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Cells and Development (CELL0007) Notes - Development and Stem Cells
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Explore development and stem cells with these tailored notes designed for Year 1 students in the Cells and Development (CELL0007) module at University College London. Delve into the intricate world of stem cells, unravel the wonders of fertilization, explore the dynamics of gastrulation, and contem...

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  • November 30, 2023
  • 39
  • 2020/2021
  • Lecture notes
  • Dr amanda cain
  • All classes

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Fertilisation to Gastrulation
Fertilisation to Gastrulation
• Lecture
o Model organisms
o Eggs, sperm and fertilisation
o Embryonic cell cleavage
o Mid-blastula transition
• Human life cycle – major steps of embryo development
o Embryonic development
▪ Fertilisation
• Haploid egg meets with single haploid sperm → fuses
o Examples where development is initiated by the egg without fertilisation =
parthenogenesis – does not occur in mammals
• Creates a diploid genome – composed of both maternal and paternal genes
▪ Cleavage
• Period of rapid cell divisions in which the large egg is divided into smaller eggs
• Divisions generate a ball of cells = blastula – surrounding a fluid filled cavity =
blastocoel
▪ Gastrulation
• Phase of coordinated cell movements
• Generates the multi-layered structure of the embryo
o Most animals are triploblasts – composed of three germ layers
▪ Ectoderm
• Outer layer of embryo – forming skin and nervous system
▪ Mesoderm
• Middle layer of embryo – forming skeleton, muscle,
connective tissue, blood, heart, kidney
▪ Endoderm
• Inner layer of the embryo – forming digestive tract and
associated organs (lungs, liver, pancreas)
▪ Neurulation
• Formation of an internal nervous system – which is part of the surface ectoderm
following gastrulation
▪ Organogenesis
• Differentiation of the organs and tissues → becoming fully functional
▪ Metamorphosis
• Transformation of the larva into an adult organism
• Occurs late in development
• May occur major remodelling of the body

Model Organisms
• Model organisms
o Simple systems to understand more complicated ones
▪ Conversion of processes and principles across model systems
• Mouses – Mus musculus
o Advantages
▪ Easy to maintain
▪ High reproductive rate
▪ Mammal – similar to humans
▪ Genetic knockouts

,Fertilisation to Gastrulation
o Disadvantages
▪ Expensive to maintain
▪ Relatively long life cycle
▪ Mammalian development occurs in the uterus – the mouse embryo is inaccessible = difficult
to manipulate
• Chicken – Gallus gallus
o Advantages
▪ Large eggs
▪ Easy to obtain
▪ Easily observable embryos
▪ Excellent for micromanipulation
o Disadvantages
▪ Not very amenable to genetic/transgenic analysis
• Frog – Xenopus laevis
o Advantages
▪ Easy to maintain
▪ Egg laying can be induced by injecting females with gonadotrophin
▪ Large eggs – excellent for observation and micromanipulation
o Disadvantages
▪ Not very amenable to genetic/transgenic analysis
• Zebrafish – Danio rerio
o Advantages
▪ Easy to maintain
▪ High reproductive rate
▪ Transparent embryos
▪ Genetically amenable
o Disadvantages
▪ Expensive to maintain
▪ Relatively long life cycle
• Fruit fly – Drosophila melanogaster
o Advantages
▪ Easy to maintain
▪ High reproductive rate
▪ Very amenable to genetic
o Disadvantages
▪ Not a vertebrate
• Nematode – Caenorhabditis elegans
o Advantages
▪ Easy to maintain
▪ Short life cycle
▪ Transparent
▪ First animal with sequenced genome
o Disadvantages
▪ Not a vertebrate

Eggs, Sperm and Fertilisation
• Mitosis vs meiosis

,Fertilisation to Gastrulation




o




o
• Gametogenesis
o Oogenesis
▪ Begins in fetal ovary but meiosis is not
completed until fertilisation
▪ Oocytes are held in prophase I until a few
are activated each menstrual cycle
▪ Once activated held at metaphase II until
fertilisation
▪ The polar bodies contain the nuclear
material of the first and second meiotic
divisions
o Spermatogenesis
▪ Begins at puberty
▪ Huge numbers produced
• Eggs
o Vary greatly in size
▪ Size depends on nutrition required
• Mammals are an exception – as they receive nutrients via the placenta
o Stockpiled with maternal contributions
▪ Yolk protein (Vitellogenin) is 90% of protein content
▪ Protein required for household functions (metabolism, cell division, DNA replication,
transcription)
▪ RNA
▪ Lipids, glycogen
o Mammalian eggs
▪ Lack large amounts of yolk
▪ Held in metaphase II after ovulation
▪ Cortical granules just beneath the plasma
membrane
▪ Surrounded by a layer of follicle cells
derived from the ovary and a membrane –
known as zona pellucida
• Sperm

, Fertilisation to Gastrulation
o Highly specialised cells
▪ Lost most of cytoplasm
▪ Head contains haploid nucleus, centriole and acrosome
▪ Midpiece contains mitochondria and base of flagellum
• Fertilisation




o
▪ Contact between sperm and zona pellucida causes acrosome to burst
▪ Acrosome releases enzymes that digest a hole
▪ Sperm pass through and fuse
▪ Calcium wave activates end of meiosis
▪ Enzyme released to modify zona pellucida to prevent polyspermy (preventing multiple
sperm fusing)
• Mammalian fertilisation




o
▪ A
• In the mouse egg chromosomes (fc) are held on the metaphase plate of meiosis II
▪ B
• Fertilisation introduces the male pronucleus (mp) and its associated centriole
▪ C
• Two pronuclei fuse to produce the diploid nucleus with zygotic chromosomes (zc)
flanked by maternal and paternal centrioles
▪ D
• Mitosis begins and chromosomes line up on the metaphase plate
• Cytokinesis
o Cytokinesis completes mitosis and divides the cytoplasm
between both daughter cells
o A contractile ring forms beneath the plasma membrane –
containing a band of actin and myosin filaments
▪ Always forms in the same plane that was
previously occupied by the metaphase plate
o As the actin and myosin filaments slide by one another –
the ring contracts and pinches the 2 cells apart

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