EXAM3 - development of multicellular organisms II

Question 1

What are the phases of neural development?

Answer 1

• phase 1
  
  • different cell types develope independently at widely separate locations in embryo according to local program and are unconnected
• Phase 2
  
  • Axons and dendrites grow out along specfic routes setting up a provisional but orderly network of connections between various parts of nervous system
• Phase 3
  
  • continues into adult life adult life, connections are adjusted and refined thrpugh interactions with distant regions via eletric signals

Question 2

What is the origin of nervous system?

Answer 2

• Neurons are produced in association with glial cells (provide supporting framework and nutrition)
• Both cell types develope form ectoderm
• CNS = brain, spinal cord, and retina = NEURAL TUBE
• PNS = nerves, sensory neurons = NEURAL CREST

Question 3

How is the neral tube formed?

Answer 3

• Dervived from a single layer of epithelium
• starts with a neural groove on the dorsal side of embryo
• gradually deepens as neural folds become elevated
• the fold meet and coalesce in the middle line and convert the groove intoa a close NEURAL TUBE
• the center of the neural tube is the neural canal

Question 4

How does signaling and the neural tube relate to one another?

Answer 4

• Delta notch signaling controls differentiation into neurons (lateral inhibition and positive feedback)
• Signal proteins secreted from ventral and dorsal side of neural tube act as opposing morphogens causing neurons at different dorso-ventro positions to express different gene regulatory proteins
• Neurons continue to generate to give rise to greater diversity

Question 5

What is BMP and Snoic hedgehog proteins?

Answer 5

• Bone morphogenic protein “BMP” - key in pattern formation, differentiation, secretion of extracellular matrix
• Sonic hedgehog protein - depends on proteoglycans for functions, activate latent genes , and transcription regression to activation
• BOTH ARE MORPHOGENS THAT CAN OPPOSE EACH OTHER

Question 6

How is the neural crest formed?

Answer 6

• Neural crest cells originate at the dorsal end of the neural tube
• Mirgrate extensively during or shortly after closure of the neural tube or neuralation
• generation of several different cell types
  
  • neurons and glial cells of PNS
  • EPinephrine-producing cells of adrenal gland
  • many of the skeletal and connective tissue componets of the head
• fate of the neural crest cells depends on where they mirgrate to and settle

Question 7

How do neurons migrate ?

Answer 7

• Radial glial
  
  • acts as a Scaffold and extend to other end
• Migrating neuron
  
  • crawls on the radial glial

Question 8

What is the molecular mechanism of neural migration

Answer 8

• A typical immature neuron has a cell body, long axon and several short dendrites
• Tip of axon / dendrite has an irregular spikey enlargement called growth cone
• Growth cone crawls through surrounding tissue while the axon or denrite trails behind
• One of the growth cone starts migrating fast and develops axon specfic proteins - this will form axon

Question 9

What is are the characteristics of growth cones?

Answer 9

• Growth cones behavior is dictated by its cytoskeletal machinery
• growth cones move via filopodia and lamelopodia
• monomeric GTPases RHo and RAC control the assembly/dissassenbly of actin filaments which control movement of growth cone
• Growth cones withdraw cells from unfavorable surfaces and steer them towards favorable ones where they persist for longer time

Question 10

What is growth cone migration?

Answer 10

• Growth cones travel towards target cells along predictable routes
• exploit two major cues to find their way
• extracellular matrix environment; sensed by receptors present on membrane
• chemotactic factors released by neighboring cells
  
  • can be attractive or repulsive

Question 11

How are the extracellular matrix and growth cones related?

Answer 11

• Growth cones often follow a path taken by other cells - contact guidance
  
  • cause nerve fibers to be found in bundles
• Mediated by homophilic cell adhesion molcules
• two clases
  
  • immunogloblin superfamily
  • cadherin family
• MAtrix molecules favor axonal outgrowth
  
  • laminin
• or like Chondroitin sulfate proteins they can inhibit growth

Question 12

How did the spinal cord relay and recieve information?

Answer 12

• Dorsal neurons of spinal cord recieve and relay sensory informaton from SENSORY NEURONS located in the periphery of the body
• Ventral clusters of spinal cord neurons develope as MOTOR NEURONS send out long axons to connect with specificsubset of muscles
• INtermediatie location has INTER_NEUURONS that connect specific set of nerve cells to each other

Question 13

What are chemotactic factors?

Answer 13

• Secreted by cells, act as guidance factors at strategic points along path
• may be attractive or repulsive
• examples
  
  • netrin
  • slit
  • semaphorin

Question 14

What is netrins role in the commissural neuron guidance?

Answer 14

• First stage depends on secretion of netrin by cells of the floor plate
• binding of netrin to its receptor causes opening TRPC (transient receptor potential C) channels
• allow entry of extracellular calcium
• leads to activation of machery for extension of filopodia and movement of growth cone
• Non commissural neurons neural tube do not have netrin receptors so do not migrate towards floor plate

Question 15

Where do Slit and semaphorin come into play during commissural neural guidance

Answer 15

• Midline cells secrete slit
• slit receptor Roundabout present on commissural neurons
• slit repels growth cones and blocks entry t the midline
• growth cones become sensitive to another repulsive signal called semaphorin
• trapped between 2 sets of repellants growth cones travel in a narrow track

Question 16

What are neurotrophic factors

Answer 16

• Signal from target tissue regulate which growth cones synapse and where
  
  • once axonal growth cones reach eventual target cells. They then Halt, communicate, and make sunapses with target cells
• Most neurons are made in excess
• Target cells produces limited amount of neurotrophic factors needed for survival
• those that do not get enough die by programmed cell death

Question 17

What is NGF?

Answer 17

• First neurotrophic factor was Nerve Growth Factor (NGF)
• Belongs to Neurotrophins
• Receptor for Tyrosine Kinase (TrkA)
• Promotes survival of specfific sensory neurons and sympathetic neurons

Question 18

What are the effects of NGF?

Answer 18

• Short term
  
  • effect on growth cone and neurite extension. Effect is ocal, direct, rapid and independt of communication with cell body
• long term
  
  • effect on cell survival mediated by its receptor, uptake into cells via endocytosis and stimulation of downstream signaling pathways

Question 19

What activity dependent synaptic remodeling?

Answer 19

• Each axon intitally branches widely and makes multiple synapses with target cell
• profusion of weak synapsese
• network subsequently trimmed by elimination of synapses and retraction of axon branches
• accompanied by sprouting of axons to develope denser distribution of synapses that survive
• Synaptic remodeling = depending upon 2 roles that create spatial order
  
  • axons from cells in different regions of retina compete for tectal neurons
  • axons from neighboring sites which are excited at same time cooperate/collaborate to retain and stengthen synapses with tectal neurons
• Activity Dependent = synaptic remodeling depends on eletrical activity and synaptic signaling
• Ex: retinal / tectal neurons

Question 20

What is the selectivity of retinal neurons for tectal position?

Answer 20

• Posterior retinal neuron axons prefer anterior tectal neurons
• anterior retinal neurona have no preference
• this occurs because the protein EphA occurs in the retinal posterior
• (EPhrinA) is in the tectal posterior prevents association with retina posterior

Question 22

Question 23

How does the rule “neurons that fire together wire together also apply to the adult brain?

Answer 23

• Synapses are strengthed by external events that cause 2 or mre neurons to be activated at the same time
• entry of calicum through the gutamate receptor (NMDA RECEPTOR) triggers lasting change in synaptic strength
• corresponding change in physical structure of synapse
• indivdual dendrite spines remodeled, and new spines appear