Biomedical Engineering Reference
In-Depth Information
Fig. 1.2
Liver does not
regenerate spontaneously at
the anatomical site of injury.
Following excision of the
median
and
left lateral lobes
of
an adult rat liver (
broken line
shows shape of intact organ
)
only the
caudate
and the
right
lateral lobes
remained, repre-
senting about one third of the
intact organ. Three weeks after
the injury the organ enlarged
by compensatory hypertrophy
back to a total mass equiva-
lent to the initial mass of the
uninjured liver. However, the
excised lobes did not grow
back. (Adapted from Goss
1992)
We begin this survey with an internal organ, the
liver
. After surgical removal of
70 % of its mass, the liver regrows, attaining its original size often by 7-10 days.
Such regrowth is recognized as an example of compensatory hypertrophy, which
restores the optimal liver mass in relation to body size (Higgins and Anderson 1931;
Steer 1995; Santoni-Rugiu et al. 2005; Michalopoulos 2013). In this well-studied
experimental paradigm, amputation of three of five lobes of rat liver has been shown
to lead to hypertrophy of the two intact lobes until the original liver cell mass has
been restored (Bucher 1963). The three resected lobes never grow back (Michalo-
poulos and DeFrances 1997). Since the residual lobes enlarge, the restored organ
has a distinctly different shape from that of the intact organ (Goss 1992; Fig.
1.2
).
The nature of the tissues that result from healing of each lobe at the site of amputa-
tion does not appear to have been studied extensively and questions have arisen
concerning the potential of liver tissue to heal as most other organs do. However,
following chronic injury of the liver with viral or toxic agents (cirrhosis), fibrous
tissue is irreversibly deposited in areas where liver cells have died (Vracko 1974;
Seyer and Raghow 1992; Iredale 2008). Not only in chronic injury but following
acute injury as well, liver heals locally by contraction and scar formation, as in most
other organs. This unexpected conclusion emerges from a study of a well-defined
acute injury, the standard perforation in the mouse liver. The results have shown
that the healthy liver locally responded with wound contraction and scar formation
(Fig.
1.3
). In another study with the healthy mouse liver an injury was inflicted that
produced stromal disruption. A few days after the injury, the liver contracted around
the site and a fibrotic reaction ensued (Masuzaki et al. 2013; Fig.
1.4
). Although
liver is often cited in the literature as the exceptional organ in adult mammals that
regenerates completely and spontaneously following injury, the observations cited
here (Figs.
1.3
and
1.4
) define a healing process that is clearly different from spon-
taneous regeneration.
