The Organ Tree
191
No one knows why newts are so much more adept at this than all
other creatures; their eyes have no obvious structural or biochemical pe-
culiarities. Steve Smith gave us an important fact to work with when he
found two proteins in the lens that seem to prevent the iris cells from
changing into new lens cells as long as the old lens is in place. Since the
neural retina must be intact for most of the transformations to happen,
it may provide a constant electrical stimulus that goes into effect only
when the inhibitory proteins are removed by injury.
No blastema is formed; instead the cells change costume right
onstage. Furthermore, certain ingenious experiments have shown that a
wound isn't really necessary, only the interruption of the inhibitory
mechanism. Therefore, the stimulus from the neural retina probably
isn't the familiar injury current of limb regrowth. However, despite a
voluminous research literature on newt eye regeneration, no one has yet
studied its electrical aspects. This may be why we're still so far from
understanding the natural process, let alone trying to adapt it to human
eyes.
Muscle
Every muscle fiber is a long tube filled with rows of cells (myocytes) laid
end to end with no membranes between them—in effect, one multi-
nucleated cell, called a syncytium. These nuclei direct the manufacture
of contractile proteins, which are lined up side by side and visible, when
stained, as dark bands across the array of myocytes. Each muscle fiber is
surrounded by a sheath, and groups of them are bound together in bun-
dles by thicker sheaths. At the edge of each bundle are long, cylindrical
cells with huge nuclei and very little cytoplasm, called myoblasts or
spindle cells. Also along the edges, between the spindle cells, clusters of
tiny satellite cells can be seen at high magnifications.
After a crushing injury or loss of blood from a deep cut, muscle in the
damaged area degenerates. The myocyte nuclei shrivel up and the cells
die. Soon phagocytes enter to eat the old fibers and cell remnants. Only
the empty sheaths and a few spindle and satellite cells are left.
Now these remaining cells turn into new myocytes, fill up the empty
rubes, and begin secreting new contractile proteins. Although the early
part of this process proceeds without nerves, it can run to completion
only if motor nerve fibers reestablish contact with the terminals, called
end
plates,
that remain at specific distances along each
fiber sheath.
If
these end-plate areas are cut out, the nerve endings will enter, sniff