Bone Tissue Notes
Bone Tissue
Overview
Bone tissue makes up approximately 18% of human body weight.
Functions of Bone Tissue
Support: Provides a framework for soft tissues and attachment points for skeletal muscles.
Protection: Surrounds internal organs, the brain, and the spinal cord.
Movement: Skeletal muscles pull on bones to produce movement upon contraction.
Mineral Homeostasis: Stores minerals like calcium and phosphorus, contributing to bone strength. Releases these minerals into the blood when needed by other organs or muscles.
Blood Cell Production: Hemopoiesis (production of RBCs, WBCs, and platelets) occurs in the red bone marrow inside bones.
Triglyceride Storage: Triglycerides, a high-energy source, are stored in yellow bone marrow, which consists of a high concentration of adipose cells within the bone.
Bone Structure
Long Bones: Have a greater length than width (e.g., femur, humerus).
Diaphysis: The central shaft of the bone.
Epiphyses: The distal and proximal ends of the bone.
Metaphyses: Regions where the diaphysis meets the epiphyses. Contains the epiphyseal plate.
Epiphyseal Plate: A layer of hyaline cartilage responsible for bone growth.
Epiphyseal Line: Formed when the hyaline cartilage of the epiphyseal plate is replaced by bone after maturation.
Articular Cartilage: A thin layer of hyaline cartilage covering the epiphysis, reducing friction and absorbing shock in joints.
Periosteum: Dense irregular tissue surrounding the bone surface (except where covered by articular cartilage). Allows bone to grow in width, provides protection and nourishment, and serves as an attachment point for ligaments and tendons.
Sharpey’s (Perforating) Fibers: Collagen fibers that attach the periosteum to the bone.
Medullary Cavity: Space in the diaphysis containing fatty yellow bone marrow in adult long bones and red marrow in flat bones.
Endosteum: A thin membrane of bone-forming cells lining the medullary cavity.
Histology of Bone Tissue
Bone cells are widely separated by the extracellular matrix, which is rigid due to its composition:
25% Water
25% Collagen
50% Crystallized Mineral Salts (mostly calcium and phosphorus)
Calcification: Mineral salts are deposited in the collagen fiber framework, causing the tissue to harden.
Bone hardness depends on crystallized mineral salt content, while flexibility depends on collagen fiber content.
Bone Cells
Osteogenic Cells: Unspecialized stem cells that undergo mitosis and differentiation to become osteoblasts. Found in the periosteum and endosteum. They are the only bone cells that undergo cell division.
Osteoblasts: Bone-building cells that synthesize collagen to build the extracellular matrix and initiate calcification. Become trapped in the matrix and develop into osteocytes.
Osteocytes: Mature bone cells that maintain bone metabolism and nutrient/waste exchange with the blood.
Osteoclasts: Large cells formed from the fusion of approximately 50 monocytes (a type of WBC). Responsible for bone resorption.
Resorption: Breakdown of bone's extracellular matrix, essential for development, growth, maintenance, and repair.
Osteoclasts release digestive enzymes (from lysosomes) that break down the protein and mineral content of the matrix.
Osteoblasts then replace the digested matrix with a new, stronger extracellular matrix.
Compact Bone
The strongest form of bone tissue, making up 80% of the skeleton.
Located in the outer portion of bones, under the periosteum.
Blood and lymphatic vessels, and nerves from the periosteum penetrate compact bone through Volkmann's canals.
Vessels and nerves travel through Haversian (central) canals to supply each osteon.
Osteon: The basic structural unit of bone.
Concentric Lamellae: Rings of calcified extracellular matrix surrounding the Haversian canal.
Lacunae: Small spaces between lamellae that house osteocytes.
Canaliculi: Small canals radiating from the lacunae, connecting them to each other and the Haversian canal. Facilitate communication and nutrient/waste exchange between osteocytes.
Interstitial Lamellae: Fragments of older osteons, partially destroyed during bone remodeling or growth, found between osteons.
Circumferential Lamellae: Lamellae that encircle the medullary cavity just beneath the periosteum.
Spongy Bone
Does not contain osteons.
Appearance is spongy, but not in texture.
Trabeculae: The lamellae in spongy bone.
Within each trabecula, lacunae contain osteocytes, connected by canaliculi.
Trabeculae are surrounded by spaces containing red bone marrow, which is rich in blood. Osteocytes receive nutrients directly from the blood in the marrow.
Abundant in flat and irregularly shaped bones (sternum, ribs, pelvis, scapulae, vertebrae).
Lighter than compact bone.
Contains and protects the majority of red bone marrow in the body (where hemopoiesis occurs in adults).
Located in bones that are not heavily stressed. High-stress areas of bone have a high compact bone concentration (e.g., hip).
Bone Scan
Radiological diagnostic procedure involving intravenous injection of a small amount of radioactive tracer.
Tracer uptake is related to the amount of blood flow to the bone.
A gamma camera measures radiation emitted from the bones to film.
Normal Bone: Identified by a gray color, indicating uniform tracer uptake.
Hot Spots: Darker spots indicating increased metabolism due to increased blood flow. May indicate bone cancer, fractures, or abnormal growth.
Cold Spots: Lighter areas indicating decreased metabolism. May indicate osteoporosis, infections, or inflammatory arthridities (rheumatoid arthritis).
Blood and Nerve Supply
Bones are richly supplied with blood and nerves.
Abundance of blood vessels accommodates constant metabolism, remodeling, growth, and delivery of newly manufactured blood from red marrow.
Periosteal Arteries: Accompany nerves, traveling through Volkmann’s canals into the diaphysis to supply the periosteum and outer compact bone.
Nutrient Artery: Travels through the nutrient foramen (near the center of the diaphysis) and enters the medullary cavity. Divides into proximal and distal branches that supply the inner compact bone of the diaphysis, spongy bone, and bone marrow as far as the epiphyseal plates/lines.
Metaphyseal Arteries: Branches of arteries that supply the joint. Enter the metaphyses and, along with the nutrient artery, supply the red bone marrow and bone tissue of the metaphyses.
Epiphyseal Arteries: Branches of the arteries that supply the joint. Enter the epiphyses of long bones and supply the marrow and bone tissue of the epiphyses.
Veins: Nutrient vein accompanies the nutrient artery; also, epiphyseal, metaphyseal, and periosteal veins.
Nerves and free nerve endings (pain receptors) are abundant in the periosteum, making fractures and bone tumors painful.
Bone Formation
Ossification/Osteogenesis: Terms for bone formation.
Involves either the replacement of preexisting connective tissue with bone or the replacement of cartilage with bone.
Two methods: intramembranous and endochondral ossification.
Intramembranous Ossification
Seen in flat bones, mandible, and skull formation.
Development of the Ossification Center: Mesenchymal cells cluster together during fetal development, forming the ossification center. They differentiate into osteogenic cells, then into osteoblasts, which secrete the extracellular matrix.
Calcification: After extracellular matrix formation stops, cells become osteocytes in lacunae connected by canaliculi. Calcium and mineral salts are deposited, calcifying and hardening the matrix.
Trabeculae Formation: The extracellular matrix develops into trabeculae, forming spongy bone. Blood vessels grow in the spaces between trabeculae, forming red bone marrow.
Periosteum Development: Mesenchymal cells on the outside of the bone condense and form the periosteum.
The newly formed bone is remodeled into its adult size and shape.
Endochondral Ossification
Replacement of cartilage by bone, forming long bones.
Development of the Cartilage Model: Mesenchymal cells form the shape of the future bone and differentiate into chondroblasts, which secrete hyaline cartilage and produce a cartilage model. A perichondrium develops around the cartilage model.
Growth of the Cartilage Model: Chondroblasts mature into chondrocytes. Chondrocytes undergo mitosis, causing the cartilage model to increase in length (interstitial growth). The cartilage model increases in width through mitotic activity of chondroblasts in the perichondrium (appositional growth). As the cartilage model grows, chondrocytes enlarge, and the surrounding cartilage extracellular matrix begins to calcify. Chondrocytes die due to lack of nutrient diffusion across the calcified matrix, forming lacunae.
Development of the Primary Ossification Center: A nutrient artery penetrates the perichondrium and travels to the middle of the cartilage model. The blood supply stimulates osteogenic cells in the perichondrium to become osteoblasts, forming the periosteum. A primary ossification center forms as capillaries fill the disintegrating calcified cartilage, and bone tissue is deposited over remnants of calcified cartilage, forming spongy bone trabeculae.
Development of the Medullary Cavity: As the primary ossification center expands, osteoclasts break down newly formed spongy trabeculae, leaving a central cavity (the medullary cavity) in the diaphysis.
Development of Secondary Ossification Centers: Branches of the epiphyseal artery enter the epiphyses, where secondary ossification centers are located. The blood supply stimulates the formation of osteogenic cells that differentiate into osteoblasts. Osteoblasts deposit bone tissue over remnants of calcified cartilage, forming spongy bone. Unlike primary ossification centers, the spongy bone remains and is not replaced by a medullary cavity.
Formation of Articular Cartilage and the Epiphyseal Plate: Hyaline cartilage remains over the epiphyses, becoming articular cartilage. Hyaline cartilage also remains between the diaphysis and the epiphysis, forming the epiphyseal plate, which is responsible for lengthening of long bones.
Interstitial Growth
The epiphyseal plate is a layer of hyaline cartilage in the metaphysis of long bones, consisting of four zones:
Zone of Resting Cartilage: Anchors the epiphyseal plate to the epiphysis.
Zone of Proliferating Cartilage: Chondrocytes divide to replace dead chondrocytes.
Zone of Hypertrophic Cartilage: Large, maturing chondrocytes arranged in columns.
Zone of Calcified Cartilage: Mostly dead, calcified chondrocytes. Osteoclasts dissolve the calcified cartilage, while osteoblasts and vessels from the diaphysis invade the area and lay down new bone. The bone's length increases at the diaphyseal end of the epiphyseal plate.
The epiphyseal plate is ultimately replaced by bone, forming the epiphyseal line (occurs at about age 18 in females and 21 in males).
Appositional Growth
Cells in the periosteum differentiate into osteoblasts, secreting collagen and extracellular matrix until they are surrounded by it and become osteocytes. This forms bone ridges on either side of a periosteal blood vessel, forming grooves for the vessel.
The ridges fold over the vessel and fuse, enclosing the blood vessel in a tunnel. The former periosteum becomes the endosteum lining the tunnel.
Osteoblasts in the endosteum lay down more bony matrix, forming new concentric lamellae around the vessel, forming the osteon.
Osteoblasts under the periosteum deposit new outer circumferential lamellae, increasing the width of the bone.
As new bone is deposited on the outer surface, osteoclasts destroy the medullary cavity, causing it to enlarge as the bone increases in thickness.
Bone Remodeling
Involves bone resorption by osteoclasts and bone deposition by osteoblasts.
Essential for growth, removing injured bone and replacing it with new, stronger bone, making it fracture resistant.
Balance between osteoclast and osteoblast activity is crucial.
Too much osteoclast activity and too little osteoblast activity can lead to excessive loss of calcium and mineral content, causing osteoporosis, rickets, and osteomalacia.
Too much osteoblast activity and too little osteoclast activity can result in abnormally thick and heavy bone or the formation of osteophytes (bone spurs).
Factors Affecting Bone Growth
Minerals: Dietary intake of calcium and phosphorus and, to a lesser extent, fluoride, magnesium, iron, and manganese.
Vitamins:
Vitamin C is crucial for collagen formation.
Vitamins K and B12 are needed for bone protein synthesis.
Vitamin A stimulates osteoblastic activity.
Vitamin D allows calcium absorption from the GI tract into the blood.
Hormones:
Insulin-like growth factors (IGFs) control bone growth. They are produced by the liver and bone tissue in response to human growth hormone secreted by the anterior lobe of the pituitary lobe.
Thyroid hormones, estrogen, and testosterone stimulate osteoblastic activity.
Decreased estrogen secretion after menopause is a significant risk factor for osteoporosis.
Hormonal Abnormalities
Oversecretion of human growth hormone (hGH) during childhood results in gigantism.
Oversecretion of hGH during adulthood results in acromegaly.
Undersecretion of hGH during childhood results in dwarfism.
Fracture Repair
A fracture is any break in a bone.
Fracture Hematoma Formation: Broken blood vessels leak blood, forming a clot (fracture hematoma) around the fracture site (6-8 hours post-injury). Nearby bone cells die due to loss of blood supply, and the inflammatory response begins. Osteoclasts remove dead bone tissue, and white blood cells remove injured tissue (may last several weeks).
Fibrocartilaginous Callus Formation: Fibroblasts and chondrocytes from the periosteum invade the fracture site and deposit collagen fibers and fibrocartilage, respectively. This leads to the formation of a fibrocartilage callus, connecting the broken ends of the bone (takes 3 weeks).
Bony Callus Formation: As the vascular supply is repaired, osteoblasts are formed and start forming spongy bone. Osteoblasts convert the fibrocartilage callus into spongy bone, forming the bony callus (lasts 3-4 months).
Bone Remodeling: Osteoclasts resorb dead bone, and osteoblasts replace spongy bone with compact bone at the bone surface.
Types of Fractures
Pott Fracture: Occurs at the ankle and affects both bones of the leg.
Comminuted Fracture: Shatters the affected area into a multitude of bony fragments (e.g., femur fracture).
Colles Fracture: A break in the distal portion of the radius, typically from reaching out to cushion a fall.
Greenstick Fracture: Only one side of the shaft is broken, and the other is bent. Generally occurs in children whose long bones have yet to ossify fully
Transverse Fractures: Break a bone shaft across its long axis.
Spiral Fractures: Produced by twisting stresses that spread along the length of the bone.
Displaced Fractures: Produce new and abnormal bone arrangements.
Nondisplaced Fractures: Retain the normal alignment of the bones or fragments.
Epiphyseal Fractures: Occur where the bone matrix is undergoing calcification and chondrocytes are dying. Can permanently stop growth if not treated carefully.
Compression Fractures: Occur in vertebrae subjected to extreme stresses.
Blood Calcium Homeostasis
Bone is the body’s calcium reservoir, storing 99% of the body’s calcium.
Calcium is essential for muscle contraction, nerve conduction, blood clotting, and heart function.
Calcium exchange between bones and blood is hormone-regulated.
Parathyroid Hormone (PTH): Secreted by the parathyroid glands when blood calcium concentration is low. Stimulates osteoclasts to break down bone tissue, releasing calcium into the blood.
Calcitonin: Released from parafollicular cells in the thyroid gland when blood calcium concentration is high. Decreases osteoclast activity and stimulates osteoblastic activity, removing calcium from the blood.
Exercise and Bone Tissue
Wolff's Law: Mechanical stress determines bone architecture. Bone is deposited in sites subjected to stress and resorbed from sites where there is little stress.
Weight-bearing exercise increases bone strength due to increased deposition of collagen and mineral salts.
Weight-bearing exercise is the best way to prevent osteoporosis (decreased bone mineralization and density) and osteopenia (decrease in bone mass).