The Irreversibility of Organ Injury - Tissue and Organ Regeneration in Adults - page 3

Epithelial tissues comprise cells only, lacking blood circulation; the basement mem-
brane comprises ECM only, also lacking blood circulation; while the stroma com-
prises cells of various types, connective tissue as well as circulating blood. In this
volume we will follow this alternative classification of tissues.

1.3 Spontaneous and Induced Regeneration

Certain individual tissues, such as epithelial tissues, are capable of spontaneous
(unaided) regeneration following injury, even in the adult mammal; most other tis-
sues are not. A detailed account of this important phenomenon, which is unaided by
the investigator, will be presented later in this chapter and in the next one. Organs
are assemblies both of tissues that spontaneously regenerate, termed regenerative ,
and those that do not, i.e., nonregenerative .
Induced regeneration is aided by the investigator. Here, recovery of physiologi-
cal structure and function of nonregenerative tissues in an organ is brought about by
an investigator. Most organs in the adult mammal do not regenerate spontaneously
and require to be induced to do so. Studies of induced organ regeneration in the
adult mammal typically make use of external means, typically by application of the
appropriate cells, matrices, cytokines, or a combination, to the site of injury. In the
literature, the progress of induced regeneration of tissues has been often observed
only by morphological methods whereas regeneration of an organ, an assembly of
tissues, has frequently been monitored by functional methods as well. The practice
and theory of induced tissue and organ regeneration, and the general principles that
emerge from such practice, are the topics treated in this volume.

1.4 Diversity of Spontaneously Regenerative Phenomena

A survey of the literature shows use of the term “regeneration” in a variety of con-
texts (Goss 1992; Stocum 1995; Call and Tsonis 2005; Brockes and Kumar 2008).
Physiological regeneration (Hay 1966) has been used to describe the processes by
which living organisms engage in continuous self-renewal or turnover. For instance,
progenitor cells are continuously produced in the bone marrow and replace older
blood cells; basal cells are continuously produced in the innermost layer of the
epidermis and replace older cells in the outermost layer of skin. These ubiquitous
“cell turnover” phenomena (Brockes 1997) do not necessarily result directly from
acute or chronic injury. On the other hand, loss of blood from a hemorrhage or loss
of the epidermis due to a sunburn are reversible losses of tissue following injury
and the response to these losses has been referred to as reparative regeneration
(Hay 1966). Compensatory growth (or hypertrophy) is the adjustment of the mass
and corresponding function of an organ to the changed mass of an organ or to the
changed needs of the organism as a whole (Goss 1992). It does not always follow
injury: Overuse of an organ leads to hypertrophy while disuse leads to atrophy.
Examples are the reversible hypertrophy of heart muscle (Verdecchia et al. 2004)