4.3 Connective Tissue Supports and Protects

Connective tissues are the body's support system, linking and cushioning other tissues. From loose areolar tissue to dense bone, these versatile tissues perform crucial functions like energy storage, protection, and transport of nutrients and waste.

Connective tissues have unique compositions of cells, fibers, and ground substance that determine their properties. This allows them to fulfill specific roles, from the shock-absorbing abilities of cartilage to the strength and flexibility of tendons and ligaments.

Types and Functions of Connective Tissue

Types of connective tissue

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• Connective tissue proper consists of cells, fibers, and ground substance (extracellular matrix)
  • Loose connective tissue has cells and fibers loosely arranged in a gel-like ground substance (areolar tissue, adipose tissue, reticular tissue)
    • Areolar tissue fills spaces between organs and binds skin to underlying muscles (superficial fascia)
    • Adipose tissue stores energy reserves as triglycerides in adipocytes (subcutaneous layer, around organs)
    • Reticular tissue forms a fine network that supports soft organs (lymph nodes, bone marrow, spleen)
  • Dense connective tissue has a high proportion of collagen fibers with minimal ground substance
    • Dense regular connective tissue has parallel collagen fibers that resist stretching forces (tendons, ligaments)
    • Dense irregular connective tissue has interwoven collagen fibers that withstand tension from multiple directions (dermis, organ capsules)
    • Elastic connective tissue has elastic fibers that allow stretch and recoil (walls of large arteries, vocal cords)
• Supportive connective tissue provides structural support and protection
  • Cartilage has collagen and elastic fibers embedded in a firm gel-like matrix
    • Hyaline cartilage covers bone surfaces in joints and supports the nose and trachea
    • Fibrocartilage resists compression and absorbs shock (intervertebral discs, pubic symphysis)
    • Elastic cartilage maintains shape but allows flexibility (external ear, epiglottis)
  • Bone is a mineralized tissue that provides a rigid framework for the body
    • Compact bone forms the dense outer layer of bones and resists bending and twisting forces
    • Spongy bone has a trabecular network that provides lightweight structural support and houses bone marrow
• Fluid connective tissue transports cells and dissolved substances
  • Blood consists of plasma and formed elements suspended in plasma
    • Plasma is the liquid medium that transports nutrients, waste products, and hormones
    • Formed elements include erythrocytes (red blood cells), leukocytes (white blood cells), and platelets
  • Lymph is a clear fluid derived from blood plasma that drains excess interstitial fluid and transports immune cells

Functions of connective tissues

• Binding and supporting other tissues
  • Connects epithelial tissue to underlying connective tissue and muscle (basement membrane, lamina propria)
  • Provides structural framework for organs (fibrous capsules, trabeculae, septa)
• Protecting organs and tissues from mechanical damage and pathogens
  • Adipose tissue cushions and insulates vital organs (pericardial fat, perirenal fat)
  • Bone and cartilage shield delicate tissues (skull protects brain, rib cage protects heart and lungs)
• Transporting substances between tissues and organs
  • Blood carries oxygen, nutrients, hormones, and waste products throughout the body
  • Lymph transports immune cells, absorbs lipids from the digestive tract, and drains excess interstitial fluid
• Storing energy reserves for future use
  • Adipose tissue stores triglycerides that can be mobilized for energy during fasting or prolonged exercise
• Providing immune defense against pathogens and abnormal cells
  • Leukocytes in blood and lymph recognize and destroy invading microorganisms and tumor cells
• Facilitating movement of body parts
  • Bones and cartilage form joints that allow for specific ranges and types of motion
  • Tendons connect muscles to bones, transmitting forces to produce movement
  • Ligaments connect bones to other bones, stabilizing joints and guiding motion

Structure-function relationships in tissues

• Loose connective tissue has a loose arrangement of fibers in a gel-like ground substance
  • Areolar tissue fills spaces and supports neighboring tissues with a mix of collagen, elastic, and reticular fibers (lamina propria, hypodermis)
  • Adipose tissue stores energy as triglycerides in specialized cells called adipocytes (subcutaneous layer, visceral fat)
  • Reticular tissue forms a fine network of reticular fibers that supports soft organs and forms the framework of lymphoid organs (lymph nodes, bone marrow)
• Dense connective tissue has a high proportion of collagen fibers with minimal ground substance
  • Dense regular connective tissue has parallel collagen fibers that provide tensile strength and resist stretching forces (tendons, ligaments)
  • Dense irregular connective tissue has interwoven collagen fibers that provide strength and flexibility to withstand tension from multiple directions (dermis, organ capsules)
  • Elastic connective tissue has branching elastic fibers that allow for stretch and recoil (walls of large arteries, vocal folds, trachea)
• Cartilage has collagen and/or elastic fibers embedded in a firm gel-like matrix
  • Hyaline cartilage has fine collagen fibers in a gel-like matrix that provides smooth, low-friction surfaces in joints and supports the nose and trachea
  • Fibrocartilage has thick collagen fibers in its matrix that allow it to withstand compression and absorb shock (intervertebral discs, pubic symphysis)
  • Elastic cartilage has elastic fibers in its matrix that provide flexibility while maintaining shape (external ear, epiglottis)
• Bone is a mineralized connective tissue that provides structural support and protection
  • Compact bone forms the dense outer layer of bones and resists bending and twisting forces
  • Spongy bone has a trabecular network that provides lightweight support and houses bone marrow for hematopoiesis
• Blood and lymph are fluid connective tissues that transport cells and dissolved substances
  • Plasma is the liquid medium that suspends formed elements and transports nutrients, waste products, and regulatory molecules
  • Erythrocytes are specialized for oxygen transport due to their high hemoglobin content
  • Leukocytes are various types of white blood cells with specific roles in innate and adaptive immunity
  • Platelets are cell fragments that adhere to damaged blood vessels and initiate the clotting process

Extracellular matrix components and functions

• Fibroblasts are the primary cells responsible for producing and maintaining the extracellular matrix
• Proteoglycans are large molecules consisting of a protein core with attached glycosaminoglycans, which help retain water and provide cushioning
• Glycosaminoglycans are long, unbranched polysaccharide chains that contribute to the gel-like consistency of the ground substance
• Mesenchyme is embryonic connective tissue from which all other connective tissues develop