Biomedical Engineering Reference
In-Depth Information
Fig. 1.1
Amputation of newt
limbs leads to spontaneous
regeneration. Limbs were
amputated either below (
left
)
or above the elbow (
right
)
and were photographed at
the indicated times while
regenerating. (Adapted from
Goss 1992)
or the reversible increase in bone mass resulting from increased physical activity
(Frede and Lee 1983). Organ hypertrophy is a systemic response that can result
not only from functional overload but also as a response to injury, such as surgi-
cal removal of part of the organ. A well-known example involves the liver, which
responds to surgical deletion of a fraction of its mass by growth of the remainder so
as to compensate roughly for lost organ mass and function (Michalopoulos 2013).
The kidney offers another example of compensatory growth; following incapacita-
tion or removal of one of the kidneys, the remaining kidney grows to almost double
its original mass, largely compensating thereby for the mass lost by removal of the
other (Goss 1992; Fong et al. 2014). Neither the excised liver nor the kidney recover
the lost mass at the original anatomical site where the injury occurred. Basic studies
in the field have revealed that compensatory cellular hypertrophy is regulated by a
homeostatic mechanism that ensures not only elimination of aberrant cells through
cell competition but also appropriate organ-sized control (Tamori and Deng 2013).
Certain amphibians are capable of truly spectacular feats of spontaneous regen-
eration (Fig.
1.1
) . Almost perfect regeneration occurs after amputation of a limb
in many larval and adult newts and salamanders, as well as in the larvae (tadpoles)
of frogs (Wallace 1981; Stocum 1995; Tsonis 1996; Yokoyama 2008; Egawa et al.
2014). This phenomenon has been termed
epimorphic regeneration
, namely, the
replacement of an amputated appendage by direct outgrowth from the severed cross
section (Goss 1992; Stocum 1995). Limb regeneration is widespread in metazoan
phylogeny, but the adult urodeles appear to comprise the exceptional vertebrate
species that is capable of this feat (Brockes 1997). Rare instances of epimorphic
regeneration in mammals have been described (Goss 1992). An often-cited case
