Detailed Study Guide on Bones and Skeletal Tissues

PowerPoint® Lecture Slides Notes: Bones and Skeletal Tissues - Part A

Cartilage: Basic Structure, Types, and Locations

General Overview of Cartilage

• Cartilage is a resilient and flexible connective tissue that plays an essential role in the skeletal system.

• Characteristics:

  • Contains a high percentage of water.

  • Lacks blood vessels and nerves, contributing to its avascular nature.

• Perichondrium:

  • Dense connective tissue that surrounds cartilage.

  • Contains blood vessels that provide nutrients to cartilage via diffusion.

Types of Cartilage

All Cartilages

• Each type of cartilage contains chondrocytes located in lacunae and an extracellular matrix (ECM).

Three Main Types of Cartilage:

1. Hyaline Cartilage

   • Function: Provides support, flexibility, and resilience.

   • Composition: Composed of collagen fibers only.

   • Locations:

     • Articular cartilage (joints)

     • Costal cartilage (rib cage)

     • Respiratory structures (larynx, trachea)

     • Nasal cartilage (nose).

2. Elastic Cartilage

   • Function: Similar to hyaline cartilage but provides more flexibility.

   • Composition: Contains elastic fibers in addition to collagen.

   • Locations:

     • External ear (pinna)

     • Epiglottis (covers the trachea during swallowing).

3. Fibrocartilage

   • Function: Provides tensile strength and can withstand heavy pressure.

   • Composition: Contains thick collagen fibers.

   • Locations:

     • Menisci of the knee joint

     • Intervertebral discs

     • Pubic symphysis.

Structural Components of Cartilage

• Chondrocytes: Responsible for maintaining the ECM of cartilage and are found within lacunae.

• Molecular Structure (Referencing Figure 7.3)

  • Proteoglycans: Large molecules that consist of glycosaminoglycans (approximately 180 per aggrecan core protein) linked with a backbone of hyaluronan, intertwined with type II collagen fibrils.

  • Territorial Matrix (TM): The area surrounding individual lacunae, showing distinct staining characteristics.

  • Interterritorial Matrix (IM): Lies outside of the territorial matrix and has a more uniform appearance.

Summary of Cartilage Features (Referencing Table 7.1)

Growth of Cartilage

• Appositional Growth:

  • New matrix is secreted against the external face of existing cartilage.

• Interstitial Growth:

  • Chondrocytes divide and secrete new matrix, leading to expansion from within.

• Calcification of Cartilage:

  • Can occur during bone growth in youth and also in old age.

Classification of Bones

• The human skeleton is composed of 206 named bones categorized into two major groups:

  1. Axial Skeleton:

  • Includes the skull, vertebral column, and rib cage.

  1. Appendicular Skeleton:

  • Comprising bones of the upper and lower limbs as well as girdles connecting limbs to the axial skeleton.

Classification of Bones by Shape

Main Categories of Bone Shapes:

• Long Bones:

  • Longer than they are wide.

  • Examples: Limbs, metacarpals, metatarsals.

• Short Bones:

  • Cube-shaped bones.

  • Examples: Carpals, tarsals, patella.

• Flat Bones:

  • Thin, flat, and slightly curved.

  • Examples: Sternum, scapulae, ribs.

• Irregular Bones:

  • Complicated shapes.

  • Examples: Vertebrae, coxal bones.

Functions of Bones

Seven Key Functions

1. Support: Provides structural support for body and soft organs.

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

3. Movement: Acts as levers for muscle action.

4. Mineral Storage: Reservoir for essential minerals, namely calcium and phosphorus.

5. Blood Cell Formation: Hematopoiesis occurs within red marrow cavities of certain bones.

6. Triglyceride Storage: Stores fat for energy sources.

7. Hormone Production: Produces osteocalcin, a hormone regulating bone formation.

Structure of Bones

• Bones are classified as organs composed of different tissue types:

  • Bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle, and epithelial cells in blood vessels.

Gross Anatomy of Bones

Bone Textures

• Compact Bone:

  • Dense outer layer; solid and smooth.

• Spongy Bone (Cancellous):

  • Honeycomb structure made up of flat pieces called trabeculae found deep to compact bone.

Structure of Short, Irregular, and Flat Bones

• Composed of thin plates of spongy bone covered by compact bone.

• Plates are sandwiched between connective tissue membranes:

  • Periosteum: Covers outer surfaces.

  • Endosteum: Covers inner surfaces.

• Articular surfaces are covered by hyaline cartilage.

Structure of Typical Long Bone

• The structure consists of:

  • Diaphysis: The tubular shaft forming the long axis of the bone; consists of compact bone surrounding the medullary cavity.

  • Epiphyses: The bone ends; has external compact bone and internal spongy bone; articular cartilage covers joint surfaces.

  • Epiphyseal Line: Remnant of the childhood growth plate (epiphyseal plate).

Membranes of Bone

Endosteum

• A delicate connective tissue membrane lining the internal bone surface.

• Covers trabecular of spongy bone and lines the canals that pass through compact bone.

• Contains osteogenic cells capable of differentiating into other types of bone cells.

Periosteum

• A white, double-layered membrane covering the external surfaces of bones except around joint surfaces.

• Composed of:

  • Outer fibrous layer: Dense irregular connective tissue that secures the periosteum to the bone matrix through Sharpey's fibers.

  • Inner osteogenic layer: Contains stem cells (osteogenic cells) and many nerve fibers and blood vessels; serves as anchoring points for tendons and ligaments.

Hematopoietic Tissue in Bones

• Red Marrow:

  • Located within the trabecular cavities of spongy bone and the diploë of flat bones (e.g., sternum).

  • Present in the medullary cavities of newborns.

• In adult long bones, red marrow is limited to the heads of the femur and humerus instead of the entire length.

• Yellow Marrow: Can convert back into red marrow when necessary, especially during increased demand for blood cell production.

Chemical Composition of Bone

Organic Components

• Composed of:

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

  • Osteoid: The unmineralized bone matrix secreted by osteoblasts, which harbors:

  • Ground substance (proteoglycans and glycoproteins)

  • Collagen fibers, which contribute to structure and tensile strength.

Inorganic Components

• Approximately 85%: Hydroxyapatites, represented by the formula Ca{10}(PO{4}){6}(OH){2}.

  • These are primarily tiny calcium phosphate crystals situated within and around collagen fibers.

  • Responsible for the hardness and resistance to compression.

• 10%: Calcium carbonate CaCO_{3}, with lesser amounts of other minerals such as magnesium, sodium, potassium, fluoride, sulfate.

Microscopic Anatomy of Bone

Cells of Bone Tissue

• Five Major Types of Bone Cells:

  1. Osteogenic Cells: Stem cells found in periosteum and endosteum; differentiate into osteoblasts.

  2. Osteoblasts: Actively mitotic cells responsible for bone matrix formation.

  3. Osteocytes: Mature bone cells that maintain the bone matrix and act as stress or strain sensors.

  4. Osteoclasts: Multinucleated cells derived from hematopoietic stem cells; responsible for bone resorption and remodeling.

Compact Bone Structure (Osteon)

• Referred to as lamellar bone; the functional unit is the osteon (Haversian system):

  • Cylindrical structures oriented parallel to the long axis of the bone.

  • Composed of collagen fibers; when loaded, adjacent lamellae resist twisting to bolster strength.

Components of the Osteon

• Haversian Canal: Contains blood vessels and nerves within each osteon.

• Lacunae: Small cavities occupied by osteocytes.

• Canaliculi: Microscopic channels that interconnect lacunae, facilitating nutrient and waste exchange.

• Lamellae: Layers of bone matrix; may exhibit interstitial and circumferential layers that contribute to structural integrity all around the bone.

Spongy Bone Structure

• Trabeculae: Align along lines of stress to help resist mechanical forces.

• Lacks a structured osteon formation but comprises irregular lamellae and osteocytes connected by canaliculi.

• Nutrient supply via capillaries in endosteum within the trabecular network.

(Cited Figures: Figures 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7; Tables 7.1)