Cell Role in Tissue Maintenance

Cell going through anaphase Nerve cell Cell Schematic: Eukaryote ATP

General Information

  1. Blastophore --> differentiation --> cell renewal and tissue maintenance throughout life. Cells must become committed
  2. Tissues are actually made of many cell types
    1. tissue cells
    2. fibroblasts (ECM)
    3. endothelial cells (blood supply)
    4. nerve cells
    5. macrophages and other white blood cells
  3. Each cell type remains that type --> cell memory (all progeny cell are the same as their parents)
  4. What leads to stability and heretibility of differentiated state, and maintains the essential predetermined fate for all time?
    1. Cells retain memory despite exposure to new environments, but can be influenced by manipulated environments: these changes are minimal not drastic EX: liver cells can upregulate enzymes, and mamary cells can produce milk proteins
  5. Adult tissues : distinct and irreversible cell lineages
    1. even stem cells --> occur in stem families
  6. Permanent cells: no dividing and no replacement of cells but some parts of the cells are replaced
    1. heart muscle
    2. nerves: but can actually regenerate axons and dendrites
    3. auditory hair cells
    4. eye lense cells

Renewal of Cells

  1. Simple duplication
    1. Liver cells
      1. hepatocytes: specialized for exchange of metabolites between hepatocytes and blood
      2. food absorption into blood --> liver: nutrients are processed for use by other body cells
      3. essential roles in carbohydrate, lipid, and protein metabolism. Also secretes waste products and bile into gut lumen
      4. Hepatocytes: relatively long - lived (compared to gut lining) but liver cell loss --> liver cell proliferation
      5. Regulated so that cell proliferation = cell death
        1. one regulator is hepatocyte growth factor (increase when liver is damaged)
        2. Cell survival/cell death factors are regulated so that a liver of only a certain, discrete size can be maintained
        3. Too much is sometimes too much EX: in an alcoholic where hepatocyte damage is too great for regeneration to keep up. Then fibroblasts (that are interspersed in the liver) take over and liver becomes clogged with connective tissue (cirrhosis)
        4. Scar tissue can replace the renewal of the damaged issue if damage is severe
    2. Endothelial cells: line all blood vessels
      1. critical for blood supply
      2. line the entire vascular system
      3. retain the capacity for cell division and
      4. new capillaries sprout from pre-existing ones, given the propper signals EX: in cancer (angiotensin)
  2. Renewal by stem cells: replace cells which cannot divide by themselves
    1. In the gut, stem cells are protected in the crypts
    2. In the skin, burried beneath many layers
    3. In blood, more haphazard arrangement
    1. Stem cells
      1. not itself terminally differentiated, yet they are determined
      2. divide without limit
      3. divide results in two pathways for progeny
        1. more stem cells
        2. irreversible, terminal differentiation
    2. Can be unipotent or pluripotent stem cells
    3. Stem cells are hard to find and isolate
      1. non descript
      2. rare
    4. Epidermal stem cells: a subset of basal cells of epidermis
      1. Basal cell proliferation is governed by many factors including thickness of the epidermis
      2. The thinner the epidermis the more cell division
    5. Pluripotent stem cells: Blood cells
      1. There are many types that are hugely varied, but all have a limited life span
      2. All must be renewed by a common stem cell in bone marrow
        1. Erythrocytes
        2. Leukocytes
        3. Platelets
        4. The type which is produced depends on what is needed EX: if an injury or infection (white blood cells), or if an increase in altitude (red blood cells for more oxygen carrying capacity)
      3. Stem cells for blood are in the bone marrow
        1. Lots of ECM, fat cells and other stromal cells, blood cell precursors, and of course stem cells. Also megakaryocytes --> platelets
    6. In addition to proliferation signals, cells need survival signals. This is also a function of CSF's and must be balanced against programmed cell death, or apoptosis