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Stem Cells and Tissue Dynamics

Overview of Skin and Tissue Types

  • Skin is an organ composed of two main tissue types:
    • Epithelial Cells (Epidermis)
    • Connective Tissue (Dermis and Hypodermis)
    • Mechanical Strength: Provided by connective tissue, created by fibroblasts.
    • Blood Vessels: Lined with epithelial cells to ensure nutrient and oxygen supply while removing waste.
    • Neuronal Innervation: Neurons with glial cells present within the skin.

Immune and Maintenance Cells in Skin

  • Macrophages: Remove dead/damaged cells and debris.
  • Lymphocytes/White Blood Cells: Combat infections.
  • Cell Varieties: Tissues require diverse cell types that regenerate continuously.

Key Factors in Tissue Organization

  1. Cell Communication:
    • Cells communicate to adjust behavior in response to environmental stimuli.
  2. Selective Cell Adhesion:
    • Utilizes Cadherins and other adhesion molecules to ensure the correct arrangement of cells, preventing chaotic mixing.
  3. Cell Memory (Epigenetics):
    • Cells inherit gene expression patterns; daughter cells maintain the type of the parent cells (e.g., epithelial cells produce other epithelial cells).

Cell Turnover and Replacement Rates

  • Different Cell Replacement Rates:
    • Nerve Cells: Generally not replaced.
    • Bone Cells: Replaced every 10 years.
    • Red Blood Cells: Renewed approximately every 120 days.
    • Intestinal Epithelial Cells: Turnover occurs every 3 to 6 days.

Stem Cells and Their Role

  • Stem Cells:
    • Self-renewing; divide to produce both a stem cell and a precursor cell.
    • Precursor cells undergo several divisions before differentiating into terminally differentiated cells.
    • Replacement varies by tissue type.

Intestinal Stem Cells

  • Located at the crypts of the intestine.
  • Villi: Finger-like projections that increase nutrient absorption.
  • Epithelial cells in the intestine comprise:
    • Border Brush Cells: Absorptive cells.
    • Goblet Cells: Mucus-secreting cells.
  • Old cells pushed upwards and sloughed off the lining.

Skin Cells

  • Stem cells located in the basal layer of the skin.
  • Precursor cells stop dividing but differentiate into various skin types as they rise through the skin layers.
  • Dead skin cells (keratinized) are eventually shed.

Stem Cells in Medical Applications

  • Cancer Treatment:
    • Leukemia involves the destruction of blood-forming tissue.
    • Bone Marrow Transplants: Donors provide stem cells after the recipient's have been destroyed by radiation.
  • Pluripotent Stem Cells:
    • Can differentiate into any tissue type; harvested from embryos.
    • Potential to treat diseases like Alzheimer's, cirrhosis, heart issues, diabetes, etc.
  • Nuclear Transplantation:
    • A diploid nucleus is implanted into an enucleated egg cell to create a genetically matching embryo.
  • Induced Pluripotent Stem Cells (iPSCs):
    • Created from adult cells using transcription factors (OCT3/4, SOX2, KLF4).
    • Can lead to cell types necessary for treating various diseases.

Tumor Development

  • Cancer arises through repeated mutations and proliferations of cells that can survive and replicate better.
  • Tumor cells exceed normal replication protocols and may spread to other body parts (metastasize).
  • Types of Tumors:
    • Benign: Non-invasive, removable.
    • Malignant: Can invade other areas.
  • Tumors often consist of cells with various mutations indicative of their rapid growth patterns.

Conclusion

  • Understanding stem cells and their replacement mechanisms is vital in tissue maintenance and in developing treatments for various conditions related to cellular damage and tumor formation.