Basic Biology and classification of Stem Cells
Week-6
What is A Stem Cell?
• A stem cell is a cell from the embryo, fetus, or adult that has, under certain conditions, the ability to reproduce
itself for long periods or, in the case of adult stem cells, throughout the life of the organism (self-renewable)
• It also can give rise to specialized cells that make up the tissues and organs of the body (differentiation)
Unique properties of all stem cells
All stem cells—regardless of their source
—have three general properties:
› they are capable of dividing and renewing themselves for long periods(self-renewal);
› they are unspecialized;
› and they can give rise to specialized cell types.
Stem cells are unspecialized
A stem cell does not have any tissue-specific structures that allow it to perform specialized functions.
A stem cell cannot work with its neighbors to pump blood through the body (like a heart muscle cell);
it cannot carry molecules of oxygen through the bloodstream (like a red blood cell);
and it cannot fire electrochemical signals to other cells (like a nerve cell).
However, unspecialized stem cells can give rise to specialized cells, including heart muscle
cells, blood cells, or nerve cells.
Stem cells can give rise to specialized cells
The signals inside and outside cells that trigger stem cell differentiation are just beginning to be understood.
The internal signals are controlled by a cell's genes,
the external signals for cell differentiation include chemicals secreted by other cells, physical contact with neighboring cells, and certain molecules in the microenvironment.
Basic Concepts
Differentiation of Human Tissues
iPSC
Basic Concepts and Definitions
• Totipotent—Having unlimited capability.
The totipotent cells of the very early embryo have the capacity to
differentiate into extra embryonic
membranes and tissues, the embryo, and all postembryonic tissues and organs.
• All of the cells are generated from a single, totipotent cell—the zygote, or fertilized egg.
Basic Concepts and Definitions
• Pluripotent stem cell —A single stem cell that has the capability of developing cells of all germ layers (endoderm, ectoderm, and mesoderm).
• pluripotent cells have the potential to give rise to any type of cell, a property observed in the natural
course of embryonic development and under certain laboratory conditions.
• The only known sources of human pluripotent stem cells are those isolated and cultured from early human embryos and from fetal tissue that was destined to be part of the gonads.
Basic Concepts and Definitions
• Multipotent- have the capacity to self-renew by dividing and to develop into multiple specialised cell types present in a specific tissue or organ. Most adult stem cells are multipotent stem cells.
• Oligopotent - ability of progenitor cells to differentiate into a few cell types. It is a degree of potency. Examples of oligopotent stem cells are the lymphoid or myeloid stem cells.
• Unipotent—Refers to a cell that can only develop in a specific way to produce a certain end result.
• the cells in question are capable of differentiating along only one lineage
Stem cell division
Symmetric and Asymmetric division
• Stem cells can divide by either asymmetric or symmetric modes of division.
• The balance between these two modes is controlled by developmental and environmental signals to produce appropriate numbers of stem cells
and differentiated daughters.
Stem cells
Differentiated cells Niche
(Morrison 2006)
SYMMETRIC DIVISION
THE EMBRYO STEM CELL DIVIDES TO YIELD
TWO IDENTICAL TOTIPOTENT
DAUGHTER CELLS
ASYMMETRIC DIVISIONTHE TISSUE PROGENITOR
CELL GIVES RISE TO ONE DAUGHTER CELL WHICH REMAINS A PROGENITOR CELL AND ONE DAUGHTER CELL WHICH BEGINS THE
PROCESS OF DIFFERENTIATION LEADING TO TERMINATION
EMBRYONIC STEM CELLS
TISSUE STEM CELLS
LOGARITHMIC EXPANSION
CONSTANT CELL NUMBER
TRANSIT
AMPLIFYING CELLS
Stem cells and symmetric cell division
• Development
• Wound healing and regeneration
• Outcome is “increase in the number of stem cells”
Stem cell plasticity
It has been described as the unexpected potential of cells from bone marrow, muscle or brain to give rise to cells of tissues other than the tissue of origin.
Examples include bone-
marrow-derived cells or even purified hematopoietic stem cells that can give rise to endothelium, skeletal
muscle, cardiac muscle or hepatocytes.
«The capacity of adult bone marrow cells to convert to cells of other tissues,
referred to by many as stem cell plasticity,»
TRENDS in Cell Biology (2002) 12:502
Stem cell plasticity refers to the ability of some stem cells to give rise to cell types, formerly considered outside their normal repertoire of
differentiation for the location where they are found.
Stem cell plasticity of mesenchymal bone marrow cells
• MSCs can be cultured ex vivo for several passages and differentiate at the single-cell level into
mesodermal cell types, including osteoblasts,
chondroblasts, adipocytes, fibroblasts and skeletal myoblasts.
• When introduced in vivo, MSCs differentiate into the same array of cell types.
• Several recent reports have shown that MSCs might acquire characteristics of cells outside the
mesoderm, including endothelium, neuroectoderm and endoderm.
Plasticity of hematopoietic bone marrow cells
• More than 80% of studies reporting adult stem cell plasticity have been performed using bone marrow (BM), or peripheral blood (PB) enriched for HSC.
• Differentiation not only into hematopoietic cells but also cells with characteristics of skeletal muscle, cardiac muscle,
endothelium, neuroectoderm, skin
epithelium and endodermal cells, including hepatocytes, gastrointestinal epithelium and lung epithelium, has been described.
• most studies also did not fulfill the third criterion of stem cells, namely functional differentiation in vivo.
Plasticity of skeletal muscle cells
• It has been shown that the transplantation of muscle cells or muscle side population cells, could give rise to hematopoietic cells following lethal irradiation,
and could compete with BM-derived HSCs, a criterion commonly used to demonstrate the presence of
functional HSCs.
• The same cell that gives rise to hematopoietic cells also gives rise to myoblasts.
Plasticity of neural cells
• Murine and human NSCs cultured ex vivo for several population doublings could
differentiate into hematopoietic cells.
Possible explanations for the plasticity
• Stem cells for a given tissue might exist in an unrelated organ.
• Plasticity might be caused by the transplanted cells fusing with a host cell of a different lineage, leading to transfer of the genetic information of the transplanted cell to the host-derived cell.
• Lineage conversion also could theoretically occur via dedifferentiation of a tissue-specific cell to a more primitive, multipotent cell and subsequent redifferentiation along a new lineage pathway
• Plasticity might occur via de- and re-differentiation, as is seen in cloning or in limb regeneration in amphibians.
• multiple stem cell or progenitor cell populations, including HSC and nonhematopoietic mesenchymal stem cells, endothelial precursors, and/or muscle progenitors, suggesting the possibility that distinct stem or progenitor cells could be contributing, consistent with their intrinsic lineage commitment and developmental potential, to each of the different lineage outcomes observed
• Cells with pluripotent characteristics might persist even after the initial steps of embryological development
Wagers 2004
Niche hypothesis
In 1978, Schofield proposed the “niche”hypothesis to
describe the physiologically limited microenvironment that supports stem cells
Variety of coculture experiments in vitro and by bone marrow transplantation supported niche hypothesis.
Stem Cell Niches
Stem Cell Niche
• Stem cell niche as “a specific location in a tissue
where stem cells can reside for an indefinite period of time and produce progeny cells while self-
renewing”
• The likely existence of microenvironmental factors produced by niche stromal cells has long cautioned that some aspects of stem cell biology may be
difficult to deduce from purified stem cells
The importance of stem cell niches
• The potential of stem cells in regenerative medicine relies upon removing them from their natural habitat, propagating them in culture, and placing them into a foreign tissue environment. To do so, it is essential to understand how stem cells interact with their micro- environment, to establish and
maintain their properties.
• Exactly when and how most somatic stem cell niches develop is still a mystery. And there is considerable variation in niche design.
Stem cells and their niches
Xie 2007
Proposed niche types.
A) Simple niche
A stem cell (red) is associated with a permanent partner cell (green) via an adherens
junction (blue). The stem cells divides asymmetrically to give rise to another stem cell and a differentiating daughter cell (orange).
Simple Niches
• In several tested cases, adherens junctions are involved , while interactions with extracellular matrices are suspected to play a roleBone
morphogenetic protein (BMP) in Drosophila germ- line stem cells (GSCs), directly controls growth and inhibits differentiation
• Wnt signaling may play a similarly direct role in regulating stem cells in the mouse intestinal crypt
• Niches must ensure that daughter cells differentiate appropriately as they leave the niche.
• Seemingly simple niches may exhibit complex temporal behavior. For example, it may be possible to support new stem cells without maintaining any special, pre-existing stromal architecture (‘empty niches’).
• Finally, some tissues may have the capacity to support stem cells without any anatomical specializations beyond a large expanse of basement membrane. The basement membrane of mammalian epidermis or seminiferous tubule may fall in this category.
Proposed niche types.
B) Complex niche
Two (or more) different stem cells (red and pink) are
supported by one or more partner cells (green). Their activity is coordinately
regulated to generate multiple product cells (orange and yellow) by niche regulatory signals.
Complex niches
• Some niches appear to be more complex than the relatively simple ones.
• Subventricular zone (SVZ) neural stem cells closely associate with and sometimes specifically contact other astrocytes, neuroblasts, ependymal cells, endothelial cells and a factor- rich basal lamina.
• Complexity may also arise when multiple stem cells reside within a niche
Proposed niche types.
C)Storage niche
Quiescent stem cells are maintained in a niche until activated by
external signals to divide and migrate (arrows).
Storage Niches
• the ‘storage niche’, may contain quiescent stem cells
• Storage niches may simply be normal niches that are located in favorable, damage-resistant regions or they may contain unique
mechanisms to facilitate the safe maintenance of quiescent cells.
Programming daughter cells
i. Niches with active stem cells must contain routes for progeny cells to exit.
ii. A cell should leave the niche when it reaches a location that cannot itself support a stem cell because one or more critical adhesive or signaling factors is no longer present.
iii. Even before it has done so, the daughter cell may begin to differentiate. Thus, niches are likely to contain specific
structural features and mechanisms designed to ensure appropriate daughter cell movement and to initiate
differentiation.
Niches in different organisms
• Studies regarding stem cells and their location/niche in other genetic model systems, including Drosophila and
Caenorhabditis elegans
provide clues since locating and identifying stem cell
niches in mammals has been extremely difficult due to
their complex anatomic structure.
Cell, 116: 769
Stem cell niches in mammalians
– the epithelial stem cell location in the bulge area of hair follicles,
– and the intestinal stem cell near the crypt base.
– In 2003, osteoblastic cells, primarily those lining the trabecular bone
surface, as the key component of the HSC niche
– In the neural system, the stem cell niche was found in endothelial cells located at the base of the
subventricular zone (SVZ) and
subgranular zone (SGZ) Cell, 116: 769
The Hematopoietic Stem Cell Niche
• Bone marrow serves as the pioneer system for studying stem cell
• However, the way in which HSCs interact with their local environment to promote stem cell maintenance has not been clear
HSC niche.
• The HSC niche is located primarily on the surface of trabecular bone, where a small subset of spindle-shaped N- cadherin-positive osteoblastic cells (indicated as SNO cells) are the key component of the HSC niche.
• N-cadherin and β-catenin form an adherens complex at the interface between stem cells and niche cells, assisting stem cells in attaching to the niche.
• Multiple growth factors and cytokines are involved in stem-niche
interaction. (SCF/Kit, Jagged/Notch, SDF-1/CXCR4, and Ang1/Tie2)
Annu. Rev. Cell Dev. Biol. 2005
Recent developments in HSC niche
• A subset of osteoblastic cells (N-cadherin+CD45−) to which HSCs physically attach in the bone marrow are identified,
• N-cadherin/β-catenin adherens complex between HSCs and osteoblastic cells have been identified,
• Jagged1, generated from osteoblasts, influences HSCs by signaling through the Notch receptor have been shown
• A number of Ncadherin+ osteoblastic lining cells control on the number of HSCs have been demonstrated
• In vitro coculture of HSCs with osteoblasts can expand the HSC population
• depletion of osteoblasts leads to loss of hematopoietic tissue.
• In addition, N-cadherin is a key target of
Angiopoietin-1 (Ang-1)/Tie-2 signaling that maintains HSC quiescence
The niche
that regulates the birth and differentiation of blood-forming HSCs
• Depleting osteoblasts of a
receptor for bone morphogenetic protein (BMP) caused a doubling in both the osteoblast population and the stem-cell population.
• expansion of the stem-cell
population when they increased the numbers of osteoblasts by using parathyroid hormone (PTH).
Nature 425, 836–841 (2003)
HSC niche (unknowns)
• BMP4 is expressed in osteoblastic cells, but which type of receptor is expressed in HSCs is unknown.
• The Wnt signal is important for stem cell self-renewal, but which Wnts are
present in the niche is unknown.
• The same is true for FGF and hedgehog.
In vitro data suggest they affect HSC behavior; however, whether they are present as niche signals is unknown.
• Different types of stromal cells (illustrated as different colors and shapes) may regulate stem cell activation, proliferation, and
differentiation by secreting different microenvironmental signals.
• Finally, maturated blood cells migrate and infiltrate into blood vessel.
Annu. Rev. Cell Dev. Biol. 2005
The Epithelial Stem Cell Niche
• Skin, provides an excellent system for studying the molecular mechanisms that regulate stem cell self- renewal, proliferation, migration, and lineage
commitment
• Each hair follicle is composed of a permanent
portion, which includes sebaceous glands and the underlying bulge area
• The bulge area functions as a niche, where epithelial stem cells (are located and maintained
Epidermal stem cells
• Stem cells are located in the bulge region of the hair follicle beneath the sebaceous gland.
• Upon activation, stem cells undergo division; the daughter cells retained in the bulge remain as stem cells while other
daughter cells migrate down to become hair-matrix progenitors responsible for hair regeneration.
• Stem cells can also migrate
upward and convert to epidermal progenitors that replenish lost or damaged epidermis.
Annu. Rev. Cell Dev. Biol. 2005
The Intestinal Stem Cell Niche
• ISCs are located above the Paneth cells. Telomerase, Tcf4, EphB3, P- PTEN, P-Akt, 14-3-3ζ , Noggin, and Musashi-1—are expressed
• Mesenchymal cells adjacent to the ISCs function as the niche.
• BMP4 expressed from the niche influences the ISCs.
• Wnt signaling is present throughout the crypt
• Noggin is proposed to be a molecular switch coordinating with Wnt
signaling to fully activate stem cells by overriding BMP restriction signaling
• Wnt signaling plays a positive role in promoting ISC activation/self-renewal
• in contrast, BMP signaling restricts ISC activation/self-renewal
• in intestine overriding the restriction of BMP activity by
Noggin as well as by active Wnt signaling are required to fully activate stem cells and support ongoing regeneration
The Neural Stem Cell Niche
• NSCs can be isolated from various regions in the adult brain and peripheral nervous system.
• the subventricular zone (SVZ) and the
subgranular zone (SGZ) of the hippocampus region are the primary and well-characterized regions in which NSCs reside and support
neurogenesis in the adult brain
The subventricular zone (SVZ)
• Astrocytes (B) lining the
ependymal cells (E) function as NSCs; they give rise to transient amplifying cells (C) (green), which further
produce neuroblast cells (A) (blue).
• Endothelial cells in the blood vessel/ laminar maintain
contact with astrocytes, which regulate NSC self-renewal and proliferation by generating different types of signals.
Curr.Opin. Genet. Dev 12:543
The subgranular zone (SGZ)
• Astrocytes (B) directly attach to the blood vessel and receive signals from the
endothelial cells that direct NSCs to undergo self-renewal,
proliferation (D), and differentiation (G).
Curr.Opin. Genet. Dev 12:543
Signals generated from NSC niche
• BMPs and their antagonist Noggin, FGFs, IGF, VEGF,TGFa.
• BMP favors astrocyte lineage fate
• Noggin favors neurogenesis
• Overexpression of beta catenin leads to
expansion of NSC, through the activation of Wnt signalling.
The Germ Line Stem Cell Niche in Mice testes
• The GSCs in mice are single cells that are
located in the periphery of seminiferous tubules and that have the ability to selfrenew and generate a large number of
differentiated gametes
Cross section of mouse testis.
• A germ line stem cell (GSC) (red) directly contacts a Sertoli cell’s (purple) basement membrane (gray) secreted by myoid cells (pink), and specialized region
(green), which together may form a putative GSC niche. Myoid cells (pink) may also participate in
niche function, as they are close to GSCs.
• The differentiated
spermatogonial cells (yellow) are germ-line cysts that move
through different domains
formed by Sertoli cells toward the lumen, where mature sperm are released.
Adult stem cell niche model
Walker 2009
Common Features, Structures, and Functions of the Stem Cell Niche
• The stem cell niche is composed of a group of cells in a special tissue location for the maintenance of stem cells.
• The niche’s overall structure is variable, and different cell types can provide the niche environment.
• The niche functions as a physical anchor for stem cells.
• E-cadherin-mediated cell adhesion events or
integrins, may help anchor stem cells to extracellular matrixes.
Common Features, Structures, and Functions of the Stem Cell Niche
• The niche generates extrinsic factors that control stem cell fate and number.
• Many signal molecules (hh, Wnts, BMPs FGFs, Notch, SCF, Ang-1 , SCF, and LIF) have been shown to be
involved in regulation of stem cell behavior, and among these, the BMP and Wnt signal pathways have emerged as common pathways for controlling stem cell self-renewal and lineage fate from
Drosophila to mammals.
Common Features, Structures, and Functions of the Stem Cell Niche
• The presence of signaling components of multiple conserved developmental regulatory pathways in stem cells supports the ideas that stem cells retain the ability to respond to these embryonic regulatory signals and that orchestration of these signals is
essential for proper regulation of stem cell self- renewal and lineage commitment.
Common Features, Structures, and Functions of the Stem Cell Niche
• The stem cell niche exhibits an asymmetric structure.
Upon division, one daughter cell is maintained in the niche as a stem cell (self-renewal);
• the other daughter cell leaves the niche to proliferate and differentiate, eventually becoming a functionally mature cell.
Challenges in Stem Cell Research
• The difficulties in isolating various types of stem cells, working with the cells in the laboratory, and proving
experimentally that the cells are true stem cells.
• Most of the basic research discoveries on embryonic and adult stem cells come from research using animal models,
particularly mice.
What Kinds of Research Might Be Conducted with Stem Cells?
• Transplantation Research—Restoring Vital Body Functions
• therapeutic potential in;
– Parkinson's disease, diabetes, chronic heart disease, end-stage kidney disease, liver
failure, cord injury, multiple sclerosis, Parkinson's disease, and Alzheimer's disease and cancer.
What Kinds of Research Might Be Conducted with Stem Cells?
• Basic Research Applications
• Embryonic stem cells will undoubtedly be key
research tools for understanding fundamental events in embryonic development and may explain the causes of birth defects and approaches to correct or
prevent them.
• Another important area of research that links developmental biology and stem cell biology is understanding the genes and molecules, such as
growth factors and nutrients, that function during the development of the embryo so that they can be used to grow stem cells in the laboratory and direct their development into specialized cell types.
What Kinds of Research Might Be Conducted with Stem Cells?
• Therapeutic Delivery Systems
• Stem cells are already being explored as a vehicle for delivering genes to specific tissues in the body.
• Stem cell-based therapies are a major area of investigation in cancer research. For many years,
restoration of blood and immune system function has been used as a component in the care of cancer
patients who have been treated with
chemotherapeutic agents. Now, researchers are trying to devise more ways to use specialized cells derived from stem cells to target specific cancerous cells and directly deliver treatments that will destroy or modify them.
Landmarks in the relatively new field of stem cell science
In 1981, two groups described the isolation of
pluripotent ES cells from the inner cell mass (ICM) of murine blastocysts.
In 1994, Sell and Pierce proposed that blocked
differentiation (so-called maturation arrest) underlies cancer stem cell proliferation and tumour growth.
In 1998, Thomson et al described the isolation of
pluripotent ES cells from the ICM of human blastocysts.
In 2006, the Yamanaka group demonstrated that adult somatic cells can be reprogrammed back to
pluripotency by the addition of four factors, generating induced pluripotent stem (iPS) cells.
