Neurodevelopment across the lifespan 1 Flashcards
What are the stages of prenatal development?
- Germinal stage (conception - 2 weeks).
- Embryonic stage (3-8 weeks).
- Fetal stage (9-38 weeks).
What happens during the germinal stage?
The nuclei of the egg & sperm fuse to form a zygote. Zygote starts to divide after 12 hours of fertilisation by a process called cleavage, to form a cluster of homogeneous cells - morula. The morula continues to divide to form a blastocyst. As continue to travel down the fallopian tube, cell division continues. Around day 7 blastocyst is implanted into uterine wall.
What happens during the embryonic stage?
Gastrulation = the formation of our nervous system. Uneven rate of cell development forms 3 distinct layers: ectoderm, mesoderm, and endoderm. Ectoderm develops more rapidly, and in the middle have neural crests that come together and fuse to form a neural tube, which is the beginning of the nervous system. The front will become the frontal lobe, the back will become the spinal cord.
What are facts about the vertebrate brain?
At birth, the human brain weighs 350g. By the first year, the brain weighs 1000g. The adult brain weighs 1200-1400g. Have the neurons we will ever have at birth – most cells are created at 7 months of gestation.
What are the basic processes of neurodevelopment?
- Cell birth/ Proliferation (Neurogenesis and Gliogenesis)
- Cell migration
- Cell differentiation and maturation
- Synaptogenesis
- Cell death and synaptic pruning
- Myelination (myelogenesis)
What happens during stage 1 (cell birth/proliferation)?
At its peak, 250,000 neurons are born per minute. The neural tube is the brain’s nursery - it is lined with stem cells. Stem cells along the inner surface of the neural tube form progenitor (precursor) cells. Precursor cell can either be a neuroblast (will become a nerual cell) or a glioblast (some kind of glial cell).
What happens during stage 2 (migration)?
Refers to the movement of the newly formed neurons and glia to their final destinations. Occurs with the help of chemical signals (attract neurons to arrive at particular place) and physical support coming from glial cells.
Migration with the help of glia - Axons extend to outer parts of the tube – immature neurons use this to find their way and migrate to the outer layers. This is called scaffolding – they provide scaffolding to migrate in particular way so the first neurons that migrate stay closer to inner surface, and other cells use these cells to support them.
What happens during stage 3 (differentiation and maturation)?
Once at their destination, primitive neurons begin to express particular genes that will allow them to become a particular type of cell. They start to form an axon and dendrites that will give them their distinctive shape.
Dendritic spines - Have extentions on them. Dendritic spines are swelling on the spine of the neuron. Point is to increase surface area of the dendrites. These are the points that other neurons are going to make contact – therefore creates more space for them to make contact. Broca’s area is the language area of the brain, and as a child gets older see much more dendritic branches.
Stem cells can take up the signals of the location of the area they are in, and take up the subsequent function. Cannot change mature cells like this.
What happens during stage 4 (synaptogenesis)?
Refers to the formation of synapses between neurons. Once differentiated, are going to make contact with similar neurons in the same area. Form synpases with a lot of other cells at the start, but then this is pruned (synaptic pruning) – only strong relationships are maintained. The growing end of the axon is called a growth cone. Growth cones develop extensions known as filopodia (fine extensions that are looking for other neurons to make contact) which make contact with another cell. Contact is guided by chemical signals. Chemicals either attract or repel other neurons. Not all relationships are maintained - lots of competition (neural darwinism).
What happens during stage 5 (cell death and synaptic pruning)?
proteins secreted by target cells promote the survival and growth of neurons – survival signal. If a neuronal axon does not receive a survival signal, it will degenerate and die through apoptosis = Programmed Cell Death (PCD). The combination of overproduction of neurons and apoptosis enables the exact matching of the number of incoming axons to the number of receiving cells. To avoid apoptosis and survive needs signals that attract.
Connections used need to be strengthened and maintained.
What is the difference between apoptosis and necrosis?
Apoptosis = Genetic code that is carried by cells – if things are going well, this is inhibited. If things neurons don’t get a survival signal, they are going to activate this code and result in neuronal death. Is a silent death that takes place when neurons activate genetic programming, and is a way that the neuron dissolves itself – does not do anything to neurons around it. Necrosis = Loud, powerful, damaging death that occurs after injury or too much excitatory, that causes damage in near-by cells.
What happens during stage 6 (myelination)?
Refers to the process by which glia produce the fatty sheath that covers the axons of neurons. Myelin speeds up the transmission of neural impulses. First occurs in the spinal cord and then in the hindbrain, midbrain and forebrain (back-to-front). Slow process – it occurs gradually for decades, depending on the region i.e. in the cortex it continues until 18 years of age and in the prefrontal cortex much later than that (early-mid twenties).
