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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.