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 are the three types of stem cell division modes?
Asymmetric self-renewing division
Symmetric differentiating division
Symmetric self-renewing division
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 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.
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 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 adult hematopoietic cells?
Hematopoietic stem cells (HSCs) are multipotent and give rise to all the various cells in the blood
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
