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The terms apoptosis, programmed cell death, and
physiological cell death have become commonplace in
the scientific literature. Publications delineating various aspects of the cell death process in various experimental systems have grown at an exponential rate over
the last decade. The fundamental tenet emerging as a
result of this intensive effort is that cell death represents an indispensable process in all multicellular animals. In the developing embryo, cell death occurs as a
programmed event (programmed cell death) whereby
individual cells or groups of cells die in a spatially and
temporally defined fashion. This mechanism ensures in
part that morphogenesis proceeds in an orderly fashion. In adult tissues, cell death also functions in complementary fashion to mitosis, ensuring that the correct numbers and types of cells are maintained in the
animal. In many cells, death appears to require triggering and execution of a genetic program. Great
strides have been made of late by the molecular cloning
and characterization of genes which are functionally
involved in the cell death program. Although our understanding of the underlying molecular mechanisms
are still rudimentary, many of these genes appear to be
structurally and functionally conservedthroughout evolution (Schwartz and Osborne, 1993). Clearly, much
remains to be learnt about this fascinating process and
will thus likely be the focus of intensive investigation
for many years to come.
As described throughout this issue, one of the most
prevalent forms of cell death is a morphological and
biochemical process known as apoptosis, which entails
chromatin condensation and margination, followed by
membrane blebbing, fragmentation of the cell into
membrane-bounded structures (called apoptotic bodies), and phagocytosis by neighbors or professional
phagocytes. During this process, nuclear DNA is often
fragmented at internucleosomal sites which can be detected by conventional agarose gel electrophoresis. Unfortunately, the term apoptosis has ofien been used interchangeably (and incorrectly) with programmed cell
death, thus creating a great deal of confusion in the
scientific literature. As outlined above and recently by
others as well (Majno and Joris, 19951,programmed
cell death occurs primarily in a developmental context.
However, there are many examples in which nonapoptotic programmed cell death also occurs in animal cells,
which until recently (Clarke, 1990;Schwartz and Osborne, 1993) had been ignored by the cell death community. Therefore, a primary aim of this topical issue
of Mic;oscopy Researih and TechnQue is to distinguish
apoptotic from nonapoptotic morphologies in selected
model systems. For instance, nonapoptotic programmed cell death is characteristically seen during
insect metamorphosis when entire organs are removed
and the organism undergoes extensive remodelling.
This point is discussed eloquently by Bowen et al.
(blow-fly salivary gland model) and Zakeri et al. (tobacco hornworm labial gland model). As discussed by
these authors, nonapoptotic cell death during insect
metamorphosis likely involves expression of lysosomal
(Zakeri et al.) and nonlysosomal (Bowen et al.) hydrolases. Interestingly, tissue regression in these systems
occurs in the absence of phagocytic involvement. By
contrast, three other model systems of programmed cell
death in which apoptosis appears to be the predominant (although not exclusive) morphology are also presented in this issue by L a w n et al. (colonial ascidiam), Ishizuya-Oka (amphibian metamorphosis), and
Hurle et al. (developing vertebrate limb). Lastly, in the
remaining part of this issue, four additional articles
depict model systems in which cell death by apoptosis
can be triggered by physiological (Oberhammer et al.;
Odaka and Ucker; Laster and Mackenzie) and nonphysiological (Vaux et al.) stimuli. Also described in
these articles are a variety of current methodologies
used in evaluating morphological (enzyme histochemistry, confocal and fluorescence microscopy, transmission and scanning electron microscopy) and biochemical (in situ end-labeling, in situ nick translation, gel
electrophoresis) changes which occur during apoptotic
and nonapoptotic forms of cell death. It is thus hoped
that the information provided in this issue will enable
the reader of Microsco~yResearch and Technique to
attain a more comprehensive understanding of the
multiple morphologies underlying cell death and that
the reading proves as enjoyable as was the task of assembling this topical issue.
Departments of Pediatrics, and
Microbiology, Immunology and Molecular Genetics
The Albany Medical College
Albany, New York 12208
Clarke, P.G.H. (1990) Developmental cell death: Morphological diversity and multiple mechanisms. Anat. Embryol., 181:195-213.
Majno, G., and Joris, I. (1995) Apoptosis, oncosis, and necrosis. An
overview of cell death (Review). Am. J. Pathol., 1463-15.
Schwartz, L.M.,and Osborne, B.A. (1993) Programmed cell death,
awutosis and killer genes. Immunol. Today, 14582-590.
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