Brain Development in Utero

Neurulation

Formation of the beginnings of the spinal cord and brain.

Begins with formation of neural plate.

Neural tube begins to close.

Cells along the tube differentiate along the tube.

Neuronal proliferation

Begins with stem cells differentiating into neuroblasts

Neural cells in the neural tube divide by mitosis

Cell nuclei oscillate between the inner and outer zones of the neural tube during mitosis.

Neuroblasts then migrate to their final locations in the developing CNS.

Leads to excess neurons

Neuronal migration

Migrate to different areas of the body depending on location in neural tube

Cells from neural crest migrate to peripheral nervous system and form clusters in brain

They do this by using cell-adhesion moleculesSome travel by radial glial cellshttps://

www.youtube.com/watch?v=ZRF-gKZHINk&safe=active

https://www.youtube.com/watch?v=SczOfOXY17U&safe=active

Apoptosis

Apoptosis is programmed cell death.This occurs as the embryo has too many

neurons.Neurons synapse onto target cells (receptors)

and compete with each other for growth factors (trophic factors)

Apoptosis is different from necrosis in that the cells dies without exposing its contents to surrounding cells.

Synaptogenesis

This is the formation of synapses between neurons.

This involves the formation of a neurotransmitter release site on the presynaptic membrane

A receptive field on the postsynaptic membrane is also formed.

The two membranes are also aligned and specialised for their specific neurotransmitter.

Animal Models

Animal models can help as they allow researchers to observe the development of brains in animals who are born with incomplete brain development e.g. ferrets.

Complex human processes typically function on a foundation of simpler processes and structures present in all animals.

Animal models have helped understand the visual cortex structure and mechanism, as shown by Hubel and Wiesel in the 1960s.

Hubel and Wiesel

Hubel and Wiesel inserted electrodes into the visual cortex of kittens.

Found that individual cells do not respond to light but to edges.

Cells only fired whenan edge was in specificlocationThese were called “simple” cells“Complex” cells which can detect an edge with no

prejudice to contrast were also found“Hyper-complex” cells were found to be able to detect

more complex lines such as perpendicular lines.

Ocular Dominance Columns

Hubel and Wiesel also discovered the presence of ocular dominance columns in the visual cortex.

These are stripes of neurons that respond more to inputs from one eye than the other.

They are thought to be relevant to binocular vision These columns are formed at birth but have a

special critical “window” or sensitive period in which they can be modified.

Hubel and Wiesel found that by covering one eye during this period the columns relating to that eye shrink in size.

This response is so strong that if both eyes are covered, the columns desegregate.

This is evidence for the existence of a critical period in which multiple stimuli must be experienced for full development of the brain.

Ethics

Ethical issues about the use of animals have been raised as some say that it is inhumane to experiment on developing brains of animals.

The experimentation may lead to the animal being permanently impaired in some aspect e.g. vision

One argument could be said that the experiments can allow us to understand complex mental health problems in humans and help us find more effective methods of treatment.

This debate has lead committees globally to enforce guidelines on how experiments with animals must be conducted, including issues such as care of the animals, its environment and whether there are no other alternatives to using animals.