Stem Cells (Elias) Flashcards
What is embryonic development?
It is a timed controlled process whereby a single celled, unspecialised zygote divides and selectively activates expression of genes to produce a complex organism composed of many cell types
Most cells in multicellular organisms have the same genome/DNA. Genes must be turned on and off differentially during development
What drives embryonic development?
- Proliferation and growth
- Differentiation
- Morphogenesis
What is proliferation and growth?
Proliferation:
- The zygote undergoes successive cell divisions to produce billions of cells that comprise the adult organisms
Growth:
- Growth of the developing embryo results from an increase in both the number and size of cell
Both are balanced by programmed cell death (apoptosis)
What is a zygote?
Fertilised egg cell that results from the union of a female gamete (egg, or ovum) with a male gamete (sperm)
What are totipotent cells?
A totipotent cell is a single cell that can give rise to a new organism, given appropriate maternal support
A totipotent cell is one that can give rise to all extra-embryonic tissues, plus all tissues of the body and the germline
What happens after the zygote is formed?
Once the zygote is formed, it begins mitotic divisions to produce more cells, which are totipotent
As the cells divide, their developmental potential decreases and their cell fate becomes determined
What is commitment?
After differentiation, commitment occurs
Commitment is the process whereby a cell becomes firmly committed to just one of the several developmental pathways that are open to it before expressing the phenotype of the differentiated cell type
It is the commitment of a cell to a certain fate
A cells developmental fate has become restricted
Occurs in 2 stages:
- Specification
- Determination
What is specification?
Specification is where a cell is capable of differentiating autonomously when placed in a developmentally neutral environment (culture dish)
What is determination?
Determination is where a cell is capable of differentiating autonomously even when placed in a non-neutral environment or moved to another region of the embryo
What are the 3 strategies of specification?
1) Autonomous (Mosaic) specification
- Cells develop only according to early fate
- Characteristic of most invertebrates
2) Conditional (Regulative) specification
- Cell fate depends on context
- Characteristic of vertebrates (and some invertebrates)
3) Syncytial Specification
- Cell fate depends on exposure to cytoplasmic determination in a syncytium
- Characteristic of most insects
Overview of autonomous specification
Characteristic of most invertebrates.
The cell “knows” very early what it is to become without interacting with other cells.
Cell fate is determined by the specific cytoplasmic morphogenic determinants (proteins and RNA) apportioned to each cell as the fertilized egg divides.
If cleavage patterns are invariant, then cell fates will be invariant. Blastomere fates are generally invariant.
Gives rise to mosaic development. Cells cannot change fate if a blastomere is lost.
What are blastomeres?
In biology, a blastomere is a type of cell produced by cell division (cleavage) of the zygote after fertilisation
Blastomeres are committed at a very early stage in mosaic development
If split, each dissociated blastomere pair forms original structures
Each blastomere contains positional information in the form of specific proteins and genes
Overview of conditional specification
Characteristic of all vertebrates and few invertebrates
Specification by interactions between cells. Relative positions are important
Variable cleavages produce no invariant fate assignments to cells
Massive cell rearrangements and migrations precede or accompany specification
Capacity for “regulative” development: allows cells to acquire different functions
Mechanism of conditional specification
Cell fate depends on interactions with neighbouring cells:
- Cell-to-cell contacts
- Secreted signals (paracrine factors)
- Physical properties of the microenvironment (mechanical factors)
Embryonic cells can change fates to compensate for missing parts = Regulation
Conditional specification produces Regulative Development
What is syncytium?
It is nuclear division without cell division; results in cytoplasm with many nuclei
A cell with at least 2 nuclei
This embryo is called syncytial blastoderm
Overview of syncytial specification
Characteristic of most insect classes
Begins before fertilization. Maternal messages are key
Specification of body regions by interactions between cytoplasmic regions prior to cellularization of the blastoderm
Variable cleavage produces no rigid cell fates for particular nuclei
After cellularisation, both autonomous and conditional specification are seen
What is superficial cleavage?
Nuclear division without cell division
Cells form later from invaginating membrane of egg
Nuclei line up at membrane
What is the importance of morphogen gradients in drosophila melanogaster?
Has 2 maternal messages:
- Bicoid - anterior
- Nanos - posterior
Bicoid and Nanos proteins are morphogens
Each morphogen establishes a gradient throughout the embryo (like a diffusion gradient)
Bicoid:Nanos ratio determines anterior-posterior identity
Cells identity depends on their position in multiple gradients
What is a stem cell?
An undifferentiated cell of a multicellular organism which is capable of giving rise to indefinitely more cells of the same type, and from which certain other kinds of cell arise by differentiation
A cell is classified as a stem cell when it satisfies three criteria:
- Undifferentiated or unspecified
- Have the ability to self renew
- Mature and differentiate
What are the three types of stem cell division modes?
Asymmetric self-renewing division
Symmetric differentiating division
Symmetric self-renewing division
What is the defining characteristic of totipotency?
The ability of a single cell, such as the zygote or the 4 to 8 cell embryo, to generate both the embryo itself and the extraembryonic tissues, which support the embryo’s development.
Which cells possess pluripotency, and what can they differentiate into?
Inner cell mass (ICM) cells or embryonic stem cells possess pluripotency
They can differentiate into cells representing all three germ layers (ectoderm, mesoderm, and endoderm), giving rise to various cell types in the embryo proper
What is the defining characteristic of multipotency?
The capacity of adult or somatic (resident) stem cells to differentiate into a limited range of cell types, typically within a specific tissue or organ
They play essential roles in organogenesis during embryo development and tissue regeneration in the adult organism
What is the inner cell mass (ICM)?
The inner cell mass (ICM) is a cluster of cells found within the blastocyst during early embryonic development
It is located inside the blastocyst, surrounded by the trophectoderm
Cells within the ICM are pluripotent
The ICM gives rise to the fetus, contributing to the formation of all three germ layers
How is the inner cell mass (ICM) established?
Asymmetric cell divisions represent the key initiating event
Asymmetrical division perpendicular to apicobasal axis occurs in the trophectoderm cells
Unequal segregation of the asymmetrical division of trophectoderm cells results in non-identical daughter cells and therefore a pluripotent stem cell is formed
How is pluripotency maintained in ICM cells?
Controlled using pluripotency factors/genes:
- Nanog
- Sox2
- Oct4
Oct4, Sox2 and Nanog drive a pluripotency gene expression network to maintain ICM
Oct4, Sox2 and Nanog must be differentially repressed during development to allow ICM cells to give the epiblast
How is Hippo signaling regulated?
Hippo signaling is regulated by cell density and cell-cell adhesion
It plays a crucial role in controlling cell fate and tissue growth
How does apical polarity contribute to maintaining cell fate in trophectoderm cells?
Apical polarity in trophectoderm cells leads to the apical localisation of proteins like partitioning defective (PAR) and atypical protein kinase C (aPKC),
These proteins recruit and inhibit Angiomotin (AMOT)
Inhibition of AMOT leads to the activation of Yap/Taz-TEAD-mediated gene expression of Cdx2, promoting trophectoderm fate
What role does Hippo signaling play in the maintenance of the inner cell mass (ICM)?
Hippo signaling is activated in the ICM through cell-to-cell-mediated interactions
This activation leads to the phosphorylation of Angiomotin (AMOT), which then interacts with the E-cadherin-Catenin adherens junction complex
The Hippo signaling kinase Lats1/2 is recruited and activated, resulting in the repression of Yap-Taz-Tead transcriptional complex
This repression leads to the inhibition of Cdx2 and the activation of Oct4, promoting pluripotency in the ICM.
How are human embryo stem (hES) cells isolated?
hES cells are derived from embryos that develop from eggs that have been fertilized in vitro
hES cells are never derived from eggs fertilized inside of a woman’s body
Protocols for hES cell culture were optimized from mouse ES cells
Pluripotent (endoderm, mesoderm and ectoderm)
mES cells versus hES cells?
- mES cells are the most immature, undifferentiated with greatest potential for pluripotency
- mES cells are naive
- hES cells display some maturation towards the epiblast lineage
- hES cells are primed or ready for differentiation
What are the therapeutic limitations of hES cells?
Difficult to differentiate uniformly and homogeneously into a target tissue
Immunogenic – embryonic stem cells from a random embryo donor are likely to be rejected after transplantation
Tumorigenic – capable of forming tumors or promoting tumor formation
Degenerative diseases are complex genetic disorders involving interactions of many genes with environmental factors
Morally objectionable, because the human embryo (life) must be destroyed in order to harvest its stem cells
Can pluripotency be induced? What did Gurdon do?
Yes it can
He did an experiment on somatic nuclear transfer
- Elimination of the nucleus of a frog egg
- Replaced it with nucleus from a skin cell taken from a tadpole
- The modified egg developed into a normal tadpole
- Subsequent nuclear transfer experiments have generated cloned mammals
What are the similarities of ES cells and induced pluripotent stem (iPS) cells?
Like ES cells, iPS cells can be propagated indefinitely
Like ES cells, iPS cells can form cell types representative of all three germ layers
Like ES cells, iPS cells can generate entire embryos
Like ES cells, iPS cells are pluripotent
What is microcephaly?
It is a congenital disease characterised by a significant reduction in brain size
Patient-derived cerebral organoids are smaller
Caused by a mutation in the gene for CDK5RAP2, a protein regulating the mitotic spindle function
Neural stem cells exhibit abnormally low levels of symmetric divisions
Leads to premature neuronal differentiation
Results in depletion of the stem cell pool
What cells can be used to cure sickle cell anemia?
iPS cells can be used to treat sickle cell anemia
Method:
Generation of autologous iPS cells
Correction of the hemoglobin mutation
Differentiation of the iPS cells into hematopoietic stem cells (HSCs)
Transplantation of HSCs in the mouse cured its sickle-cell phenotype
What are the four major medical uses for iPS cells?
- Making patient-specific iPS cells for modelling diseases (e.g. autism, Dawn syndrome, diabetes…)
- Combining gene therapy with patient-specific iPS cells to treat diseases
- Using patient-specific iPS cell-derived progenitor cells in transplantation medicine without the complication of immune rejection
- Using differentiated cells derived from patient-derived iPS cells for screening drugs and toxicity testing
What are adult stem cells?
Adult stem cells are undifferentiated cells.
They are found in small numbers in most adult tissues.
They are finite may not live as long as ES or iPS cells in culture.
They are also called “somatic stem cells”
They are multipotent in nature and give rise to unipotent progenitor cells.
They give rise to a closely related family of cells within the
tissue.
What are adult hematopoietic cells?
Hematopoietic stem cells (HSCs) are multipotent and give rise to all the various cells in the blood
What is the otogeny of hematopoietic stem cells?
In embryonic development, hematopoiesis (the formation of blood cells) occurs in two sequential phases: primitive and definitive
- Primitive hematopoiesis in the embryonic yolk sac
- Definitive hematopoiesis in the aortic portion of the aorta-gonad-mesonephros (AGM)
HSCs migrate through the developed vasculature to the fetal liver
Homing where HSCs migrate through the circulatory system and find their tissue-specific niche in the developing bones
HSCs express CXCL4 receptor, which senses the chemokine CXCL12 expressed by osteoblasts and stromal cells, guiding it to its destination
Adhesion proteins (e.g. E-selectins and VCAM1
(Vascular Cell Adhesion Molecule 1)), also support HSC homing to the niche
The endosteal hematopoietic niche (long term HSCs)
HSCs in the endosteal niche that are adhered to osteoblasts are long-term HSCs
Long-term HSCs are typically quiescent. They sustain long-term hematopoiesis
Quiescence is maintained by Angiopoietin-1 and Thrombopoietin, secreted by osteoblasts
The perivascular hematopoietic niche (short term HSCs)
In the perivascular niche, short-term active
HSCs can be seen associated with blood vessels at oxygen-rich pores
Short-term HSCs interact directly with stromal cells including the CAR cells (yellow;
CXCL12-abundant reticular cells) and mesenchymal stem cells
Short-term HSCs and their derived progenitor cells are mobile and migrate into the blood stream, which can be stimulated by sympathetic connections
