Understanding Brain Dissection and Histology Part 6

Neurobiology Exam 1 Caudalize: Change the developmental trajectory of the cells so that they will adopt a more caudal (posterior) cell fate. Retinoic Acid (RA): Tells the cells to become caudal cells instead of rostral cells. The higher the concentration of RA, the more caudal the brain region’s fate will be. ! High RA = Spinal Cord Mid RA = Mid/Hindbrain Low RA = Forebrain • The “default mode” of ectodermal cells is not just to become neural tissue, but actually Rostral Neural Tissue. • No BMP = Neural Tissue — No RA = Rostral Neural Tissue Hox Genes: Group of related genes that control the body plan of an embryo along the rostralcaudal axis. Family of transcription factors which are involved in the patterning of the brain, spinal cord and other body parts. Essential to RC patterning. Hox Gene Array: Corresponds to the spatial pattern of expression; meaning the order of genes from front to back correlates to front to back segments. Mutate/knock out a single gene will affect corresponding body segment. • Hox genes result in nested expression in the hindbrain; Caudal hindbrain regions express many different Hox genes simultaneously, while more rostral segments express fewer. • The expression of the gene comes from the NUMBER of genes being expressed at a certain point as opposed to a concentration gradient of the genes. Dorsoventral Patterning: CNS is patterned this way; Spinal cord is as well via the following: Neurons that send their axons to muscles lie ventrally (motor neurons) Neurons that receive input from the sensory nerves lie dorsally (sensory neurons) Ventralizing: To change the developmental trajectory of cells so that they will adopt a more ventral cell fate. Neurobiology Exam 1 Sonic Hedge Hog (SHH): Secreted protein that can centralize cells in the embryonic spinal cord and brain. • SHH is expressed along the entire VENTRAL edge of the entire brain and sends its signal throughout the entire neural tube. • Boundary formation due to ventralization involves two classes of SHH transcription factors that create complementary expression domains: Class I - Induced by SHH: Concentration dependent: dorsal expression boundary meaning it is TURNED ON by different concentrations. Class II - Repressed by SHH: Concentration dependent: ventral expression boundary meaning it is TURNED OFF by different concentration. • Mutual expression of these SHH transcription factors influences the cell; either one TF can be expressed, but not both resulting in individual cells expressing only one of the two TF. • BMP now will be used in DORSALIZING the embryo. In early development it expressed the ectoderm to skin. Now there is a gradient between BMP (dorsally) and SHH (ventrally). The dorsoventral domains are established by the opposing concentrations of these two factors. • Where do neurons come from? Progenitor Cells: a cell that is still dividing and will eventually differentiate due to only being able to divide a certain number of times. Rapid proliferation of progenitor cells causes the bulges to form in the neural tube of the embryo which go on to form the ventricles of the brain. Ventricular Zone: Thin layer of cells which surrounds large fluid filled ventricles of the developing brain. The border has progenitor cells which divide a ton. Neurobiology Exam 1 Radial Cell/Glia: A type of neural progenitor cell that has a radial process that extends to the brain surface which are capable of generating neurons and glial cells. Each cell body moves up and down within its radial process. Radial Migration/Neurogenesis Mechanism Steps: 1.) The cell body of the radial cell moves up and down within the process. 2.) When the cell body is near the top/external brain’s surface, the progenitor cell duplicates its DNA and is in S phase. 3.) Cell body sinks down to the ventricular surface and the cell divides mitotically into one daughter cell which will differentiate into a neuron and one daughter cell that is a progenitor/ radial cell. 4a.) One daughter cell exits the cycle as a new born neuron. 4b.) The other cell re-enters the cycle as a new born progenitor. 5a.) The neuron daughter cell migrates out of the ventricular zone. 5b.) The progenitor daughter cell repeats the process of migration and division. 6.) Toward the end of embryonic neurogenesis, both daughter cells become neurons. 7.) Rapid decrease in progenitor cells. 8.) Newborn neuron cells migrate radially away from the ventricular surface. 9.) Formation of a separate layer called the mantle zone (MZ). • What is neurogenesis Neurogenesis: Process of generating neurons and some glial cells from progenitor cells; timing depends on transcription factors acting indirectly on a few key molecules. B-Catenin: Protein with diverse functions, one of which is to delay the onset of neurogenesis; In early development B-Catenin is expressed throughout the embryo but as it proceeds its ability to be expressed is slowly pushed to one side. Increase B-Catenin = Increased progenitor division = increased brain size. Delta-1: Protein that helps initiate neurogenesis. High Delta-1 = cell leaves radial to become neuron. Birth Date: Time at which cell stopped dividing and began to differentiate toward a mature cell fate. Sub-Ventricular Zone: A layer of dividing cells that lies between the ventricular zone and the mantle zone in some parts of the embryonic nervous system. Formed by some of the baby progenitor cells migrating out of the ventricular zone without becoming neurons. Sub-Ventricular Zone Creation Mechanism Steps: 1.) One daughter cell becomes another progenitor in the ventricular zone, and the other becomes an intermediate progenitor which migrates into the sub ventricular zone. Neurobiology Exam 1 2.) Intermediate progenitors divide again and gives birth to neurons and non-radial glia which form the laminae of the neocortex. 3.) Newborn neurons then hitch a ride on the radial cell processes and move to expand the mantle zone. • As development proceeds cells migrate out of the VZ to form more superficial layers, collectively referred to as the mantle zone. As the MZ thickens, it subdivides into multiple layers including the internal and outer sub ventricular zones, the subplate, and the cortical plate. ! • In the NEOCORTEX the newer cells move/migrate past the older cells until they reach the outer layer of the cortical plate where they end migration and begin differentiation. • Oldest Cells/Born First = Deep Cortical Layers • Younger Cells/Born Later = Superficial Layers! • In most regions of the brain the younger cells DO NOT migrate past old cells. Reelin: A protein that regulates the migration of young cortical neurons along the radial processes of radial cells. Cell Fate Factors to Consider: Time when cell is born affects its position and fate Specified order and timing in neurogenesis Neurons before glia; oligodendrocytes before astrocytes With time progenitors express a mix of transcription factors whose expression varies with age • • • • Lecture Summary • Embryo’s ectoderm gives rise to both skin and nervous system, Nervous system is default fate • Reostrocaudal patterning in the hindbrain and spinal cord involves Retonoic Acid (RA) and • • • • • Hox Genes. Without these caudalizing signals the neural tube tends to form rostral brain regions. Floor plate cells secrete SHH which induces some transcription factors and represses others in a concentration-dependent manner • Mutual repression between transcription factors creates distinct dorsoventral compartments with sharp boundaries. • It’s hypothesized that Bone Morphogenic Protein (BMP) is a dorsalizing agent which counteracts the ventralizing agent of SHH. Progenitors are the rapidly dividing undifferentiated cells The process of ceasing to divide and then becoming a neuron is called neurogenesis When leaving the cell radial cycle, young cells migrate away from their birth place along other radial cells’ processes. Where and when the brain cells are born affects which transcription factors they express which affects their fate. Neurobiology Exam 1 Fine Tuning Brain Development • What is an axon growth cone and why is it awesome Growth Cones: the tip of a growing axon. • Axons grow out of neuron cell body while it is migrating • Growing axons are filled with long and slender microtubules and tipped in actin which supports the growth cones. • As growth cones move forward, the axon behind it elongates. Filopodia: Slender, often transient extension of axon’s growth cone filled with actin filaments and linked to microtubules near the growth cone’s center. Able to move if they adhere to something in the external environment. This movement comes from the retraction of the growth cone toward the filopodia (like climbing a rope). Actin Subunit = Extends Filopodia Myosin Molecules = Retract Filopodia Axonal Growth: Extension of filopodia by actin moves cone out; retraction pulls it forward. The direction can change caused by substrate “stickiness” and filopodia directionality. Substrate Stickiness: A growing axon will turn toward a location that offer the best foothold. Filopodia Directionality: Filopodia do not always extend equally in the all directions. Growth Cone Movement: Direction of movement is a product of how likely the cone is to extend it filopodia in a particular direction and how well the filopodia stick to its surroundings. • How do axons find their targets/growth cone guidance Axon Chemotaxis: Movement of the axon towards the source of a diffusible substance along a concentration gradient. Used primarily in early development Different guidance cues arise at different stages of their journey Netrin: Guidance factor/chemical which causes an axon to grow toward the highest concentration of Netrin. Semaphorin: Repellant factor/protein that tends to repel growing axons by causing the filopodia of their growth cones to collapse on the side with the highest concentration of Semaphorin causing the growth cone to turn away from the molecules. Commissural Neuron: A neuron whose axon crosses to the opposite side of the brain or spinal cord. Neurobiology Exam 1 Axon Reprogramming of Commissural Neurons Mechanism: 1.) Commissural Axon grows ventrally towards the Netrin source (floor plate). 2.) When they get into the floor plate they start making a membrane receptor called Robo that is activated by Silt secreted by the floor plate. 3a.) Robo receptors make the growth cone less sensitive to Netrin. 3b.) Robo receptors make the growth cone repulsed by Silt 4.) Indifferent to the Netrin and repulsed by the Silt the axon grows away from the floor plate. Positive Chemotaxis (Axon/Floor Plate): Axons are attracted to the molecule creating the chemical gradient; that is they are attracted to the floor plate due to Netrin. ! ! Negative Chemotaxis (Axon/Floor Plate): Axons are repulsed by a molecule; that is they begin making Robo receptors and then are repulsed by the molecule Silt, causing them to grow away. Axon Guidance Molecules: Broad class of molecules that influences the direction of axon outgrowth through a variety of mechanisms. As axons grow, their responses may change. • How do synapses form Synapse Formation: Process through which synapses are created. Pre-Syn must be located directly across from Post-Syn. Begins with pre- and post- synaptic cells extending filopodia “feelers” that contact each other. Once contact has been established, the pre- and post- synapse processes communicate. If the exchanged signals are positive, then pre- and post- synaptic specializations begin to form. Over time these molecular interactions strengthen the bond between the pre- and post- synaptic cells, and a mature synapse is formed. Post-Synaptic specializations are laid down as the synapses emerge. Dendritic Filopodia: Slender and often transient processes on dendrites which develop into spines or retract soon after forming. Dendritic Tiling: Dendrites of neurons of the same type of cell repel one another during development and therefore tend not to overlap. This ensures dendritic branches are spread far and wide and not clumped. This spreading maximizes a neuron’s ability to gather inputs from many different axons far and wide. Purkinje Cells: Cerebellar granule axons pass through purkinje cells at right angels; granule cells are major source of info input because so many touch them. Adhesion Molecules: Ensure budding synapses don’t get pulled apart. Neurobiology Exam 1 Synapse Formation in Steps: • Axons and dendrites confirm whether the axon’s neurotransmitter is a good “match” for post cell. If they are compatible, the formation continues in 3 key steps. 1.) Adhesion: Axon filopodia sticks together with the dendritic filopodia due to adhesion molecules. ! 2.) Induction: Bidirectional Molecular Interactions (meaning both axon and dendrites are interacting and neither is passive in the process) cause pre and post synaptic specializations. 2a.) Post synaptic proteins accumulate on the dendritic side. 2b.) Pre synaptic synaptic vesicles and various proteins needed for transmitter release accumulate on the axon side. 3.) Maturation: Young synapses become larger and accumulate a greater variety of pre and post synaptic proteins causing the strength of the synapse to increase considerably. • As synapse matures more connections/inputs are made which increase the ESPS and IPSP and where it was slow before, it begins to get faster. • AMPA receptors (recruited to the Glutamate synapse) are put in place relatively late compared to the NMDA receptors which have been there for a while. • How can a neural circuit be fine-tuned Nearly 1/2 of all neurons created in development die off before complete maturation of the nervous system. Developmental Neuron Death: Death of neurons as a part of normal developmental causes rather than injury or disease. Apoptosis: Form of neuron death that proceeds in an orderly fashion, recycling some neuronal components and minimizing harm to neighboring cells. Numerical Matching: Idea that the number of neurons in one brain region can be “matched” to number in other interconnected brain region. The overproduction of neurons followed by apoptosis is easier than trying to proliferate the exact right amount in each side. Number of neurons projecting to a set limb muscle and the number of sensory neurons carrying input from the limb is matched during development to the amount of limb tissue available for innervation; achieved by modulating the extent of neuron death. Example 1: Staggerer mice whose Purkinje cells have died early in development leading to the death of granule cells that only exist to attach to them. Suggests that granule cells that fail to make connections commit apoptosis. Failure to find your purpose = Apoptosis. Example 2: Experiments showed that by removing a wing which subtracts neurons/wing tissue leads to less wing motor neurons. Adding leg adds neurons/leg tissue leads to more leg motor and sensory neurons. Neurobiology Exam 1 Nerve Growth Factor (NGF): First trophic factor to be discovered. Trophic Factor: Neuron survival and growth promoting factors which allow for numerical matching. Young neurons rely on neurotropic factors released by the inputs and their targets. They compete with one another for NGF The more NGF that is available for uptake, less competition and more neurons survive If demand for NGF exceeds supply, neurons die • Are new neurons added to adult brains In mammals, new neuronal cells are added to the olfactory bulb and dentate gyrus (part of the hippocampus) in adulthood. Lecture Summary: • Axon growth cones can be guided by molecules expressed in gradients. They must be attracted or repulsed by these guidance factors and their preferences may change throughout growth. • Synapse Formation: When dendrites and axons touch and are compatible, synapses form. • Neural development involves a surprising amount of apoptosis and axon pruning due to trophic factors and activity-dependent competition. • Adult Neurogenesis: Most neurons in adult brains are irreplaceable, but adult neurogenesis is evident in olfactory bulb and dentate gyrus. " Neurobiology Exam 1 Protecting The Brain How does the brain protect itself from physical trauma! Combination of skull and cranial vault which distributes compressive forces along the surface Cranial Vault: Space inside of skull that is occupied by the brain. Meninges: Membranes surrounding the brain and spinal cord. Cerebrospinal Fluid: Salty, watery substance with a density nearly identical to nervous system tissue. Allows brain to float and allow CSF to absorb movements; gathers neuronal waste products and carries them to the brain’s surface. Ventricles: Bath brain in CSF from within. They are connected to one another. Lateral Ventricles: Largest; paired in the cerebral hemispheres. 3rd Ventricle: Slit between sides of diencephalon. 4th Ventricle: Lies within medulla. Cerebral Aqueduct: Tube through midbrain. Choroid Plexus: Produces CSF. It is thin, highly folded, highly vascularized tissue that extends into the ventricles. Hydrocephalus: Condition in which there is an accumulation of CSF within the brain. • Any blockage in the flow of CSF from the choroid plexus to the subarachnoid space increases intraventricular pressure. Neurobiology Exam 1 Cerebrospinal Circulation: Gathers neuronal waste and carries it to the brain’s surface. 1.) CSF secreted by cells in choroid plexus. 2.) CSF flows into the ventricles (both forebrain and hindbrain). 3.) Exits brain through small holes in 4th ventricle; move into subarachnoid space. 4.) Leaves subarachnoid space through arachnoid granulations which protrude into the veins and sinuses. 5.) Recycled into the venous sinuses. • Common Brain Traumas: • Blow to the head or rapid acceleration/deceleration • Subdural hematoma (blood in subarachnoid space) • Concussion How does the brain protect itself from toxins and pathogens Lumen: a vessel’s interior. Endothelial Cells: line the INTERIOR surface of blood vessels and lymphatic tissue. Epithelial Cells: line the EXTERIOR surface of blood vessels. Parenchyma: the tissue of the brain (excluding ependymal cells and blood vessels) Ependyma: a thin membrane of glial cells lining the ventricles of the brain and the central canal of the spinal cord. Tight-Junctions: A place where the cell membranes of two cells come into contact and are held together by special molecules. Blood-Brain Barrier: barrier to the flow of large hydrophilic molecules from the blood into the brain; separates the neural tissue from the blood to keep out certain molecules. • Tight-Junctions between CAPILLARIES & ENDOTHELIAL CELLS ! • Restrict flow of large hydrophilic molecules (glucose, amino acids) and water through extracellular space ! • Allows small lipophilic molecules, oxygen, carbon dioxide to leave the capillaries and enter the brain’s parenchyma (tissue of the brain) Blood-CSF Barrier: barrier to the flow of large hydrophilic molecules from the blood into the Cerebrospinal fluid. Unidirectional (CSF to Blood) so no danger of intruders from blood. • Tight-Junctions between CHOROID PLEXUS & EPITHELIAL CELLS • CSF flows freely between ventricles and brain parenchyma via gaps in ependyma cells Neurobiology Exam 1 Arachnoid Barriers: Very thin layer of cells between subarachnoid space and veins of the sinuses, usually formed by tight-junctions and transporter molecules between its cells. Arachnoid Granulations: Mushroom like protrusions of the arachnoid membrane into veins or sinuses through which CSF is recycled into the blood. Giant Vacuoles: Large organelles inside the cells of the arachnoid barrier that carry CSF from the subarachnoid space into the venous blood. Barriers Between Blood, Brain and Ventricles Diagram: How does the nervous system respond to an attack! Innate Immune Response: relatively rapid and nonselective response of the body to foreign invaders (inflammation). Adaptive Immune Response: relatively slow response of the body to foreign invaders, involving the production of antibodies and other processes. Lymphocytes: White blood cells. B-Cells are an example. • Each B-cell responds to a specific antigen • Responds to foreign invader by making antibodies against the pathogen/antigen • Antibodies bind to the antigen stimulating macrophages and “killer cells” which remove the invader Neurobiology Exam 1 Immune Privileged: Idea that foreign substances in brain do not generate an immune response because the immune cells cannot pass the various barriers. ! Astrocytes: Star glial cells; multiple jobs to include secreting inflammatory molecules, controlling local blood flow, recycle neurotransmitters, form glial scars. Also secrete molecules that kill invading white blood cells and neutralizes any immune effector that comes into the brain parenchyma. Microglia: Smaller star glial cells; multiply in response to brain injury, migrate to the site of injury and secrete inflammatory molecules while engulfing cellular debris. Circumventricular Organs: locations generally near the midline and ventricles where the blood brain barrier is imperfect which allows for WBC to enter the CSF and parenchyma which boosts the amount of cells available to fight a serious immune threat; also allows neurons to sense the blood composition and initiate appropriate responses. Necrosis: a form of cell death were the cell bursts, spilling its contents. Excitotoxicity: Excessive stimulation of neurons by the spilled guts and neurotransmitters of a cell that has undergone necrosis (e.g. by glutamate) that leads to excessive neural firings and in some cases cell death. ! Astrocyte-Mediated Potassium Buffer: some astrocytes have K+ channels which allows K+ to flow into the astrocyte; this helps to ensure the K+ concentration is lower outside of the neurons than inside. Glial Scars: a barrier of reactive astrocytes that forms around damaged brain tissue, limiting the spread of damage to the brain. It repels growth cones which limits axon regeneration in the area. ! ! • How do neurons get their energy • Brains are energy expensive organs and picky about the energy/metabolic fuel. • Neurons generate ATP by employing glycolysis and oxidative phosphorylation. • Neurons feed their metabolic pathways almost exclusively with glucose which is supplied by the extracellular environment mainly from the blood. ! • Astrocytes can store glucose in the form of glycogen. Glucose Transporter: membrane spanning protein which transports glucose into the cells. Endothelial cells lining blood vessels and glial/neural cell membranes loaded with them. Neurobiology Exam 1 Glycolysis: Chemical conversion of glucose into pyruvate, resulting in a net gain of 2 ATPs per 1 Glucose molecule. Oxidative Phosphorylation: Conversion of pyruvate into carbon dioxide and water, resulting in a net gain of 30 ATP in the process. Lecture Summary: • The brain is physically protected by the cranial vault, meninges, and the CSF. • The brain is protected from pathogens and toxins by Blood-Brain, Blood-CSF, Arachnoid barriers. • Role of Astrocytes in brain protection: reduce the spread of brain damage, mainly by forming glial scars. • Role of Microglia in brain protection: moving to damaged areas, neutralizing threats, clearing cellular debris. • Removal of sensory input to cortical areas can result in significant remapping of the remaining inputs, especially when the affected individuals are young. Excessive cortical rewiring is linked to phantom pain. • Neurons derive most of their energy from glucose, which they obtain from blood or from storage in the astrocytes.