Biomedical Engineering Reference
In-Depth Information
2.2 Irreversible thermal
alterations in tissues
and necrosis occur over essentially similar heating times and
temperature ranges and are often observed together. In fact, one
of the early criticisms of the TUNEL assay work was that the
original study included some necrosis mislabeled as apoptosis. (8)
More recent TUNEL techniques have successfully addressed this
issue. Apoptosis and necrosis are both relatively low tempera-
ture “slow” thermal processes and are not usually observed after
short-term higher temperature heating. Triggering an apoptotic
cascade is one hypothesis for the clinical effectiveness of tumor
hyperthermia therapy. (7) Bhowmick et al. (9) report Arrhenius
coefficients for combined apoptosis and necrosis in human pros-
tate (see Table 2.1).
Moderate heating also induces alterations in tissue microvas-
culature. Moderate heating increases the inter-endothelial cell
gaps in capillaries, leading to edema—extravasation of plasma
proteins. Green et al. (10) and others (11,12) have studied the migra-
tion of high molecular weight fluorescent-tagged molecules in
the dorsal skin flap model. Fluorescent dye and vital stain tech-
niques are also very useful in cell damage studies, and provide
quantitatively measurable markers of thermal damage. (13 -17) At
slightly higher temperatures the endothelial cells die and blood
in the vessels coagulates; hemolysis of red blood cells and the
clotting cascade result. Vascular disruption is one of the pri-
mary differences between low temperature thermal injury in
cells and those in tissues and organs; propagation of cell death
downstream from vascular damage may occur for many days
after the initial thermal insult, occurring even in unheated tis-
sue volumes as a result of disruptions in local perfusion. Similar
thermal damage occurs in tubular endothelial cells, for example,
in breast, pancreas, and the testes. (4)
More thorough reviews of thermally induced tissue alterations
may be found in either of the topic chapters (2,3) and in two recent
papers by homsen (4) and by Godwin and Coad. (5) The brief over-
view given here will serve to introduce the wide spectrum and
complexity of the intricate cascade of thermally induced pathol-
ogy, both the acutely observable immediate effects and delayed
effects. A short summary of representative healing responses is
also included. It should be appreciated that many tissue ther-
mal responses, such as hemorrhage and the healing response,
are only observable in vivo in perfused tissues and will not be
observed in vitro .
2.2.1 Low temperature tissue Effects
Cells and tissues can survive moderate heating for limited times
depending on their functional architecture and internal adap-
tive mechanisms. Protective mechanisms include, for example,
the evolution of heat shock proteins—shepherd molecules that
assist damaged proteins to refold into their native state confor-
mation. The development of heat shock proteins protects the cell
from moderate thermal damage, and makes the cell somewhat
robust to repeated heat treatment. Other nonlethal changes are
secondary to accelerated metabolism or reversible inactivation
of enzymes, or the like. Governing thermal mechanisms in cells
and cell cultures are often different than those observed in organs
and tissue structures—the tissue structure support mechanisms
somewhat ameliorate thermal alterations, and often a substan-
tial volume of an organ can be thermally deactivated and it still
may be able to perform to required levels.
Low temperature heating first results in cell swelling, which is
largely reversible. The lowest temperature irreversible alterations
are apoptosis and necrosis. (6) Apoptosis, or programmed cell
death (PCD), is a form of cellular suicide in which the cell dies
in response to either an intrinsic or extrinsic trigger. (7) Intrinsic
triggers include the presence of a virus, or tissue differentiation
during embryonic development; extrinsic triggers include heat,
radiation, and circulating cytokines, such as tumor necrosis fac-
tor (TNF). Apoptotic processes comprise complex biochemical
cascades that shut down cell metabolic functions (the mitochon-
dria) and particular enzymes. (4) Markers of apoptosis include
blebbing of cell membranes (the formation of irregular buds in
the margin), nuclear shrinking and pyknosis, and chromatin
condensation. The improved TUNEL assay ( T (Terminal deoxy-
nucleotidyl transferase d U TP and N ick E nd L abeling) is now
an accepted method for identifying the DNA fragments that
result from the last stage of an apoptotic cascade. (8) Apoptotic
processes may continue to occur over several days after heating.
Necrosis is cell death due to traumatic injury from external
agents—chemicals, poisons, and of course, heat and radiation.
Markers include membrane disruption, mitochondrial degener-
ation, or other morphological changes. Heat-induced apoptosis
2.2.2 Higher temperature tissue Effects
Higher temperature heating ranges from “heat fixation” of
tissues—deactivation all life processes without significant alter-
ation of morphology—up to significant structural disruption
resulting in amorphous masses. For this discussion, we limit the
consideration to temperatures below the boiling point, at which
steam evolution dominates other thermodynamic forces. Blanching and Heat Fixation
Heat-fixed tissue is morphologically similar to viable tissue, but
the complete absence of reperfusion (in contrast to a cryotherapy
lesion) and metabolic processes give it a blanched appearance
grossly; consequently, it is often referred to as “white coagula-
tion.” At higher temperatures the cellular and tissue structure
is visibly disrupted. In successful ablation lesions the central
zone next to the applicator frequently exhibits heat fixation and
substantial coagulative necrosis and denaturation of the cellular
and extracellular matrix proteins that resists standard wound
healing. (5) The central lesion is surrounded by coagulative necro-
sis that undergoes classical wound healing. The outermost lesion
site ( in vivo ) is a transition zone of intermixed processes in
which substantial extravasation of red blood cells (hemorrhage)
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