Developmental Neuroscience. Halo response of an embryonic chick ganglion after incubation with nerve growth factor. (Courtesy of Rita Levi-Montalcini). Embryonic and Fetal Development of the Human Brain. Actual Size. Actual Size. Photographs of Human Fetal Brain Development.
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Halo response of an embryonic chick ganglion after incubation with nerve growth factor. (Courtesy of Rita Levi-Montalcini)
Lateral view of the human brain shown at one-third size at several stages of fetal development. Note the gradual emergence of gyri and sulci.
(a) At 18 days after conception the embryo begins to implant in the uterine wall. It consists of 3 layers of cells: endoderm, mesoderm, and ectoderm. Thickening of the ectoderm leads to the development of the neural plate (inserts).(b) The neural groove begins to develop at 20 days.
(c) At 22 days the neural groove closes along the length of the embryo making a tube. (d) A few days later 4 major divisions of the brain are observable – the telencephalon, diencephalon, mesencephalon, and rhombencephalon.
8 stages are sequential for a given neuron, but all are occurring simultaneously throughout fetal development
1. Mitosis 2. Migration 3. Aggregation and 4. Differentiation
5. Synaptogenesis 6. Death 7. Rearrangement
At early stages, a stem cell generates neuroblasts. Later, it undergoes a specific asymmetric division (the “switch point”) at which it changes from making neurons to making glia
Note that differentiation is going on as neurons migrate.
Radial glial cells act as guide wires for the migration of neurons
Growth cones crawl forward as they elaborate the axons training behind them. Their extension is controlled by cues in their outside environment that ultimately direct them toward their appropriate targets.2. Migration
The fine threadlike extensions shown in red and green are filopodia, which find adhesive surfaces and pull the growth cone and therefore the growing axon to the right.
Scanning electron micrograph of a growth cone in culture. On a flat surface growth cones are very thin. They have numerous filopodia
Ramon y Cajal drew these growth cones showing their variable morphology
There are extrinsic and intrinsic determinants of neurons’ fate.
Development of the cerebral cortex
The ventricular zone (VZ) contains progenitors of neurons and glia. 1st neurons establish the preplate (PP); their axons an ingrowing axons from the thalamus establish the intermediate zone (IZ). Later generated neurons establish layers II-VI. After migration and differentiation there are 6 cortical layers.
Like neurons move together and form layers
Axons (with growth cones on end) form a synapse with other neurons or tissue (e.g. muscle)
Target cells release a chemical that creates a gradient (dots) around them. Growth cones orient to and follow the gradient to the cells. The extensions visible in c are growing out of a sensory ganglion (left) toward their normal target tissue. The chemorepellent protein Slit (red) in an embryo of the fruit fly repels most axons.
Release and uptake of neurotrophic factors
Neurons receiving insufficient neurotropic factor die
Axonal processes complete for limited neurotrophic factor
Time-lapse imaging of synapse elimination
Two neuromuscular junctions (NM1 and NMJ2) were viewed in vivo on postnatal days 7, 8, and 9.
Greek – “teratos” – wonder or monster
“genos” - birth