix secretion of chondrocytes.

  3. Development of Primary Ossification Center: In the diaphysis, chondrocytes hypertrophy and the surrounding cartilage matrix calcifies. Blood vessels penetrate, bringing osteoprogenitor cells into the area, which differentiate into osteoblasts. Bone tissue replaces most of the cartilage here.

  4. Development of Medullary Cavity: Osteoclasts break down the newly formed spongy bone in the center of the diaphysis, forming the medullary cavity.

  5. Development of Secondary Ossification Centers: These centers appear in the epiphyses, where similar processes of cartilage calcification and bone formation occur.

  6. Formation of Articular Cartilage and Epiphyseal Plate: Hyaline cartilage remains as articular cartilage on the epiphyses and as the epiphyseal plate (growth plate) between the diaphysis and epiphyses.

• Epiphyseal Plate (Growth Plate):

  • Lengthwise growth of bone is dependent upon cartilage growth at this region.

  • New hyaline cartilage is continuously produced and then replaced by bone from the diaphyseal side.

  • The epiphyseal plate narrows over time until it completely ossifies, forming the epiphyseal line in adults, indicating the cessation of lengthwise growth.

• Zones of the Epiphyseal (Growth) Plate (from epiphysis to diaphysis):

  1. Zone of Resting Cartilage: Nearest the epiphysis; small, scattered chondrocytes anchor the plate to the epiphyseal bone.

  2. Zone of Proliferating Cartilage: Chondrocytes divide rapidly and arrange into columns, forming new chondrocytes.

  3. Zone of Hypertrophic Cartilage: Chondrocytes enlarge and mature.

  4. Zone of Calcified Cartilage: Chondrocytes die, and the matrix around them calcifies. This calcified cartilage is then replaced by bone on the diaphyseal side, leading to lengthwise growth.

Bone Remodeling

• A continual process involving both bone deposition (building) and bone resorption (breakdown).

• Continues throughout adulthood.

• Occurs at both the periosteal and endosteal surfaces of a bone.

• Increased Bone Mass:

  • Results from weight-bearing activities and mechanical stress.

  • Can increase total bone mass throughout an individual's lifetime.

• Decreased Bone Mass:

  • Results from the removal of mechanical stress.

  • Leads to demineralization and bone loss.

Fracture and Repair of Bone

• A fracture is a break in a bone; many different types can occur.

• The healing process involves four steps:

  1. Reactive Phase: An early inflammatory phase characterized by the formation of a fracture hematoma (a blood clot) at the site of the break.

  2. Reparative Phase - Fibrocartilaginous Callus Formation: Fibroblasts and chondroblasts invade the hematoma and produce collagen fibers and fibrocartilage, forming a fibrocartilaginous (soft) callus that bridges the broken ends of the bone.

  3. Reparative Phase - Bony Callus Formation: Osteoprogenitor cells develop into osteoblasts, which produce spongy bone. The fibrocartilaginous callus is converted into a bony (hard) callus of spongy bone.

  4. Bone Remodeling Phase: The final step where the bony callus is remodeled over several months. Excess material on the exterior and within the medullary cavity is removed, and compact bone replaces spongy bone where appropriate, restoring the bone to its original shape and strength.

Common Fractures

• Open (Compound): The broken bone pierces the skin.

• Comminuted: The bone fragments into several pieces (often more than two).

• Greenstick: A partial fracture where one side of the bone is broken and the other side is bent (common in children).

• Impacted: One end of the fractured bone is driven forcefully into the other end.

• Pott: A fracture of the distal end of the fibula, often accompanied by a fracture of the medial malleolus of the tibia and damage to the ankle joint.

• Colles: A fracture of the distal end of the radius, where the distal fragment is displaced posteriorly (often caused by falling on an outstretched hand).

Bone's Role in Calcium Homeostasis

• Bones store $99\%$ of the body's calcium.

• When blood calcium levels drop, the parathyroid gland secretes Parathyroid Hormone (PTH).

• PTH stimulates osteoclasts to increase the rate of bone breakdown.

• This process causes calcium to be released from the bone matrix into the bloodstream, thus raising blood calcium levels.

Effects of Aging on Bone

• Osteopenia:

  • A condition of reduced bone mass that occurs to some extent in all people with age.

  • Typically begins as early as age $35-40$.

  • Characterized by a decline in osteoblast activity, while osteoclast activity remains at previous levels.

  • Women tend to lose more of their skeletal mass every decade than men due to hormonal changes (e.g., menopause).

• Osteoporosis:

  • A more severe condition characterized by a significant reduction in bone mass, making bones porous and brittle.

  • This reduction is sufficient to compromise normal bone function and greatly increases the risk of fractures.