6A. The Developing Brain Flashcards
3 layers that form during embryonic development
Ectoderm (outer), mesoderm (middle), endoderm (inner)
Nerve tissue arises from…
Ectoderm. Signals from mesoderm trigger this in a process called neural induction. Subsequent signaling interactions refine the nerve tissue into the basic
categories of neurons or glia (support cells), and then into subclasses of each cell type.
Fate of a developing cell is determined by…
Its proximity to various sources of signaling molecules. The concentration of signaling molecules decreases farther from the source, creating gradients throughout the brain.
Sonic hedgehog
Signaling molecule secreted from mesodermal tissue lying beneath developing spinal cord. When exposed to this signal, adjacent nerve cells are converted into a specialized class of glia. Cells farther away become motor neurons. Even lower concentration promotes formation of interneurons.
Early divisions of cells during proliferation are…
Symmetric, resulting in two identical daughter cells that are both capable of further division
Later divisions of cells during proliferation are…
Asymmetric, giving rise to only one daughter cell that keeps proliferating and a second that progresses towards its ultimate fate as a neuron or glia.
After the initial proliferative process is complete…
Only a few neural stem and progenitor cells remain within the brain, and neurogenesis in adulthood is limited to a few regions of the brain, such as those involved with memory
Proposed cause of microcephaly
Protein defects causing a premature switch from symmetric to asymmetric divisions
Excessive proliferation causes…
Megalencephaly
Migration - definition
New neurons travel from inner surface of embryonic brain, where they formed, to their long-term locations in the brain.
When does migration begin?
3-4 weeks after conception.
What happens 3-4 weeks after conception?
Ectoderm begins to thicken and build up along midline of embryo. Cells continue to divide, neural plate grows, followed by formation of parallel ridges that rise along either side of the midline. Ridges extend from the head end along the future spinal cord, eventually folding towards each other and fusing into a hollow neural tube. Head end thickens into 3 bulges.
What happens 7 weeks after conception?
First signs of eyes and hemispheres appear. New neurons produced and move from neural tube’s ventricular zone (inner surface of tube) towards border of marginal zone (outer surface). After neurons stop dividing, they form an intermediate zone where they gradually accumulate as the brain develops.
Most common guidance mechanism for migration
Radial glia. Accounts for 90% of migration in humans. Project radially from intermediate zone of cortex. Neurons use these glia as scaffolding, inching along glial projections until they reach their final destinations.
Process of radial migration occurs in what manner?
Inside-out manner. Cells that arrive earliest (oldest ones) form deepest layer of cortex, while late-arriving (youngest) neurons form outermost layer.
Which three stages of brain development occur internally during fetal development?
Induction, proliferation, migration
Where are the longest human axons?
Periphery, extending from lower spinal cord to toes
How does a developing axon grow?
By the extension of its growth cone, an enlargement at the tip of the axon that explores the environment to seek out its destination. Guided to its destination by molecular cues in its environment, some of which are on the surfaces of cells, while others are secreted into areas near the growth cone.
Types of cues for growth cones
Attractive and repellent
Families of proteins that are signalling molecules
Netrin, semaphorin, ephrin
Signalling molecules are common to many organisms. True or false?
True. Common to worms, insects, and mammals. As a result, simpler animals are useful experimental models.
Where was netrin first discovered, and how did that discovery apply to human research?
In worms, where it was found to guide neurons around the worm’s nerve ring. Later, vertebrate netrins were found to guide axons around the mammalian spinal cord
Formation of synapse
On presynaptic side, axon terminal becomes specialized for releasing neurotransmitter, stocking itself with neurotransmitter packets and proteins that enable those packets to be held in place then released.
On postsynaptic side, receptors on dendrite that respond to neurotransmitters begin to dot membrane.
Astrocytes play an important role
Possible cause of autism
Defects in molecules that promote proper synapse formation
Neurotransmitter used by motor neurons
Acetylcholine (fixed)
When certain immature neurons are maintained in a culture dish with no other cell types, they produce…
Norepinephrine
When certain immature neurons are cultured with specific cells, such as cardiac tissue, they produce…
Acetylcholine
Myelination increases the speed of signals by ___ times
100 times
Name of gaps in myelin sheath
Nodes of Ranvier
Saltatory conduction
Saltatory = leaping. Alternating pattern of insulation and nodes allows electrical signals to travel faster, jumping from one node to the next
What fraction of the neurons generated during development survive in an adult, and why?
Half. Creates a more efficient system
How are populations of neurons removed?
Apoptosis. Programmed cell death. Activated if a neuron fails to receive enough trophic factors. Each type of trophic factor supports the survival of a distinct group of neurons
Nerve growth factor
Maintains survival of sensory neurons
What is the pruning of excess connections dependent on?
Relative activity of each connection
Role of astrocytes in pruning
Aid the formation of eye-specific connections by engulfing unnecessary synapses.
