Nurs 1002- Obj #6

Nurs 1002-Obj #6

Bones and Skeletal Tissue

Introduction

The skeletal system provides the framework for the human body, supporting movement, protecting organs, and serving as a reservoir for minerals. Understanding bone anatomy and physiology is essential for comprehending bone diseases and the process of bone remodeling.

Major Structures of Long Bone

Gross Anatomy of Long Bones

• Diaphysis: The tubular shaft forming the long axis of the bone, composed of compact bone surrounding the medullary (marrow) cavity, which contains yellow marrow in adults.

• Epiphyses: The ends of long bones, consisting of compact bone externally and spongy bone internally. Articular cartilage covers joint surfaces.

• Epiphyseal Line: Remnant of the epiphyseal plate (growth plate) where bone growth occurs during childhood.

• Membranes:

  • Periosteum: Double-layered membrane covering external bone surfaces (except joints). The outer fibrous layer is dense irregular connective tissue; the inner osteogenic layer contains bone-forming cells. Rich in nerve fibers and blood vessels.

  • Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone and canals of compact bone. Contains osteogenic cells.

• Bone Marrow:

  • Red Marrow: Site of hematopoiesis (blood cell formation), found in trabecular cavities of spongy bone in flat bones (e.g., sternum) and heads of femur and humerus in adults.

  • Yellow Marrow: Fat storage, found in the medullary cavity of long bones in adults. Can convert to red marrow if needed (e.g., in anemia).

Bone Markings

• Projections: Sites of muscle, ligament, and tendon attachment (e.g., tuberosity, crest, trochanter, spine).

• Surfaces: Involved in joint formation (e.g., head, facet, condyle).

• Depressions and Openings: Conduits for blood vessels and nerves (e.g., foramen, groove, notch).

Principal Constituents of Bone Tissue

Cartilage Types and Functions

• Hyaline Cartilage: Provides support, flexibility, and resilience; most abundant type; contains collagen fibers. Found in articular (joints), costal (ribs), respiratory (trachea, larynx), and nasal cartilages.

• Elastic Cartilage: Similar to hyaline but contains more elastic fibers. Found in external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers; has great tensile strength. Found in intervertebral discs and menisci of knee.

Bone Tissue Composition

• Organic Components:

  • Cells: Osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts.

  • Osteoid: Organic bone matrix secreted by osteoblasts, composed of ground substance and collagen fibers, providing tensile strength and flexibility.

• Inorganic Components:

  • Hydroxyapatites (mineral salts): Mainly calcium phosphate crystals, accounting for bone's hardness and resistance to compression. Make up about 65% of bone mass.

Functions of Bones

• Support: Framework for the body and soft organs.

• Protection: Protects brain, spinal cord, and vital organs.

• Movement: Levers for muscle action.

• Mineral and Growth Factor Storage: Reservoir for calcium, phosphorus, and growth factors.

• Blood Cell Formation (Hematopoiesis): Occurs in red marrow cavities.

• Triglyceride (Fat) Storage: Stored in yellow marrow for energy.

• Hormone Production: Osteocalcin regulates insulin secretion, glucose homeostasis, and energy metabolism.

Classification of Bones by Shape

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

• Short Bones: Cube-shaped (e.g., wrist and ankle bones); sesamoid bones (e.g., patella) form within tendons.

• Flat Bones: Thin, flat, slightly curved (e.g., sternum, scapulae, ribs, skull bones).

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

Skeleton Organization

• Axial Skeleton: Long axis of body; includes skull, vertebral column, and rib cage.

• Appendicular Skeleton: Bones of upper and lower limbs, and girdles attaching limbs to axial skeleton.

Types and Functions of Bone Cells

Bone Cell Types

• Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), rich in collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as mechanosensors.

• Bone-Lining Cells: Flat cells on bone surfaces; help maintain bone matrix.

• Osteoclasts: Large, multinucleated cells derived from hematopoietic stem cells; responsible for bone resorption (breakdown).

Bone Development and Ossification

Overview of Ossification

• Ossification (Osteogenesis): Process of bone tissue formation, beginning in the embryo and continuing through childhood and adolescence.

• Types of Ossification:

  • Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most of the skeleton (long bones, ribs, vertebrae).

  • Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones (e.g., skull, clavicles).

Bone Growth

• Interstitial (Longitudinal) Growth: Increases bone length via division of chondrocytes in the epiphyseal plate.

• Appositional Growth: Increases bone thickness by addition of new bone tissue by osteoblasts beneath the periosteum.

• Epiphyseal Plate Closure: Occurs at the end of adolescence when the plate is replaced by bone, ceasing longitudinal growth (around age 18 in females, 21 in males).

Bone Remodeling and Repair

• Bone Remodeling: Continuous process of bone deposition and resorption, regulated by hormonal and mechanical factors.

• Bone Repair: Involves hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling.

Homeostatic Functions of Bones

Calcium Homeostasis

• Parathyroid Hormone (PTH): Released in response to low blood calcium; stimulates osteoclasts to resorb bone, releasing calcium into the blood.

• Calcitonin: Produced by the thyroid gland in response to high blood calcium; inhibits bone resorption.

• Vitamin D: Essential for calcium absorption from the gut.

• Consequences of Imbalance: Hypocalcemia (low calcium) causes hyperexcitability; hypercalcemia (high calcium) causes nonresponsiveness and can lead to kidney stones.

Hormonal Regulation of Bone Growth

• Growth Hormone: Stimulates epiphyseal plate activity in childhood.

• Thyroid Hormone: Modulates growth hormone activity for proper bone proportions.

• Sex Hormones: Promote growth spurts and induce epiphyseal plate closure at puberty.

• Imbalances: Excesses or deficits cause abnormal skeletal growth.

Mechanical Stress and Bone Remodeling

• Wolff's Law: Bones grow or remodel in response to the demands placed on them (e.g., weight-bearing exercise increases bone density).

Bone Disorders

Common Bone Diseases

• Osteoporosis: Bone resorption exceeds deposition, leading to fragile bones and increased fracture risk.

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

• Paget's Disease: Excessive and haphazard bone deposition and resorption, resulting in structurally abnormal bone.

Bone Fractures

• Types:

  • Open (Compound): Bone ends penetrate the skin.

  • Closed (Simple): Bone ends do not penetrate the skin.

  • Displaced: Bone ends are out of normal alignment.

  • Nondisplaced: Bone ends retain normal position.

  • Complete: Bone is broken all the way through.

  • Incomplete: Bone is not broken all the way through.

• Fracture Repair Stages:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

Summary Table: Types of Cartilage

Type	Main Features	Locations
Hyaline	Support, flexibility, resilience; collagen fibers	Joints, ribs, nose, trachea, larynx
Elastic	Similar to hyaline, more elastic fibers	External ear, epiglottis
Fibrocartilage	Thick collagen fibers, great tensile strength	Intervertebral discs, menisci of knee

Key Equations

• Calcium Homeostasis (simplified):

$\text{PTH} \uparrow \Rightarrow \text{Osteoclast activity} \uparrow \Rightarrow [\text{Ca}^{2+}]_{\text{blood}} \uparrow$

$\text{Calcitonin} \uparrow \Rightarrow \text{Osteoclast activity} \downarrow \Rightarrow [\text{Ca}^{2+}]_{\text{blood}} \downarrow$

Additional info: Some explanations and context have been expanded for clarity and completeness based on standard Anatomy & Physiology textbooks.