1. Option A – Neurobiology and behavior

2. Embryology – The study of development of organisms from a single, fertilized cell.
The study of embryology encompasses the development of gametes, fertilization and the development of embryos and fetuses. ?
This process aims to understand development in humans, and as such uses the most close approximations for human subjects in experimentation – organisms from the phylum Chordata.
Scientists began using frog embryos as a model organism, but around 1900, began to use chick embryos instead, due to their closer evolutionary relationship with humans.
1800
1900
Time
One of the benefits of these studies is that we now have a much stronger understanding of the neural development of embryos in general.

3. Annotation of a diagram of embryonic tissues in Xenopus, used as an animal model, during neurulation.
First thing about development – All structures appear in a specific order, and receive signals from previously established tissues.
After fertilization of the egg in a frog embryo, three tissue layers develop. These go on to form different tissues in the adult frog –
Ectoderm
Brain
Nervous
Endoderm
Lining of gut and other organs
Mesoderm
Skeletal
Reproductive
Circulatory
Excretory
Muscular
Embryonic tissue of Xenopus
Identify these tissues on the diagram above. Note the placement of each with respect to the whole embryo.
Try and draw this tissue without using your notes!

4. Neurons are initially produced by differentiation in the neural tube.
The neural tube of embryonic chordates is formed by infolding of ectoderm followed by elongation of the tube.
At approximately 3 weeks into development, the growing embryo begins to develop the nervous system from the ectoderm.
3 Weeks
This infolding of the ectoderm will eventually lead to the formation of the different portions of the nervous system.
Neurons are initially produced by differentiation in the neural tube.
Time
8 Weeks
The notochord will eventually disintegrate and only compose our intervertebral discs.
The neural crest will eventually become portions of our peripheral nervous system (PNS).
Neural canal becomes spinal cord.

5. Incomplete closure of the embryonic neural tube can cause spina bifida.
Spina bifida is a disorder in which the neural tube forms incompletely, most often in the lumbar region of the spine. X
This leads to a small protrusion on the lower back of the baby.
The potential ramifications are:
Fluid in the brain
Mental Retardation
Growth abnormalities
Loss of control of autonomic functions
Fortunately, the symptoms can now be treated often with surgery after birth. (Or even before in some cases)
Great series on Spina bifida and its implications for parents.

6. Immature neurons migrate to a final location.
The neurons in the developing embryo move to where they will be used through a process known as neuronal migration.
Cells formed earlier in neuronal development migrate small distances using glial fibers as a scaffold on which to climb.
Neurons created later in the process will move much more distance, which causes the brain to add on to itself in layers as it develops.
Brains at different stages of development -
Animation Live Feed
Check out the clips to the left to see just how hard these neurons work to get where they are needed in the brain!

7. An axon grows from each immature neuron in response to chemical stimuli.
These axons grow to form the different regions of the brain, and form synapses with one another.
Many times the neurons form multiple synapses as they grow.
Some axons extend beyond the neural tube to reach other parts of the body.
These axons go on to form portions of the peripheral nervous system, and are recruited by signals.
To the right, see how cell-adhesion molecules (CAMs) induce the formation of a synapse with tissue away from the nervous system.
How is this an application of chemotaxis?

8. X Synapses that are not used do not persist.
Much in the same way that you try all of your possible passwords when you forget which one you used, the developing neurons try all available synapse formations with those neurons around them.
These connections are moderated by CAMs, and are held in place once determined favorable. X
Neural pruning involves the loss of unused neurons.
However, these synapses do not stick around!
Synapses that are not used during this critical period are degenerated during neural pruning.
At age 2-3, you have approximately 15,000 synapses, twice the amount you have in adolescence and adulthood!
This partly has to do with how adult synapse formation is a much more complex.

9. Sweet TED Talk on Brain Scans
The plasticity of the nervous system allows it to change with experience.
Sweet TED Talk on Brain Scans
Modern research has proven that experience changes the brain, and has shown that the brain exhibits both functional and structural plasticity.
Structural Plasticity
Remodeling of synaptic elements e.g. spine strengthening/weakening, taxi drivers have enlarged hippocampus
Functional Plasticity
Recovery of function, due to existing neurons moving to fill in the gaps left behind, e.g. learning to use arm after stroke
Events such as strokes may promote reorganization of brain function.
For a long time, we treated people with brain injuries as non-functioning people, and there is currently a paradigm shift to now refer to these people as being in a dormant state, implying that they can be awoken, given the proper stimulus.
Essential Idea: Modification of neurons starts in the earliest stages of embryogenesis, and continues to the final years of life.

10. After this subunit, can you do each of these?
Challenge Questions
After this subunit, can you do each of these?
Redraw the picture of Xenopus from memory to include each of the tissue types.
Outline the differentiation and migration of immature neurons.
Describe spina bifida.
Explain neural pruning.
Annotate the table of tissues in Xenopus with the eventual fate in the adult frog.
Compare and contrast functional and structural plasticity of the brain.