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substratum-bound molecules or cell surface molecules of other cells. Most convenient culture systems that produce an active growth cone have been used for this purpose at one time or another, but several have been particularly amenable. For example, vertebrate retinal ganglion cells show clear cell-cell attractions and repulsions in culture (Cox et al., 1990) and their growth cones respond to specific extracellular signaling molecules involved in axonal guidance in vivo (Mann and Holt, 2001), as do those of vertebrate sensory neurons (Nakamura et al., 2000). Large invertebrate neurons, such as those of the snails Aplysia (Schafer et al., 2002) and Heliosoma (Chapter 9), have also been useful because of their large size and ability to recapitulate in culture their patterns of in vivo outgrowth. Studies of growth cone guidance and navigation within intact or partially dissected embryos have been carried out successfully in transparent embryos such as zebrafish (Whitlock and Westerfield, 1998), as well as in Drosophila (Godenschwege et al., 2002) and the grasshopper, Schistocerca americana (Isbister and O'Connor, 2000; Chapter 10). The former two systems have the advantage of genetic manipulation.

2. Protrusive Activity

As with crawling and navigation, many neuronal culture systems have been used for the study of cytoskeletal, membrane, and signaling events in the protrusive activity of the growth cone. The extreme thinness of the growth cone, along with its complex behavior and response to signaling molecules, makes it as attractive as the best lamellipodia of nonneuronal cells. Invertebrate neurons with

Table II

Basic Biological Properties of Cultured Neurons

Name

Cell body diameter

Neurite diameter

Growth rateb

Chick sensory (DRG)

20-25 ^m

0.5-1 ^m

40 ^m/h

Chick sympathetic

10-15 ^ma

0.4 ^m

40 ^m/h

Rat sympathetic (SCG)

20-25 ^ma

0.-1 ^m

40 ^m/h

Chick ciliary

23 ^mc

0.5-1.5 ^m

25-35 ^m/h

Chick spinal motor

20-50 ^mc

1-2 ^m

25 ^m/h

Chick forebrain

15 ^m

0.5-1 ^m

5-10 ^mJhd

Rat hippocampal

20-25 ^m

0.2-1 ^m

4-8 ^mJhd

Rat cortical

15-20 ^m

0.2-1 ^m

4-8 ^mJhd

Xenopus spinal neurons

10-20 ^m

0.4-3 ^m

2-4 ^m/hf

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