Chapter 9 Neurodevelopment (Prenatal Development) Flashcards
What are three general ideas on Neurodevelopment?
- There is an amazing nature to neurodevelopment
- Experiences (enrichment) play an important role on neurodevelopment
- Errors in neurodevelopment can be dire
Define zygote
from fertilization to two weeks
-single diploid cell formed from joining of egg and sperm
Define embryo
from two weeks to 8 weeks
-after implantation in uterine wall (endometrium)
When does the embryo become the fetus?
9 weeks to birth
Define totipotent
able to develop into any class of cell in the body (zygote is totipotent)
Define pluripotent
able to develop into many (but not all) classes of cell
Define multipotent
able to develop into dif cells of ONE class (more specialized)
Define unipotent
can develop into only one type of cell
What are two criteria for embryonic stem cells?
- Almost unlimited capacity for self-renewal (splits into one stem cell and one specialized cell with each division)
- Ability to develop into many dif types of cell (can be totipotent, pluripotent or multipotent)
What three things MUST happen during prenatal development?
- Cells must DIFFERENTIATE
- Cells must make their way to appropriate area and align with other cells (MIGRATION AND AGGREGATION)
- Cells must establish functional relationships with other cells (SYNAPTOGENESIS)
What are five stages of development?
- Induction (formation) of neural plate
- Neural Proliferation
- Migration and Aggregation
- Axon Growth and Synapse Formation
- Neuron Death and Synapse Rearrangement
When does stage 1 (induction of neural plate) occur?
What does this stage include?
- 3 weeks after conception
2. Chemical signals from the Mesoderm layer induce the development of the neural plate
What is the neural plate?
small patch of Ectodermal tissue on the dorsal surface of the embryo
Cells that eventually become the CNS originate where?
The neural tube
Cells that eventually form the PNS originate from where?
The neural crest
What happens during Phase 2 (Neural Proliferation)?
- The neural plate folds to form the neural groove which then fuses to form the neural tube
How is Neural Proliferation organized?
Chemically by Roof Plate and Floor Plate
What will the inside of the neural tube form?
Cerebral Ventricles and spinal cord
At 40 days after conception what is visible at the end of the neural tube?
Three Swellings:
- Forebrain
- Midbrain
- Hindbrain
What is the ventricular zone?
Where most cell division occurs in the neural tube
-adjacent to the ventricle (fluid filled centre)
What does phase three (migration and aggregation) consist of?
Migration: mvmt of new cells to appropriate location
Aggregation: Neurons align with other neurons
There are two types of migration, what are they?
- Tangenitial
2. Radial
What is tangenitial migration?
Moving “in” to another part of tube at right angle to Radial Migration
What is Radial Migration?
Moving from centre (ventricular zone) outward
What are two methods of migration?
- Somal Translocation
2. Glial-Mediated Migration
Describe somal translocation
- an extension grows (like a hand searching in the dark) from the cell and the cell body moves along it
- slow and meandering - extension tests environment for ‘cues’
- Can be Radial or Tangential
Describe Glial-Mediated Migration
- A temporary network of radial-glial cells develop in the neural tube
- cells can move into position along radial-glial cells (like climbing up scaffolding)
- More directed than somal translocation
- Only Radial Migration
Is cell migration faster in CNS or PNS? Why?
CNS because have shorter to travel than PNS
Migration occurs in an inside-out pattern, what does this mean?
Cells go through already formed lower layers of cortex before reaching final destination
*occurs in organized waves from deep tissue to more superficial layers
What is Aggregation?
- Neurons must align with other neurons to form connections
- Aided by Cell-Adhesion Molecules (CAM’s)
How is Aggregation accomplished?
Cell adhesion molecules:
- act as a beacon that attracts certain cell types
- located on the surface of cell and allows cells to attach to eachother
Gap Junctions are very important in neurodevelopment, why?
allow “huddle of neurons” to communicate with eachother before synaptogenesis has occured
- pass cytoplasm betwn cells
- connexin instead of axon
What happens in Stage four (Axon Growth and Synapse Formation)?
- each growing axon and dendrite develops a “growth cone” (exagerrated terminal button)
- CAM’s and Tropic Molecules guide growth
What are tropic molecules?
produced by target cells being sought by axons
-specific to certain areas of the brain
What are the two hypotheses for how synapses form?
- Chemoaffinity hypothesis
2. Topographical gradient hypothesis
What is the Chemoaffinity Hypothesis?
Each postsynaptic surface releases a specific chemical label that attracts axons to it
–some axons follow very indirect routes, suggesting that growth cones may be guided by a number of growth cones
What are pioneer growth cones?
First cones to travel a new route in developing nervous system
What is Fasciculation?
Tendency fro axons to grow along paths established by Pioneer Growth cones
What is the Topographical Gradient Hypothesis?
Axons growing from one topographic surface to another are arranged according to layout of cell bodies on original surface
*Suggests that there is a chemical gradient in the environment that axons follow
What chemicals are apart of the chemical gradient depicted in the topographical gradient hypothesis? Which directions do they account for?
Ephrin A: Medial Lateral
Ephrin B: Dorsal Ventral
Define Synaptogenesis
Formation of new synapses
-requires glial cells / astrocytes
What happens in the fifth stage (Neuron Death and Synapse Rearrangement)
because 50% more neurons are produced than are needed, neuron death must occur
There are two mechanisms to achieve neuron death, what are they?
- Apoptosis
2. Necrosis
What is Apoptosis?
Programmed cell suicide (active)
Cells have an “expiration date”
-packages self into vesicles = clean and organized
What is Necrosis?
Passive cell death
- from malnutrition
- messy = contents of cell “spill” and can cause inflammation
What are the consequences if
(a) apoptosis occurs before they are programmed?
(b) apoptosis fails to occur
(a) Autoimmune Disease
(b) Tumours
How is Synapse Rearrangement achieved?
Target cell releases Life-Preserving Chemicals to pre-synaptic cell that they want to maintain a connection with
- reward for correct connection
- leads to increased selectivity of transmission
