Growth and Guidance of Axons

Question 1

Intro - general

Answer 1

• embryonic cells divide, forming a ball of cells, which becomes organised into two layers hypoblast and epiblast
• epiblast cells form a three-layered embryo through gastrulation: endoderm, mesoderm, ectoderm
• Endoderm is most important for this topic as it forms the future epidermis and also the nervous system

Question 2

What happens before nerve fibres can grow

Answer 2

• in vitro cultures ectoderm cells are similar to those in vivo but lack key extrinsic cues and signals & lack mesoderm layer —> here cells undergo neural differentiation, suggesting that this is the default mode
• when cultured with direct cell-to-cell interactions, the same cells become epidermal cell instead

—> due to BMPs, specifically BMP4

—> BMP4 must be inhibited in order for cells to become neurons, which is done by noggin and chordin, which diffuse from the mesoderm to the ectoderm

• after neural differentiation, the neural plate develops and later turns into the neural tube with three vesicles which form the forebrain, midbrain, and hindbrain

Question 3

How do neurons grow and find their targets?

Answer 3

• Axons of neurons extend and grow via the growth cone of axons
• growth cone, discovered by Cajal, is a specialised structure on the developing axon and has high motility, constantly exploring the extracellular environment
• it is both a sensory transducer that receives directional cues from the environment and a motor structure that drives axon growth
• three compartments
  
  • central core: rich in microtubules, mitochondria and other organelles
  • filopodia are long extensions that project from the body
  • lamellipodia are motile and give the growth cones its characteristic appearance

Question 4

Filopodia

Answer 4

• filopodia sense environmental signals via receptors
• due to heir length, they can sample environments far in advance
• when a signal is encountered, the growth cone reacts accordingly
  
  • the advancement happens under the control of cellular motors
  • actin is polymerized at the extension of the axon, driving the axon forward

Question 5

Involvement of calcium

Answer 5

• concentration of calcium is regulated by activation of receptors on filopodia, which modulates motility through the organization of the cytoskeleton
• motility of the growth cone has an optimal range of ca+ concentration: set point
• filopodia receptor activation leads to Ca+ concentration gradient, which may be the basis for direction of growth

Question 6

Changes in intracellular proteins play a crucial role in axonal growth

Answer 6

• in vitro neurons
• if growth factors are applied to one side of the growth cone, leads to neuronal growth in that direction, IF cAMP levels are normal
• if cAMP levels are decreased, the same stimulus acts as a repellent

Question 7

Molecular cues for axonal growth (including Sperry)

Answer 7

• can either promote or inhibit axonal growth
• growth cone contains receptors that are able to sense, interpret, and respond to cues accordingly
• if negative cues are detected, the actin bundles collapse and therefore move the growth cone away from that cue

Chemospecificity- Sperry Frog experiments

• the chemospecificity hypothesis was put forth by Sperry based on experiments conducted in the 1960s with frogs. He the optic nerve of the eye and rotated the eye by 180 degrees in its socket before the nerve could regenerate.
• Then let the nerve fibers regenerate and see how the frog reacted to flies. Indeed, the reactions were exactly opposite: if a fly was presented on the ground, he would jump up.
• this experiment led to the conclusion that recognition between axons and their targets relied on molecular matching rather than functional validation and refinement of random connections

—> the nerve reconnected in the same way as the eye was previously and did not adjust to being rotated by 180 degrees —>this shows that axonal growth relies on molecular matching