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LECTURE 13
DR HACK
How does an action potential start?
Signalling to muscle cells illustrates general
principle. Sensory receptors are gated ion
channels or cause gated ion channels to open.
How does an action potential start?
Signalling to muscle cells illustrates general
principle. Sensory receptors are gated ion
channels or cause gated ion channels to open.
Opening of channel initiates an action potential
or causes release of synaptic vesicles.
Key points: membrane potential and
action potential
• Membrane potential depends on active
transport of ions to create a concentration
gradient. This leads to diffusion down
concentration gradient, countered by resulting
charge difference across membrane.
• Opening of voltage-gated channels changes
membrane potential, causing action potential
that propagates an electrical signal.
Key points: synapses
• At synapses, opening of voltage-gated Ca2+
channels triggers exocytosis of synaptic
vesicles, releasing neurotransmitter.
• Neurotransmitter binds to and stimulates
opening of chemically gated ion channel.
• Opening of chemically gated ion channel
initiates an action potential.
• In muscle, this leads to Ca2+ signalling via
release of Ca2+ from sarcoplasmic reticulum.
Development
The processes by which a multicellular
organism forms from a single cell.
Key processes of development
• Determination – sets the fate of cells.
• Differentiation – the process by which
different types of cells arise.
• Morphogenesis – organisation and
spatial distribution of differentiated cells
• Growth – increase in body size by cell
division and cell expansion.
Underlying cellular processes
• Proliferation (growth).
• Specialisation (determination and
differentiation).
• Interaction.
• Movement.
Stages in development: plant
Stages in development: starfish
Stages in development:
Drosophila
1. Embryonic development:
about one day from fertilisation
of egg.
2. Larva hatches from egg.
3. Three larval stages take 5
days.
4. Insect pupates.
5. Adult emerges about 9 days
after fertilisation; is a few mm
long.
Stages in
development:
frog
See also video on Blackboard.
Stages in embryogenesis
1.
2.
3.
4.
Fertilisation.
Blastulation.
Gastrulation.
Organogenesis.
Fertilisation
LIFE 10e Photo 43.5. Early stage of
insemination: multiple sperm attached
to sea urchin egg. SEM. © Gerald
Schatten/BPS.
LIFE 10e Photo 43.6. Human egg
about to be fertilised; single
spermatozoon passing through
zona pellucida. LM. © R.
Yanagimachi, U. Hawaii/BPS.
Fertilisation
Fertilisation initiates embryogenesis:
• Blocks entry of additional sperm.
• Stimulates ion fluxes across membrane.
• Changes egg pH.
• Triggers completion of meiosis.
• Increases egg metabolism and protein
synthesis.
• Leads to cell division.
See also video on Blackboard.
Contribution of egg
Most of embryo’s requirements come from egg,
including mitochondria.
Egg cells are typically large. Major contributor to
size is reserves of yolk.
Amphibians: ~1 mm diameter.
Birds: several cm diameter.
Drosophila: eggs 0.5 mm long, adults 2.5 mm.
But mammalian eggs are relatively small: 0.1
mm.
Which of the following have a
placenta?
More than one answer might be correct.
a.
b.
c.
d.
e.
Amphibians.
Birds.
Insects.
Mammals.
Reptiles.
a
b
c
d
e
Contribution of egg
Most of embryo’s requirements come from egg,
including mitochondria.
Egg cells are typically large. Major contributor to
size is reserves of yolk.
Amphibians: ~1 mm diameter.
Birds: several cm diameter.
Drosophila: eggs 0.5 mm long, adults 2.5 mm.
But mammalian eggs are relatively small: 0.1
mm.
Mammals do not need large amounts of yolk.
Yolk
Contains reserves of proteins, lipids, vitamins,
and minerals.
Predominant constituent is lipoproteins.
Lipoproteins are synthesised as precursor
vitellogenin outside egg: in vertebrates,
synthesis is in liver.
Taken into egg by
–
What is the process by which the egg
takes up vitellogenin?
a.
b.
c.
d.
Active transport.
Phagocytosis.
Pinocytosis.
Receptor-mediated
endocytosis.
e. Receptor-mediated
exocytosis.
a
b
c
d
e
Endocytosis in egg yolk formation
Yolk
Contains reserves of proteins, lipids, vitamins,
and minerals.
Predominant constituent is lipoproteins.
Lipoproteins are synthesised as precursor
vitellogenin outside egg: in vertebrates,
synthesis is in liver.
Taken into egg by
– receptor-mediated endocytosis
– but not broken down immediately.
Contribution of sperm
Besides DNA, sperm contributes the centriole.
The centriole forms the centrosome, which
organises the mitotic spindle.
Establishment of polarity
Multicellular organisms are asymmetric: they
have polarity.
http://commons.wikimedia.org/wiki/File%3AAnatomical_Directions_and_Axes.JPG
(public domain)
Establishment of polarity
Polarity originates in the egg,
which is asymmetric.
Example: amphibian egg has
two distinct hemispheres: animal
(top) and vegetal (bottom).
Hemispheres can be distinguished because
animal hemisphere is pigmented, vegetal
hemisphere is not.
• Animal hemisphere contains the nucleus and
sperm binding sites.
• Vegetal hemisphere contains most of the yolk.
Establishment of polarity
Establishment of polarity
Following fertilisation, outer cortical cytoplasm
rotates towards site of sperm entry.
Establishment of polarity
The rotation is visible as it creates a band called the
grey crescent.
Establishment of polarity
The rotation creates the dorsal-ventral axis.
See also animation on Blackboard.
Ventral
Dorsal
Cleavage
After fertilisation, the zygote is transformed into
a mass of cells by a series of cell divisions:
rapid DNA replication and mitosis but no
growth, so that cells become progressively
smaller.
Pattern of division varies among organisms.
Cleavage patterns
Complete cleavage
Whole egg divides into cells. Typically occurs
when there is relatively little yolk.
In frogs, cells in animal hemisphere are smaller
than those in vegetal hemisphere, which has
more yolk.
Blastula formation
In animals with complete
cleavage, the blastula is
a ball of cells with a
central fluid-filled cavity,
the blastocoel.
What kind of cell-cell connections
would seal the outermost layer of
cells together?
a.
b.
c.
d.
Desmosomes.
Gap junctions.
Plasmodesmata.
Tight junctions.
-%
-%
a
-%
b
c
-%
d
Blastula formation
In animals with complete
cleavage, the blastula is
a ball of cells with a
central fluid-filled cavity,
the blastocoel.
Outermost layer of cells
forms tight junctions.
Blastula formation
Cells in different regions
of the blastocyst form
three germ layers that
give rise to specific
tissues and organs:
• Ectoderm
• Mesoderm
• Endoderm
Incomplete cleavage
Yolk mass does not divide. Discoidal cleavage
occurs in fish, reptiles and birds. Embryo forms
disc of cells on top of yolk: blastodisc.
Superficial cleavage
Occurs in insects. Initial nuclear divisions occur
without cell division, giving a multinucleate
syncytium. Nuclei migrate to periphery, then
cell division occurs around nuclei, leaving a
central space filled with yolk.
Blastulation in Drosophila
Fertilised egg (400 µm x 160 µm) 
multinucleate syncytium 
nuclei migrate to periphery 
Blastulation in Drosophila
Fertilised egg (400 µm x 160 µm) 
multinucleate syncytium 
nuclei migrate to periphery 
cell boundaries form 
cellular blastoderm: single layer – except
for pole cells (germ cell precursors) at
rear – of about 6000 cells surrounding
central yolk-filled cavity.
Timing
• Early nuclear divisions occur every 8 min.
• Cellular blastoderm forms by 195 min after
fertilisation.
Blastulation in mammals
Cell divisions are much slower in mammals
than in other animal groups and are not
synchronous. After 8-cell stage, cells form a
compact mass joined by tight junctions.
Blastulation in mammals
Cells separate into two
groups:
• Inner cell mass forms
the embryo.
• Outer cells form
trophoblast.
Trophoblast cells
secrete fluid to form
blastocoel.
Embryo at this stage is
called a blastocyst.
Blastulation in the mouse
Implantation
Mammalian egg
is released from
ovary into
oviduct, where
fertilisation takes
place.
As early cell
divisions occur,
embryo migrates
along oviduct.
Implantation
Trophoblast
adheres to
lining of uterus
(endometrium)
and embryo
implants in
endometrium.
Embryonic stem cells
Cells from inner cell
mass are pluripotent
and can be cultured:
embryonic stem (ES)
cells.
• Can be transplanted
and integrated into a
different blastocyst.
• Can be genetically
modified.
Embryonic stem cells
ES cells can be induced to differentiate into
different cell types by suitable treatments.