Developmental Neurobiology

Developmental Neurobiology and Developmental Neurobiology (English Developmental Neuroscience and Developmental Neurobiology ) is engaged in the development and maturation of the nervous systems of various animals. Frequently used model organisms include a. the chicken ( Gallus gallus ), the zebrafish ( Danio rerio ), the small fruit fly ( Drosophila melanogaster ), the nematode ( Caenorhabditis elegans ) and the clawed frog ( Xenopus laevis ). The development of precursor cells and stem cells into nerve cells and the development of complex parts of the nervous system (pattern formation) are of interest. The regeneration of nerve cells is also the subject of research and is therefore partly of direct medical relevance.

Brief summary of the evolution of the vertebrate nervous system

The development of the nervous system begins with gastrulation (from the Latin gastrum "bulbous clay vessel") (see embryogenesis ), the transition from two-leaved embryoblasts to three-leaved form. The ectoderm forms an epithelial-like layer of cylindrical cells. The outermost layer, located at the back of the embryo, develops into the ectoderm, which thickens to form the neural plate from which the nervous system develops. The process of neurulation begins shortly after this cell layer is formed . Cells at the edge of the neural plate divide so strongly that invagination ( invagination ) and finally constriction occurs. Between the neural tube and the overlying ectoderm, neural crest cells develop , which lead to the spinal ganglia . The part of the ectoderm that lies outside the neural tube later forms the skin layer. The olfactory (olfactory sense) and auditory (hearing) epithelium as well as some peripheral ganglia develop from the tissue adjacent to the neural tube. At the edges of the neural plate, the neural folds arise, which then form the neural crest from which the peripheral nervous system develops ...

From the neural tube, three vesicles ( prosencephalon , mesencephalon and rhombencephalon ) form in the anterior ( anterior or rostral ) part . The spinal cord arises from the posterior ( posterior or caudal ). The brain develops through progressive subdivision of the vesicles: Telencephalon (later including cerebral cortex , hippocampus ) and diencephalon (becomes thalamus , hypothalamus , retina ) and the rhombencephalon divides into metencephalon and myelencephalon emerge from the prosencephalon . The mesencephalon does not change to the same extent and remains a single vesicle.

While the early development is mainly characterized by chemical signals, the refinement of the synaptic connections through electrical activity plays an increasingly important role with increasing age of the embryo. Especially in mammals, this form of development is not yet complete immediately after birth. In humans, it does not end until puberty .

Even the adult brain is still capable of astonishing plasticity in the context of learning processes .

Key steps and mechanisms of brain development

Cell formation, migration and cell death

The cells of the nervous system arise from precursor cells ( progenitor cells ), which come from the neural tube or the neural crest. First of all, a large number of progenitor cells arise through symmetrical division (= two identical offspring) in the early stage of development, which then generate neurons and glial cells in the late embryo through asymmetrical division (= two different offspring). A number of signal pathways ensure that histogenesis proceeds correctly .

Brain regions with different layers, each containing different cell types, are usually created by the fact that the precursor cells always produce cells of one type in a fixed sequence for a certain period of time, which migrate into the corresponding layers. This gradually creates levels of cells that are then networked with each other and with other parts of the brain.

Not all cells that are born and migrate to certain brain regions survive. It is a basic principle that there is an excess of cells and only a part of them is still present in the adult animal ( apoptosis ). Whether a neuron survives or not often depends on the availability of neurotrophins , while those cells survive that contribute better to the functioning of the system.

Axonal growth and axonal pathfinding

For a functioning network, nerve cells must be connected to one another. Since specific connections are required between certain areas, some of which are far apart, there are a number of mechanisms that guide the growing processes of the newborn neurons to their intended destination. At the tip of these processes is the growth cone , which is able to perceive and react to the chemical stimuli in its environment. These stimuli can be soluble substances that attract or repel the axon (similar to chemotaxis ) and thus guide it over greater distances or prevent it from growing into certain areas. Molecules bound on the cell surface can also have such effects if the growth cone comes into direct contact with them. This is of particular importance when axons grow along so-called pioneer axons (which have already covered at least part of the way). A prominent example of axononal navigation is the emergence of the connection between the retina and the downstream visual centers.

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Scott F. Gilbert: "Developmental Biology", 7th Edition.
Eric Kandel : James H. Schwartz, Thomas M. Jessel. Principles of Neural Science. 4th edition. McGraw-Hill Companies New York 2000
Michael J. Zigmond, Floyd E. Bloom, Story C. Landis, James L. Roberts, Larry R. Squire. (Editor) "Fundamental Neuroscience," Academic Press, San Diego 1999