[Chemotropic molecules as a mechanism for the orientation of axon growth and nerve migration during the development of the nervous system of mammals].
OBJECTIVE
In 1890 Santiago Ramon y Cajal proposed one of the most widely accepted hypotheses as to how nerve connections are established during development, the well known chemotropic hypothesis . However, it was not until 90 years later that firm data was obtained regarding the mechanisms involved in such a complex process. At the present time, an avalanche of scientific publications support the premonitions of this hypothesis an give information regarding the mechanisms involved in the formation of nerve circuits. The aim of this review is to give an up to date view of the subject.


METHODS
After describing the chemotropic phenomena involved in the general development of the nervous system, we consider the chemotropic molecules identified to date. Special emphasis is given to the families which have been most extensively studied (semaphorines, netrines, slits). In another section we describe the role of these chemotropic molecules in the process of neural migration. Finally, we review the neurological syndromes and disorders in which some of these molecules are involved.


CONCLUSIONS
The chemiotropic hypothesis of Cajal, according to which, during development, connections between neurons are established because of the gradients of diffusible substances present in the environment, which guide the growth cones of the axons, has recently been confirmed by many experiments. The long range activity of these secreted molecules is which have been secreted is therefore accepted as one of the main mechanisms which contribute to the formation of the complex nerve circuitry. Its importance in neuroembriological disorders starts to be recognized.
de Castro Soubriet
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Servicio de Neurobiología-Investigación; Hospital Ramón y Cajal, Madrid, 28034, España. fernando.castro@hrc.es
D000818:Animals; D001369:Axons / Q000502:physiology*; D002630:Chemotactic Factors / Q000502:physiology*; D002633:Chemotaxis; D006801:Humans; D008322:Mammals / Q000254:growth & development*; D009414:Nerve Growth Factors / Q000502:physiology*; D009420:Nervous System / Q000254:growth & development*
D002630:Chemotactic Factors; D009414:Nerve Growth Factors