Connective Tissue

• Connective tissue is found everywhere in the body, but the proportion present in tissues varies
• Major functions of connective tissue include support and binding, holding body fluids, defending the body against infection, and storing nutrients as fat
• All connective tissue arises from the mesenchyme layer, an embryonic tissue
• Connective tissue varies in vascularity, from rich vascular supply to avascular (e.g., cartilage)
• The living cells of connective tissue are distributed within a matrix of non-living extracellular substances
• The extracellular matrix of connective tissue allows it to bear weight, withstand tension, and abrasion
• Connective tissue is made of ground substance, fibers, and cells
• The composition and arrangement of extracellular elements in connective tissue vary, leading to diverse types of connective tissues
• Ground substance in connective tissue is an unstructured material that fills the space between cells and contains fibers that support the tissue
• Ground substance in connective tissue is composed of interstitial fluid, cell adhesion proteins, and proteoglycans
• Interstitial fluid in ground substance acts as a molecular sieve for nutrient diffusion between capillaries and cells
• Cell adhesion proteins in ground substance allow connective tissue cells to attach to the extracellular matrix
• Proteoglycans in ground substance consist of a protein core with attached large polysaccharides called glycosaminoglycans (GAGs)
• Collagen fibers in connective tissue are tough and have high tensile strength, able to withstand great longitudinal stress
• Elastin fibers in connective tissue are elastic and found where greater elasticity is needed, like in skin, lungs, and blood vessel walls
• Reticular fibers in connective tissue form delicate networks that support small blood vessels and soft tissue of organs
• Major types of connective tissue cells include fibroblasts, chondroblasts, osteoblasts, and hemocytoblasts
• Other cell types in connective tissue include adipocytes, white blood cells, mast cells, and macrophages
• Connective tissue proper includes loose connective tissue like areolar tissue, white adipose tissue, brown adipose tissue, and reticular connective tissue
• Areolar tissue is a universal packing material between other tissues, with functions like supporting and binding other tissues, defending against infection, and storing nutrients as fat
• White adipose tissue functions as a shock absorber, insulation, and energy storage site, located under the skin, around kidneys, and in the abdomen
• Brown adipose tissue generates heat using lipids and is found in babies and adults in specific locations
• Reticular connective tissue forms delicate networks that support free blood cells in lymphoid organs
• Dense regular connective tissue contains closely packed collagen fibers running parallel to the direction of pull, providing resistance to pulling forces
• Connective tissue - Tendons:

  • Attach bones to bones
  • Withstand great tensile stress when pulling force is applied in one direction
  • Location: tendons, most ligaments, aponeuroses
• Dense irregular connective tissue:

  • Same structural elements as the regular variety but with thicker and irregularly arranged bundles of collagen fibers
  • Function: resists tension from many directions, provides structural strength
  • Location: dermis, fibrous joint capsules, fibrous coverings of some organs, submucosa of the digestive tract
• Elastic connective tissue:

  • Dense irregular connective tissue containing high amounts of elastin
  • Function: allows tissue to recoil after stretching
  • Location: walls of large arteries, within certain ligaments associated with the vertebral column, within the walls of the bronchial tube
• Connective tissue - Cartilage:

  • Stands up to tension and compression, tough but flexible
  • Lacks nerve fibers and is avascular, up to 80% water
  • Receives nutrients by diffusion from blood vessels located in the perichondrium surrounding it
  • Chondroblasts: predominant cell type in growing cartilage, produce new matrix until the skeleton stops growing at the end of adolescence
  • Chondrocytes: mature cartilage cells typically found in small groups within cavities called lacunae
  • Perichondrium: covers the surface of most cartilage, except at epiphyseal plates
• Hyaline Cartilage:

  • Most abundant cartilage in the body, consists of bluish-white ground substance (chondroitin sulfate) with fine collagen fibers
  • Function: supports and reinforces, serves as a resilient cushion, resists compressive stress, provides smooth surfaces for movement
  • Location: forms most of the embryonic skeleton, covers the ends of long bones in joint cavities, forms costal cartilages of the ribs, cartilages of the nose, trachea, and larynx
• Elastic Cartilage:

  • Similar to hyaline cartilage but with more elastic fibers
  • Function: maintains the shape of a structure and allows flexibility
  • Location: external ear, epiglottis
• Fibrocartilage:

  • Intermediate between hyaline cartilage and dense regular connective tissues
  • Consists of chondrocytes scattered among thick bundles of collagen fibers
  • Function: tensile strength allows it to absorb compressive shock, support, and fusion
  • Location: intervertebral discs, pubic symphysis, menisci of knee joint, portions of tendons that insert into cartilage
• Connective tissue - Bone:

  • Osteoblasts: bone-building cells, produce and surround themselves in matrix
  • Osteocytes: mature bone cells that maintain bone tissue, entrapped in matrix secretions
  • Osteoclasts: reabsorb and remodel bone, part of the normal development, growth, maintenance, and repair of bone
  • Compact bone: arranged in osteons
  • Function: support (structural framework for the body, provides attachment for tendons), protection (protect internal organs from injury), assistance in movement (when muscles contract they pull on bones), mineral homeostasis (store minerals e.g., calcium, phosphorus), blood cell production (red bone marrow produces blood cells)
• Connective tissue - Blood:

  • Classified as a connective tissue because it develops from mesenchyme
  • Consists of blood cells surrounded by a nonliving fluid matrix called blood plasma
  • Red blood cells are the most common cell type, also contains white blood cells and platelets
  • Function: transport vehicle for respiratory gases, nutrients, wastes, and other substances
• Elastin allows tissues to stretch and recoil.
• The extracellular matrix is composed of collagen, elastin, proteoglycans, glycoproteins, and fibronectin.
• Collagen provides tensile strength to the connective tissue.
• Reticular Fiber is thin and branching, forming networks or meshworks.
• Collagen is strong and flexible, providing structural support.