Connective Tissue Overview

Connective Tissue Definition

• Connective tissue is a fibrous tissue providing structural support for organs.
• It is found throughout the body and contains an extracellular matrix that surrounds living cells.
• It originates from mesoderm and is abundant and widely distributed.
• It connects or binds cells and tissues.
• It is highly vascular except for cartilage and tendons, which have limited blood supply and nerves.

Functions of Connective Tissue

• Protection: Encloses organs as a capsule, separates organs into layers.
• Connection: Connects tissues (tendons and ligaments).
• Support and movement: Provides structure and allows for movement (bones).
• Storage of energy: Stores energy as fat (adipose tissue).
• Thermal Insulation: Provides thermal insulation (fat).
• Transport: Transports substances through the blood.
• Protection: Protects against pathogens (bone, immune cells).
• Metabolic functions: Plays a role in metabolic functions.

Structural Elements of Connective Tissue

• Consists of three main components: ground substance, fibers, and cells.

Ground Substance

• Extracellular matrix, a non-living material between cells.
• Clear, semi-fluid gel.
• Composed of polysaccharides and proteins.
• Thickness varies depending on tissue type.
• Can be liquid, gel-like, or rock-hard.

Fibers

• Collagen fibers: Provide strength, primarily composed of collagen type 1.
• Elastic fibers: Allow for stretching and recoiling, made of randomly coiled proteins.
• Reticular fibers: Fine fibers forming a supporting network, composed of collagen type III.

Cells

• Primary cell types:
  • Connective tissue proper: fibroblast
  • Cartilage: chondroblast
  • Bone: osteoblast
  • Blood: hemocytoblast (actively mitotic)
• Each cell type exists in immature ("blast") and mature ("cyte") forms.

Chief Function and Cell Type

• Embryonic source of all connective tissue cells: Mesenchyme
• Structural support: Fibroblasts, chondroblasts, osteoblasts
• Defense and immune: Plasma cells, lymphocytes, neutrophils, eosinophils, basophils, mast cells, macrophages
• Metabolic: Adipocytes
• Energy storage: Adipocytes
• Thermal insulation: Adipocytes

Types of Connective Tissues

• Classified based on components and structural characteristics:

Connective Tissues Proper

• Gelatinous matrix.
• Two main categories: Loose Connective Tissue and Dense Connective Tissue.

Loose Connective Tissue (Areolar Tissue)

• Gel-like ground with both elastic and collagen fibers.
• Contains various cell types and all three fibers.
• Soft and flexible, acts as packing material between tissues and organs.
• Location: Subcutaneous layer of skin, between muscles.
• Function: Protection, binds muscles together.

Dense Connective Tissue

• Fewer cells with more collagen fibers.
• Subcategories:
  • Dense regular connective tissue:
    • Regularly arranged collagen fibers for strength.
    • Located in tendons (muscle to bone) and ligaments (bone to bone).
    • Provides attachment.
  • Dense irregular connective tissue:
    • Irregularly arranged collagen fibers.
    • Forms sheets where tension is exerted in various directions.
    • Provides structural strength.
    • Location: Dermis of skin.

Connective tissues with special properties

• Adipose tissue:
  • Little collagen and elastic fibers.
  • Large cells filled with lipids (adipose cells).
  • Highly vascular.
  • Location: Subcutaneous tissues, breast, around the kidney.
  • Function: Energy store, reduces heat loss, protects organs.
• Elastic connective tissues:
  • Primarily composed of elastin fibers.
  • Strong and elastic, allows for recoil.
  • Location: Walls of elastic arteries, lungs, vocal ligaments.
  • Function: Elasticity needed for accommodating changes in blood pressure.
• Reticular connective tissues:
  • Branching reticular fibers forming a soft internal skeleton.
  • Found in lymphoid organs.
  • Location: Lymph nodes, spleen, liver, bone marrow.
  • Function: Provides support and strength, forms the stroma (framework) of soft organs.
• Mucous connective tissue:
  • Also known as embryonic tissue, found primarily during embryonic life.
  • Abundant matrix composed of Wharton's Jelly.
  • Few collagen fibers and fibroblasts.
  • Location: Umbilical cord.
  • Function: Supports blood vessels of the umbilical cord.
• Vascular C.T:
  • Distinctive collections of cells in a fluid matrix.
  • Matrix is different from other connective tissues; it is not produced by the cells.
  • Matrix = plasma, composed of water, salts, and dissolved proteins.
  • Contains:
    • Lymph: Formed as interstitial fluid is collected into lymphatic vessels and returned to the blood vessels.
    • Blood: Blood cells float in a liquid matrix called plasma.
• Types of Blood cells:
  • Erythrocytes (red blood cells)
  • Leukocytes (white blood cells):
    • Granulocytes (neutrophils, basophils, eosinophils)
    • Agranulocytes (monocytes, lymphocytes)
  • Platelets: Cell fragments aiding in blood clotting.
• Location: Blood system.
• Function: Respiratory, nutritional, thermoregulation, excretory, defensive, transports hormones, regulates metabolism.

Supporting Connective Tissues

• Either cartilage or bone.

Cartilage

• Composed of chondrocytes (cartilage cells) located in a matrix containing collagen (gum-like).
• Cells are found in lacunae within the matrix.
• Fibers can be elastic or non-elastic (reticular).
• Lacks blood vessels, cells depend on diffusion for nutrients and oxygen.

Types of Cartilage:

• Hyaline cartilage:
  • Most common and abundant.
  • Glassy appearance.
  • Supports and resists compressive stress.
  • Location: Nose, trachea, ends of long bones, thoracic cage.
  • Function: Protects bone against friction, keeps airways open, covers ends of bones as articular cartilage (absorbs compressive forces at joints).
• Fibrocartilage:
  • Highly compressible and resists tension.
  • More fibrous than hyaline cartilage (bundles of collagen).
  • Located where hyaline cartilage meets ligaments or tendons, where strong support and heavy pressure tolerance are needed.
  • Location: Intervertebral disks, spongy cartilage of knees.
  • Function: Support, protects bone against friction.
• Elastic cartilage:
  • Maintains shape while allowing flexibility and elasticity.
  • Contains more elastin fibers than hyaline cartilage.
  • Coiled fibers amongst collagen bundles.
  • Recoil to original shape when bent.
  • Located where strength and stretch ability are needed.
  • Location: Ear pinna, epiglottis.
  • Function: Maintains shape and allows flexibility.

Bone

• Hard connective tissue consisting of cells and a mineralized matrix.
• Osteoblasts secrete the matrix called lamellae.
• Types:
  • Soft bones (skull).
  • Compact bones (femur).