The C. elegans germ line proliferates both during larval development and in adult
worms. Indeed, the germ line is the only proliferative tissue in the adult worm. In
contrast to the almost invariably somatic C. elegans development, germ-line
development is much more variable. As in mammalian tissues, populations of germ
cells respond to growth-factor regulation and are subject to stochastic events,
including the elimination of many cells by programmed cell death, to maintain tissue
homeostasis. Furthermore, as in mammalian tissues, C. elegans germ cells, but not
somatic cells, can respond to environmental stress, such as genotoxic stress, or to the
stress caused by bacterial infections, with programmed cell death (see below). The
only cell type within the germ line capable of undergoing programmed cell death is
the meiotic pachytene stage (Gumienny et al., 1999; Gartner et al., 2000). Within
the adult C. elegans hermaphrodite ovotestis, germ cells are only partially surrounded
by a plasma membrane and are therefore part of a syncytium (Hall et al., 1999;
Seydoux and Schedl, 2001). By convention, these partially enclosed nuclei are referred
to as cells. Within the germ line, germ cells progress through various stages of
differentiation (Figure 1). At the distal end of the U-shaped ovotestis, mitotic
precursor cells are generated throughout the worm’s life. Upon passage through the
transition zone, germ cells cease to divide and begin meiosis. The most abundant
population of meiotic cells is the pachytene stage of meiosis I, residing between the
transition zone and the bend of the gonad. Upon leaving the pachytene, germ cells
progress into the meiotic diplotene, cellularize, undergo the final stages of oogenesis,
and complete meiosis after fertilization upon passage through the spermatheca (Hall
et al., 1999; Seydoux and Schedl, 2001) (Figure 1). Under normal growth conditions,
approximately 50% of female germ cells are doomed to programmed cell death
(Gumienny et al., 1999). These cells are eliminated rapidly by the corpse-engulfment
machinery, resulting in a steady-state level of 0–4 morphologically apoptotic cells
(Gumienny et al., 1999).
4.
Identification and quantification of programmed cell death in
C. elegansCell death in C. elegans can be readily observed in living animals by standard
Normasky optics (Sulston and Horvitz, 1977). The first sign of impending death is
a decrease in the refractivity of the cytoplasm that occurs concomitant with a slight
increase in the refractivity of the nucleus. Soon afterwards, both nucleus and
cytoplasm become increasingly refractile until they resemble a flat disk. After about
10–30 min, this disk starts to disappear, and the nucleus of the dying cell decreases
in refractility, begins to appear crumpled, and finally vanishes within less than 1 h
(Figure 2) (Sulston and Horvitz, 1977). The morphology of the corpses as well as the
kinetics of their disappearance is similar in somatic and germ cell apoptosis (Figure 2).
However, as germ cells are only partially surrounded by a plasma membrane, the first
step in germ cell death is the full cellularization of the apoptotic cell (Gumienny et
al., 1999).
PROGRAMMED CELL DEATH IN C.ELEGANS 163