Lecture 36 - Development of Multicellular Organisms II

Question 1

What are the 3 phases of neural development?

L36 S6

Answer 1

Phase 1:

• different cell types develop independently throughout the embryo
• unconnected

Phase 2:

• axons and dendrites grow along specific routes
• provisional network of connections form

Phase 3:

• connections are adjusted and refined
• continues into adulthood

Question 2

Where do the CNS and PNS come from?

L36 S8

Answer 2

• both derived from ectoderm
• CNS from neural tube
• PNS from neural crest

Question 3

What is a growth cone?

L36 S18-19

Answer 3

• irregular spiky enlargement on the on the tips of a growing axon or dendrite
• growth occurs through polymerization and depolymerization of actin filaments using monomeric GTPases (Rho and Rac)

Question 4

What factors direct growth cone migration?

L36 S22

Answer 4

• contents of extracellular matrix; laminin favor growth and chondroitin inhibits
• chemotactic factor released by neighboring cells either attract or repel growth cone

Question 5

Why do commissural neurons grow towards the floor plate of the neural tube?

L36 S29

Answer 5

• floor plate secretes netrin which is an attractive chemotactic factor
• only commissural cells have the receptor for netrin (TRPC), resulting in them growing towards the floor plate

Question 6

What happens to commissural neurons when they reach the midline?

L36 S30

Answer 6

• midline cells secrete slit, a repellant
• growth cones turn away from midline and become sensitive to another repellant, semaphorin
• this causes axons to grow in a narrow track

Question 7

What happens to neurons after they reach their target cell?

L36 S33

Answer 7

50% of them die due to limited amount of neurotrophic factors, which are required for survival, made by target cell

Neurotrophic factors:

• tyrosine kinase (TrkA)
• nerve growth factor (NGF)

Question 8

What are the short term and long term effects of NGF?

L36 S36

Answer 8

Short term:

• effects growth cones
• response is local and rapid

Long term:
-effects cell survival

Question 9

What is synaptic remodeling?

L36 S41

Answer 9

• synapsing neurons are strengthened if they excite the cell they synapse with (this is caused by spines being produced by entry of calcium)
• if they do not excite the cell, the synapse weakens or is eliminated