Connective Tissue

• Connective Tissue - provides a matrix that supports and physically connects other tissues and cells together to form organs of the body
• Interstitial fluid - gives metabolic support to cells as the medium for diffusion of nutrients and waste products
• Extracellular matrix (ECM) - major constituent of connective tissue; consists of different combinations of protein fibers (collagen and elastic fibers) and ground substance
• Ground substance - a complex of anionic, hydrophilic proteoglycans, glycosaminoglycans (GAGs), and multiadhesive glycoproteins (laminin, fibronectin, etc.)
• Mesenchyme - connective tissue that forms the mesoderm and endoderm
• Mesenchymal cells - can differentiate into bone, cartilage, and muscle cells; "spindle-shaped"
• Fibroblasts - produce collagen and elastin fibres
• Loose Connective Tissue - more prevalent in the body than dense connective tissue and characterized by a loose, irregular arrangement of connective tissue fibers and abundant ground substance
• What predominates in the Loose Connective Tissue?

  • Collagen Fibers
  • Fibroblasts
  • Adipose cells
  • Mast Cells
  • Plasma cells
  • Macrophages
• Dense Connective Tissue - contains thicker and more densely packed collagen fibers, with fewer cell types and less ground substance
• Dense irregular connective tissue - exhibit a random and irregular orientation, present in the dermis of skin, in capsules of different organs, and in areas that need strong binding and support
• Dense regular connective tissue - contains densely packed collagen fibers that exhibit a regular and parallel arrangement, primarily found in the tendons and ligament
• two most common cell types in the connective tissue:

  • Fibroblasts
  • Fibrocytes
• In both dense connective tissue types, fibroblasts are the most abundant cells, which are located between the dense collagen bundles.
• Fibroblast - to denote the active cell
• Fibrocyte - to denote the quiescent cell
• Active fibroblast - has more abundant and irregularly branched cytoplasm, containing much RER and a well developed Golgi apparatus, with a large, ovoid, euchromatic nucleus and a prominent nucleolus
• Quiescent cell - smaller than the active fibroblast, usually spindle-shaped with fewer processes, much less RER, and a darker, more heterochromatic nucleus
• Fireblasts are targets of many families of proteins called Growth Factors
• Fibroblasts involved in wound healing, sometimes called Myofibroblasts
• three distinctive types of connective tissue fibers:

  • collagen
  • elastic
  • reticular
• The primary function of the fibrous components within the connective tissue:

  • to provide strength and resistance to stretching and deformation
• Collagen fibers - tough, thick, fibrous proteins that do not branch, are the most abundant fibers and found in almost all the connective tissues of all organs
• Type I collagen fibers - most common and very strong; found in skin, tendons, ligaments, and bone
• Type II collagen fibers - found in hyaline and elastic cartilage and the vitreous body of the eye; provide resistance to pressure
• Type III collagen fibers - forms meshwork in liver, lymph node, spleen, and hemopoietic organs
• Type IV collagen fibers - found in basal lamina of basement membrane; associated with hemidesmosomes
• Reticular fibers - consist mainly of type III collagen, are thin, and form a delicate netlike support framework in the liver, lymph nodes, spleen, hemopoietic organs, and other locations where blood and lymph are filtered, also support capillaries, nerves, and muscle cells.
• Reticular fibers - become visible only when the tissue or organ is stained with silver stain
• Elastic fibers - found in abundance in the lungs, bladder wall, and skin
• Elastic fibers - thin, small, branching fibers that are capable of stretching and returning to their original length. They have less tensile strength than collagen fibers and are composed of microfibrils and the protein elastin.
• In the walls of the large vessels, the smooth muscle cells synthesize the elastic fibers
• In other organs, fibroblasts synthesize elastic fibers
• Loose Connective tissue (spread). Stained for cells and fibers; High Magnification

  A) Mast Cells

  B) Fibroblasts

  C) Collagen fibers

  D) PLasma cell

  E) Elastic fibers

  F) small lymphocyte

  G) Capillary with erythrocytes

  H) Large lympocyte

  I) Mast cell

  J) Elastic fibers

  K) Plasma Cells\

  L) Macrophage with ingested particles

  
• White Adipose (Fat) Cells - Most common type of adipose tissue with wider distribution, occur singly or in groups and contain single or unilocular lipid droplets
• Brown Adipose Cells - Exhibits more limited distribution, cells smaller than white adipose cells; store fat as multiple lipid droplets, best developed in hibernating animals.
• Brown Adipose Cells - in newborns or animals emerging from hibernation, generates body heat; As an adaptation to cold environment, cell numbers and tissue increase
• Norepinephrine from sympathetic nervous system promotes hydrolysis of lipids
• Macrophages - Most numerous in loose connective tissue and ingest bacteria, dead cells, cell debris, and foreign matter
• Macrophages - are antigen-presenting cells to lymphocytes for immunologic response and derived from circulating blood monocytes