Growth cone

from Wikipedia, the free encyclopedia
Immunofluorescence image of a growth cone

As growth cones (engl. Growth cone ) the specialized leading end of a sprouting axons (extension of a nerve cell hereinafter), examined with the aid of this way to his destination. Growth cones are always needed when a new axon sprouts and looks for its way: in the embryonic development of the nervous system , in (occasionally) later newly formed neurons that are still without synaptic connection, in regenerating connections and in the expansion of connections ( connectivity ) of a neuron.

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

As a rule, nerve cells have a (sometimes branched) process that carries the excitation away from the cell ( efference ) and forms synaptic connections with other excitable cells . The dimensions of these processes, known as axons or neurites, can reach considerable proportions, for example those of central motor neurons ( Betz cells ) in humans are over a meter long, i.e. 10,000 times longer than the cell body itself. The connections, however, are typically significantly shorter.

However, a newly formed nerve cell initially has none of these connections. In order to create new links, it needs a specially designed structure that shows it the way to the target cells. This structure is called a growth cone.

construction

A growth cone consists of a rounded to conical expanded cytoplasmic nucleus and two types of processes: on the one hand, these are thin so-called filopodia - narrow, finger-shaped protuberances of the cell membrane that can reach up to 50  µm in length. The wider lamellipodia are located between them . The growth cone is amoeboid and constantly scans its immediate surroundings with its appendages. If it encounters attractive or repulsive signaling signals, the growth of the axon aligns itself in direction and speed accordingly. Attraction leads to an increase in the length of the axonal process. If the repulsive factors predominate, the axon can also be partially regressed in order to then look for another path.

function

A distinction can be made between different mechanisms of control by means of control signals in the environment: One possibility is chemotaxis based on increasing and decreasing substance concentrations in the environment . Messenger substances (for example netrin , ephrin or semaphorin ) bind to cell membrane receptors in the area of ​​the growth cone. The signal cascades triggered inside the cell can influence the cytoskeletal structure of the axon and thus its alignment. One and the same messenger substance can - depending on the specific properties ( differentiation pattern ) of different nerve cells - have an attractive or repulsive effect.

Another mechanism is contact through structural proteins in the extracellular matrix , which can either promote or inhibit further growth. In artificially created environments it was investigated how sprouting axons prefer or avoid certain macromolecules as lead structures. The growth-promoting guidance mediated by selective attachment has been proven , for example, for type IV collagen and fibronectin . Other molecules act to inhibit axonal growth.

Substances localized on cell surfaces can also act in this way. Glial cells in particular often take on the role of “guide” in embryonic development (e.g. the radial glia ). Finally, axons often follow other, already formed nerve cell processes, which can be understood as pioneer axons.

The receptors on the membrane surface of the growth cone, which are important in this context, belong to the groups of integrins , cadherins and the immunoglobulin superfamily .

literature

Web links