BIO 201: Human Anatomy & Physiology I - Bones & Bone Tissue

BIO 201: Human Anatomy & Physiology I - Bones & Bone Tissue

Introduction

  • Instructor: Curt Burney

  • Date: 17 September 2025

  • Course Content: Focuses on bones and skeletal tissues; to aid students prepare for further learning in human anatomy and physiology.

Learning Objectives

  • List the structures of the skeletal system.

  • Compare and contrast compact and spongy bone.

  • Identify the types and locations of cartilage within the skeletal system.

Overview of the Skeletal System

  • Components of the Skeletal System:

    • Bones of the skeleton.

    • Cartilage.

    • Ligaments (bone-to-bone connections).

    • Tendons (muscle-to-bone connections).

    • Other connective tissues (CT).

Bones

  • Primary Functions:

    • Rigid framework of the body.

    • Other functions include supporting structures, movement, and protection of vital organs.

  • Types of Bone:

    • Compact Bone:

    • Also known as dense or cortical bone.

    • Comprises 80% of bone mass.

    • Spongy Bone:

    • Also known as cancellous or trabecular bone.

    • Located internal to compact bone; porous in appearance.

    • Comprises 20% of bone mass.

Cartilage

  • Characteristics:

    • Semirigid connective tissue (CT).

    • More flexible than bone.

  • Types of Cartilage:

    • Hyaline Cartilage:

    • Attaches ribs to the sternum.

    • Covers ends of some bones and found within growth plates; serves as a model for bone formation.

    • Fibrocartilage:

    • Weight-bearing cartilage that withstands compression, found in intervertebral discs, pubic symphysis, and menisci of the knee.

Functions of Bone

  1. Support: Provides a framework for the body and soft organs.

  2. Protection: Protects vital organs, such as the brain and spinal cord.

  3. Movement: Acts as levers for muscle action.

  4. Mineral and Growth Factor Storage: Serves as a reservoir for calcium and phosphorus, along with growth factors.

  5. Blood Cell Formation (Hematopoiesis): Occurs in red marrow cavities of certain bones.

  6. Triglyceride Storage: Fat acts as an energy source and is stored in bone cavities.

  7. Hormone Production: Osteocalcin secreted by bones helps regulate insulin secretion, glucose levels, and metabolism.

Classification of Bones by Shape

  • Long Bones:

    • Length greater than width (e.g., femur).

  • Short Bones:

    • Cube-shaped bones found in the wrist and ankle; sesamoid bones form within tendons (e.g., patella).

  • Flat Bones:

    • Thin, flat, slightly curved bones (e.g., sternum, scapulae, ribs, cranial bones).

  • Irregular Bones:

    • Complex shapes (e.g., vertebrae, hip bones).

Bone Structure

  • Organs: Bones are considered organs because they comprise various types of tissues.

  • Three Levels of Structure:

    • Gross anatomy, microscopic anatomy, and chemical composition.

Gross Anatomy of Bones

  • Compact Bone:

    • Dense outer layer appearing smooth and solid.

  • Spongy Bone:

    • Composed of small, needle-like or flat pieces of bone called trabeculae.

    • Open spaces between trabeculae are filled with red or yellow marrow.

Typical Long Bone Structure

  • Diaphysis:

    • Elongated cylindrical shaft, provides leverage and weight support.

    • Comprises compact bone surrounding the central medullary cavity.

    • Features thin spicules of spongy bone extending inward towards each end of the shaft.

  • Medullary Cavity:

    • Hollow cylindrical space containing red bone marrow in children, transitioning to yellow bone marrow in adults.

  • Epiphysis:

    • Knobby region at each end of the bone, including proximal (close to body) and distal (far from trunk) epiphysis.

    • Comprises outer compact bone and inner spongy bone with articular cartilage covering joint surfaces.

    • Epiphyseal Line:

    • Remnant of the epiphyseal plate, where bone growth occurs during childhood.

Internal Structures of Long Bones

  • Endosteum:

    • Delicate CT membrane covering the internal bone surface.

    • Covers trabeculae and lines canals through compact bone.

  • Periosteum:

    • Tough sheath covering the outer bone surface except at joints.

    • Anchors blood vessels and nerves; serves as an attachment site for ligaments and tendons.

Blood Supply and Innervation

  • Highly Vascularized:

    • Vessels enter from the periosteum via nutrient foramen, which serves as an entry point for arteries and exit for veins.

    • Nerves innervate the bone, periosteum, endosteum, and marrow cavity, mainly providing sensory functions.

Hematopoietic Tissue in Bones

  • Red Marrow:

    • Found in trabecular cavities of spongy bone and flat bones (e.g. sternum) in adults.

    • In newborns, all spongy bone and medullary cavities contain red marrow.

    • Yellow marrow may convert back to red if anemia occurs.

  • Yellow Marrow:

    • A product of red marrow degeneration that occurs as children mature; composed mainly of fat.

Bone Markings

  • Markings: Sites for muscle, ligament, and tendon attachments, areas for joint formation, or conduits for blood vessels and nerves.

  • Types of Markings:

    1. Projections: Outward bulges due to muscle pull or joint modifications.

    2. Depressions: Bowl- or groove-like cuts that can allow for vessel/nervous passageways, or play a role in joints.

    3. Openings: Holes or canals serving as pathways for blood vessels and nerves.

Clinical Application

Bone Marrow Transplant:

  • Why: Bone marrow destroyed by radiation/chemotherapy, or abnormally functioning marrow.

  • Harvested cells are injected into the bloodstream of the recipient, migrating to normal locations for red bone marrow.

Microscopic Anatomy of Bone

  • Bone Connective Tissue:

    • Composed of cells and a predominantly extracellular matrix.

    • Four Types of Cells:

    1. Osteoprogenitor cells: Stem cells developing into osteoblasts.

    2. Osteoblasts: Bone-forming cells.

    3. Osteocytes: Mature bone cells maintaining the bone matrix.

    4. Osteoclasts: Large, multinucleated cells involved in bone resorption.

Ese of Osteoblasts and Osteoclasts

  • Osteoblasts: Secretes osteoid, a semisolid organic form of bone matrix composed chiefly of collagen, comprising about 90% of bone protein. They are actively mitotic.

  • Osteoclasts: Function in bone resorption by releasing enzymes that digest organic matrix components, while hydrochloric acid dissolves calcium and phosphate.

Osteon Structure

  • Compact Bone Composition:

    • Composed of osteons (Haversian systems), the basic functional and structural units of mature compact bone.

    • Each osteon has concentric rings of bone matrix (lamellae) and contains central (Haversian) canals through which blood vessels and nerves run.

Spongy Bone Composition

  • Appears disorganized but is structured along lines of stress to better resist those pressures through a meshwork of trabeculae and interconnected canaliculi, supported by capillaries.

Chemical Composition of Bone

  • Organic Components:

    • Composed of osteoid produced by osteoblasts, containing collagen and glycoproteins, making up about 1/3 of bone matrix.

    • Resilience is due to sacrificial bonds in collagen molecules that disperse energy and prevent fractures.

  • Inorganic Components:

    • Composed of minerals, primarily hydroxyapatite (Ca10(PO4)6(OH)2), forming salt crystals around collagen fibers, responsible for bone hardness and resistance to compression.

Bone Formation and Resorption

  • Ossification: The process of bone formation and development, occurring from fetal stages through adulthood.

  • Two Types of Ossification:

    • Intramembranous Ossification: Develops from membrane; produces flat bones of the skull, some facial bones, mandible, and the central part of the clavicle.

    • Endochondral Ossification: Most bones formed this way, replacing hyaline cartilage models (e.g., long bones).

Procedure of Bone Formation

  1. Intramembranous Ossification Steps:

    • Formation of ossification centers, calcification of osteoid, formation of woven bone and periosteum, replacement of woven bone with lamellar (mature) bone.

  2. Endochondral Ossification Steps:

    1. Formation of a hyaline cartilage model.

    2. Bone collar formation around the diaphysis as cartilage begins to calcify and develop cavities.

    3. Periosteal bud invades cavities leading to spongy bone formation.

    4. Secondary ossification centers appear in epiphyses; bone replaces cartilage except for articular cartilages and epiphyseal plates.

Bone Growth

  • Growth Mechanisms:

    • Interstitial Growth: Lengthwise growth at the epiphyseal plate through zones of proliferation and hypertrophy.

    • Appositional Growth: Growth in thickness due to secretion of bone matrix beneath the periosteum.

Zone Descriptions in the Epiphyseal Plate

  1. Resting Zone: Nearest to the epiphysis, stabilizes epiphyseal plate.

  2. Proliferation Zone: Chondrocytes rapidly divide, facilitating lengthening.

  3. Hypertrophic Zone: Older chondrocytes enlarge; cartilage matrix becomes calcified.

  4. Calcification Zone: Matrix becomes calcified leading to chondrocyte death.

  5. Ossification Zone: New bone formation begins as osteoblasts replace cartilage.

Hormonal Regulation of Bone Growth

  • Growth Hormones:

    • Important for stimulating growth at epiphyseal plates.

  • Thyroid Hormones: Modulates growth hormone activity ensuring proper proportions.

  • Sex Hormones: (estrogens and androgens) promote growth spurts but lead to epiphyseal plate closure.

Homeostasis of Blood Calcium Levels

  • Parathyroid Hormone (PTH): Secreted in response to low calcium levels, increasing absorption, promoting resorption and vitamin D activation.

  • Calcitonin: Released by the thyroid to lower blood calcium levels by inhibiting osteoclast activity and enhancing calcium loss in urine.

Bone Fractures

  • Classification:

    • By position: nondisplaced vs displaced

    • By completeness: complete vs incomplete

    • By open vs closed status.

  • Types of Fractures:

    • Comminuted, compression, spiral, epiphyseal among others; each with specific implications on management and healing.

Healing Process of Bone Fractures

  1. Formation of a hematoma.

  2. Formation of a fibrocartilaginous callus.

  3. Formation of a bony callus.

  4. Bone remodeling.

Aging and Bone Health

  • Age-related changes affect bone density, strength, and resilience, with osteoporosis being a significant condition where bone resorption exceeds deposition.