Lecture 14 Target Selection Apoptosis 03 20 2015 Continue adding cels onto the outside Add cells posteriorally what used to be the middle is no longer the middle Continuously evaluating environment and cmompensating for ephrin isgnals and its partners What WAS the middle of the old axon eads environment and moves posteriorally Shown very clearly through experimentation Control nasal side goes posterior temporal goes anterior Remove half the tectum axons change orientation where old red retinal axon used to think is middle resets and compresses the entire map where the middle is just changing where it is in relation to nasal and temporal plasticity of axons still reading environment Entrie tectum eph gradient is different and there is competition for where they will end up red competes with Ephrin gradient and resets its position to end up in middle of tectum In contrast remove hald of the retina just the temporal and middle of the retine but remove the nasal you remove competition of nasal retinal cells so the axon reads the axon ephrin gradient and goes all the way to the bottom because there is no more competition DON T need firing for this to occur after axon reaches target they reestablish their connectivity Neurons that wire together fire together if the neurons that are similarly innervating a tissue ex all firing arm then they are normally found together Human brain in 1950s descriptive map of homunculus human motor cortex matches with peripheral target Our brain does not have pain sensors so you don t nee dcomplete anesthetia to do brain surgery can just anesthetize the skull cap and person can still be awake stimulate parts of motor cortex and ask patient what they are feeling Activate one region of motor cortex and they would feel like something is touching their leg or arm mapped out entire region Intricate feeling and sensitivity is needed in our hands the size representing the hand in the motor cortex is relatively large A mole that always feels with its snout snout has huge representation Whiskers of mouse take over like half of motor cortex Our backs are very small portion of overall cortex Our mouth head and hands represent large portions Amputated limb still sense that it s present phantom limb Plasticity sprouting of fibers from neighboring regions if you have no limb then no firing from missing tissue so that region of the ortex shrinks no support for needing to be there neighboring regions start sprouting into that structure Ex hand region is right next to head If you amputate the hand then touching to the face can make you feel it in the arm that s not there Good example of plasticity in nervous system Continuous stimuluation firing feels like it s clenched pain Each whisker vibrissa has certain area it innervates in cortex Some mouse with extra whisker extra barrel field possibility of adding cortical region with new peripheral tissue Experimentally ablate a whisker or early neural progenitor of whisker prenatally removed whisker B3 neighboring whiskers expand and take over that barrel field Fuse two together two barrel fields fuse together and think that they re one Axons will find target and they are always reading their target if missing they will take over CHAPTER 7 Neuron Cell Death Apoptosis will focus on this Classic death Prevents other cells from being damaged Autoohagy Cytoplasm starts recruiting cellular components into lysosome and breaks it down so you end up with bag of broken down cell components Necrosis instead of a nice simple death with it all internalized into cell membrane you get explosion of cell and it can damage neighboring cells Spinal cord injury is so damaging because damaging compoinents that cause necrosis are bursted out or Overexcitation causes cells to die Motor neuron innervating target muscle fiber gets input from surrounding glial cells of oligodendrocytes or astrocytes which secrete signals to support its survival that is called paracrine signaling cells that are in close proximity but NOT direct contact send signals Nerves innervating upon the target afferent derived fibers any nerves innervating the motor neuron can supply trophic factors to keep it alive Schwann cells in periphery also support survival of neurons Blood derived blood system supplies support signals so they often grow along vasculature Target derived retrograde signaling to make sure neurons doesn t die All of these factors decide whether a cell will survive or die first it condesnses nuclear DNA very tightly and chromatin segregate from one another crescent shapes this is pyknosis Pyknotic nuclei can be seen that means this is happening and cell is preparing for death Things that can be recycled for other cells break off in apoptotic bodies macrophages and microglia and other cells engulf them migrate into CNS later in development they are the macrophages of the CNS they respond to dying neuron that has a tag on it microglia respond to cues that dying cell is releasing engulf apoptotic body and continue to break it down enzymes can add onto 3 prime end of DNA molecule tag allows you to visualize end of fragmented DNA once it starts to break down put flourscent tag on end of DN fragments and get nice image of dying cell TUNEL cell BrdU is incorporated into new DNA but you can also tag end of DNA that is being fragments with TUNEL Tag cells that are dividing and cells that are dying You can t just count cells before and after development because proliferation and death keep happening you can look at proliferation and death at the same time like this When you have the most growth the most proliferation you get the most death Black dots on bottom left cell death A lot of the dying cells where the cell proliferation takes place Yellow BrdU aka just undergone S phase but also die before they differentiate into neurons a lot of motor neurons generated early in development by the time it s adult you lose about 70 of originally generated neurons you see massive spike in cell death When losing neurons you get spike in apoptosis Easier to remove neurons then to add on later our limbs have more targets that need to be innervated more neurons necessary to innervate limbs than trunk External development open up egg take out limb bud close up egg and see what happens Remove limb bud on side with missing limb you have DRG with fewer neurons Add limb bud via graft to middle of trunk DRG sending neurons to that limb bud has more neurons More peripheral target tissue