Growth cone

As a growth cone ( engl. growth cone ) the specialized front end of an axon sprouting is ( extension of a nerve cell ) called, with the aid of this looking the way to his destination. What is needed growth cone whenever a new axon aussprosst and his path searches: in the embryonic development of the nervous system (in some cases ) at later newly formed neurons that are still without synaptic junction where regenerating compounds and in the expansion of links (connectivity ) of a neuron.

The possible existence of growth cones has been postulated end of the 19th century by Santiago Ramón y Cajal.

Introduction

Usually have nerve cells a (sometimes branched ) extension, which the excitation of the cell leads away ( efferent ) and enters into with other excitable cells synaptic connections. The dimensions of these so-called axon or neurite extensions can reach significant proportions, for example, of central motor neurons ( Betz cells) in humans over a meter long, so 10,000 times longer than the cell body itself, the compounds, however, are typically much shorter.

However, a newly formed nerve cell is initially none of these compounds. To create new links, it requires a specially designed structure that assigns the path to the target cells. This structure is referred to as the growth cone.

Construction

A growth cone consists of a roundish to conical distended cytoplasmic core and two kinds of extensions: on the one hand, these are thin so-called filopodia, - narrow, finger-like protrusions of the cell membrane, which can reach up to 50 microns in length. Between them, the broader lamellipodia are located. The growth cone is amoeboid mobile and constantly scans with its processes its immediate surroundings. If he hits it on attractive or repulsive guidance signals, the growth of the axon is directed in direction and speed accordingly. Attraction leads to an increase in length of axonal extension. With predominance of repulsive factors, the axon can also be partially regressed, only to find another way.

Function

It can be distinguished by guidance signals in the environment different mechanisms of control: One possibility is geared to increasing and decreasing concentrations in the area chemotaxis. It messengers bind ( for example, netrin, ephrin or semaphorin ) to cell membrane receptors in the region of the growth cone. The resulting inside the cell triggered signaling cascades can Cytoskelettstruktur the axon and thus influence its orientation. The same neurotransmitter can - depending on the specific properties ( Differenzierungsmusterm ) of various nerve cells - be attractive or repulsive.

Another mechanism is contact management through structural proteins in the extracellular matrix that can either promote or inhibit further growth. In artificial environments has been studied as sprouting axons prefer or avoid certain macromolecules as lead. Mediated selective adhesion, growth-promoting leadership is busy, for example, collagen type IV and fibronectin. Other molecules cause an inhibition of axonal growth.

On cell-surface localized substances may act in this manner. Especially glial cells take in embryonic development is often the job of the " Signpost " (eg the Radiärglia ). Finally, axons often follow other, already formed nerve cell processes, which can be understood as Pionieraxone.

In this context, the important receptors on the membrane surface of the growth cones are in the groups of integrins, cadherins and the immunoglobulins superfamily.