Cell Specialization & Stem Cells

• Cell Specialisation


  Cell specialisation is a fundamental process in the development of multicellular organisms.
• From Unspecialized to Specialized Cells

  After fertilization, a single-celled zygote undergoes rapid cell division to form a ball of unspecialized cells. These cells then differentiate into specialized cells with specific functions. This process is called differentiation.
• The differentiation process is largely controlled by gene expression, which is influenced by chemical gradients within the developing embryo. These gradients act like a coordinate system, telling cells where they are and what they should become.
• In a developing fruit fly embryo, the gradient of the protein Bicoid determines which end will become the head. Higher concentrations of Bicoid activate genes for head development, while lower concentrations allow tail development.
• The "stemness" of a cell can be thought of as a spectrum:

  • Totipotent stem cells (e.g., zygote) → can form all cell types
  • Pluripotent stem cells (e.g., embryonic stem cells) → can form most cell types
  • Multipotent stem cells (e.g., hematopoietic stem cells) → can form multiple cell types within a lineage
  • Unipotent stem cells (e.g., skin stem cells) → can form only one cell type
• Properties of Stem Cells

  Stem cells are unspecialized cells with two key properties:

  1. Self-renewal: They can divide and produce more stem cells indefinitely.
  2. Potency: They can differentiate into various cell types.
• The ability of stem cells to divide endlessly is due to the enzyme telomerase, which prevents the shortening of telomeres during cell division.
• Stem Cell Niches in Adult Humans

  Stem cell niches are specialized microenvironments that maintain and regulate stem cells in adult tissues. These niches play a crucial role in tissue homeostasis and repair.
• Bone Marrow Niche


  The bone marrow niche supports hematopoietic stem cells (HSCs), which give rise to all blood cell types.

  Function:

  • Maintains HSCs in a quiescent state
  • Promotes HSC self-renewal when needed
  • Regulates HSC differentiation into various blood cell lineages
• Hair Follicle Niche


  The hair follicle niche contains epithelial stem cells responsible for hair growth and skin regeneration.

  Function:

  • Maintains hair follicle stem cells during the resting phase
  • Activates stem cells to initiate hair growth
  • Regulates the balance between stem cell maintenance and differentiation
• Types of Stem Cells: Totipotent, Pluripotent, and Multipotent

  Stem cells can be classified based on their potency, or their ability to differentiate into different cell types:

  1. Totipotent Stem Cells
  2. Pluripotent Stem Cells
  3. Multipotent Stem Cells
• Totipotent Stem Cells


  • Can form ALL cell types, including embryonic and extraembryonic tissues
  • Example: Zygote and early blastomeres (up to 4-cell stage)
• Pluripotent Stem Cells


  • Can form ALL cell types of the embryo proper, but not extraembryonic tissues
  • Example: Inner cell mass of blastocyst, embryonic stem cells
• Multipotent Stem Cells


  • Can form MULTIPLE cell types, but within a specific lineage
  • Example: Hematopoietic stem cells in bone marrow
• A totipotent cell can form a complete organism. A pluripotent cell can form any cell in the body but not a complete organism. A multipotent cell, like a hematopoietic stem cell, can form all types of blood cells but not other tissue types.
• In early-stage animal embryos, cells start as totipotent but quickly become pluripotent as the embryo develops. Adult tissues generally contain multipotent or more restricted stem cells.