Developmental Exam One Flashcards
Embryonic development
- Egg and sperm undergo fertilization to make zygote
- Zygote undergoes cleavage/mitotic cell division to become a blastocyst/blastula
- Blastula undergoes gastrulation to become a gastrula
- Organogenesis
- Gamete formation from mature being
How does an egg become a functional adult?
Differentiation, pattern formation, morphogenesis, growth and reproduction
Differentiation
Process of unspecialized cells becoming specialized
Morphogenesis
Organization of cells into functional structures via cell division, cell migration, apoptosis, composition changes, growth via cell division and shape changes
growth
cell division
reproduction
gamete formation and fertilization
What are the embryonic germ layers
They are the layers of cell organization which make cell tissues
They are the endoderm, mesoderm and ectoderm
Endoderm
Inner layer
Tissue or organ linings
Mesoderm
Middle layer
Make muscle, bone connective tissue and red blood cells
Ectoderm
Outer most layer
Makes skin and the nervous system
Dr. Kniss’s favorite
Phylotypic stage
Stage of development where related organisms look the most similar
Ernst Haeckel’s drawings
Made drawings of embryos of many different species to show they were similar
Later found out he fudged most of the drawings to be similar to prove his hypothesis
Fate map
diagram that shows where adult structure are derived in the early embryo
Can tag a group of cells and let the cells grow to see where the cells end up in the organism
Explain his lineage example with chimeras
Took a quail embryo which has a different embryo than a chick embryo since they look different
Quail specific proteins can be labeled with anti-bodies
A piece of the Quail embryo is taken out and donated to the chick embryo to see where it is located in the embryo.
More effective than tagging the cells with a GFP.
Cell specification
How cells become committed to a specific fate
Differentiation
Generation of specialized cell types
Steps of specifying identity
- Cell specification- how cells become committed to a role
- Determination- can differentiate autonomously
- Differentiation
How do cells become commited?
- Autonomous specification- received a determinant from mother cell. Determinant can be protein or mRNA
- Conditional specification- The conditions it has found itself in
How do cells become committed part two?
Cell signaling!
Induction!
Induction
Close range influence of one cell population on another
Inducers
Cause induction on other cells. Can be tissue or cells to cause induction
Responder
Receives signal. Is tissue/cells being induced
Competence
Ability to respond to an inductive signal
Instructive induction
A signal from an inducing cell is necessary to initiate a change in a responding cell
Epithelium
Tightly linked, sheet or tube of cells
Talks to mesenchyme
Mesenchyme
Loosely packed, unconnected cells; separated by extracellular matrix (ECM)
Talks to epithelium
Regional specificity of induction
When a specified cell can be added to another cell that is the inducer which changes the specificity of the responder cell.
Ex. Mesenchyme instructed epidermal epithelium as talked about in class
Permissive interaction/induction
When responding cells have been specified but need the correct environment to differentiate
Ex. Rat heart
1. Only ECM and connective tissue (shell of the heart) to begin. Trying to make conditions to make heart cells.
2. Pumped undetermined cardiac cells which developed into beating heart.
How are signals passed between inducer and responder? How do we get signal transduction?
Paracrine Interaction, Juxtracrine Interaction.
Paracrine Interaction
Passes molecule to another cell to relay a signal.
Juxtracrine Interaction
When cell communicate by physical contact with one another
Paracrine factors
What is being passed between cells.
ex. morphogen
Diffusible molecules that can determine the fate of a cell based on the concentration of the morphogen. Can measure this on a morphogen gradient.
ex. Higher level of morphogen might activate different genes than a lower level of morphogen.
Steps of signal transduction
- Reception- extracellular signals are received at the membrane/across the membrane
- Transduction- Information is relayed (transduced) within the cell.
Ex. Signal transduction cascade - Response- Change in gene expression, change in cell shape/structure, more signaling…
Major families of paracrine factors
FGF-fibroblast growth factor, Hedgehog, Wnt, TGF-beta superfamily.
RTK
Receptor Tyrosine Kinase
Receptor protein located in the cell membrane
Forms “dimers” when bound by ligand via paracrine signal.
General steps of RTK pathway
- Paracrine ligand binds extracellular part of RTK receptor.
- Binding and dimerization induces shape change in receptors which activates intracellular tyrosine-kinase.
- Tyrosine kinases phosphorylate tyrosine’s on their receptor partner.
aka. autophosphorylation/cross phosphorylation - a. Activated receptors could phosphorylate other proteins. OR b. Adaptor protein might be able to bind the activated receptors.
Two types of Wnt Signal Transduction
Canonical, noncanonical
Canonical Wnt Signal Transduction
General/most common
Receptor- Transmembrane protein (e.g. frizzle, LRPs)
Ligand- Wnt family member
Output- beta-cadherin transcription factor
Goal- Allow beta-cadherin to enter nucleus and promote expression of wnt target genes
What would happen if there is NO wnt signal for canonical wnt signal transduction?
A degradation complex forms in the cytoplasm: beta-cadherin, APC, GSK-3 and Axin.
GSK-3 phosphorylates beta-cadherin which leads to beta-cadherin degradation via proteosome.
Therefore there would be no expression of wnt target genes.
What happens if there is a wnt signal for cononical wnt signal transduction
Wnt binds receptor(s) and forms complex which recruits/binds GSK-3, Axin and other components.
This prevents GSK-3 from phosphorylating beta-cadherin which prevents beta-cadherin degradation.
Therefore wnt target gene expression os promoted.
Examples of Juxtacrine signaling
Cell adhesion molecules (cadherins)
Ephrins ligands
Notch proteins
Eph receptors
Mechanism of notch activity
Inactive state: no receptor-ligand binding
a repressor (txn’1 repressor) is bound to promoter of Notch target genes
Active state: receptor-ligand binding. Shape change of notch receptor (Notch Intracellular Domain (NICD)).
The NICD can then be cleaved by a protease and the cleaved NICD enters the nucleus.
Repressor is displaced and txn’1 is activated which causes expression of the Notch target gene.
How do cells undergo morphogenesis
cell division, migration, programmed cell death, growth, shape changes and cell composition changes
morphogenesis
organization of cells into functional structures via coordinated cell behavior
How was it determined cells exhibit a cell sorting ability in development?
In 1955 Townes and Holtfreter placed embryonic tissues in an alkaline solution which dissociated the tissues into single cells. They then mixed different cell types together such as the mesoderm, ectoderm, endoderm. They found each type sorted itself out into their own region. The epidermal cells moved to the periphery and the mesodermal cells stayed inside the cell.
Did another experiment with ectodermal lineages of epidermis and neural plate cells. The epidermis did the same thing as the other experiment. The neural plate cells formed a neural tube like structure.
Therefore selective affinities change during development. For cell division to occur, cells must interact differently with other cell populations at specific times.
How do cells sort themselves?
What forces direct cell movement during morphogenesis?
1964, Steinburg
His hypothesis was that cells sort based on differences in adhesion. Weakly adhering cells will move to the outside than strongly adhering cells.
Combined neural retina cells and heart cells in culture dish and waited.
Showed certain cell types migrate centrally when mixed with some cell types but then go peripherally when combined with others.
Cells with greater cohesion segregate inside of those with less cohesion.
Cells interact so as to form an aggregate with the smallest interfacial free energy. Cells rearrange themselves in the most thermodynamically stable pattern.
Therefore the early embryo can be viewed as existing in an equilibrium state until there is change in the adhesive properties of the cell membranes.
The thermodynamics differences could be caused by different types of adhesion molecules
Steinberg’s differential adhesion hypothesis
Cells rearranged to achieve/maximize adhesive interactions which require differential adhesion and cell motility
Trypsin
An enzyme most commonly used to cleave cell surface protein connections that result in dissociated cells in culture
Selective affinity hypothesis
Cells have a positive affinity for some cells and a negative affinity for other cells. Selective affinities must change during development.
Differential adhesion hypothesis
Cells rearrange to maximize adhesive interactions. Requires differential cell adhesion and cell motility.
Types of cell adhesion molecules
Calcium independent: Integrins, Amino-globulin superfamily (IgCAMs)
Calcium dependent: Cadherins, Selectins
Homophilic adhesion
Between like molecules
Heterophilic adhesion
between different molecules
Cadherin
Dependent adhesion molecules
transmembrane proteins
bind to other cadherins
can interact with other molecules like adhesion junctions
Plays a role in neural tube formation.
Anchored to cells by catenins
Types of Cadherins
E-cadherin, N-cadherin, P-cadherin, R-cadherin and Protocaherins
E-cadherins
epithelial; all early mammalian cells
Mediates zebrafish epiboly (epithelial cell spreading)
Expressed more highly in cells of outer layer during epiboly
N-cadherins
Nervous system
P-cadherins
Placenta (vagina cadherin)
Helps placenta stick to uterus
R-cadherins
Retina
Protocadherins
Do not attach to cytoskeleton
Highly expressed in the nervous system
Play large role in dendrite development and neural circuit formation
Neural circut
a population of neurons interconnected by synapses to carry out a specific function when activated.
Epiboly
Sliding, moving, spreading during gastrulation
To enclose deeper cell layers
Thinning of epiblast over time by radical intercalation
Radical intercalation
Cells exchange places throughout the thickness of a multilayered tissue, can drive tissue spreading, or epiboly. Radial intercalation occurs during frog and fish gastrulation, as well as during thinning of the ventral mesoderm in Drosophila and development of skin in frogs
a process that occurs when cells in a multilayered tissue exchange places, which can cause the tissue to thin and spread.
Caused by chemotaxis
Can be caused by tissue stress or cell protrusive activity
Intercalation can be a powerful way to expand a tissue sheet. When cells move between each other, the tissue’s shape changes, creating a longer but thinner array of cells.
Chemotaxis
When cells move towards the tissue’s external layer. The chemoattractant in this case is the complement component C3a, which is usually associated with the immune system.
Cell protrusion
Cell protrusions are outward extensions of the plasma membrane of individual cells that function in sensing the cell environment and in making initial, dynamic adhesions to extracellular matrix and other cells.
Neural tube formation related to cadherins
Epidermal and neural tissues separate during neural tube formation
In the neural plate, all cells express e-cadherin
Neural tube precursors begin to express n-cadherin which causes the layers to separate
In an n-cadherin mutant, neural tube cannot separate from the epidermis
Epithelial-Mesenchymal Transition (EMT)
When an epithelial cell becomes a mesenchyme cell
Signal from paracrine factors breaks cell adhesion of epithelial cell and the basement membrane is dissolved
The cell is released from the basement membrane and is now a mesenchyme cell
EMT during deleopment
During neural crest formation, neural tube cells leave to be other cells in different areas.
During mesoderm formation EMT happens in which new mesenchyme cells from the epithelial layer, create the mesoderm
Catenins
complex of proteins which anchor cadherins inside the cell
Cadherin-catenin complex
Forms adherence junctions that help hold epithelial cells together
How to block cadherin function?
Using antibodies
They will bind and inactivate cadherin. Cadherin synthesis will be blocked
How to block cadherin synthesis?
Antisense RNA. Will bind to cadherin messages and prevents their translation.
This can prevent the formation of epithelial tissues and cause the cells to disintegrate
Functions of caherin?
Adhere cells together, help assemble actin cytoskeleton, initiate and transduce signals that can lead to changes in a cells gene expression
What do E-cadherin mutants cause during development?
Their epiboly fails to be completed
How was it determined how cells sort themselves based on the amount of cadherin on their surface?
Mixed cells with different levels of P-cadherin expression
Was found the cells with more P-cadherin and surface cohesion migrated internally compared to the cells which expressed less P-cadherin
Therefore cadherin-dependent sorting is directly correlation with surface tension
The surface tensions of cells are linearly related to the amount of cadherin expressed on the surface of the cells
Heterotypic aggregates
The relative amounts of different cadherin types still predict cell sorting in vitro
What happens when an ephrin on one cell binds with an Eph receptor on another cell?
Signals are sent to each of the cells
Function in the formation of blood vessels, neurons, and somites
Ephrins
Are seen when cells are being told to migrate or where boundaries are forming
Types of differentiation potential (potency)
Totipotent, pluripotent and multipotent
Totipotent
Capable of all
cell with potential to become any/all cell types
Pluripotent
Cells capable of becoming all cells except extra-embryonic tissue
all cell types of the embryo
Multipotent
Limited to be a subset of cell types
specific cell types of a tissue
How to test for totipotency?
Enucleate a host egg
Isolate donor nucleus and transfer it to a host egg
Called reproductive cloning
Reproductive cloning
Want to clone donor nucleus
Briggs and King 1952
Did this on bull frogs
Took a cell and removed the nucleus, then they added a donor nucleus and a full bullfrog was made
They concluded the blastula stage nucleus can direct development to tadpole stage
Complete embryo was made
How affects cloning sucess?
Age of donor nucleus because cell fates become restrictive
Older nuclei are less able to direct proper development
John Gurdon 1960s
Donor nucleus from tailbud stage intestinal cells was able to complete the development of an enucleated egg into a cloned adult frog
1.5% success rate
The donor eggs came from non-albino frogs
The parents of nucleus donor are albino
Why did Gurdon get criticism?
Intestine cells are a mix of differentiated and partially differentiated and un-differentiated cells
Primordial germ cells (future gamete) migrate through/near the tadpole intestines
John Gurdon 1975
Donor: adult foot webbing. Differentiated epithelial cells
1st try: clones survived until gastrulation
2nd try: donor cells from the footed gastrula clone.
Subsequent rounds: continued “serial” transplantation
Dolly the sheep 1997
variation of methods by Briggs/Kings/Gurdon
Stomatic cell nuclear transplant
1/434
Embryo transferred to surrogate mother (Scottish blackface sheep) after the mothers nucleus was taken out and replaced with a Finn-Dorset lamb nucleus from udder cells.
Dolly was a Finn-Dorset lamb genetically identical to nuclear donor
Therefore the nuclei of adult somatic cells in vertebrates contain all the genes needed to generate an adult organism
Who won the nobel prize in physiology/medicine in 2012?
Gurdon and Yamanaka for the discovery that mature cell can be reprogrammed to become pluripotent
All genes present in differentiated cells
Genes can be restored to a pluripotent state
Egg cytoplasm can regulate gene expression?
How are genes regulated?
transcriptionally and translationally
Transcriptional regulation
promoters, enhancers, etc…
chromatin state
alternative splicing
Translational regulation
“masking”- when cells make RNA and store it away so they can mask it when they want to
mRNA stability
mRNA localization
Things that are on a gene
enhancer sequences, promoter, transcriptional start site, gene A.
Role of transcriptional activator
Binds to enhancer site to make the process more efficient.
Binds to RNA pol II and helps with initiating transcription. Interacts with specific subunits of RNA pol II
There are tissue specific enhancers
DNA sequences that regulate gene expression in a tissue-specific manner
Abnormalities can lead to cancer
Closed vs. open chromatin
Condensed chromatin: methyl groups are on histone tails. Less access to gene regulatory sequences
“open” chromatin: acetyl group on histone tails. Greater access to regulatory sequences
nucleosome
8 histones
DNA methylation
the “5th base”
prevents transcription factor access
Adds a methyl group which prevents the binding of transcription factors
makes the gene inactive when “5th base” is present
repression of transcription
heterochromatin
tightly packed chromatin
euchromatin
loosely packed chromatin
exons
part of gene which codes protein
introns
part of gene that has nothing to do with the amino acid sequence of the protein
Alternative pre-RNA splicing
mechanism which allows a large variety of proteins to come out of one gene
Mediated by spliceosomes which bind to splice sites or place adjacent to them
A sequence that is an exon in one cell could be an intron to another
What defines a stem cell?
unspecified (un-differentiated)
capable of differentiating
Capable of self renewal (via mitosis)
Regulated by their environment (their “niche”)
Inner cell mass can induce ___________.
Teratomas which is a tumor with cell types of all three germ layers
hematopoiesis
process of creating blood cells
Stem cell progression
totipotent: zygote
pluripotent: cells of ICM (inner cell mass)
multipotent: Tissue specific stem cells
progenitor
precursors
differentiated cells
Two types of stem cell divisions
self-renewal via symmetric cell division
or
differentiation via cell division
Stem cell asymmetry
allows sc’s to self renew while also producing cells committed to differentiate
Example of asymmetric division
NB divide asymmetrically and in doing so they renew themselves and generate a progenerator cell called the GMC that will make neurons or glia cells
How does asymmetric division work?
When neuroblasts divide more proteins are transferred to the GMC while others stay in the neuroblasts
The “basal” proteins ae self fate determinants that promote differentiation
Stem cell niche
where stem cell lives
Niche regulates self renewal, survival, process of differentiation
Regulation via the stem cell niche
- cytoplasmic determinants
- chemical signaling
- physical/mechanical interactions
Adult human stem cell niche
Bone marrow, brain, eyes, gonads, gut, skin, teeth, heart, liver, lungs, muscle
