Developmental Biology Flashcards
What are the advantages & disadvantages of using mice as a model organism?
Chicken?
Frog?
Zebrafish?
Fruit fly?
Nematode? (worm)
- easy maintenance, high reproductive rate, mammal, genetic knockout
- expensive to maintain, long life cycle
- easy maintenance, high reproductive rate, mammal, genetic knockout
- large eggs (embryos), easy obtain & observe embryos, great for micromanipulation
- not emenable for genetic/transgenic analysis
- large eggs (embryos), easy obtain & observe embryos, great for micromanipulation
- easy maintain, induce egg laying by injecting females with gonadotrophin, large eggs (observation & micromanipulation)
- not amenable to genetic/transgenic analysis
- easy maintain, induce egg laying by injecting females with gonadotrophin, large eggs (observation & micromanipulation)
- easy maintain, high reproductive rate, transparent embryos, genetically amenable, vertebrates
- expensive to maintain, long life cycle
- easy maintain, high reproductive rate, transparent embryos, genetically amenable, vertebrates
- easy maintain, high repro rate, amenable genetics
- not a vertebrate
- easy maintain, high repro rate, amenable genetics
- easy maintain, 3 day life cycle, transparent, first animal with fully sequenced genome
- not a vertebrate
- easy maintain, 3 day life cycle, transparent, first animal with fully sequenced genome
How are oocytes (immature eggs) held/what is the first step of Oogenesis?
What happens once they have been activated?
Fertilised?
What do polar bodies contain?
Primary oocyte in foetal ovary- held in prophase I until activated in menstruation (2n)
Secondary oocyte (oocyte II) formed at metaphase II (n due to meiosis- first polar body formed n)
Forms 2n Ootid which undergoes meiosis again (2nd polar body formed) to form n ovum which eventually goes into zygote 2n
Nuclear material of 1st & 2nd meiotic division
When does spermatogenesis begin?
What happens?
Why are mouse eggs significantly smaller than others?
What are eggs stockpiled with?
Puberty
Primary spermatocyte 2n undergoes 2 rounds of meiosis
meiosis I = secondary spermatocyte n formed
meiosis II = spermatid n formed
Mammals receive nutrients via placenta (externally) & other animals can’t- other eggs are big as need nutrients in them
Maternal goodies: yolk protein 90%, proteins for household functions (metabolism, cell division etc), RNA, lipids, glycogen etc
How do mammalian eggs differ? 5 ways
To what does egg size depend?
What specialised structures do sperm have? 5
What happens on fertilisation with the egg & sperm in humans?
How is polyspermy prevented?
- Small- 100 micrometre diameter
- Lack lots yolk
- Held in metaphase II after ovulation
- Contain cortical granules below membrane
- Zone pellucida = layer follicle cells from ovary (outside layer)
Yolk content
Cytoplasm = lots of it is lost
Acrosome = enzymes at tip of head to break down zone pellucida
Mitochondria = in midpiece for moving flagella
Centriole (base of head)
Flagella
- Contact with sperm & zona pellucida = acrosome bursts releasing enzymes to digest a hole
- Sperm pass through & fuse
- Calcium waves activate end of meiosis
Enzymes released by acrosome modify zona pellucinda to prevent sperm getting through
What happens to the embryo after fertilisation has occurred?
How does this occur?
What 4 things does a fertilised egg must do?
What are the 2 types of cleavage of embryos?
What are cleavages not accompanied by?
Cytokinesis (completes mitosis)
Contractile ring of actin & myosin forms beneath plasma membrane on same plane that used to be the metaphase plate. Actin & myosin slide past eachother = ring contracts & pinches cell apart
- Produce trillions cells mitosis
- Instruct cells on cell-type = specification & determination
- Organise these into tissues & organs = morphogenesis
- Give cells characteristics for their function = differentiation
Holoblastic = entire cell cleaved during each division Meroblastic = only part of the egg is cleaved
Growth
What type of cleavage do mammalian cells undergo?
What are the 2 sub-types of meroblastic cleavage? What organisms undergo these?
How do cleavage divisions occur? 2
What modifications are made to the cell cycle in synchronous/early embryos? Why?
What changes to the cycle in the blastula stage of embryos?
What is the transition called between the 2 changes? What happens at this stage?
What occurs to the maternal gene products & zygotic gene products after this point?
Holoblastic (due to lack of yolk)
- Discoidal: restricted to disc of yolk-free cytoplasm at the animal pole (top)- only cells on top cleave. Zebrafish
- Superficial: yolk free cytoplasm covering the egg surface. Eggs have concentrated yolk in centre with yolk free periplasm- nuclei migrate to the egg’s periplasm & undergo nuclear division & cleave outside of egg
Synchronous & rapidly
Remove G1 & G2 (gap phases), shorten S-phase (DNA replication is faster), & in some remove cytokinesis (only nuclei divide)
- so only M & S
To make cell division faster in early embryos
Gaps re-introduced & becomes asynchronous divisions = blastula stage & transcribes own zygotic genes
Mid-blastula transition- embryo undergoes its own zygotic transcription (turns on genes & direct to cell types)
Levels of maternal gene products decrease (transcribed from mother’s genome & stockpiled in egg) and levels of zygotic gene products increases (from maternal & paternal)
What are the 2 poles in embryos and how are they different?
What is gastrulation?
What does it result in?
What are the 3 types of gastrulation movements?
How are the poles of the embryo established in gastrulation?
Animal pole (small rapidly dividing cells) & vegetal pole (large yolky slowly dividing cells)
Movements where single layers of blastula is reorganised into multilayered gastrula
Formation of 3 germ layers: ectoderm, mesoderm, endoderm
Ingression, invagination, involution
What is ingression?
What is invagination?- where does it originate & what does it form?
In what cells does it happen?
What does buckling of the vegetal plate depend on?
What does elongation/forming the tube require? How does it form the tube?
What cells form at the tip of the archenteron & what do they do?
What does the archenteron & blastophore become?
What happens to the outside of the cell where invagination occurred?
Cells in the thickening vegetal pole migrate individually from surface/plate into the cell
Cells from thickening vegetal pole folds/’buckles’ inwards from the blastophore to form the archenteron (tube)
Sea urchins & c elegans
Convergent extension-
lamellipodia converge: perpendicular to invagination & intercalculate with eachother (form sheets side on)
extend: elongated parallel (stretched up) to form long & narrow archenetron
Apical contraction of actin-myosin complex relative to the basal surface (shortens the apical surface)
Archenteron = gut, blastophore = anus
Secondary mesenchyme cells- produce pseudopodia to pull the archenteron to the animal pole and fuse to it
cells patch it up
What is involution?
In what cells does it occur?
How does it occur?
In what sequence do the different germ layers move & what do they form?
In-turning of expanding outer layer of cells
Fish & frogs
Blastoderm sits on the yolk- cells on the outside involute inside & migrate up
- Endoderm (inmost): gut/digestive tract, liver, lungs, pancreas, gall bladder
- Mesoderm: muscle, blood, heart, kidney, gonads
- Ectoderm (outside): skin & nervous system
How does a mammalian blastocyst form?
What is special about the inner cells?
What are the inner cells subtypes & what can these go on to form?
What does the trophoblast form?
- Fertilised egg undergoes holoblastic cleavage
- At ~16 cells/morula stage cells undergo compaction & minimise contact (cells in centre = inner cells, peripheral cells = trophoblast)
- Cavity forms blastocoel forms in centre of embryo = blastocyst
Pluripotent (form any cell type)
Epiblast = 3 germ layers, amniotic membrane, extraembryonic mesoderm (forms placental vasculature) Hypoblast = extraembryonic endoderm (Heuser's membrane/yolk sac)
Placenta
Implantation:
How does a blastocyst penetrate the uterine wall?
What happens to the membranes?
Attaches & trophoblast divides/proliferates to form syncytiotrophoblast which penetrates the uterine wall
Membrane divides & forms epiblast & hypoblast layers- epiblast forms amniotic membrane & hypoblast forms Heuser’s membrane
What is the most common gastrulation method in mammals?
What occurs during gastrulation in mammals?
What cells are responsible in forming the 3 germ layers in mammals?
What do the cells migrating through the node form?
What do the cells ingressing through the primitive streak form?
Ingression
Epiblast cells
- Ingression occurs first by the epiblast cells entering via primitive streak individually
- The endoderm ingresses & displace hypoblast = embryonic endoderm (gut)
- Then mesoderm = embryonic mesoderm
- Remaining cells in epiblast make up ectoderm
Axial mesoderm, prechordal mesoderm & notochord
Form paraxial by condensing either side of notochord
What is a somite?
What process is this?
How are they formed?
In response to signals, what do somites divide into & what do they each form?
How quickly do somites form in zebrafish, chicks & humans?
When paraxial mesoderm either side of the neural tube divides
Morphogenesis
In pairs & from rostral to caudal (top to bottom)
Sclerotome = axial skeleton Myotone = skeletal muscle Dermatome = connective tissue of dermal layer of dorsal skin
30 min, 90 min, 5 hours
Which cells are responsible for forming the neural plate in neurulation?
How does this occur?
What does this go on to form?
How does the neural tube close?
What can often occur in this?
Ectoderm (exterior)
Cells on neural plate elongate from rostral to caudal & form pseudostratified columnar epithelium/neural tube
Spinal cord & brain of central nervous system
Apical (exterior side of layer of ectoderm/neural plate) constriction of the neural plate like invagination- layer is rich in actin & myosin
neural tube closure defects- lead to open spines & congenital conformations
In vertebrates, what forms on the dorsal roof of the neural tube?
What process is this?
What type of cells are these and what can they form?
What stage in vertebrates is reached after neurulation?
What is shared between vertebrates at this stage?
Neural crest cells
Morphogenesis
migratory multipotent stem cells- form mesoderm & ectoderm tissues (key role in vertebrate evolution)
Phylotopic stage- display basic vertebrate body plan & all quite similar
Common molecular anatomy
What is the idea of preformation (typically believed before a microscope)?
Idea of epigenesis?
What did Aristotle believe in BC?
What did William Harvey believe in 1600s?
What did Antonie van Leeuwenhoek believe?
What did Marcello Maplighi do? (breakthrough)
In the 19th century the debate swung in favour of epigenesis. Why?
Organs & tissues already positioned in fertilised egg & enlarge in embryonic development
Organs & tissues added over time in development with complexity increasing over time.
Believed in epigenesis & that embryos form from menstrual blood & semen
Embryos form from female eggs & male semen & epigenesis (complexity of embryo increases over time)
Could see nerves arteries & veins in embryos- animals are performed in sperm
First person observe chick development under a microscope- but still concluded eggs were preformed
Powerful microscopes, dyes to stain embryos, egg & sperm are single cells (can’t contain tissues/organs)
What is the idea of developmental commitment of a fertilised egg?
What kind of stem cell is a fertilised egg?
What is pluripotency? What stem cell does it produce?
What is oligopotency? What stem cell does it produce?
What are the stages of commitment?
What are the 3 mechanisms of commitment?
Progressive restriction of developmental potential- increasing differentiation means decreasing flexibility
Totipotent (all cell types)
stem cell that can give rise to all cell types & produced a multipotent stem cell
When a multipotent stem cell can form a few mature cell types- forms a unipotent stem cell
- Speciation (commitment to a fate that can be changed if cells are moved into a diff environment) - plastic: fertilised egg->multipotent
- Determination (commitment to a fate which can’t be changed) multipotent->unipotent
- Differentiate (acquiring final functional characteristics)
Localised determinants, embryonic inductions, morphogens
What is the idea of localised determinants?
What did August Weismann propose in his theory for localised determinants?
What other theory did he propose that was incorrect?
How did Roux test Weismann’s theory? What was wrong with it?
How did Conklin test the theory?
What does the yellow cytoplasm contain?
What happens if you inject this into other cells?
What experiment was it that made people believe that localised determinants was the only method?
What are PAR proteins used for?
Asymmetric cell division in a fertilised egg cell gives rise to daughter cells with different proteins/materials/”determinants”
Germ plasm theory: inheritance takes place in germ cells containing a germ plasm (contains determinants) & that determinants specify the fates of somatic cells.
Genetic info can’t pass from soma to germ plasm & onto the next generation = Weismann barrier
Killed a blastomere of a 2-cell frog egg- concluded that it resulted in half the frog cell being grown. Wrongly concluded that the frog egg was a mosaic of different determinants- as frogs undergo regulative development (killing cells makes its smaller)
Saw yellow cytoplasm/crescent in some eggs & follow its development- only shown in some of the cells and soon developed muscle cells in larvae- proposed the yellow cytoplasm is a muscle determinant (true)
Macho-1 mRNA- asymmetrically localised in the cell.
Macho-1 is a transcription factor & binds to elements upstream of genes required in muscles
Induces them to become muscle cells too
C.elegans and their invariant lineage- but they also have inductions it is not the only mechanism
Regulators of asymmetric division & polarity
What is the idea of embryonic inductions?
How was Roux’s experiments proven wrong? (frogs/sea urchins) What did this demonstrate?
What is required?
What is the competence of the cell?
How did Nieuwkoop prove inductions in amphibian embryos?
How did he prove which germ layer induced it?
How did the animal cap assay identify mesoderm inducing factors (MIF)/signals?
What 2 MIFs were significant as a signalling pathway?
Cells receive non-cell autonomous signals (from different cell) instructing them to commit- cell’s fate changed from signals from an adjacent group of cells
Separate the 4-cell sea urchin cells & the individual cells still formed larvae, just smaller- regulative demonstration. Each of the 4 cells has potential to give rise to larvae (haven’t been determined yet)- due to cell signalling
Signalling cell, responding cell
Ability of a cell to respond to a signal
Combining the animal ectoderm & vegetal endoderm meant that a mesoderm was induced by the vegetal fragment
Using albino & non-albino cells- found out that the ectoderm (animal/top) were the responding cells
Transforming growth factor ß (TGFß) in fibroblast growth factor (FGF) family and Nodal
Grow animal ectoderm in culture with and without MIFs- if the mesoderm was induced the tissue/notochord undergoes convergent-extension (if it’s not present remains a ball)
How does morphogen gradients work?
What is the model?
What does the notochord do to the neural plate?
What does the signal concentration probably do in those cell types?
Morphogen/signalling is secreted from cells & as a result travels down a concentration gradient & decreasing in concentration through specialising neurone (& other) cells- where high conc induces 1 type & lower conc induces a different type
French Flag model 1969
Secretes (source) sonic hedgehog & induces nearest cells to form floorplate and also express sonic hedgehog- establishing a gradient, then inducing motor neurones etc. diffusing across neural tube & induces fates on conc dependent manner
Turns on different transcription factors
What is an adult?
What is differentiation?
What did Cajal suggest?
What did Thomas Hunt Morgan suggest?
What is a terminal/differentiated gene battery?
How many coding genes does the human genome contain? How many of these are transcribed by all cells?
What are the 2 different types of gene batteries?
Diversity of differentiated cell types
Cells acquiring their functional characteristics
Major cell types can be divided into subtypes
- Genes = unit of heredity & carried on chromosomes
- Initial differences in cytoplasmic regions affect the activity of genes which inturn affect the cytoplasm = gradual elaboration & differentiation of regions in embryo
- “different batteries of genes come into action as development proceeds”
Genes expressed in specific tissue & transcribed by only few cell types
20,000- 6-7% where housekeeping genes needed to maintain basic cell functions
Major cell type batteries (neurone) and subtype (motor neurone) batteries
What are the 2 wrong hypotheses for why differentiated cells express different genes?
What was Davidson’s hypothesis? What is the main basis?
What are the 3 epigenetic mechanisms associated with turning genes on & off?
What are the genes involved?
- Gene loss: observation that invertebrate embryos lose chromosomes in development. Wrong: somatic nuclear transfer experiments show genome of differentiated cells are intact
- Gene amplification: observation that amphibian nuclei contain amplified copies of rRNA genes. Wrong: amplified genes not found in most differentiated cells.
Gene regulatory networks:
controlled by enhancer elements which bind to tissue specific TFs which interact & activate general TFs like RNA polymerase II. Progressive turning on & off of genes via transcription
Methylation cytosine = silencing
Methylation histones = activation & silencing
Acetylation = activation
H3K4m3 = activated methylated histone
H3K9ac = activated acetylation
H3K9m3 = silence methylated histone
5-methyl cytosine = silence methylated cytosine
What is determination?
What happens once these drivers switch on the specific genes?
How does subtype specification occur?
What 2 examples of terminal selector genes are there?
What are the steps for differentiation of skeletal muscle?
Selector genes/master regulators regulate entire differentiation gene batteries- early specification events lead to expression of key differentiation drivers (= determination)
The cell is committed to that cell type- with the exception of plasticity & status of differentiation
Terminal selector genes specific to the sub-type regulate entire differentiation gene batteries- e.g neurone expresses Ascl1 gene which expresses specific terminator genes (genes of that specific cell subtype)
MyoD (muscles) Ascl1 (neurones)
- Holoblastic cleavage & ingression of mesoderm
- Mesoderm specification through induction
- Somites form
- Myotome is determined through induction leading expression of MyoD
- MyoD drives differentiation of MyoD by regulating skeletal muscle gene battery
How did Ernst Hadorn determine determination stability? What is this due to?
How did Gurdon prove that the genome can be reprogrammed?
What is reprogramming the cell fate dependent on?
Put the leg disc of a fly into its abdomen- determined 99% a leg, 1% was abdomen. Determination is very stable but occasional trans-determination: plasticity
Somatic cell nuclear transfer- transplanting nuclei from cells in the diff stages of development in enucleated cells- e.g taking graft cells from an embryo & putting it into another will change its fate
More differentiated = lower plasticity
