Osseous Tissue: Structure, Function, and Development

Osseous Tissue

Key Definitions

• Hematopoiesis: The process of blood cell formation, which occurs primarily in the red bone marrow of certain bones.

• Lamella: Concentric rings of bone matrix within an osteon of compact bone.

• Trabecula: Irregular, lattice-like structures of spongy bone that provide structural support and house bone marrow.

• Mesenchyme: Embryonic connective tissue from which all connective tissues, including bone and cartilage, are derived.

• Comminuted: A type of bone fracture where the bone is broken into three or more pieces.

Types of Cartilage and Locations

• Hyaline Cartilage: Most abundant; found in articular surfaces, costal cartilages, nose, trachea, and larynx.

• Elastic Cartilage: Contains elastic fibers; found in the external ear and epiglottis.

• Fibrocartilage: Highly compressible with great tensile strength; found in intervertebral discs, pubic symphysis, and menisci of the knee.

Functions of Bone

• Support: Provides a framework for the body and supports soft tissues.

• Protection: Shields vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Acts as levers for muscles to produce movement.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphorus.

• Blood Cell Formation: Hematopoiesis occurs in red marrow cavities.

• Fat Storage: Yellow marrow stores triglycerides as an energy reserve.

• Hormone Production: Osteocalcin is produced by bones and helps regulate bone formation and glucose metabolism.

Classification of Bones

• Axial Skeleton: Bones of the skull, vertebral column, and rib cage.

• Appendicular Skeleton: Bones of the limbs and girdles (shoulder and pelvic girdles).

Bone Shapes and Examples

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Flat Bones: Thin, flattened, and usually curved (e.g., sternum, scapulae, ribs, skull bones).

• Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

• Short Bones: Cube-shaped (e.g., carpals, tarsals).

Gross Anatomy of Bones

Long Bone Structure

• Epiphysis: Ends of the bone; contains articular cartilage, compact bone, spongy bone, and red bone marrow.

• Diaphysis: Shaft of the bone; contains the medullary cavity (yellow marrow), compact bone, and spongy bone.

• Periosteum: Double-layered membrane covering the bone; contains Sharpey's fibers (anchor periosteum to bone) and nutrient arteries.

Flat Bone Structure

• Compact Bone: Outer layers.

• Spongy Bone (Diploe): Internal layer between compact bone layers.

Bone Markings

Projections (Sites of Muscle and Ligament Attachment)

• Tuberosity, Crest, Trochanter, Line, Tubercle, Epicondyle, Spine, Process

• Head, Facet, Condyle, Ramus

Depressions and Openings (Passage for Vessels and Nerves)

• Groove, Fissure, Foramen, Notch, Meatus, Sinus, Fossa

Microscopic Anatomy of Bone

Compact Bone

• Osteon (Haversian System): Structural unit of compact bone.

• Lamellae: Concentric, interstitial, and circumferential layers of bone matrix.

• Osteocytes: Mature bone cells in lacunae.

• Canaliculi: Small channels connecting lacunae for nutrient and waste exchange.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Perforating (Volkmann's) Canals: Connect blood vessels and nerves of periosteum to central canal.

• Periosteum: Outer membrane covering bone.

Spongy Bone

• Trabeculae: Lattice-like structures providing support.

• Endosteum: Membrane lining internal bone surfaces.

Flat Bone

• Compact Bone: Contains osteons.

• Spongy Bone (Diploe): Contains trabeculae.

Bone Cells

• Osteocytes: Mature bone cells maintaining bone matrix.

• Osteoblasts: Bone-forming cells; secrete osteoid (unmineralized bone matrix).

• Osteoprogenitor (Stem) Cells: Mitotically active stem cells in periosteum and endosteum.

• Osteoclasts: Large cells that resorb or break down bone matrix.

Chemical Composition of Bone

• Organic Components (Osteoid): Includes collagen fibers and ground substance, providing flexibility and tensile strength.

• Inorganic Components (Minerals): Mainly hydroxyapatite (calcium phosphate crystals), providing hardness and resistance to compression.

Ossification (Bone Formation)

Endochondral Ossification

• Bone forms by replacing hyaline cartilage (cartilage model derived from mesenchyme).

• Bony collar forms around diaphysis.

• Primary ossification center develops in diaphysis; cartilage calcifies and dies.

• Vascular invasion brings osteoblasts and osteoclasts.

• Secondary ossification centers form in epiphyses.

Intramembranous Ossification

• Bone develops directly from mesenchyme.

• Mesenchymal cells cluster and differentiate into osteoblasts.

• Osteoid is secreted and calcifies, forming osteocytes.

• Spongy bone forms first, followed by compact bone.

Bone Growth and Remodeling

• Length Growth (Interstitial): Occurs at the epiphyseal plate; when growth stops, plate becomes epiphyseal line.

• Diameter Growth (Appositional): Bone increases in thickness.

• Wolff's Law: Bone grows or remodels in response to mechanical stress.

Hormonal Regulation of Bone Growth

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, increasing intestinal absorption, and reducing kidney excretion of calcium.

• Calcitonin: Lowers blood calcium by inhibiting osteoclasts and increasing calcium excretion by kidneys.

• Growth Hormone: Stimulates bone growth, especially at epiphyseal plates.

• Thyroid Hormones: Modulate activity of growth hormone, ensuring proper bone proportions.

• Testosterone and Estrogens: Promote growth spurts and end growth by inducing epiphyseal plate closure.

Bone Fractures

Types of Fractures

• Closed (Simple): Bone does not penetrate skin.

• Open (Compound): Bone ends penetrate skin.

• Comminuted: Bone fragments into three or more pieces.

• Compression: Bone is crushed.

• Depressed: Broken bone portion is pressed inward.

• Impacted: Broken bone ends are forced into each other.

• Spiral: Ragged break from twisting forces.

• Greenstick: Bone breaks incompletely, common in children.

Treatment of Fractures

• Reduction: Realignment of broken bone ends (closed or open).

• Immobilization: By cast or traction to allow healing.

Stages of Fracture Healing

1. Inflammation: Hematoma forms at fracture site.

2. Soft Callus Formation: Fibrocartilaginous callus forms.

3. Hard Callus Formation: Spongy bone replaces soft callus, then compact bone forms.

4. Bone Remodeling: Mature bone is restored to original shape.

Bone Diseases

• Osteomalacia and Rickets: Softening of bones due to vitamin D or calcium deficiency; rickets occurs in children.

• Osteoporosis: Bone resorption outpaces deposit, leading to fragile bones.

• Paget's Disease: Excessive and disorganized bone remodeling, resulting in weak bones.

Summary Table: Types of Bone Cells

Cell Type	Function	Location
Osteoprogenitor	Stem cells; differentiate into osteoblasts	Periosteum, endosteum
Osteoblast	Bone formation; secrete osteoid	Bone surfaces
Osteocyte	Maintain bone matrix	Lacunae within bone
Osteoclast	Bone resorption	Bone surfaces

Key Equations

• Hydroxyapatite Formula (Main Mineral in Bone):

$Ca_{10}(PO_4)_6(OH)_2$

• Wolff's Law (Conceptual): Bone adapts to the loads under which it is placed.

Example: The femur thickens in response to increased weight-bearing activity, such as in athletes.

Additional info: This guide expands on the outline by providing definitions, explanations, and examples for each listed topic, ensuring a comprehensive review for exam preparation.