Ch 5

• Ex: of “RBC Progenitors” in Zebrafish

  • Spatiotemporal expression of transcription factors in zebrafish hematopoietic stem cell (HSC) development
  • “hematopoietic stem cell”
• Progenitor Cells


  • not capable of unlimited self-renewal; can only divide a few times before differentiating
• Stem Cell Concept

  1. Asymmetrical cell division: A dividing stem cell can produce another stem cell while also producing a cell committed to undergoing differentiation

  2. Population asymmetry- stem cell has the ability to divide symmetrically to produce either two stem cells (thus increasing the stem cell pool by one) or two committed cells (thus decreasing the pool by one). This is called symmetrical renewing or symmetrical differentiating

  3. In many organs, stem cell lineages pass from multipotent stem cells capable of forming numerous cell types, to a committed stem cell that makes one or very few types of cells, to a progenitor cell that can proliferate for multiple rounds of divisions but is transient in its life & is committed to becoming a particular type of differentiated cell
• Potency Defines a Stem Cell

  • totipotent-can produce all cell types
  • pluripotent-can produce all cells of the embryo (but not those that will form the placenta)
  • multipotent- can generate cell types w/ restricted specificity for the tissue in which they reside
• Stem Cell Regulatory Mechanisms

  1. stem cell niche- microenvironment surrounding a stem cell

  2. multiple external & internal molecular mechanisms influence stem cell behavior
• “restricted specificity”
• Establishment of the inner cell mass (ICM), which will become the embryo, in the mouse blastocyst
• ICM cells are pluripotent
• morula cluster of cells formed through cell division very early in the embryonic development that occurs after the formation of a zygote through fertilization but before the blastocyst stage
• epiblast- or primitive ectoderm, is the primitive cell layer from which all three germ layers (ectoderm
• In outer cells, apically positioned partitioning proteins inhibit Hippo Signaling, leading to an active Yap-Taz-Tead transcriptional complex, upregulation of cdx2, and trophectoderm fate
• Activated Hippo signaling

  Represses the Yap-Taz-Tead transcriptional complex, resulting in maintenance of pluripotent ICM development via Oct14
• Symmetrical division


  Distributes cytoplasmic determinants evenly in daughter cells
• Expression of Cdx2-TF in ICM

  Decreases as epiblast differentiates
• TF's important for transient pluripotency of ICM

  • Oct14
  • Nanog
  • Sox2
• Morula is a cluster of cells formed through cell division very early in embryonic development after the formation of a zygote through fertilization but before the blastocyst stage
• Cdx2-TF upregulation in outer cells of morula

  Promotes trophectoderm differentiation and represses epiblast development
• Epiblast or primitive ectoderm


  The primitive cell layer from which all three germ layers (ectoderm, mesoderm, and endoderm) are derived during gastrulation
• Asymmetrical division


  Segregates daughter cells to inside and outside of the embryo
• 1. cadherin activates the Hippo pathway in the ICM
• Cell-to-Cell Interactions


  Foundation for specification of layers
• Regulation of MSC Development

  1. MSCs can be "coaxed" to form different tissues dependent on paracrine factors & cell matrix molecules within the stem cell niche

  2. It is not known whether this range of potency is found in our body
• Most Adult Stem Cells are Restricted to Forming a Few Cell Types
• Regulation of HSCs

  • Residence within endosteal or perivascular niche requires different forms of regulation
  • Within the endosteal niche, HSCs are in intimate contact with osteoblasts
  • Manipulation of osteoblasts can increase or decrease HSCs
  • The niche also contains sinusoidal microvessels
  • The c-Kit receptor is a marker for HSCs & progenitors, which are in direct contact with the sinusoidal microvasculature in the niche
• Stem Cells Fuel Diverse Lineages in Adult Blood

  1. Within your blood, 100 billion cells are replaced daily

  2. All blood cell types are derived from HSCs (RBC’s, immune cells, etc)

  3. The "niche hypothesis" states that stem cells reside in and are controlled by specialized "microenvironments"

  4. Within niches exist differing populations; i.e., more active or quiescent cells
• Pluripotent stem cells from the early embryo


  • ESCs arise from culturing the ICM and require Oct4, Sox2, & Nanog for pluripotency
  • Embryonic germ cells are derived from primordial germ cells that have not yet reached the gonads
• MSC Differentiation

  • It is influenced by the elasticity of the matrices upon which the cells sit
• MSCs

  • Mesenchymal stem cells (MSCs) or bone marrow-derived stem cells (BMDCs) are multipotent stem cells that originate in bone marrow
  • They give rise to numerous bone, cartilage, muscle, & fat lineages
  • They can be found in numerous tissues other than bone marrow such as umbilical cords and baby teeth
• Embryonic germ cells

  Derived from primordial germ cells that have not yet reached the gonads
• How do we use iPSCs to study diseased human tissues?
• Inducing Differentiation from ESCs

  Human ESCs cultured in confined micropatterned discs demonstrate a pattern of differential gene expression similar to that seen in the early embryo
• Protocol for curing a “human” disease in a mouse using iPSs plus recombinant genetics

  1. Generate iPSCs from the mouse

  2. Correct hemoglobin mutation; then differentiate the iSPCs into HSCs

  3. Implantation back into mouse cured the disease
• Organoids mimic embryonic organogenesis
• Major Medical Uses for iPSCs

  • Making patient-specific iPSCs for studying patient pathology
  • Combining gene therapy with patient-specific iPSCs to treat disease
  • Using patient-specific, iPSC-derived progenitor cells in cell transplants without complications of immune rejection
  • Using differentiated cells derived from patient-derived iPSCs for screening drugs
• Physical Constraints of Environment in Which ESCs are Cultured Can Influence Differentiation

  Constraining growth area correlates to early embryo
• Maintaining ESCs in Culture


  ESCs in culture can be coaxed with paracrine & TFs to differentiate into the cell types of each germ layer
• Rudimentary organs can be grown from pluripotent stem cells
• Organoids: Human Organogenesis in a Culture Dish


  Differential gene expression results in cells with different cell adhesion molecules that confer self-organizing properties