Bone Tissue
Bone Tissue Study Notes
Introduction to the Skeletal System
The skeletal system is comprised of multiple connective tissues, which include:
Bone
Cartilage
Connective tissue
Bones serve as the primary organ of the skeletal system with several functions:
Provides framework for the body
Facilitates movement
Offers protection to vital organs
Stores minerals and helps maintain homeostasis
Types of Bone
Compact Bone
Also known as cortical bone
Made of osteons, providing bone strength
Accounts for 80% of bone mass
Spongy Bone
Also referred to as cancellous or trabecular bone
Located internally to compact bone
Appears porous which reduces bone weight and density
Provides a location for hemopoietic tissue in many areas
Cartilage and Connective Tissue
Cartilage
A semirigid connective tissue that is more flexible than bone
Hyaline Cartilage
Functions include:
Attaches ribs to the sternum
Forms articular cartilage at bone ends
Constitutes the cartilage in the epiphyseal plate of growing skeletons
Serves as a model for bone formation in fetal and infant skeleton
Fibrocartilage
A weight-bearing cartilage capable of resisting compression.
Located in:
Intervertebral discs
Pubic symphysis
Menisci of the knee
Dense Regular Connective Tissue (DRCT)
Provides structural connections:
Ligaments (connect bone to bone)
Tendons (connect muscle to bone)
Distribution of Cartilage in the Skeleton
Adult Skeleton:
Hyaline cartilage found in bone sockets of:
Shoulder
Arm
Wrist
Fingers
Femur
Knees
Ankle
Toes
Fibrocartilage located in:
Thorax
Intervertebral discs
Pubic symphysis
Juvenile Skeleton:
Notable presence of cartilage at the epiphyseal plate in the humerus
Bone as a Tissue
General Functions of Bone:
Provides body support and protective functions
Acts as levers for muscle attachment and movement
Responsible for hematopoiesis (blood cell production)
Stores minerals (calcium and phosphate) and energy resources
Classification of Bone by Shape:
Long Bones: Length greater than width (e.g., femur, humerus)
Short Bones: Length nearly equals width (e.g., carpals, tarsals)
Flat Bones: Thin, flat surfaces (e.g., cranial bones)
Irregular Bones: Complex shapes (e.g., vertebrae, pelvis)
Coverings and Linings of Bone
Periosteum:
A tough double-layered connective tissue lining the external surface of bones
Comprises:
Outer fibrous layer
Inner dense irregular connective tissue (DICT) layer
Protects bone from surrounding structures, anchors blood vessels/nerves, and serves as an attachment site for tendons/ligaments
Responsible for bone growth in width
Endosteum:
A thin connective tissue lining the internal surfaces of the bone
Lines the medullary cavity, Haversian canals, Volkmann’s canals, etc.
Contains:
Osteoprogenitor cells
Osteoblasts
Osteoclasts
Actively involved in bone growth, repair, and remodeling
Gross Anatomy of Long Bone
Diaphysis:
The shaft of the bone
Medullary Cavity:
The hollow center housing bone marrow
Metaphysis:
The transition region from diaphysis to epiphysis
The epiphyseal plate is made of hyaline cartilage and converts to bone as length increases, forming the epiphyseal line post-growth
Epiphyses:
The proximal and distal ends of long bones
The proximal epiphysis may contain hemopoietic tissue (e.g., femur)
Articular Cartilage:
A layer of hyaline cartilage located on the epiphyseal surface
Gross Anatomy of Other Bone Classes
Short, flat, and irregular bones have distinct anatomical structures:
External surface features:
Comprised of compact bone covered with periosteum
Internal make-up: entirely spongy bone
Absence of medullary cavity
Blood Supply and Innervation of Bone
Blood Supply:
Highly vascularized, especially within spongy bone
Blood vessels enter from periosteum
Main vessels include Nutrient Artery and Nutrient Vein for nutrient supply and waste removal
Nervous Innervation:
Nerves accompany blood vessels through foramina
Innervates:
Bone
Periosteum
Endosteum
Marrow cavity
Mainly comprised of sensory neurons
Bone Marrow
Red Marrow (myeloid tissue):
Contains reticular connective tissue, developing blood cells, adipocytes
Primary function: hematopoiesis
Conversion in aging:
In children: found in spongy bone and medullary cavity of long bones
In adults: found in skull, vertebrae, ribs, sternum, ossa coxae, proximal epiphyses of humerus and femur
Yellow Marrow:
Acts as an energy storage site
In conditions of severe anemia, it may revert back to red marrow to meet RBC demands
Clinical View: Bone Marrow Transplant
Bone marrow transplants may be required for individuals with destroyed red marrow due to:
Radiation
Chemotherapy
Abnormal marrow functionality
Hematopoietic Harvest:
Donor: red bone marrow is harvested primarily from the hip or the sternum
Recipient: harvested cells are injected into the venous system to relocate them appropriately
Match Importance: Ensuring antigen and MHC alignment between donor and recipient to prevent transplant rejection.
Microscopic Anatomy of Bone
Bone consists of:
Cells
Extracellular matrix (ECM)
Cell Types in Bone:
Osteoprogenitor Cells
Osteoblasts
Osteocytes
Osteoclasts
Organic Components of Bone Matrix
The organic component termed osteoid is produced by osteoblasts, containing:
Collagen
Semisolid ground substances of proteoglycans (including chondroitin sulfate)
Glycoproteins suspending and supporting collagen fibers
These organic components provide:
Tensile strength: resistant to twisting and stretching
Overall flexibility
Inorganic Components of Bone Matrix
Comprises mainly salt crystals:
Calcium phosphate: ext{Ca}3( ext{PO}4)_2
Calcium hydroxide: ext{Ca(OH)}_2
Calcium phosphate and calcium hydroxide react to yield hydroxyapatite: ext{Ca}{10}( ext{PO}{4}){6}( ext{OH}){2}
Other incorporated salts:
Sodium, magnesium, sulfate, fluoride
Functions:
Provides compressive strength
Crystals deposit around the collagen fiber long axis in the ECM, hardening the matrix and accounting for rigidity.
Mineral Deposition
Calcification (Mineralization):
The process of mineral deposition occurs as hydroxyapatite crystals deposit in the bone matrix.
Initiated when calcium and phosphate ion concentrations reach critical levels (hypercalcemia).
Necessary Substances:
Calcitonin
Vitamin D
Vitamin C
Calcium
Phosphate
Results in increased density (hardening) of bone.
Mineral Reabsorption
A process in which the bone matrix is destroyed by substances from osteoclasts and released into adjacent extracellular space, acting during hypocalcemia.
Mechanism:
Proteolytic enzymes from lysosomes of osteoclasts digest organic compounds like collagen and proteoglycans.
Hydrochloric acid (HCl) dissolves inorganic minerals.
Results:
Liberated calcium and phosphate ions enter the bloodstream, leading to decreased bone density but increased blood calcium availability.
Clinical View: Osteitis Deformans
Known as Paget Disease, caused by imbalanced osteoclast and osteoblast activity.
Characterized by:
Excessive osteoclast activity followed by excessive osteoblast activity.
Resultant bone deposited is poorly formed and unstable, increasing deformation and fracture susceptibility.
Components of Compact Bone
Comprises small cylindrical structures called osteons (Haversian systems), the functional and structural unit of compact bone.
Components of Osteons:
Central (Haversian) Canal: Contains blood vessels.
Concentric Lamellae: Layers of bone matrix.
Osteocytes: Mature bone cells found in lacunae.
Lacuna: Small spaces housing osteocytes.
Canaliculi: Small channels that connect lacunae to central canals.
Perforating (Volkmann) Canals: Connect central canals across the bone.
Circumferential Lamellae: Layer surrounding the outer edges of the bone.
Interstitial Lamellae: Remains of osteons that have been partially remodeled.
Components of Spongy Bone
Characterized by a latticework of narrow rods and plates of bone called trabeculae.
Bone marrow fills spaces between trabeculae, creating a meshwork resistant to multi-directional stress.
Microscopic features include:
Parallel lamellae of bone matrix,
Osteocytes in lacunae,
Canaliculi present for nutrient diffusion to osteocytes.
Microscopic Anatomy of Hyaline Cartilage
Composed of chondrocytes within a glassy matrix of protein (collagen) fibers, proteoglycans, and chondroitin sulfate.
Lacks inorganic salts, granting resilience and flexibility.
Contains a high water percentage (60%-70%) making it highly compressible.
Avascular and nerve-free, receiving nutrients from perichondrium.
Components of Hyaline Cartilage
Chondroblasts: Derived from mesenchymal cells, producing the cartilage matrix.
Chondrocytes: Mature cells maintaining the cartilage matrix, housed in lacunae.
Perichondrium: Surrounds hyaline cartilage, providing structure and nutrients; absent on articular cartilage surfaces.
Growth of Cartilage
Types of Cartilage Growth:
Interstitial Growth: Increases length internally through mitotic activity in chondrocytes within lacunae.
Appositional Growth: Increases width by stem cells in perichondrium giving rise to chondroblasts at the periphery.
Bone Formation
Ossification: Refers to the formation and development of bone connective tissue, beginning at 8-12 weeks of embryonic development, continuing through childhood and adolescence.
Types include:
Intramembranous Ossification: Bone growth from within the membrane; begins with ossification centers from mesenchyme, resulting in woven bone and ultimately lamellar bone.
Endochondral Ossification: Replaces hyaline cartilage with bone, especially in long bones.
Bone Growth and Remodeling
Interstitial Bone Growth: Occurs lengthwise, dependent on cartilage growth at the epiphyseal plate through childhood and adolescence.
Zones of Epiphyseal Plate:
Zone of Reserve (Resting)
Zone of Proliferation
Zone of Hypertrophy
Zone of Calcification
Zone of Ossification
Appositional Bone Growth: Increases bone width through periosteum osteoblast activity and osteoclast activity along the endosteum.
Bone Remodeling: A dynamic process of adding new bone and removing old bone, influenced by mechanical stress and hormonal stimulation.
Mechanical Stress and Hormonal Influence on Bone
Mechanical stress increases in weight-bearing movements and exercises, which enhance bone density. Conversely, absence can lead to reduced density.
Hormonal Influence:
Promotes growth via:
Growth hormone
Thyroid hormone
Calcitonin
Sex hormones (Estrogen & Testosterone)
Inhibits growth via:
Glucocorticoids
Serotonin
Parathyroid hormone
Calcitriol
Calcium Homeostasis
Maintains a normal calcium range (8.9-10.1 mg/dL), crucial for:
Muscle contraction initiation
Neurotransmitter exocytosis
Stimulation of heart pacemaker cells
Blood clotting
Increasing Blood Calcium:
Parathyroid Hormone (PTH): Secreted by parathyroid glands during hypocalcemia. Stimulates osteoclasts and renal retention of calcium.
Calcitriol: Active form of Vitamin D promoting intestinal absorption of dietary calcium.
Decreasing Blood Calcium:
Calcitonin: Released from thyroid gland during hypercalcemia, stimulating osteoblast activity and inhibiting osteoclasts, increasing renal calcium loss.
Effects of Aging on Bone
Aging impacts bone strength and density due to:
Decreased tensile strength and increased brittleness.
Increased demineralization due to osteoclast overactivity in post-menopausal women.
Clinical View: Osteoporosis: Characterized by significant bone mass loss, increased fracture risk, particularly in wrist, hip, and vertebral column post-menopause.
Bone Fracture and Repair
Types of Fractures:
Stress fracture
Pathologic fracture
Simple fracture
Compound fracture
Classification of Bone Fractures:
Multiple types including avulsion, comminuted, complete, compound, compression, depressed, displaced, epiphyseal, greenstick, hairline, impacted, incomplete, linear, oblique, pathologic, Pott, spiral, transverse.
Fracture Healing Process:
Hematoma Formation: Increased blood flow
Fibrocartilaginous Callus Formation: Collagen and fibrocartilage deposition
Bony Callus Formation: Temporary bony collar uniting fragments
Bone Remodeling: Osteoclasts and osteoblasts reshape the healed area.