Osseous Tissue, Bone Structure, and Connective Tissue: ANP Study Guide

Osseous Tissue and Bone Structure

Functions of the Skeletal System

The skeletal system is essential for structural support, protection, movement, mineral storage, blood cell production, and endocrine regulation.

• Support: Provides a framework for the attachment of soft tissues and organs.

• Protection: Shields delicate organs (e.g., ribs protect heart/lungs, skull protects brain).

• Leverage: Bones act as levers for muscle action, enabling movement.

• Storage of Minerals: Bones store calcium and phosphate, maintaining their levels in body fluids.

• Blood Cell Production (Hematopoiesis): Red bone marrow produces red and white blood cells and platelets.

• Endocrine Function: Bones help regulate calcium levels for muscle contraction.

Bone Classification

The adult skeleton consists of 206 bones, divided into two main groups:

• Axial Skeleton (80 bones): Includes skull, hyoid, sternum, rib cage, vertebral column, sacrum, and coccyx.

• Appendicular Skeleton (126 bones): Includes limbs and girdles (pectoral and pelvic) attaching limbs to the axial skeleton.

Connective Tissue: Bone

Overview of Connective Tissues

Connective tissues are the most abundant tissues in the body, supporting and protecting other tissues.

• Origin: All connective tissues derive from mesenchyme.

• Components: Specialized cells, extracellular protein fibers, and ground substance.

• Specialized Cells:

  • Fibroblasts: Produce connective tissue proper.

  • Chondrocytes: Produce cartilage.

  • Osteocytes: Produce bone.

  • Hemocytoblasts: Produce blood.

• Extracellular Protein Fibers:

  • Elastic fibers: Stretchy and recoil after distortion.

  • Collagen fibers: Strong, resist stretching.

  • Reticular fibers: Form branching networks.

• Ground Substance: Fills space between cells and fibers; varies from gel-like to rigid.

• Matrix: Combination of ground substance and fibers.

• Vascularity: Many connective tissues are highly vascular and contain sensory receptors.

Supporting Connective Tissues

These tissues have less diverse cell populations and denser matrices, providing protection and support.

• Cartilage: Solid, rubbery matrix with chondrocytes; surrounded by perichondrium.

  • Hyaline cartilage: Stiff, flexible support; reduces friction. Found in ribs, articular surfaces, trachea, nose.

  • Elastic cartilage: Flexible; found in ear and epiglottis.

  • Fibrous cartilage: Strong; found in intervertebral discs, meniscus, pubic symphysis.

• Bone: Solid, crystalline matrix with osteocytes; surrounded by periosteum.

  • Compact bone: Organized into osteons.

  • Spongy bone: Network of trabeculae, filled with marrow.

Comparison of Cartilage and Bone

Feature	Cartilage	Bone
Cell Type	Chondrocytes	Osteocytes
Matrix	Gel-like, flexible	Rigid, calcified
Vascularity	Avascular	Highly vascular
Surrounding Membrane	Perichondrium	Periosteum
Repair Ability	Poor	Good

Bone Structure

Gross Anatomy of Long Bones

Long bones are structured to transmit forces and have a rich blood supply.

• Diaphysis: Tubular shaft, mainly compact bone.

• Epiphyses: Ends of bone, mainly spongy bone covered by compact bone and articular cartilage.

  • Proximal epiphysis: Closest to origin.

  • Distal epiphysis: Furthest from origin.

• Metaphysis: Narrow zone connecting diaphysis and epiphyses; contains epiphyseal plate (growth plate).

• Medullary Cavity: Central cavity containing bone marrow (red in children, yellow in adults).

• Membranes:

  • Periosteum: Dense irregular tissue covering bone, attached by Sharpey's fibers.

  • Endosteum: Internal membrane lining canals and medullary cavity.

• Nutrient Foramen: Tunnel for blood vessels into bone shaft.

• Articular Cartilage: Hyaline cartilage covering epiphyses; avascular, relies on diffusion.

Microscopic Anatomy of Compact Bone

• Osteon: Basic unit, concentric lamellae around central canal.

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

• Perforating Canals: Connect central canals of adjacent osteons.

• Lamellae: Rings of matrix; types include circumferential, interstitial, and concentric.

• Lacunae: Spaces housing osteocytes.

• Canaliculi: Tiny channels connecting lacunae and central canal for nutrient/waste exchange.

Bone Cells

• Osteocytes: Mature bone cells maintaining matrix; participate in repair.

• Osteoblasts: Immature cells producing new bone matrix (osteogenesis/ossification).

• Osteoprogenitor Cells: Stem cells in periosteum/endosteum; differentiate into osteoblasts.

• Osteoclasts: Multinucleated cells digesting bone matrix (osteolysis/resorption); derived from monocytes.

• Bone Lining Cells: Maintain bone surfaces and matrix.

Chemical Composition of Bone

• Organic Osteoid: Collagen fibers (1/3 bone weight); provide flexibility.

• Mineralization: Addition of calcium and phosphate hardens bone.

• Alkaline Phosphatase: Enzyme on osteoblasts facilitating mineral binding.

• Inorganic Hydroxyapatites: Mineral salts (2/3 bone weight); calcium phosphate and calcium hydroxide form hydroxyapatite crystals.

• Protein-Crystal Combination: Provides strength and resistance to shattering.

Bone Formation and Development

Osteogenesis/Ossification

Bone formation begins during embryonic development via two processes:

• Endochondral Ossification: Most bones form from hyaline cartilage models.

  1. Cavitation: Chondrocytes enlarge, matrix calcifies, cells die, blood vessels grow, perichondrium converts to osteoblasts.

  2. Invasion: Blood vessels invade shaft, forming primary ossification center; periosteal bud brings osteoblasts.

  3. Medullary Cavity Formation: Osteoclasts break down spongy bone, forming cavity.

  4. Epiphyses Formation: Secondary ossification centers form at bone ends; spongy bone fills epiphyses.

• Intramembranous Ossification: Flat bones form directly from mesenchyme.

  1. Matrix Formation: Mesenchymal cells cluster, secrete matrix, differentiate into osteoblasts.

  2. Woven Bone and Periosteum Formation: Osteoid accumulates, forms trabeculae.

  3. Compact Bone Plate Formation: Remodeling produces osteons; periosteum forms.

Bone Growth

• Longitudinal Growth (Length): Occurs at epiphyseal plate via five zones:

  1. Resting Zone: Inactive cartilage.

  2. Proliferation Zone: Rapid cell division, plate thickens, bone lengthens.

  3. Hypertrophic Zone: Chondrocytes enlarge, matrix deteriorates.

  4. Calcification Zone: Matrix calcifies, chondrocytes die.

  5. Ossification Zone: Osteoblasts ossify cartilage spicules, forming spongy bone.

  Epiphyseal plate is replaced by bone (epiphyseal line) in early adulthood, ending growth in height.

• Appositional Growth (Width): Osteoprogenitor cells under periosteum form new osteons; osteoclasts enlarge medullary cavity.

Fractures and Bone Repair

Repair Process

1. Hematoma Formation: Blood vessels tear, clot forms.

2. Fibrocartilage Callus Formation: Capillaries and cells invade, fibroblasts secrete collagen, osteoblasts form spongy bone.

3. Bony Callus Formation: Osteoblasts/osteoclasts migrate, tissue calcifies into bony callus.

Fracture Classification

• Skin Penetration:

  • Simple (closed): Bone breaks cleanly, does not penetrate skin.

  • Compound (open): Bone protrudes through skin.

• Orientation:

  • Transverse: Perpendicular to bone axis.

  • Linear: Parallel to bone axis.

• Position:

  • Non-displaced: Bone ends retain position.

  • Displaced: Bone ends misaligned.

Types of Fractures

• Pathologic Fracture: Caused by underlying disease (e.g., vertebrae).

• Stress Fracture: Small crack or severe bruising, often from overuse.

Exercise, Nutrition, and Hormones Affecting Bone Tissue

Exercise

• Lack of exercise: Leads to loss of bone mass.

Nutrition

• Calcium: Must be obtained from diet; dairy and leafy vegetables are sources.

• Vitamin D: Required for calcium absorption in the intestine.

Hormones

• Growth Hormone (GH): Stimulates chondrocyte proliferation, increases calcium retention, and osteoblastic activity.

• Thyroxine: Promotes osteoblastic activity and bone matrix synthesis.

• Sex Hormones (Estrogen, Testosterone): Promote osteoblastic activity, bone matrix production, adolescent growth spurt, and conversion of epiphyseal plate to line.

• Calcitriol: Active vitamin D, produced by kidneys, stimulates calcium and phosphate absorption.

Calcium Homeostasis: Skeletal System and Other Organ Systems

Bone Remodeling

Bone undergoes continuous deposition and resorption, regulated by osteoblasts and osteoclasts.

• Deposition: Osteoblasts deposit new matrix (osteoid seam).

• Resorption: Osteoclasts digest matrix, release minerals (osteolysis).

Hormonal Regulation

• Low Blood Calcium: Parathyroid hormone (PTH) released.

  • Bones: Stimulates osteoclasts to release calcium.

  • Intestine: Enhances calcitriol effects, increases calcium absorption.

  • Kidneys: Increases calcitriol production, stimulates calcium reabsorption.

• High Blood Calcium: Calcitonin released by thyroid.

  • Bones: Inhibits osteoclasts, osteoblasts continue depositing calcium.

  • Intestine: Decreases calcium absorption.

  • Kidneys: Inhibits calcium reabsorption, increases excretion.

Role of Skeleton in Homeostasis

• Calcium Reserve: Skeleton maintains normal calcium ion concentration in body fluids.

• Other Ions: Skeleton also helps balance other ions (e.g., phosphate, magnesium).

Example: When dietary calcium is low, PTH increases bone resorption to maintain blood calcium levels, while calcitonin acts to lower calcium when levels are high.

Additional info: Bone remodeling is a lifelong process, adapting to mechanical stress and metabolic needs. Disorders such as osteoporosis result from imbalances in bone deposition and resorption.