Development Lecture 1 Flashcards
Define embryogenesis
the process from a single cell (fertilised egg) until the fully-grown larva
What are the six stages of embryogenesis?
- fertilisation 2. cleavage 3. gastrulation 4. neurulation 5. organogenesis 6. metamorphosis
Why are Xenopus laevis a model organism? (2 advantages 4 egg specific advantages, 2 embryonic advantages)? What type of egg do they have?
- can be maintained in the lab 2. can induce egg laying all year round with hCG eggs: 1. accessible at all stages of development 2. large for micro manipulation 3. lay thousands of eggs at a time 4. easy to maintain embryos: 1. easy to inject with mRNA, DNA, dyes 2. can be used to manipulate gene expression MESOLECITHAL EGGS
What type of eggs to these model organisms have? 1. Xenopus laevis 2. gallus gallus 3. caenorhabditis elegans (nematode) 4. mus musculus 5. Drosophila
- mesolecithal 2. telolecithal 3. isolecithal 4. isolecithal 5.centrolecithal
Why is the gallus gallus used for modelling development? (4 points)
- large –> micromanipulation 2. amniotes, have amniotic membrane (like humans) 3. easy to obtain 4. accessible at most stages of development (except first 24 hours when in oviduct)
Why is danio rerio used to model development? (4 points)
- easy to to large genetic screens - so can be used to identify mutations that effect development 2. large number of embryos produced 3. embryo is transparent - can use microscopy 4. embryo is accessible at all stages of development
Why is the caernorhabditis elegans used to model development? (3 points)
- precisely defined cell-lineage (has only 959 somatic cells and they arise from the zygote in the same way every time) –> good model for mosaic development 2. can ablate individual cells using layers to observe the effects on development 3. the entire genome has been sequenced, and similarities with drosophila and humans detected
What did ablating specific cells in C. elegans (nematode) embryo show?
this showed that cell interactions are also required for development and embryos are capable of some regulation
Why is mus musclus used for development? (4 points)
- high rate of reproduction 2. mammal 3. easy to handle 4. transgenesis (inserting foreign genes into genome) - used to model human diseases -ve: embryos are VIVIPAROUS - development occurs inside uterus and so inaccessible and difficult to manipulate
Why is drosophila used for development? (3 points)
- fully sequenced genome 2. more than 50% genes similar to humans 3. homologs are identified and their role in development is determined -ve: cannot perform micro manipulations on embryo
What are the four types of eggs, and which animal has each?
- mesolecithal - amphibians 2. telolecithal - fish, birds, reptiles 3. isolecithal - mammals 4. centrolecithal - anthropods, insects
What are the disadvantages of using the drosophila for development?
cannot perform micro manipulations on embryo
What two factors does the size of an egg depend on?
- the amount of nutrition that the mother provides before the egg is laid 2. the size of the larval organism it must produce
Why are mammalian eggs abnormally small?
The embryo continues to receive nutrition from it mother during metal development
What molecules are found in eggs? (5 points)
- yolk protein - nutrition 2. proteins require of cellular functions, e.g enzymes for metabolism 3. polymerases - DNA, RNA synthesis 4. ribosomes - protein synthesis 5. lipids, glycogen
What are orthologues, how are they used for development?
Orthologues are the same genes from divergent animals e.g. Pax6 (vertebrate) and eyeless (ectopic) - they both induce ectopic eyes when expressed in the wings of drosophila
Which orthologues when expressed in the eyes of drosophila, while induce ectopic eyes?
Pax6 (vertebrate) eyeless (ectopic)
Where are yolk proteins synthesised?
maternal organs and then transported to the egg
Why are all egg contents referred to as maternal products?
They are formed by action of the mothers genome
What is the germinal vesicle?
The egg nucleus
Where are the egg contents synthesised?
In the oocyte In insect by ovarian nurse cells
Where are mammalian eggs produced?
In the foetal ovary
what are polar bodies?
Due to unequal meiosis, two polar bodies are formed, which are the discarded chromosomes
At what stage of meiosis sis a mammalian egg held at prior to ovulation?
Prophase I
What stage of meiosis is the mammalian egg held at before fertilisation ?
Metaphase II
What stage of meiosis are insect eggs held at prior to ovulation?
Metaphase I
Describe the structure of sperm
The head contains: 1. haploid nucleus 2. centriole 3. acrosome The mid-piece: 1. the base of the flagellum (containing tubulin) 2. mitochondria for powering the movement
What happens at fertilisation?
- contact between sperm head and zona pellucida cause the acrosome to burst 2. enzymes released from acrosome digest the zona pellucida 3. sperm fuse with the egg plasma membrane to produce a diploid nucleus 4. corticle granules release their contents, which change the zona pellucida to prevent polyspermy
What is parthenogenesis?
The invitation of development without sperm
What are gynomorphs?
mouse embryos that are created with only female chromosomes - poorly developed placental tissue, poorly formed embryo that dies
what are andromorphs?
mouse embryos which are created with only male chromosomes - abnormal growth of placental tissue - poorly formed embryo which dies
What are hydatidiform moles?
Similar to andromorphs, but human
What is genomic imprinting?
it is where some genes are silenced due to their paternal origin by methods such as methylation it prevents parthenogenesis from occurring in mammals
Where does fertilisation occur?
In the ampulla of the fallopian tubes
What is a partial hyatidiform mole?
It is an embryo with two sets of male chromosomes and one set of female chromosomes
What is a complete hyatidiform mole?
It is an embryo with two sets of male chromosomes
True or false: non-mammalian eggs have a zona pellucida
false
What induces the acrosome reaction?
contact between the sperm and zona pellucida
What is ZP3?
ZP3 is the sperm receptor protein on the zona pellucida of the mouse egg these prevent sperm from other species fertilising the eggs
What is the acrosome reaction?
the acrosome releases enzymes that digest a hole in the zona pellucida this allows the sperm to penetrate through the plasma membrane
What induces the cortical reaction?
penetration of the sperm through the plasma membrane, causes a rise in cytosolic calcium in the egg the calcium causes the cortical granules to fuse with the plasma membrane and release their contents into the extracellular space these enzymes modify the sperm receptors in the egg so that sperm can no longer bind and therefore prevents polyspermy
What is the problem of polyspermy?
malsegragation of the chromosomes during mitosis
What is the ‘fast block’ to polyspermy?
non-mammalian species change the charge on the plasma membrane on the egg so that further sperm cannot fertilise the egg
What is cleavage?
the rapid, synchronous cell division, in which one large fertilised zygote divide into smaller blastomeres
Why were blastomeres important in determining the different stages of mitosis?
the divisions are frequent and synchronous the embryos are readily available
What is cytokinesis and how does it work?
The division of cytoplasm, resulting in two daughter cells 1. a contractile ring of actin and myosin filaments forms beneath the plasma membrane 2. the ring forms in the same plane as the metaphase plate 3. the actin and myosin filaments slide past each other, causing contraction of the ring, pinching the two cells apart
Why does the first cleavage division take longer than subsequent cleavage division?
the male and female haploid nucleus have to find each other and fuse to form the zygotic nucleus
How are cleavage divisions so fast?
- gap phase are omitted 2. S phase is modified
How long do cleavage division take for: 1. drosophila 2. C elegans 3. danio rerio 4. Xenopus laevis
- 8 mins 2. 20-40 mins 3. 15-20 mins 4. 25-30 mins
How are cell division sped up in xenopus laevis?
- no Gap phases 2. S phase is modified so that there are more replication origins than nuclei at later stages of development
Why must the mother supply the egg with a large number of maternal RNAs?
cleavage nuclei is mostly dedicated to DNA synthesis (zygotic transcription is silenced) and gene transcription is very difficult and doesn’t really occur
Which gap phases are left out of mammalian cell cycle during cleavage?
NONE!
How is cell cycle length controlled in cleavage division?
- the ratio of DNA to cytoplasm - at a critical ratio the cell cycle is slowed
How is Xenopus laevis cell cycled slowed and from what duration to what duration?
at a critical ratio of DNA to cytoplasm, the cell cycle slows from 25mins to 2-4hours g1 and g2 is introduced zygotic transcription starts
The number of cleavage divisions is usually around…
12
What type of cleavage do sea urchins undergo?
Holoblastic
What type of cleavage do mammals undergo?
Holoblastic
What animals undergo holoblastic cleavage? (points)
- sea urchins 2. ascidians 3. C elegans 4. Xenopus laevis 5. mammals
What is the duration of cleavage divisions in mammals?
~20hours
What type of cleavage do isolecithal and mesolecithal eggs undergo?
holoblastic
How does a mammalian 16-32 cell embryo form a blastocyst?
- compaction - the blastomeres maximise their contacts with each other 2. otuer layer of cells forms a simple epithelium called the trophoblast 3. trophoblasts pump water into extracellular space in the centre of the embryo to form a fluid filled cavity, called the bllastocoel 4. a small population of non-trophoblastic cells at on pole of the embryo form the inner cell mass
Put these in the correct order in mammalian development: trophoblast, inner cell mass, blastocoel, compaction
compaction: outer layer form a TROPHOBLAST and pumps water into extracellular space to form the fluid filled BLASTOCOEL. a small population of cells in the embryo form the INNER CELL MASS
How do Xenopus laevis embryos develop from a single cell until a blastula?
- first cleavage - vertical, along the animal-vegetal axis this creates left and right halves 2. second cleave - vertical, but at right angle to the first creates four quarters 3. third cleavage - horizontal creates four animal and four vegetal blastomeres 4. fourth cleavage - vertical creates two layer of vegetal and animal blastomeres (4 layers of eight cells) 5. the four layers of eight cells is referred to as a MORULA 6. from this point, cleavage planes become more variable 7. water is pumped into the centre of the embryo to form a blastocoel and the embryo is know called a blastula
At what point does Xenopus cleavage division become less synchronous?
morula - 4 layers of eight cells, after the fourth cleavage
What is the nature of mammalian cleavage?
- quickly becomes asynchronous and so odd number of blastomeres are formed - approx. 20 hours
What is holoblastic division?
Complete cleavage of the egg
What animals undergo meroblastic division?
telolecithal (birds, fish, reptiles) - discoidal centrolecithal (insects, anthropods) - superficial
Why do telolecithal and centolecithal eggs undergo _ cleavage?
meroblastic cleavage large amounts of yolk which would be difficult to cleave
How do telolecithal eggs cleave?
discoidal cleavage - only a small, relatively yolk free region is cleaved
How are danio rerio eggs cleaved?
meroblastic, discoidal - cleavages 1-5 are along animal vegetal axis to produce a layer of 32 cells sitting above the yolk - cleavage 6 is horizontal so that two layer of 32 cells are produced
How are gallus gallus eggs cleaved? What are the cleaved cells collectively called?
discoidal, meroblastic - blastoderm
what is superficial cleavage?
Centrolecithal eggs 1. fusion of male and female nuclei deep within the yolky centre 2. first nine division happen within yolky centre with NO CYTOKINESIS 3. the multinucleated cell is called a SYNCTIUM 4. 90mins, 9th division - nuclei migrate to periperhy of egg to form the SYNCTIAL BLASTODERM 5. After 3 more nuclei divisions, membranes grow between the nuclei to form the cells = CELULLAR BLASTODERM 6. first cells are located at the posterior pole of the embryo to form germ cells
When does zygotic genome transcription start in amphibians?
midbastula stage = 1. blastomeres increase the length of the cell cycle by introducing G1, G2 and a longer S phase 2. cell divisions become asynchronous and blastomeres become motile
How is brachyura used as a molecular marker for mesoderm?
brachyrury is transcribed when zygotic genome transcription becomes and the maternal genome is destroyed this can be detected when there are no other discernible microscopic differences
How are zygotic genes used by them embryo?
- to determine cell fate - to determine mesoderm, ectoderm, endoderm cells - regulate the next phase, gastrulation
When do amniotic embryo (gallus gallus, humans) begin gene transcription?
two cell stage
