Tissue & Cell 35 (2003) 213–222
Morphological and ultrastructural analysis of Turritopsis
nutricula during life cycle reversal
E.C. Carla’, P. Pagliara, S. Piraino, F. Boero, L. Dini∗
Department of Biological and Environmental Science and Technology, University of Lecce, Via per Monteroni, 73100 Lecce, Italy
Received 13 December 2002; received in revised form 28 February 2003; accepted 7 March 2003
The hydrozoa life cycle is characterized, in normal conditions, by the alternation of a post-larval benthic polyp and an adult pelagic
medusa; however, some species of Hydrozoa react to environmental stress by reverting their life cycle: i.e. an adult medusa goes back to the
juvenilestageof polyp. This very uncommon life cyclecouldbe considered as some sort ofinvertedmetamorphosis.Amorphological study
of different stages during the reverted life cycle of Turritopsis nutricula led to the characterization of four different stages: healthy medusa,
unhealthy medusa, four-leaf clover and cyst. The ultrastructural study of the cellular modiﬁcations (during the life cycle reversion of T.
nutricula) showed the presence of both degenerative and apoptotic processes. Degeneration was prevalent during the unhealthy medusa
and four-leaf clover stages, while the apoptotic rate was higher during the healthy medusa and cyst stages. The signiﬁcant presence of
degenerative and apoptotic processes could be related to the occurrence of a sort of metamorphosis when an adult medusa transforms itself
into a polyp.
© 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Hydrozoa; Apoptosis; Lectin-binding sites
The hydrozoa life cycle is generally characterized by the
alternation of a post-larval benthic polyp and an adult pelagic
medusa; but in some species a reduction or a suppression of
polyp or medusa stage can be observed (Boero and Bouillon,
1993). In a complete cycle, the mature medusa releases ga-
metes into the water where fertilization occurs; while in
species without the medusa stage, gametes develop in spe-
cial structures (gonophores). After a short period of free life,
the resulting planula larva settles and metamorphoses into
a benthic polyp. During metamorphosis there are running
programs which initially strive to reduce all body parts that
are no longer necessary. Afterwards, a distinct turning point
is reached, followed by the subsequent development of fea-
tures, ﬁnely represented by a primary polyp (Seipp et al.,
2001). This polyp buds other polyps and forms a colony,
but can also bud medusae that detach themselves from the
colony to spend a free life, until the gametes are released,
after which they die.
∗Corresponding author. Tel.: +39-0832-298614;
E-mail address: firstname.lastname@example.org (L. Dini).
In a normal life cycle, stress (all factors causing change
in a biological system), which is potentially injurious and
whose effects may be lethal or sub-lethal, is considered to
be a disadvantage for animals. However, stress is not always
harmful and may trigger very different biological responses;
indeed a variety of stress responses have been measured
in biological systems exposed to various kinds and intensi-
ties of stress factors (Karlin and Brocchieri, 1998; Kregel,
Among Hydrozoa, some species seem to be able to re-
act to stress in an unexpected way: the medusa drift back
ontogenetically to a polyp, generally considered to be a
juvenile stage (Boero et al., 1997). Turritopsis nutricula
was the ﬁrst described Hydrozoan species able to revert its
life cycle (Bavestrello et al., 1992). The authors claimed
that the reversion occurred only in young medusae, and not
in sexually mature animals. The medusa, under stressed
conditions, develops back to the polyp stage. During this
reversion cellular reorganization is needed; differentiated
cells return to an undifferentiated stage to change their fate
(Piraino et al., 1996). Indeed, by applying different types
of stresses (changes in temperature, salinity, O2concentra-
tion, lack of food), Piraino et al. (1996) induced reversion
and demonstrated that a set of undifferentiated cells in the
0040-8166/03/$ – see front matter © 2003 Elsevier Science Ltd. All rights reserved.
214 E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222
transforming medusa produced a polyp; this property has
also been observed during indirect development, when un-
differentiated set aside cells retain the potential to divide
indeﬁnitely and thus to build large structures (Davidson
et al., 1995; Peterson et al., 1997). However, different
mechanisms of differentiation can be evoked to explain the
reverted life cycle in T. nutricula: i.e. some differentiated
cells can also participate in polyp reconstruction, by the
activation of a process similar to metamorphosis.
Metamorphosis generally occurs in those species charac-
terized by indirect development. During this period, in which
a larva transforms itself into an adult, degenerative as well as
apoptotic processes have been described for a wide range of
species: amphibians and insects represent good models for
this (Vogt, 1842; White et al., 1994; Truman, 1984; Sanders
and Wride, 1995). Apoptosis or programmed cell death is a
process fundamental for correct development as well as for
the maintenance of the organism’s integrity by destroying
cells that are potentially harmful. This process may be im-
portant to redeﬁne the body structure of the polyp generated
during the reverted life cycle of T. nutricula.
The present paper describes a morphological study of dif-
ferent stages during the reverted life cycle of T. nutricula,
giving evidence for degenerative and apoptotic processes, as
well as the capacity of the mature medusa to revert.
2. Materials and methods
Hydrozoa colonies were collected by scuba diving near
the Cheradi Isles (Mar Piccolo of Taranto, Italy) during
late summer and autumn (from August to November) 1999.
Colonies were kept in aerated sea water and identiﬁed in the
laboratory by a stereoscope (Leica MZ12).
Fertile colonies of T. nutricula were isolated and kept
in ﬁltered sea water (FSW) at 22◦C. The newly generated
medusae were isolated and fed with Artemia naupli. Daily
changes of FSW were performed. Different stresses were
applied to verify the medusae’s capacity to react by reorga-
nizing itself into a polyp. Umbrella excision with scissors,
raising the temperature to 30 ◦C for 30min or oxygen reduc-
tion by adding to large a quantity of nauplia were applied.
2.2. Transmission electron microscopy (TEM)
Different stages of T. nutricula ﬁxed in 2.5% glutaralde-
hyde sea water solution for 2h at ice temperature were
washed three times in FSW. Post-ﬁxation was performed
with 1% OsO4in FSW for 1h. Samples were then washed
several times in FSW and dehydrated with increasing con-
centration of alcohol for 30min each at room tempera-
ture, then embedded in epoxy resin. Ultra-thin sections were
stained with uranyl acetate and lead citrate and examined
under a Philips CM12 TEM.
2.3. Scanning electron microscopy (SEM)
Different stages of T. nutricula were ﬁxed in 2.5% glu-
taraldehyde and post-ﬁxed in 1% OsO4in FSW as described
above. They were dehydrated with acetone, desiccated with
2Critical Point Dryer 020 (Balzer, Liechtenstein), spat-
tered with gold (Coated 040 Balzer) and observed under a
Philips XL50 scanning microscope.
2.4. Confocal microscopy
Different stages of T. nutricula were ﬁxed in 4% buffered
paraformaldehyde and embedded in parafﬁn; 3-m thick
sections were analyzed by using FITC-conjugated lectins.
The following lectins were used (the concentrations and the
sugar speciﬁcities are given in brackets): Ulex europaeus
(40g/ml, ␣-fucose); Concanavlin-A (ConA; 40g/ml,
mannose); Ricinus (2g/ml, ␤-gal). All lectins were pur-
chased from Sigma (St. Louis, MO, USA).
Surface sugar localization was investigated by using a
Nikon PCM 2000 microscope (Nikon, Japan) with Plan
Fluor objectives (Nikon, Japan). Confocal microscopy was
performed utilizing a confocal laser scanning head Nikon
PCM 2000 based on a Nikon Eclipse 600 microscope. Ac-
quisition and visualization are completely computer con-
trolled through the EZ 2000 software (Coord-Nikon, The
2.5. Light microscopy
Semi-thin sections from the same embedded TEM sam-
ples were stained with toluidine blue and silver staining.
For a better evaluation of apoptotic nuclei, silver staining of
sections was also performed, by using a mixture of silver
nitrate (3m/ml) and hexamethylenetetramine (75g/ml).
Turritopis nutricula was able to survive after different
types of stress, by a reorganization of adult tissues into a ju-
venile form. The most reproducible stress able to trigger life
cycle reversion was the exposure of the medusae to higher
temperatures (30◦C) than normal (22◦C). Under these con-
ditions, 80% of medusae activated life cycle reversion.
By means of stereoscopy observation, we identiﬁed four
easy to distinguish stages during the inverted life cycle
and we named them: healthy medusa, unhealthy medusa,
four-leaf clover and cyst. The light micrograph in Fig. 1
shows their very different macroscopic morphology.
Each stage had the following characteristics. The healthy
medusa had a bell-shaped umbrella with long tentacles and
swam actively (Fig. 2a). The unhealthy medusa was not able
to swim and maintained its tentacles in a retracted position;
the transparency typical of the normal healthy medusa was
lost (Fig. 3a). The four-leafed stage was characterized by the
E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222 215
Fig. 1. Life cycle of Turritopsis nutricula: (a) healthy medusa; (b) unhealthy medusa; (c) four-leaf clover; (d) cyst; (e) polyp. The double arrow indicates
the point of reversion of the cycle.
absence of tentacles, by the reduction of the sub-umbrella
cavity which showed a number of lobes and many degenera-
tive processes (Fig. 4a and b). The cyst stage had a spherical
shape and a smooth surface (Fig. 5a). It was able to attach
to the substrate and rapidly gave rise to a polyp morphologi-
cally similar to those generated during the normal life cycle.
By means of light, ﬂuorescent, scanning and transmis-
sion electron microscopy the microscopic morphology of
the four stages was observed in order to analyze the mor-
phological modiﬁcations occurring at cellular level. In the
healthy medusa (Fig. 2), a large sub-umbrella cavity with a
central manubrium was observed (Fig. 2a and b). The um-
brella was delimited externally by an ectodermal layer, made
up of ﬂat cells interconnected by tiny cytoplasmic protru-
sions (Fig. 2c, arrow). Mitochondria, glycogen droplets, and
rough and smooth endoplasmic reticula were abundant in
these cells. Epithelial muscle striated cells were detected as
well (Fig. 2d). The gonads and the gastric cavity in commu-
nication with the radial channels were recognized (Fig. 2e).
The ultrastructural investigation of the manubrium revealed
its highly complex organization, with many morphologically
different cytotypes: in particular, muscle cells with ﬁbers ra-
dially oriented and glandular cells (Fig. 2f). Apoptotic cells
were also present in the manubrium (Fig. 2g and h).
In the unhealthy medusa (Fig. 3), the tentacles and
sub-umbrella cavity were reduced. The umbrella cells
showed many large empty vacuoles (Fig. 3c); many sarcom-
eres and mitochondria were still present in the epithelial
muscle cells, although a dramatic alteration of their mor-
phology was observed (Fig. 3d). Even more modiﬁcations
were revealed in the manubrium: cells lost cellular contacts
but maintained their junctional contacts (Fig. 3e, arrow-
head). The ectodermal cells had few cytoplasmic organelles;
endodermal cells were round; large electron dense vacuoles
were present in their cytoplasm (Fig. 3f). Some of these
cells showed clear signs of degeneration (Fig. 3g), probably
due to lytic phenomena, while other cells showed typical
features of apoptosis (Fig. 3h and i), However, the number
of apoptotic cells was lower than in the healthy medusa
(Table 1). Intracellular content and mesoglea were lost.
In the four-leaf clover stage (Fig. 4), the morphological
damage described for the unhealthy medusa became more
evident. Indeed, stress induced the total disappearance of
Apoptosis and necrosis rate in the different stages of reverted life cycle
Stage Apoptosis (%) Necrosis (%)
Healthy +++ −−
Four-leaf clover ++++ −+
Cyst +++ ++
216 E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222
E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222 217
tentacles (Fig. 4a) and a further regression of the umbrella,
delimiting an irregular and more restricted sub-umbrella cav-
ity (Fig. 4b). Some of the umbrella cells had a round shape
and maintained cellular contacts (Fig. 4c), while other cells
were highly damaged showing many empty vacuoles in-
side the cytoplasm (Fig. 4d). The apoptotic rate was higher
growing (Fig. 4e and Table 1). Conversely, the muscle cells,
with radially oriented ﬁbers (Fig. 4e, arrow), and the gonads
(Fig. 4f) were morphologically unchanged.
The medusae that lost all tentacles and the sub-umbrella
cavity that became spherical were deﬁned cysts (Fig. 5).
Under SEM, this type showed a smooth and regular sur-
face (Fig. 5a). Light microscopy observation revealed a
two-layered spherical structure (Fig. 5b). The external cell
layer mainly consisted of two different cytotypes, which
were regularly distributed along the surface. In Fig. 5b,it
is possible to distinguish cells (probably gland cells “gc”,
mainly localized in the lower left part of the cyst) with
many fenestrae and a lesser quantity of small cytoplasmic
inclusions; cells rich in cytoplasmatic inclusions (probably
zymogenic cells, “zc”) were mainly concentrated in the
upper part of the cyst. The probable initial outline of the
stolon, seen as a protrusion of the left part of the cyst, is
shown in Fig. 5b (asterisk). In contrast, the internal layer
had a less regular organization and a great number of mor-
phologically different cells could be recognized. A speciﬁc
spatial distribution, probably related to the intense reorga-
nization of this stage, was observed for cells containing a
large number of zymogenic granules in the cytoplasm. The
cells surrounding the stolon’s outline are packed extremely
tightly. These cells are probably involved in the process
of stolon formation. The ultrastructural TEM investigation
showed that the cells of the external layer re-established
contacts (Fig. 5c and d) lost during the previous stages. The
fenestrate cells of the superﬁcial layer are in contact with
each other and are divided from the inner cellular layer of
the cyst by compact and homogeneous substances, probably
mesoglea (Fig. 5c, mes). Interaction among cells was seen
to resume in the inner layer of the cyst, while isolated cells
could still be recognized in the cyst center (Fig. 5e). Many
of these cells showed condensed chromatin, as evidenced
by silver staining (Fig. 5f) and conﬁrmed by TEM obser-
vation (Fig. 5g), thus indicating the presence of an intense
apoptotic rate. Gonads (Fig. 5i) were also present at this
stage as well as muscle tissue (Fig. 5h) in the cysts with
By using FITC-conjugated lectins, we studied saccharide
residue exposure on T. nutricula cell surfaces during the
polyp, healthy medusa and unhealthy medusa stages. Sig-
Fig. 2. Healthy medusa. (a) Stereoscopic image of a medusa, showing the section plane of panel b; (b) semi-thin section of a medusa stained with
toluidine blue (magniﬁcation 20×); (c) umbrella cells with tiny touching protrusions (arrow) (magniﬁcation 6500×); (d) epithelial and muscle cells with
clearly visible sarcomeres (arrow) (magniﬁcation 7000×); (e) semi-thin section of manubrium showing gonads; (f) high magniﬁcation of gonads at E.M.
(magniﬁcation 5000×); (g) manubrium cells, “gl” a glandular cell, “mc” transversal section of muscle cell (magniﬁcation 3000×); (h) apoptotic cell (the
nucleus with condensed chromatin) inside the manubrium (magniﬁcation 5000×).
niﬁcant differences between the healthy and the unhealthy
medusae were observed (Fig. 6). ConA, Ricinus communis
and Ulex binding sites were found in all the studied stages,
but differences in distribution and ﬂuorescence intensity
were observed by a confocal microscope. Fluorescence was
quite intense in the gonads of unhealthy medusae while the
gonads of healthy medusae were labeled to a much lesser de-
gree. The cells lining the radial channels were also strongly
labeled and the labeling increased in the unhealthy medusae.
In the umbrella, the cells of healthy medusae showed faint
labeling with small scattered clusters of ﬂuorescence. Con-
versely, the labeling increased in the unhealthy medusae as
did the number of ﬂuorescent spots. The labeling observed
in the polyp was homogeneously distributed throughout the
tissues, with ﬂuorescence intensity comparable to that of the
The uncommon life cycle of T. nutricula involves the
transformation of an adult medusa to a polyp, which some
authors consider a juvenile stage (Boero et al., 1997). This
event represents a reverted development involving a crucial
process characterized by drastic modiﬁcations, comparable
to a sort of metamorphosis. During metamorphosis a large
part of the cell population acquires the fully differentiated
characteristics of adult cells (Truman, 1984; Weis and Buss,
1987). Furthermore, this process, occurring in many verte-
brate and invertebrate species, is often accompanied by de-
generative and apoptotic processes, well described for the
resorption of amphibian tail (Sanders and Wride, 1995).
Our ultrastructural study of cellular modiﬁcations during
the life cycle reversion of T. nutricula has shown the pres-
ence of both degenerative and apoptotic processes. Degen-
eration is prevalent during the stages of unhealthy medusa
and four-leaf clover, while apoptotic rate is high during the
stages of healthy medusa and cyst. The presence, even in the
healthy medusa, of apoptotic events is not surprising due to
its role in preserving cellular homeostasis in the majority of
animals (Wyllie et al., 1980; Gupta, 1996). Apoptosis also
plays an important role in many developmental processes,
such as cell differentiation, oogenesis (Sommer et al., 1998),
organogenesis, as well as in the establishment of body struc-
tures (Jacobson et al., 1997; Sanders and Wride, 1995). The
existence of apoptosis and its morphogenetic role during
body reorganization (from larva to polyp) of Hydractinia
echinata was ﬁrst described by Seipp et al. (2001).InT.
nutricula’s reverted life cycle, the presence of degenerative
218 E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222
E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222 219
Fig. 4. (a) SEM micrograph of a four-leafed medusa showing the absence of tentacles (magniﬁcation 2000×); (b) stereoscopic image of a four-leafed
medusa; (c) round-shaped cells (magniﬁcation 3000×); (d) extensive vacuolization of ectodermal cells (magniﬁcation 2000×); (e) apoptotic cell (asterisk)
and radially sectioned muscle ﬁbers (arrow) (magniﬁcation 3000×); (f) gonads (magniﬁcation 2000×).
Fig. 3. Unhealthy medusa. (a) Stereoscopic image of an unhealthy medusa; (b) SEM micrograph of an unhealthy medusa showing the retraction of
tentacles (magniﬁcation 2000×); (c) manubrium cells showing two cells that have began to detach (arrow) (magniﬁcation 4000×); (d) umbrella cells with
a damaged cytoplasm. An epithelial muscle cell shows a reduction of the striated ﬁbers (arrows) (magniﬁcation 4500×); (e) ectodermal manubrium cells;
the cytoplasm has no intracellular organelles (magniﬁcation 5000×); (f) endodermal manubrium cells with cytoplasm rich in electron dense granules
(magniﬁcation 3500×); (g) endodermal manubrium cells. The cells are round and have lost contact with each other. Some show signs of necrosis (asterisk)
(magniﬁcation 5000×); (h) and (i) apoptotic cells inside the manubrium; h: initial stage of apoptosis with chromatin condensation and enlarged nuclear
cisternae (magniﬁcation 5000×); i: late/necrotic stage of apoptosis (magniﬁcation 6500×).
220 E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222
E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222 221
Fig. 6. Confocal images of Turritopsis nutricula (polyp, healthy and unhealthy medusa stages) sections labeled with ﬂuorescent FITC-conjugated lectins.
as well as apoptotic processes supports our idea that this
inversion is a sort of metamorphosis, utilized to reorganize
The cyst stage appears the most intriguing stage of the
entire reverted cycle of T. nutricula; this stage has a mor-
Fig. 5. (a) SEM micrograph of a cyst showing the almost smooth surface (magniﬁcation 3000×); (b) semi-thin section of a cyst. Two cellular layers are
shown. On the external layer the outline of the stolon is visible (see Section 3 for details) (magniﬁcation 1500×); (c) TEM micrograph of fenestrated cells
of the external layer. Mesoglea can be seen (mes) (magniﬁcation 3000×); (d) vacuolated and isolated cells of the central part of the cyst (magniﬁcation
2000×); (e) an apoptotic cell in secondary necrosis in the central part of the cyst (magniﬁcation 2000×); (f) silver methenamine staining of a cyst.
Many nuclei with condensed (apoptotic) chromatin are visible, in particular in the central region; (g) TEM micrograph of fenestrated cells of the external
layer with an intact and beam-shaped nucleus (magniﬁcation 2000×); (h) differentiated muscle cells with clearly visible sarcomeres in the part of the
cyst which is budding the stolon (magniﬁcation 2000×); (i) gonads (magniﬁcation 2000×).
phological organization similar to that of the planula larva:
cells are organized in two different layers. Therefore, it could
be hypothesized that cellular modiﬁcations (degeneration,
apoptosis and differentiation) during the cyst stage are simi-
lar to those characterizing the physiological metamorphosis
222 E.C. Carla’ et al./Tissue & Cell 35 (2003) 213–222
from planula to polyp; it follows that the cyst stage is not a
quiescent stage but more likely the most active one; indeed,
the high apoptotic rate could be a mechanism for cell selec-
tion, necessary for body reorganization and the development
of polyp tissues and structures.
Apoptotic process described in healthy medusae and in
the other stages, mainly cyst, may have different purposes.
In the healthy medusa, apoptosis is the normal process to
maintain cellular homeostasis; conversely in the cyst, apop-
tosis leads to dramatic body reorganization, a process similar
to what has previously been described for higher animals,
such as Drosophila (Jiang et al., 1997) or amphibians (Tata,
1993), in which apoptosis is used to remove typical larval
features, while simultaneously preserving those cells that
build up adult structures. Intriguing are the data concerning
the permanence of gonads throughout all stages. This rep-
resents clear evidence that both the young and the mature
medusa is able to revert its life cycle; further studies are
in progress to investigate their potential involvement in the
reconstruction of polyp structure.
Cellular morphological modiﬁcations that take place dur-
ing the reversion of the life cycle of T. nutricula underline,
of course, the various biochemical and molecular changes
affecting all compartments of the cell. This idea is supported
by the data obtained with a confocal microscope and re-
ported here concerning the analysis of the distribution of
lectin-binding sites. With this instrument many thin sections
through the sample were analyzed and very clean images
obtained by a virtual elimination of out-of-focus ﬂuores-
cence, thus increasing clarity and contrast within the im-
ages (Diaspro, 2002). The high exposure of surface sugar
residues was observed in the unhealthy medusa, thus indicat-
ing that reorganization of the cell surface takes place during
this stage, most likely in order to abolish cellular contacts
and prepare the cellular reorganization of the cyst stage.
In conclusion, the presence of degenerative as well as
apoptotic processes, during the reverted life cycle of T. nu-
tricula have been shown. These morphological data support
the idea that when a medusa transforms itself into a polyp,
a sort of metamorphosis occurs.
Bavestrello, G., Sommer, C., Sarà, M., Hughes, R.G., 1992. Bi-directional
conversion in Turritopsis nutricula. In: Bouilllon, J., Boero, F., Cicogna,
F., Gili, J.M., R.G. (Eds.), Aspects of Hydrozoan Biology, vol. 56.
Sci. Mar. pp. 137–140.
Boero, F., Bouillon, J., 1993. Zoogeography and life cycle patterns of
Mediterranean Hydromedusae. Biol. J. Linn. Soc. 48 (3), 239–266.
Boero, F., Bouillon, J., Piraino, S., Schmid, V., 1997. Diversity of
hydromedusan life cycle: ecological implications and evolutionary pat-
terns. In: Proceedings of the 6th International Conference on Coelen-
terate Biology 1995. pp. 53–62.
Davidson, E.H., Peterson, K.J., Cameron, R.A., 1995. Origin of bilate-
rian body plans: evolution of developmental regulatory mechanisms.
Science 270, 13919–13925.
Diaspro, A., 2002. Confocal and Two-Photon Microscopy: Foundations,
Applications and Advances. Wiley.
Gupta, S., 1996. Apoptosis/programmed cell death. A historical perspec-
tive. Adv. Exp. Med. Biol. 426, 1–9.
Jacobson, M.D., Weil, M., Raff, M.C., 1997. Programmed cell death in
animal development. Cell 88, 347–354.
Jiang, C., Baehrecke, E.H., Thummel, C.S., 1997. Steroid regulated pro-
grammed cell death during Drosophila metamorphosis. Development
Karlin, S., Brocchieri, L., 1998. Heat shock protein 70 family: multiple
sequence comparisons, function, and evolution. J. Mol. Evol. 47 (5),
Kregel, K.C., 2002. Heat shock proteins: modifying factors in physiolog-
ical stress responses and acquired thermotolerance. J. Appl. Physiol.
92 (5), 2177–2186.
Peterson, K.J., Cameron, R.A., Davidson, E.H., 1997. Set-aside cells in
maximal indirect development: evolutionary and development signiﬁ-
cance. Bioessays 19, 623–631.
Piraino, S., Boero, F., Aeschbach, B., Schmid, V., 1996. Reversing the life
cycle: medusae transforming into polyps and cell transdifferentiation in
Turritopsis nutricula (Cnidaria, Hydrozoa). Biol. Bull. 190, 302–312.
Sanders, E.J., Wride, M.A., 1995. Programmed cell death in development.
Int. Rev. Cytol. 163, 105–173.
Seipp, S., Schmich, J., Leitz, T., 2001. Apoptosis—a death-inducing
mechanism tightly linked with morphogenesis in Hydractinia echinata
(Cnidaria, Hydrozoa). Development 128 (23), 4891–4898.
Sommer, R.J., Eizinger, A., Lee, K.Z., Jungblut, B., Bubeck, A., Shlak,
I., 1998. The Pristionchus HOX gene Ppa-lin-39 inhibits programmed
cell death to specify the vulva equivalence group and is not required
during vulval induction. Development 125, 3865–3873.
Tata, J.R., 1993. Gene expression during metamorphosis: an ideal model
for post-embryonic development. Bioessays 15, 239–248.
Truman, J.W., 1984. Cell death in invertebrate nervous systems. Ann.
Rev. Neurosci. 7, 171–188.
Vogt, C., 1842. Untersuchungen über die Entwicklungsgeschichte der
Geburtshelferkröte (Alytes obstetricans). Solothurn, Jent & Gassmann.
Weis, V.M., Buss, L.W., 1987. Ultrastructure of metamorphosis in Hy-
dractinia echinata. Postilla 199, 1–20.
White, K., Grethner, M.E., Abrams, J.M., Young, L., Farrell, K., Steller,
H., 1994. Genetic control of programmed cell death in Drosophila.
Science 264, 677–683.
Wyllie, A.H., Kerrr, J.F.R., Currie, A.R., 1980. Cell death: the signiﬁcance
of apoptosis. Int. Rev. Cytol. 68, 251.