HHIS L4 & 5

• Connective tissue is specialized to physically support and connect other tissues and maintain the water required for metabolite diffusion to and from cells
• Connective tissue cells are scattered throughout the extracellular matrix, which consists of large protein fibers and glycoproteins attached to the basement membrane
• General characteristics of connective tissue:

  • Most abundant tissue in the body
  • Major constituent of connective tissue is the extracellular matrix (ECM) containing large protein fibers (elastic, collagen, reticular) and non-fibrous areas of unstained ground substance rich in various GAGs and water
• All connective tissue has 3 components: cells, ground substance, and fibers

  • Cells of connective tissue originate from embryonic mesenchyme
• Functions of connective tissue:

  • Responsible for attachment of one tissue to another (e.g., ligaments, tendons)
  • Transport of fluids and materials (e.g., blood)
  • Defending the body from invading microorganisms through the immune system
  • Storage of materials (e.g., fat)
  • Establishing a structural framework for the body (e.g., bones)
• Cells of connective tissue:

  1. Fibroblasts:

  • Major cells of connective tissue proper
  • Elongated, irregularly shaped cells with oval nuclei that synthesize and secrete most components of the ECM
  • Targets of growth factors influencing cell growth and differentiation
  • Involved in wound healing and sometimes called myofibroblasts
• Cells of connective tissue (cont.):

  2. Fibrocyte (quiescent cell):

  • Inactive form of fibroblast
  • Smaller than active fibroblasts, usually spindle-shaped with fewer processes and less RER

  3. Adipocytes (fat cells):

  • Very large cells specialized for storage of triglycerides, predominant in adipose tissue

  4. Mesenchymal cells:

  • Embryological tissue from which all types of supporting/connective tissue are derived
  • Relatively unspecialized and capable of differentiation into all supporting tissue cell types
• Macrophages:

  • Phagocytic cells that differentiate in connective tissue from precursor cells called monocytes
  • Function in ECM turnover, phagocytosis, antigen presentation, secretion of growth factors, cytokines, and other agents
• Mast cells:

  • Originate from blood cell precursors and release various vasoactive agents and substances during inflammatory and allergic reactions
• Plasma cells:

  • Short-lived cells that differentiate from B lymphocytes and secrete specific antibodies
• Leukocytes:

  • Provide surveillance against bacterial invaders and stimulate tissue repair
• Ground substance in connective tissue:

  • Amorphous transparent material filling the space between cells and fibers
  • Mixture of glycoproteins and complex carbohydrates with water-binding ability
• Glycosaminoglycans (GAGs/Mucopolysaccharides):

  • Long polymers of repeating disaccharide units, including hyaluronate, chondroitin, dermatan sulfate, keratan sulfate, heparan sulfate, and heparin sulfate
• Hyaluronate (hyaluronic acid/hyaluronan) is a type of GAG
• Hyaluronate (hyaluronic acid/ hyaluronan) is the most abundant, largest, and most ubiquitous
• Proteoglycans consist of a core protein to which are covalently attached various numbers and combinations of the sulfated GAGs
• Aggrecan is a large core protein heavily bound with chondroitin and keratan sulfate chain
• Perlecan is a key proteoglycan in all basal laminae
• Fibronectin has binding sites for collagens & certain GAGs, and forms insoluble fibrillar networks throughout connective tissue
• Laminin has binding sites for integrins, type IV collagen, & specific proteoglycans, providing adhesion for epithelial & other cells (component of the basal lamina)
• Collagen is the main fiber type found in most supporting tissues & is the most abundant protein in the human body
• Type I collagen is the main structural collagen found in fibrous supporting tissue, skin (dermis), tendons, ligaments, and bone
• Type II collagen is the main structural collagen of hyaline cartilage
• Type III collagen forms the delicate branched 'reticular' supporting meshwork prominent in highly cellular tissues such as the liver, bone marrow, and lymphoid organs
• Type IV collagen is a network/mesh-forming collagen and is an important constituent of basement membranes
• Type VII collagen forms special anchoring fibrils that link extracellular matrix to basement membranes
• Reticular fibers originate from Collagen III, stain poorly in H&E preparations, and form a delicate supporting framework for many cellular organs such as endocrine glands, lymph nodes, bone marrow, and liver
• Elastic fibers are arranged as fibers and/or discontinuous sheets in the extracellular matrix where they provide stretching and elastic recoil properties
• Connective Tissue Proper is usually classified as loose or dense according to the amount of collagen and ground substance present
• Loose Connective Tissue (Areolar Tissue) has relatively more ground substance than collagen, surrounds small blood vessels, and has a delicate consistency
• Dense Connective Tissue has fewer cells, mostly fibroblasts, and a clear predominance of bundled type I collagen fibers over ground substance
• Dense Irregular Connective Tissue is filled primarily with randomly distributed bundles of type I collagen, providing resistance to tearing from all directions as well as some elasticity
• Dense Regular Connective Tissue features bundles of essentially parallel type I collagen, providing great strength (but little stretch) in binding together components of the musculoskeletal system
• Reticular Tissue consists of delicate networks of type III collagen and is most abundant in certain lymphoid organs where the fibers form attachment sites for lymphocytes and other immune cells
• Mucoid Tissue is a gel-like connective tissue with few cells found most abundantly around blood vessels in the umbilical cord, with abundant ground substance (Hyaluronan) and sparse collagen fibers