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Chapter FiveDevelopment of the Human Brain

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Prenatal Development

http://www.healthofchildren.com/images/gech_0001_0003_0_img0218.jpg

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Early Differentiation Within the first week following

conception, the human embryo has divided into three germ layers: Ectoderm Mesoderm Endoderm

Inducing factors Chemicals which differentiate the ectoderm

layer into skin and nervous tissue.

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The Closing of the Neural Tube

notochord

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Overview of Neural Development

1. Neurogenesis2. Migration3. Differentiation4. Synaptogenesis5. Apoptosis & Myelination6. Refinement of synapses

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1. Neurogenesis and 2. Migration

Ventricular zone: A layer of cells that line the inside of the

neural tube, creates new neurons Some daughter cells remain in the

ventricular zone and continue to divide.

Other daughter cells migrate away from the ventricular zone along radial glia.

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1. Neurogenesis

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2. Migration

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Resulting Cortical Layers

http://www.psych.ndsu.nodak.edu/mccourt/Psy460/Central%20visual%20pathways/cortical%20layers.gif

http://thebrain.mcgill.ca/flash/i/i_02/i_02_cl/i_02_cl_vis/i_02_cl_vis_3a.jpg

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3. Differentiation

Daughter cells differentiate into neurons or glia.

The neural tube differentiates in two directions:

between the dorsal and ventral halves.

along the rostral-caudal axis.

Both genes (producing chemicals) and different types of input can be responsible for differentiation

11Copyright © 2004 Allyn and Bacon

3. DifferentiationThe developing brain

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The Growth Cone

From Bridgman, P.C. and Dailey, M.E., “The organization

of myosin and actin in rapid frozen nerve growth cones,”

Journal of Cell Biology, 108, pp. 95–109

4. Synaptogenesis

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Growth Cones Respond to External Cues

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A Synapse Is Formed

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5. Apoptosis

Significant numbers of new neurons die during the development process.

Neurons appear to compete for nerve growth factors (NGF), and those that fail to obtain this stimulation die.

Synapses follow a similar pattern of overproduction followed by "pruning."

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Visual Synapses Peak at 1 Year of Age

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5. Myelination

Myelination occurs from the spinal cord rostrally towards the forebrain.

Sensory systems are myelinated before motor systems.

Although the majority of myelination occurs very early in childhood, human myelination is not complete until about the age of 20 years.

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Experience Affects Development

Plasticity Ability of nervous system to change

Critical period Segment of time during development

in which experience can change system; after this time, experience causes little or no change in system

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Input Organizes the LGN

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Disorders of Development As the neural tube closes early in

development, mistakes may occur, leading to spina bifida and anencephaly.

Genetic disorders affecting development include trisomy 21, or Down syndrome, fragile X and phenylketonuria.

Environmental toxins, including alcohol, tobacco and other legal and illegal substances, can interfere with the normal course of development.

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Fetal Alcohol Syndrome May Produce Physical and Intellectual Abnormalities

Courtesy Dr. Sterling K. Clarren