In-Depth Notes on Cellular Communities and Cell Cycle

Cellular Communities and Cell Cycle

Multicellular Organisms

• Multicellular organisms are organized into tissues.
  • Vertebrates have four main tissue types:
    • Nervous
    • Muscle
    • Epithelial
    • Connective
• Tissues consist of cells and extracellular matrix (ECM).
  • ECM is secreted by cells.
  • Cell adhesion can be:
    • Direct (cell junctions)
    • Via the ECM

Plant Cell Walls

• Plant cells have tough external walls, a type of ECM.
  • The composition is controlled by the plant cell.
  • Variability in strength:
    • Thick and hard = wood
    • Thin and flexible = leaf
• Cellulose microfibrils provide tensile strength.
  • Interwoven with other polysaccharides and structural proteins.
  • Structure resists compression and tension.
• Plant cell elongation is orientation-dependent on cellulose microfibrils.
  • Cells control how they lay down the cell wall.
  • Turgor pressure affects growth and shape direction.

Animal Connective Tissue

• Animal connective tissue consists largely of ECM; it carries mechanical loads.
• Cells in other tissues (nervous, epithelial, muscular) are closely joined with little ECM.
• Major component: Collagen provides tensile strength.
  • Fibroblasts produce ECM, including collagen.
  • Procollagen precursors mature outside the cell; it assembles into collagen fibers.
  • Fibroblasts orient and compact collagen for specific tissue structure.
  • Example patterns: skin (woven) vs. tendons (parallel).
• Cells use fibronectin to adhere to collagen.
  • Integrins connect ECM to the cytoskeleton inside the cell.
  • Active integrins enhance binding properties during adhesion processes.
• Glycosaminoglycans (GAGs) fill ECM, forming huge macromolecules that resist compression.

Animal Epithelial Cells

• Epithelial sheets can have various packing styles: simple, stratified, cuboidal, squamous, or columnar.
  • Epithelium is polarized, resting on a basal lamina that supports the structure.
  • Functionally specialized cell types can be found lining organs (e.g., intestines).
• Tight junctions create barriers to solute diffusion between cells.
  • Contains sealing strands of occludin and claudin proteins.
• Cytoskeleton-linked junctions (e.g., adherens junctions) help bind epithelial cells together.
  • Cadherin molecules interact with actin filaments to maintain structure.
• Desmosomes link keratin filaments of adjacent epithelial cells.
• Gap junctions provide communication channels between cells, regulated by external signals.

Stem Cells

• Tissues like skin are composed of various cell types.
• Stem cells can generate differentiated cells or other stem cells, involved in tissue renewal.
• Epithelial lining of the intestine continuously renews through stem cells which migrate upwards.
• Epidermis regeneration relies on stem cells located in the basal layer.
• Embryonic stem cells are pluripotent, capable of becoming different cell types.
• Nuclear transplantation may facilitate cloning or stem cell generation for therapeutic purposes.
• Induced pluripotent stem cells (iPS) are created from adult fibroblasts via genetic manipulation.

Cancer

• Cancer is characterized by uncontrolled cell growth due to dysregulation of the cell cycle.
• Mutations can accumulate, altering cell behavior and leading to tumor formation.
• Proto-oncogenes encourage the cell cycle while tumor suppressor genes inhibit it; mutations can disrupt their functions.
  • p53 gene mutations are common in cancers, compromising DNA damage response.

Cell Cycle

• The cell cycle encompasses cell growth and division, divided into two main phases:
  1. Interphase - preparation phase (G1, S, G2)
  2. Mitotic phase - division phase (karyokinesis and cytokinesis)
• Some cells enter a quiescent state (G0 phase) and may remain non-dividing permanently or return to the cycle.
• The timing of phases varies among cell types; e.g., a complete human cell cycle might last 24 hours.
• External signals (e.g., hormones) and internal checkpoints regulate the cycle's progress.
• Positive regulators (like cyclins) promote progression, while negative regulators (like p53) act to halt the cycle if necessary.
• Errors in the cycle can lead to mutations that result in cancer, highlighting the importance of regulation in cellular proliferation.