Nonregenerative Tissues - Tissue and Organ Regeneration in Adults - page 29

Fig. 2.2 The dermis is a nonregenerative tissue. Following excision of the epidermis and of the
dermis to its full thickness ( left ), the wound edges contract and close with simultaneous formation
of scar tissue ( S ) in place of a physiological dermis ( right ). Adipose tissue (fat) is shown under-
neath the dermis or scar. The epidermis that forms over scar is thinner and lacks undulations (rete
ridges)

tion that leads to formation of a new epidermis is an intrinsic property of migrating
keratinocytes rather than being a property of the type of injury.
We conclude that, during healing processes in the skin, regeneration of the epi-
dermis occurs spontaneously and therefore cannot be used as a reliable indicator of
a successfully induced regenerative outcome. These considerations limit the value
of the epidermocentric viewpoint in an analysis of induced regeneration of skin and
focus instead attention to the stroma.

2.1.2 The Axonocentric Viewpoint in the Study of Nerve Wound

Healing

There is a profound topographic difference between the organization of tissues in
the skin and the peripheral nerve trunk. Whereas tissues in skin are layered in a
largely planar configuration, in a nerve trunk the tissues are wrapped around each
other concentrically in a cylindrical arrangement. A peripheral nerve trunk measures
about 1 mm in diameter for a typical rat sciatic nerve and comprises one or more
bundles (fascicles), each consisting of many elementary conducting units (nerve
fibers). Its main function is transmission of electrical signals from the spinal cord to
the periphery. Many mature nerve fibers comprise an axon surrounded by a sheath
of the protein myelin (myelinated axon), the later provided by the wrappings of
many Schwann cells around the axon perimeter, and a tubular basement membrane
that lines the external surface of Schwann cells. Other nerve fibers lack myelin
(nonmyelinated axons). An axon is a long, fiber-like extension (cytoplasmic pro-
cess) of a nerve cell (neuron). Individual nerve fibers are surrounded and supported
by “nonneuronal” tissues arranged cylindrically around the fibers. Proceeding from
a nerve fiber radially toward the periphery of the nerve trunk, we encounter the
following nonneuronal tissues: the endoneurium, comprising a loose stroma (endo-
neurial stroma) and specialized blood vessels that establish a blood-nerve barrier;