Genetics of Apoptosis

2.

Cell death in Cnidaria and Porifera

Cnidaria are the phylogenetically oldest Eumetazoa. The phylum includes the first
multicellular animals whose body plans are based on two important principles that
define the body plans of all higher animals. These are first the formation of epithelial
layers that later in phylogeny give rise to tissues and organs and second the presence
of pluripotent stem cells that, in cnidarians, give rise to nerve cells, nematocytes, gland
cells, and germ cells.
For developmental biologists, the most prominent cnidarian is Hydra. It is, in fact,
the oldest model organism in developmental biology. Experiments carried out in the
eighteenth century by the Swiss scientist Abraham Trembley revealed the almost
unlimited capacity of Hydra to regenerate. This property led to more refined
experiments to study pattern formation and stem-cell differentiation in Hydra
(Gierer, 1974) and, more recently, to analyze the molecular parameters of
development and regeneration (Broun et al., 1999; Technau and Bode, 1999;
Hobmayer et al., 2000; Smith et al., 2000). Recent work indicates that the cnidarians,
including Hydra and Hydractinia, can also teach us a few lessons about the evolution
of apoptosis and its role in growth regulation in multicellular animals, egg and sperm
development, and metamorphosis.

2.1

Apoptosis in Hydra regulates cell numbers

Hydra reproduces asexually by budding, or sexually by fusion of male and female
gametes. In laboratory culture, however, the preferred mode of reproduction is
budding. Under these conditions, the number of buds an animal produces depends
strictly on feeding. If animals are well fed, their numbers double every two to three
days. The same is true for total cell numbers in such animals. If Hydra is starved,
however, no buds are formed, and cell numbers remain roughly constant for a few
days before beginning a slow decline.
Comparison of cell-proliferation rates in fed and starved animals revealed that they
did not differ significantly (Bosch and David, 1984). This implied production of
‘excess’ cells under starvation conditions where the cell number did not increase in
size; indeed, it could be shown that cells were eliminated from tissue by phagocytosis
under these conditions. Phagocytic vacuoles filled with condensed cells containing
apoptotic nuclei were found in epithelial cells (Bosch and David, 1984). The number
of epithelial cells with phagocytic vacuoles depended directly on the feeding regimen
and was about sevenfold higher in starved animals than in fed animals.
Closer examination of the apoptotic process in Hydra has been achieved by
inducing cell death with colchicine or wortmannin. Rapidly cycling interstitial cells
are particularly sensitive to colchicine treatment (Campbell, 1976) and were
eliminated from treated tissue by apoptosis. Phagocytized corpses stained brightly
with acridine orange and exhibited pyknotic nuclear morphology with DAPI staining,

152 GENETICS OF APOPTOSIS