8. Bone Tissue - MH - A&P I Sp2025

Lecture Overview

Lecture 8: Bone Tissue, BIO 111C - Fundamentals in Anatomy & Physiology I

Instructor: Dr. Michael Ha, M.D., Visiting Assistant Professor of Natural Sciences, University of District of Columbia

Previous Lecture Recap

Key Topics from Last Time:

  • Skin

  • Accessory Structures

  • Skin Pathology

  • Skeletal System Components

  • Bone Formation

  • Bone Histology

  • Matrix Organisation

The Skeletal System

Composition: The human skeletal system is made up of 206 bones that provide the structural framework for the body, categorized into the axial and appendicular skeletons.Each bone acts as a living tissue organ, facilitating various body functions.

Functions:

  1. Support: Provides a rigid framework to support the body and cradle vital organs.

  2. Movement: Provides attachment points for muscles, facilitating movement through leverage.

  3. Protection: Safeguards critical organs (e.g., skull protects the brain, ribs protect the heart and lungs).

  4. Mineral Storage: Stores essential minerals, primarily calcium and phosphorus, which can be released into the bloodstream as needed.

  5. Energy Storage: Yellow marrow, found in the medullary cavity of long bones, stores triglycerides for energy.

  6. Hemopoiesis: Red bone marrow is responsible for producing blood cells, including red blood cells, white blood cells, and platelets.

Types of Skeletal Tissue

  • Compact Bone:

    • Dense connective tissue appears smooth and solid in structure.

  • Spongy Bone:

    • Less dense, consisting of a lattice structure that houses bone marrow and supports the trabecular architecture.

  • Cartilage: This semi-rigid connective tissue includes three main types:

    • Hyaline Cartilage: Found at joints, providing a smooth surface for movement.

    • Fibrocartilage: Strong and durable, found in intervertebral discs and pubic symphysis.

    • Elastic Cartilage: Flexible and resilient, found in the ear and epiglottis.

  • Ligaments: Dense regular connective tissue that connects bone to bone, providing stability to joints.

  • Tendons: Connect muscle to bone, facilitating movement through muscle contractions.

Classification of Bones

  • Long Bones: Characterized by a length greater than width; includes femur, tibia, and humerus, primarily function in movement and support.

  • Short Bones: Equal in length and width; includes carpals and tarsals, providing stability and support with limited motion.

  • Flat Bones: Thin and slightly curved; includes the skull, sternum, and scapula, offering protection and extensive areas for muscle attachment.

  • Irregular Bones: Complex shapes that don’t fit into other categories; includes vertebrae and certain skull bones, providing various functions depending on their locations.

Gross Anatomy of Long Bones

  • Diaphysis: The tubular shaft of a long bone, providing leverage and weight support.

  • Epiphysis: The enlarged ends of long bones; composed of spongy bone and covered with articular cartilage to absorb shock and reduce friction at joints.

  • Metaphysis: Transitional zone between diaphysis and epiphysis; contains the epiphyseal plate (growth plate) in growing children and the epiphyseal line in adults after growth has ended.

Periosteum Structure

  • A dense layer of vascular connective tissue that covers the outer surface of bones, except for the ends covered by articular cartilage.

    • Outer Fibrous Layer: Provides protection and serves as an attachment point for tendons and ligaments.

    • Inner Cellular Layer: Contains osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells), crucial for bone growth and repair.

Articular Cartilage and Medullary Cavity

  • Articular Cartilage:

    • Reduces friction and absorbs shock, found at joint surfaces to facilitate smooth movement.

  • Medullary Cavity:

    • The central cavity of long bones, houses red bone marrow in children (for blood cell production) and yellow marrow in adults (for fat storage).

  • Endosteum:

    • A thin membrane lining the medullary cavity; is involved in bone growth and repair through its osteogenic activity.

Anatomy of Short, Flat, and Irregular Bones

  • Comprised of an outer layer of compact bone with an internal network of spongy bone (diploe), these bones lack a medullary cavity, supporting diverse functions such as muscle attachment and protection.

Blood Supply and Innervation

  • Bones are highly vascular, with blood vessels entering through the periosteum.

  • Nutrient Foramen: Tiny openings that allow nutrient arteries and veins to penetrate the bone, supplying essential nutrients.

  • Nerves accompany blood vessels, innervating the bone, periosteum, endosteum, and marrow, playing a critical role in bone health and response to injuries.

Bone Marrow Types

  • Red Bone Marrow:

    • Hematopoietic tissue is responsible for producing blood cells, including red blood cells, white blood cells, and platelets. Contains reticular connective tissue and fat.

  • Yellow Bone Marrow:

    • Fat-rich tissue that replaces red marrow in long bones as a person ages and serves as an energy reserve.

Bone Formation and Histology

  • Ossification: The process of laying down new bone material by osteoblasts; begins during embryonic development (8th to 12th week).

Types of Ossification:

  1. Intramembranous Ossification: Involves the direct formation of bone from mesenchyme and is responsible for forming flat bones of the skull and the clavicle.

  2. Endochondral Ossification: Involves the transformation of a hyaline cartilage model into bone, occurring predominantly in long and short bones of the skeleton.

Intramembranous Ossification Process

  1. Formation of ossification centers in the mesenchyme.

  2. Osteoblasts secrete osteoid, which undergoes calcification.

  3. Formation of trabecular structures.

  4. Development of the periosteum surrounding the newly formed bone.

Endochondral Ossification Process

  1. Formation of a hyaline cartilage model mimicking the future bone shape.

  2. Growth of the cartilage model, contributing to lengthening of the bone.

  3. Development of the primary ossification center in the diaphysis, where calcification starts.

  4. Formation of the medullary cavity as the cartilage continues to be replaced by bone.

  5. Development of secondary ossification centers in the epiphyses after birth.

  6. Formation of articular cartilage on joint surfaces and the epiphyseal plate for growth.

Factors Affecting Postnatal Bone Length

  • Genetic factors influencing the overall shape and size of bones.

  • Environmental factors, including nutrition and activity level, impacting mineralization, organic matrix production, and overall bone health.

  • Hormonal influences, such as growth hormone, which significantly impacts epiphyseal growth.

Hormones & Bone Growth

Promote Growth:

  • Growth Hormone (GH): Directly stimulates growth plate activity, promoting longitudinal bone growth.

  • Insulin-like Growth Factor (IGF): Released in response to GH, further stimulates proliferation and activity at growth plates.

  • Thyroid Hormone: Enhances metabolic activities of bone cells, necessary for normal growth.

  • Sex Hormones: Estrogen and testosterone increase the rate of bone growth and remodeling, particularly during puberty, leading to height increases.

Inhibit Growth:

  • Glucocorticoids: Steroid hormones that can impair growth by affecting the epiphyseal plate and promoting bone resorption.

  • Serotonin: Neurotransmitter that may inhibit the differentiation of osteoprogenitor cells into osteoblasts.

Calcium Homeostasis

  • Parathyroid Hormone (PTH):

    • Secreted by parathyroid glands in response to low blood calcium levels; promotes the activation of osteoclasts to release calcium and phosphate into the bloodstream.

    • Stimulates the formation of calcitriol (active form of vitamin D) to enhance intestinal absorption of calcium.

  • Calcitonin (CT):

    • Secreted by the thyroid gland; decreases blood calcium levels by inhibiting osteoclast activity and promoting calcium deposition into the bone matrix.

Bone Cell Types

  • Osteogenic Cells: Undifferentiated stem cells located in the periosteum and endosteum, capable of developing into osteoblasts.

  • Osteocytes: Mature bone cells residing in lacunae, maintaining bone tissue and regulating bone matrix through signals.

  • Osteoblasts: Cells responsible for bone formation; secrete osteoid which subsequently becomes mineralized.

  • Osteoclasts: Large multinucleated cells that resorb bone, involved in bone remodeling by breaking down bone matrix.

Bone Matrix Composition

  • Organic Components: Comprise osteoids (matrix proteins) secreted by osteoblasts, primarily collagen fibers, contributing tensile strength and flexibility, along with ground substance (proteoglycans and glycoproteins).

  • Inorganic Components: Form mineral crystals (e.g., hydroxyapatite) that provide rigidity and structural strength to bone, along with other minerals like calcium phosphate, sodium, magnesium, sulfate, and fluoride.

Bone Formation Process

  1. Osteoblasts produce and secrete osteoid, largely composed of collagen.

  2. Calcification: Mineral crystals, primarily hydroxyapatite, accumulate and harden the osteoid through the addition of calcium and phosphate ions, requiring vitamin D and vitamin C for proper mineralization.

Bone Resorption Process

  • Osteoclasts dissolve the bone matrix by secreting acid (hydrochloric acid) and proteolytic enzymes, facilitating the release of minerals into circulation.

  • Balance Between Osteoblast and Osteoclast Activity: Critical for maintaining proper bone density and structure, allowing for remodeling according to mechanical stress and physiological needs.

Bone Remodeling Cycle

  1. Osteoclast Activation: Initiate bone resorption in areas where bone is underused or damaged.

  2. Osteoblast Activity: Lay down new bone matrix in remodeled areas, facilitating strengthening and adaptation to stress.

  3. This dynamic cycle adjusts bone architecture and density throughout life in response to physical activity, diet, and overall health.

Bone Matrix Organisation

  • Compact Bone: Organized into functional units called osteons (Haversian systems) that run parallel to the longitudinal axis of the bone, providing structural integrity and resistance to bending and twisting forces.

    • Osteon Structure:

      • Central Canal: Contains blood vessels and nerves, supplying nutrients to the bone cells.

      • Concentric Lamellae: Layers of bone matrix surrounding the central canal, providing strength.

      • Osteocytes are embedded within lacunae, connected through tiny channels called canaliculi, allowing communication and nutrient exchange.

Spongy Bone Characteristics

  • Spongy bone lacks the organization of osteons; instead, it contains trabeculae, which are thin, interconnected rods and plates of bone tissue.

    • The spaces between trabeculae are filled with red or yellow bone marrow, contributing to both hematopoiesis and energy storage.

Cartilage Structure

  • Cartilage is a flexible connective tissue devoid of blood vessels and nerves, allowing for various growth types:

    • Interstitial Growth: Enlargement due to chondrocyte division and matrix production.

    • Appositional Growth: Growth from the outer surface as chondroblasts in the perichondrium add new layers.

Bone Fractures Types

  • Transverse Fracture: A clean fracture directly across the bone shaft.

  • Compound Fracture: The bone protrudes through the skin, creating an open wound.

  • Oblique Fracture: A diagonal fracture angle across the bone.

  • Comminuted Fracture: Bone fragments into multiple pieces, often requiring surgical intervention.

  • Greenstick Fracture: An incomplete fracture with flexion on one side, common in pediatric bones due to their pliability.

Bone Repair Stages

  1. Hematoma Formation: Blood vessels break and pull together, leading to clot formation that serves as a temporary scaffold.

  2. Fibrocartilaginous Callus Formation: New capillaries grow into the hematoma; fibroblasts and osteoblasts start forming a fibrocartilaginous callus that bridges the gap.

  3. Bony Callus Formation: The fibrocartilaginous callus is gradually replaced by spongy bone, forming a bony callus as healing progresses.

  4. Bone Remodeling: The bony callus is remodeled over time, with osteoclasts resorbing excess tissue and osteoblasts forming compact bone, restoring original bone structure and integrity.