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
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
