The Questions of Developmental Biology

The targets of these motor neurons are specified before their axons extend into the
periphery. This was shown by Lance-Jones and Landmesser (1980), who reversed segments of
the chick spinal cord so that the motor neurons were in new locations. The axons went to their
original targets, not to the ones expected from their new positions (Figure 13.13). The molecular
basis for this specificity resides in the members of the LIM family of proteins that are induced
during neuronal migration (see Figure 13.12; Tsushida et al. 1994). For instance, all motor
neurons express Islet-1 and (slightly later) Islet-2. If no other LIM protein is expressed, the
neurons project to the ventral body wall muscles. Those neurons in the medial portion of the
MMC also express Lim-3, which distinguishes them from the other motor neurons. The lateral
pools of the LMC are distinguished by their short expression of Lim-1, while the CT motor
neurons cease to express Islet-2. Thus, each group of neurons is characterized by a particular
constellation of LIM transcription factors.
Pattern Generation in the Nervous System
The functioning of the vertebrate brain depends not only on the differentiation and
positioning of the neurons, but also on the specific connections these cells make among
themselves and their peripheral targets. In some manner, nerves from a sensory organ such as the
eye must connect to specific neurons in the brain that can interpret visual stimuli, and axons from
the nervous system must cross large expanses of tissue before innervating their target tissue. How
does the neuronal axon "know" to traverse numerous potential target cells to make its specific
connection?
Ross G. Harrison (see Figure 4.3) suggested that the specificity of axonal growth is due
to pioneer nerve fibers, which go ahead of other axons and serve as guides for them* (Harrison
1910). This observation simplified, but did not solve, the problem of how neurons form
appropriate patterns of interconnection. Harrison also noted, however, that axons must grow on a
solid substrate, and he speculated that differences among embryonic surfaces might allow axons
to travel in certain specified directions. The final connections would occur by complementary
interactions on the target cell surface: