UnitE 4

• Organisms are built from various tissues
• Each organ is composed of various types of tissues (ex: intestine)
• Tissues must organize in specific manner in order to ensure proper development of the organism
• Extracellular Matrix and Connective Tissue

  Animal and plant tissues are composed of different structures

  
• Animal tissues


  • Must be flexible (cells must continuously regenerate)
• Plant tissues


  • More rigid
• Cell wall


  The supportive matrix in plant cells

  
• Cellulose


  Long, unbranched chains of glucose; each subunit is connected via a β-1,4 linkage
• 18 cellulose molecules are held together via hydrogen bonds in a single cellulose microfibril
• The middle lamella (yellow) is rich in pectin and is the layer that cements one cell wall to another

  
• Types of animal tissues

  • Connective
  • Epithelial
  • Nervous
  • Muscle
• Connective tissues


  • Extracellular matrix is abundant (cells are more spread apart) vs the other types of tissues in which cells are joined together

  
• Animal connective tissues


  • Flexible
  • Hard and dense
• Collagen


  Most common proteins found in bones, tendons and skin in humans

  
• Mammals have > 40 types of collagen-coding genes
• Osteoblasts


  Collagen producing cells in the bone
• Fibroblasts


  Collagen producing cells in skin/tendon

  
• Genetic syndrome resulting from absence/mutations in proteinase cause skin to hyperextend
• Epithelia


  Multicellular sheets where adjacent cells are joined tightly together
• Epithelial sheets are polarized: they have a basal and apical surface

  
• Tight junctions


  Prevent water from leaking in between them by creating a seal between neighbouring cells
• Gap junctions


  Regions where two cells are separated by 2-4 nm
• Tissues are composed of many different types of cells
• All tissues require mechanical strength provided by connective tissue
• In the skin, blood vessels are lined with endothelial cells which help provide nutrients and oxygen
• Some tissues are composed of nerve cells where their axons are wrapped with Schwann cells that provide electrical insulation
• Cells are continuously balancing cell division/growth with cell death in order to allow for tissue renewal
• Factors that contribute to tissue stability

  • Cell communication
  • Selective cell adhesion
  • Cell memory
• Cadherins


  Transmembrane proteins that mediate cell-cell adhesion via homophilic binding
• Most differentiated cells that need replacement are unable to divide
• Hematopoietic stem cells


  Immature cells that can develop into all types of blood cells, including white blood cells, red blood cells, and platelets

  
• Hematopoietic stem cells are found in the peripheral blood and the bone marrow
• Extracellular signals


  Exchanged between stem cells, their progeny and other cell types to control stem-cell systems
• Wnt proteins


  Secreted in the intestinal crypt and promote the proliferation of stem cells and precursor cells
• Mutations/defects in Wnt signaling à colorectal cancers
• Transfusion of hematopoietic stem cells into a mouse with decreased blood cells due to irradiation resulted in re-population of blood cells
• Bone marrow transplantation in cancer (leukemia) patients undergoing irradiation
• Embryonic stem cells (ES cells)

  Cells derived in culture from early mouse embryos that have the ability to continuously proliferate under appropriate conditions
• Pluripotent Stem Cells


  Have the capacity to self-renew by dividing and to develop into the three primary germ cell layers of the early embryo and therefore into all cells of the adult body

  
• Induced Pluripotent Stem Cells (iPS)

  Type of pluripotent stem cell that can be derived from a somatic cell type (skin or blood cells that have been re-programmed back into embryonic pluripotent state)