Biomedical Engineering Reference
In-Depth Information
hyaline cartilage, suggesting synthesis of fibrocartilage, and indicating repair (Wor-
nom and Buchman 1992). Current efforts are directed toward restoration of part of
the subchondral bone, rather than restoration of articular cartilage per se (Ochi et al.
2004; Swieszkowski et al. 2007; Needham et al. 2014).
Transection of
skeletal muscle
has been studied extensively. The severity of
injury varies widely across surgical incisions, wartime injuries and sports-related
injuries. After a clean cut, sarcoplasm retracts over a short distance inside the sar-
colemma, leaving an empty tube that is soon filled by invading leukocytes and
macrophages. Provided that the cut ends are held closely together, budding and
union of cut fibers occurs, suggesting that effective regeneration has taken place
across the very short gap length (McMinn 1969). On the other hand, evidence of
scar formation has been reported when the distance between the cut ends has been
greater than a few millimeters (Volkman 1893; Allbrook 1962). It has been hy-
pothesized that scar formation in the gap effectively prevents connection of muscle
fibers (McMinn 1969). In contrast, injury of skeletal muscle by methods (freezing,
ischemia) that killed the cells without disorganizing the matrix, was followed by re-
population of the defect with new cells and recovery of physiological structure after
3 weeks (Vracko and Benditt 1972). Contrasts in healing behavior between injuries
of different severity are being studied with an eye toward understanding the cellular
pathways that account for the typically degenerative processes of muscle healing
(Gharaibeh et al. 2012; Shin et al. 2014).
The examples in this section illustrate the prevalence of irreversible injury in
several, distinctly different, anatomical sites in the adult mammal. No organ, not
even liver, is spared the irreversible loss of its structure and function at the site of
injury (Fig
1.5
).
1.9
Nature of Injured Tissue vs. Critical Size of Defect
Consideration must now be given to the detailed type of injury in an organ that heals
irreversibly. There is need to consider in some detail the size of wound that will be
studied as well as the nature of tissues that will be experimentally injured. It often
appears that the size of the wound, or the extent of the injury, dictates the outcome
of the healing process. In a number of organs a small wound often heals reversibly
while a wound in the same organ that is deep or large in volume may heal irrevers-
ibly. In this section, we will review tissues in different organs in order to answer the
question: What makes an injury irreversible? Is it the size of the resulting wound or
is it the identity of injured tissues?
Epithelial and endothelial tissues
in all parts of the anatomy are widely reported
to be capable of spontaneous regeneration without apparent loss of structural or
functional characteristics (Hay 1966). For example, the epidermis of skin, a cellular
tissue layer about 100 µm thick in many mammals, comprises several cell layers
at states of increasing differentiation along the distal direction. A reproducible and
reversible injury that is confined to the epidermis can be produced by tape stripping
(Pinkus 1952; Stoschek et al. 1992). In one study, when the tape was applied ten
