Stem Cell Basics Introduction to Embryonic and Adult Stem Cells

Nuclear Transfer Cloning Demonstrates Nuclear Totipotency

A General Definition All stem cells—regardless of their source—have three general properties: –they are capable of dividing and renewing themselves for long periods – they are unspecialized –they can give rise to specialized cell types.

The Process of Gastrulation Defines 3 Germ Layers from Which All Tissues are Derived

Operational Definition of ES Cell The plan of the experiment, spanning almost a decade, is diagrammed, starting at the upper left. Grafted blastocysts form malignant teratocarcinomas in ectopic sites in the host mouse. After dissociation, teratocarcinoma cells microinjected into genetically marked wild-type blastocysts gave rise to offspring that were mosaics with chimeric tissues composed of cells derived from a mixture of both the teratocarcinoma and blastocyst-derived cells

Defining Properties of an Embryonic Stem (ES) Cell Derived from the inner cell mass/epiblast of the blastocyst. Capable of undergoing an unlimited number of symmetrical divisions without differentiating (long-term self-renewal). Exhibit and maintain a stable, full (diploid), normal complement of chromosomes (karyotype). Pluripotent ES cells can give rise to differentiated cell types that are derived from all three primary germ layers of the embryo (endoderm, mesoderm, and ectoderm). Capable of integrating into all fetal tissues during development. (Mouse ES cells maintained in culture for long periods can still generate any tissue when they are reintroduced into an embryo to generate a chimeric animal.) Capable of colonizing the germ line and giving rise to egg or sperm cells. a Clonogenic, that is a single ES cell can give rise to a colony of genetically identical cells, or clones, which have the same properties as the original cell. Expresses the transcription factor Oct-4, which then activates or inhibits a host of target genes and maintains ES cells in a proliferative, non-differentiating state. Can be induced to continue proliferating or to differentiate. Lacks the G1 checkpoint in the cell cycle. ES cells spend most of their time in the S phase of the cell cycle, during which they synthesize DNA. Unlike differentiated somatic cells, ES cells do not require any external stimulus to initiate DNA replication. Do not show X inactivation. In every somatic cell of a female mammal, one of the two X chromosomes becomes permanently inactivated. X inactivation does not occur in undifferentiated ES cells

What Is an Adult Stem Cell? First, they can make identical copies of themselves for long periods of time; this ability to proliferate is referred to as long-term self- renewal. Second, they can give rise to mature cell types that have characteristic morphologies (shapes) and specialized functions. –Typically, stem cells generate an intermediate cell type or types before they achieve their fully differentiated state. The intermediate cell is called a precursor or progenitor cell. Progenitor or precursor cells in fetal or adult tissues are partly differentiated cells that divide and give rise to differentiated cells.

Adult Stem Cells Vs. Progenitor Cells

Stem Cells From One Tissue Can Differentiate Into Another Tissue

What Activates Stem Cells to Differentiate In order to change its fate, a stem cell presumably responds to key migration factors and growth factors. –Factors released by damage may induce stem cells to home to a particular tissue. –Growth and differentiation factors within the tissue determine which genes will be activated. These factors in turn alter the pattern of genes expressed by a stem cell when it resides in a new tissue. –The molecular players involved in migration and differentiation of stem cells are likely to differ depending on the tissue, degree of injury, and stem cells involved. The factors that induce stem cells to migrate may be tissue-specific or generic to injury.

What Maintains Stem Cells in The Undifferentiated State Perhaps like the differentiated state, the stem cell state requires active maintenance and is dependent on the composition of proteins and the balance of those proteins present in that cell at any given time. Given its plasticity, the stem cell state is dynamic since it can respond to a variety of signals that dictate distinct differentiated paths. Accordingly, stem cells may be protected or isolated from cues such as differentiation signals at most times. This precept raises the possibility that adult stem cells are maintained in a nonresponsive state by a constellation of as yet unidentified molecules until needed, for example for tissue maintenance (skin and blood) or in the repair of damage. (A “Stem Cell Niche”)

Two Conceptual Views of Stem Cells

Important Questions to be Answered What are the molecular correlates of the adult stem cell state? Is there a “universal adult stem cell ~ to an ES cell? –Are all tissues capable of generating stem cells which can contribute to other tissues What are the pathways followed from one differentiated state to another (how plastic are adult cells)? What are the “rules” that govern plasticity at the molecular level (e.g. what changes in gene expression are allowed and/or required)?