AMER. ZOOL., 14:833-849 (1974).
Growth, Degrowth, and Irreversible Cell Differentiation in Aurelia aurita
W. M. HAMNER
Department of Zoology, University of California, Davis, California 95616
AND
R. M. JENSSEN
Stanford Medical School, Stanford University, Palo Alto, California 94305
SYNOPSIS. Growth patterns of the Scyphomedusa Aurelia aurita from Tomales Bay.
California, were examined in the field and in the laboratory. Manipulation of growth
patterns demonstrated that degrowth and regrowth are not constrained by initial
ievelopmental stage. Although initial degrowth of certain tissues is allometric (e.g.,
gonads regress in 5 to 8 days; bell diameter decreases more rapidly at first than do
the oral arms), thereafter regression appears identical to, but reversed from normal
growth. Regrowth patterns are normal. Sexual maturation in the sea does not always
alter subsequent capacity for degrowth or regrowth to sexual maturity in the laboratory, because reproductive and somatic tissues do not always degenerate after spawning.
Gonadal tissue can be renewed and maintained in a ripe condition in the laboratory
apparently indefinitely. Sexual maturation is a size-dependent phenomenon, not an agespecific developmental event.
Spermatogenesis, once initiated, proceeds irrespective of outside events. Labeled
spermatogonial cells can continue to differentiate to form sperm even though the gonad
containing those cells, and the animal itself, show rapid degrowth. The importance of
this decoupling of developmental events is discussed. The experimental importance of
animals with flexible life cycles is emphasized.
nisms. Botanists, for example, are usually
concerned with growth because this is imThe discipline of development is nomi- portant not only in the phenomenon of
nally concerned with the three basic phe- general enlargement of the growing organomena of cell differentiation (develop- nism but because one of the most signifimental diversification of cell types, cant components of morphogenesis in
morphogenesis (the class of processes lead- plants involves aspects of differential
ing to correct form and placement of cells growth or asymmetric enlargement. The
and tissues), and growth (the general in- importance of differential growth in plants
crease in size of the organism). It is inter- is especially clear because morphogenetic
esting that botanists and zoologists em- movement, an important aspect of morphasize these three areas differently because phogenesis in animals, does not occur to a
of the nature of their experimental orga- significant extent in plants; cell walls restrict the movement of plant cells. Zoolo———-——-—————-—— — : : gists, on the other hand, when discussing
INTRODUCTION
We thank Dr. Cadet Hand of the University of
California Bodega Marine Laboratory for space, encouragement, and discussion. We thank Dr. Milton
J. Boyd for technical assistance and for suggestions
regarding the manuscript, and Dr. P. B. Armstrong for help with the autoradiography and for
thoughtfully criticizing the manuscript This work
was partly supported by NSF GB 7691 to W. M.
°
'
.
' .
°
morphogenesis, Stress the importance of
cell movement and tend to de-emphasize
the subject of growth. Most common labor a t o r y animals have inflexible life cycles,
, ' • -.•a- ,
.
,
,
a n d lt
1S d l f f i c u l t t O
investigate the pher
nomenon of animal growth experimentally
Hamner and C. Hand.
because animals usually grow larger, be833
834
W. M. HAMNER AND R. M. JENSSEN
come sexually mature, get old, and die
with discouraging precision. Students of
animal growth have partly circumvented
this difficulty with an array of powerful
and ingenious techniques (see Goss, 1964),
but one is nonetheless left with the distinct
impression from recent textbooks that the
rigid life cycle of most laboratory animals
has constrained the experimentalist and
has historically limited our knowledge of
animal growth. This impression is even
more deeply imprinted when one compares
most recent textbooks with those of 40
years ago (Huxley, 1932; Huxley and
deBeer, 1934). Then the subject of growth
was embossed into virtually every page of
the text, and entire chapters were often
devoted to unusual growth patterns in the
belief that a comparative acquaintance
with the bizarre might clarify the familiar.
But such logic did not prevail, and the
examples of growth that then seemed so
fascinating are now mostly forgotten or
ignored (but see Berrill, 1961; Kum£ and
Dan, 1968).
Among the many compelling, but long
forgotten, odd growth patterns is the phenomenon of degrowth, in which the entire
animal becomes smaller when deprived of
food, regressing until it resembles a diminutive adult.1 Whole animal size reduction
has been reported for a wide range of invertebrates, principally coelenterates and
flatworms (see especially Child, 1911, 1915;
Mayer, 1914; Huxley and deBeer, 1923;
deBeer and Huxley, 1924), and is familiar
to most invertebrate zoologists, but the
phenomenon has been reinvestigated only
recently by developmentalists (Beck and
Bharadwaj, 1972). Degrowth should be a
powerful tool for manipulating developmental events because not only can absolute size of an animal be altered, but also
the length of the entire life cycle and the
pattern of reproductive development can
be modified. Manipulation of growth in
familiar laboratory animals is certainly also
possible, but none of the experimental
i Forgotten, indeed! Beck and Bharadwaj (1972)
state: "Experimentally reversed development has
never been accomplished with a metazoan animal."
techniques currently available, such as induced compensatory hypertrophy, can
modify simultaneously characteristics as
diverse as size, longevity, and reproductive
development.
One of the last investigations of degrowth cited in the literature (deBeer and
Huxley, 1924) used the Scyphozoan medusa Aurelia aiirita as an experimental
animal. This is a most favorable organism
indeed. Aurelia medusae (Fig. 1) are relatively large (15 cm bell diameter) but can
be crowded and raised easily in the laboratory (Spangenberg, 1965). Individual
animals can also be marked, recognized,
and followed sequentially via biopsy of
selected tissues. Aurelia adults the size of
dinner plates can be regressed to the size
of a dime and then regrown to the fully
mature condition, so that size and differentiation of organs and tissues can be experimentally manipulated in the laboratory.
Furthermore natural populations can be
easily sampled in the field. We sampled
a population of Aurelia inhabiting Tomales Bay, California. These medusae are
resident, restricted to the upper end of the
bay, and accessible. Thus, life history data
could be obtained and the normal growth
sequence easily denned.
The experiments presented herein explore, specifically, the relationship between
whole animal regression and differentiation
of spermatogonial cells. Two types of experiments were conducted. Whole animal
growth studies tested the hypothesis that
degrowth and recovery were not size, time,
or reproductively dependent phenomena
and also compared the variance in population regression rates with those of marked
individuals. The second type of experiment
was concerned with differentiation of spermatogonial cells and tested the hypothesis
that whole animal regression does not
necessarily affect cellular differentiation sequences even though the organ containing
those cells and the animal containing that
organ regress rapidly. These data are discussed and some speculations regarding
cellular population regulation and sequences of differentiation within given cell
lineages are considered.
DEGROWTH AND CELL DIFFERENTIATION IN
MATERIALS AND METHODS
Medusae were individually collected by
hand from Tomales Bay, California, from
April to July 1970. Some data were also
obtained from medusae collected in May
1971. Medusae were rapidly transported in
plastic buckets to the Bodega Marine
Laboratory, Bodega Bay, California, so that
the water in the buckets did not heat and
cause the bells to deform. Animals were
maintained in 35-gal plastic garbage pails
at 16 to 18 C. Four adults (13 cm bell
diameter) or about 30 juveniles (5 cm bell
diameter) could be maintained in each
pail. They were fed brine shrimp (Artemia
salina) daily; the water was changed twice
a week and the pails thoroughly scrubbed.
Water was agitated by slowly bubbling air
into the pails; air stones were not used.
Bell diameters were measured usually
every 8 days. Four measurements were
taken adradially from each individual
across the center of the bell and the average
of the three most similar measurements
recorded. The measurements were obtained by dipping out a medusa onto a
large flat dish, placing the jellyfish on its
exumbrellar surface, and draining the
water. Animals handled this way were almost never damaged. Marked animals that
underwent sequential gonadal biopsy had
portions of the gonad removed either by
forceps or by suction from an eye dropper
inserted through the membrane of the subgenital pit into the gonadal cavity. Tissues
were immediately preserved in either
Bouin's or 5% gluteraldehyde. Tissues
were stained with Harris' haemotoxylin
and eosin.
Two autoradiographic analyses were
attempted. In the first experiment methyl
3
H-thymidine in aqueous solution (0.25
me) was added to 3 liters of sea water containing four mature male Aurelia. Brine
shrimp were added to induce feeding and
enhance penetration of the thymidine into
the gonadal cavity. The animals were
allowed to feed in this solution for 1 hr,
then the solution was removed and replaced with fresh sea water containing un-
Aurelin
835
labeled "cold" thymidine concentrated to
104 times that of the radioactive material.
The Aurelia remained in this solution for
the next 8 hr. The solution was changed
twice more in 24 hr, with radioactive sea
water waste monitored via liquid scintillation. Most of the labeled thymidine was
removed on the first wash. The testes of
individually marked medusae were serially
biopsied on clays 1, 2, 4, 8, and 16.
The second pulse-label experiment was
performed like the first, except that 0.04 me
of methyl 3H-thymidine was injected directly into each of the four gonadal cavities of five mature medusae immobilized
on the exumbrellar surface in a partly
drained large dish. The gonads were incubated for 31/9 hr. The medusae were
then placed in buckets of sea water and
allowed to feed. The water in the buckets
contained unlabeled "cold" thymidine at
approximately 104 times the concentration
of the solution injected into the gonadal
cavity. Inasmuch as the gonad and gastric
filaments of the stomach of Amelia are
contiguous tissues, the presence of a feeding current in the gut and radial canals
ensured that the labeled thymidine would
be washed out of the gonadal cavity into
the surrounding sea water, the specific activity of which was followed via liquid
scintillation. Tissues were sampled and
treated for histological examination as
noted for the previous experiment. Autoradiography of tissue sections employed
Kodak Nuclear Track Emulsion (NTB2)
with a 2-month exposure time.
RESULTS
Life cycle
The life cycle of Aurelia aurita is well
known (Hargitt, 1902; Berrill, 1949;
Spangenberg, 1965; Yasuda, 1969; Russell,
1970). Fertilized eggs develop on the oral
arms of the medusa (Fig. 1) into hollow,
undifferentiated, ciliated larvae called
planulae. When released, these disperse in
the plankton and thereafter settle on a
suitable substrate where they metatnor-
836
W.
M. HAMNER AND R.
M. JENSSEN
ciliated planula
larva
MALE QR FEMALE ADULT
ephyra
I cm
bell dia.
FIG. 1. Life history of Aurelia aurita. The four
dark crescent objects in the mature adult medusa
are the gonads. Note the extreme size increase from
the 1-cm juvenile medusa to the 15-cm adult.
phose into tiny 2 mm polyps called scyphistomae. The scyphistoma is a perennial,
asexual attached hydranth stage that reproduces continuously throughout the year
by budding. However, at particular times
of the year (February in Tomales Bay)
budding ceases, the tentacles of the polyp
shorten, and 10 to 20 constrictions develop
about its stalk, each demarking a region of
the stalk that will transform into a tiny
larval medusa by an asexual reproductive
process known as strobilation. The larval
medusae, called ephyrae, arranged like a
tall stack of tiny saucers, detach sequentially and swim away. After strobilation is
completed, the remaining polyp tissue
transforms back into a tiny scyphistoma
and the yearly asexual budding pattern
resumes. Transformation from the eightlobed ephyra into a miniature adult is
rapid; subsequent growth to the full adult
size of about 13 cm bell diameter follows,
taking about 5 months. The medusae become sexually mature in midsummer,
spawn, release planulae, and then usually
die.
The actual seasonal growth pattern of
medusae in Tomales Bay, California, for
the years 1969 and 1970 is shown in Figure 2. In 1969 strobilation occurred in late
February or early March and the population grew synchronously and rapidly until
June. The adults became reproductive,
spawned, and died. The sample taken in
July of that year shows that pre-death tissue deterioration has characteristic effects
that are reflected in bell diameter. Medusae
at this time of year show a sloughing of tissue, a shortening of the oral arms, and a
constriction of tissue so that the normally
flattened medusa becomes quite rounded.
There are irregular holes and obvious localized sites of necrosis over the entire animal; bell tentacles are reduced or absent
DEGROWTH AND CELL DIFFERENTIATION IN
Aurelia
837
normally an annual organism. However,
during the summer of 1970 in Tomales Bay
the medusae of Aurelia aurita not only did
not die, but they continued to grow (in
some cases up to bell diameters of 17 cm)
and to reproduce continuously for another
full year, so that in February 1971 there
were both sexually mature adults and
newly released ephyrae present in Tomaies
Bay. This finding was irritating, because
we had already completed some experiments in the summer of 1969 based on the
assumption that Aurelia was an obligatory
annual both in the laboratory and field,
but this type of experiment, we think, is
still of interest. Even if Aurelia medusae
don't always die as conveniently as salmon
they are, nonetheless, fascinating animals.
Their ability to degrow when starved is of
particular interest.
and the medusae are generally misshapen.
The only tissue seemingly unaffected is the
gonad, which continues to produce eggs
and sperm while the somatic tissue deteriorates.
In 1970 the annual growth pattern was
different. Apparently two periods of strobilation occurred in early 1970 because the
young medusae collected in February,
March, and April of that year had a bimodal size distribution. By May both sets
of medusae were within the same size range
and could no longer be distinguished, and
therefore we did not plot a growth curve
for the 1970 population. Secondly, the
medusae did not die in July, but continued
to live for another full year (Hamner and
Boyd, unpublished). We had previously
believed Aurelia to be an obligate annual
organism. Our previous observations for
the two prior years in Tomales Bay as well
as reports in the literature (Hargitt, 1902;
Spangenberg, 1965; Yasuda, 1969; but see
Russell, 1970) indicate that Aurelia is
In order to test the hypothesis that degrowth and recovery were dependent upon
FIG. 2. Growth pattern of Aurelia aurita in Tomales Bay in 1969 and 1970. Solid triangles are
measurements pf flattened bell diameters of animals collected via a semi-random stratified sampling
regimen in 1969. Medusae deferiorated and died in
July, 1969. The drawings in the lower right shpw
the necrosis and loss of oral arms. In 1970 two
sets of ephyrae were released, as indicated by the
solid circles. In February, March, and April 1970
the medusae had a bimodal size distribution. By
May the size classes had become undistinguishable.
Growth curve drawn for the 1969 samples only.
Growth and degrowth
838
W. M. HAMNER AND R. M. JENSSEN
neither initial size nor reproductive state,
seven groups of variously juvenile and
sexually mature medusae, of up to 40 per
group and spanning the size range of 5 to
12 cm bell diameter, were held for 120 days
without food, and the diameter of the bells
12
of each group recorded every 8 days. At
selected intervals medusae were removed
from each of the seven groups and fed on
excess brine shrimp in order to determine
if they could recover and regrow at normal
rates. The initial size range selected al-
small
10
8 <
6 •
A
A
A
A
A
A
A
1
10
20
40
TIME
FIG. 3. Developmental stage vs. degrowlh and
regrowth potential in Aurelia. "Small" refers to
size range preselected in the field. Brackets and
numbers indicate sue range and number of starved
f
. D•
60
80
100
IN DAYS
controls. On clays 8 (solid circles), 32 (solid triangles) , and 40 (solid squares), selected medusae
were fed in separate containers to see i£ they could
grow.
DEGROWTH AND CELL DIFFERENTIATION IN
Amelia
839
A4
A
A
medium
12
*
•
11
[
:
i
10
O
8
A
A
A
A
A
•
6 -
2
<
II f
18
4 •
14
UJ
CD
11
1
11
11
11
10
2 -
20
40
TIME
60
80
100
IN DAYS
FIG. 4. Symbols as in Figure 3. 22 medusae with
mean bell diameter of 8.3 cm were starved for up
to 100 days. At selected intervals members of this
group were separated and fed brine shrimp daily.
None of these animals were initially sexually
mature.
lowed us to evaluate growth and degrowth
for small medusae (5 cm bell diameter) as
well as for sexually mature large individuals (12 to 18 cm bell diameter). Data
from three of these seven groups are presented in Figures 3, 4, and 5. Group A
(Fig. 3), representing 31 individuals with
bell diameters of about 4.3 cm, were deprived of food May 13, 1970; the experiment terminated on August 18, 1970.
Whole body regression, or degrowth, began
immediately and continued for about 6
weeks until the medusae were reduced to
less than 2 cm in diameter. At this time
840
W. M. HAMNER AND R. M. JENSSEN
deformation of the bell, similar to that
observed by deBeer and Huxley (1924),
occurred, with the bell becoming round
and relatively smaller than the now extremely elongate oral arms. These very
small medusae were usually unable to feed
when deformed in this way, and deteriorated thereafter. Medusae could recover
large
12
10
U
fully as long as disproportionate regression
did not occur, and animals with bells as
small as 1.4 cm diameter often recovered
fully. Rates of recovery for medusae which
had been starved for various intervals were
comparable at each size range to the growth
of normal laboratory medusae which had
not been starved. Group B medusae (Fig.
f
r
A
UJ
<
Q
* I :
•
20
40
TIME
FIG. 5. Symbols as in Figure 3. Sexually mature
medusae of both sexes were starved until the
gonads were completely regressed. On day 40 sev-
60
IN
80
I
100
DAYS
eral were removed and fed separately until they
again became sexually mature as determined by
biopsy.
DEGROWTH AND CELL DIFFERENTIATION IN
Aurelia
841
20
3,
large
18
16
s
14
o
z
DIAME1
a.
u
12
medium
•
10
21
II
I—range—
13
2-
%r
I
confidence
limiits!
small
20
40
60
80
TIME
100
120
140
160
180
IN DAYS
FIG. 6. Lack of effect of constant size or reproductive state on longevity. Animals were maintained within prescribed size ranges by alternating
schedules of feeding and starvation. Large-sized
medusae spawned repeatedly; medium-sized animals
did not mature sexually. The six solid circles for
days 124, 156, and 170 for the "medium" group
are actually "small" animals that received 2 extra
days of food inadvertently. Numbers indicate number of medusae in each group.
4), with bell diameters of 8 to 9 cm, regressed and recovered similarly. Most of
the large medusae in group C (Fig. 5)
were initially sexually mature, as determined by biopsy of the gonad, and these
animals also regressed and recovered at
normal rates. The gonads of sexually mature animals regressed to an immature
state in 5 to 8 days.
These experiments indicate that (i) degrowth is not dependent upon initial stage
of development of animals freshly collected
from the field; (ii) onset of reproductive
activity does not necessarily affect subsequent degrowth or recovery to the sexual
condition; (iii) recovery from a degrowth
842
W. M. HAMNER AND R. M. JENSSEN
sequence is normal, both morphologically eter. It is interesting to note that some of
and temporally, over the entire size range the medusae became inordinately large in
of the medusa, except for animals below the laboratory, reaching bell diameters 4
about 2 cm bell diameter; (iv) Aurclia cm greater than did any of the animals
below 2 cm bell diameter become deformed ever collected in Tomales Bay. Furtherand often can neither recover nor feed; more, the large medusae remained sexually
nor do they metamorphose back to the mature for the duration of the experilarval ephyra condition. The deformation ment. Although several medusae spawned
observed by deBeer and Huxley (1924) spontaneously during the 130 days of obduring degrowth of Aurelia thus does in- servation, the gonad would quickly bedeed occur, but only in very small medusae. come ripe again within 2 weeks. This was
The relationship of size, reproductive determined by inspection and by regular
condition, and longevity was investigated biopsies of the gonad each week. Thus,
further. Specifically, it was of interest to spontaneous somatic deterioration and condetermine whether large, reproductively comitant loss of reproductive ability did
active medusae would remain in a ripe not occur in the laboratory, contrary to
state for many months if fed sufficiently in the observations by Spangenberg (1965),
the laboratory, or if the gonadal and so- even though senescence is perhaps a regumatic tissues would deteriorate spontane- lar sequence in the life history of the aniously as they apparently do on occasion in mal in the field.
the sea and laboratory. In addition several
Medium-sized animals were easier to
size ranges of animals were included in the maintain within prescribed size limits, and
experiment to ascertain if sexual matura- this is reflected by the reduced range and
tion was size dependent or an age-specific confidence limits of the "medium" group
in Figure 6. Furthermore, none of these
developmental event.
animals reached puberty (if it is permisAccordingly, various sized medusae that
sible to discuss puberty in jellyfish). Hence,
were approximately the same age were colit is apparent that sexual maturation is
lected from Tomales Bay, and were divided
not an age-specific developmental event in
by size of bell in the laboratory into three
Aurelia, but a size-dependent developmengroups: 2 to 5 cm, 6 to 9 cm, 11 to 16 cm
tal phenomenon.
(Fig. 6). Animals were measured at the
Small animals are more difficult to retain
times indicated, and individuals larger
than the specified size were starved until within prescribed size ranges because of
successive measurement revealed that they their rapid growth response to food on the
had regressed to the desired size range, one hand, and slow regression rate on the
whereupon they then again received food. other. After 100 days of treatment this
Animals maintained in the largest size cate- "small" group was inadvertently given sevgory were fed constantly, however, because eral extra days of food and they all grew
failure to feed for even a few days resulted quickly beyond the desired size limit. The
in rapid regression of the bell. This sensi- solid circles in Figure 6 represent the bell
tivity of size to feeding pattern in large diameters of individuals in the "small"
medusae is reflected in the great range of group that received extra food, and undersizes represented in the "large" group of score one of the difficulties in working
Figure 6. Although the medusae were experimentally with medusae in this size
usually fed every day, occasionally the mass range. Besides their small size, animals
cultures of Artemia would become syn- only 3 to 4 cm in diameter are excessively
chronous and sufficient food would be un- thin and delicate, easy to damage, and, in
available, but also some medusae do not all, not as well suited for laboratory' use
eat every day even though food is avail- as are the larger, more robust medusae.
able, and these factors may have contribTo ascertain if animals could be followed
uted to the large variance in bell diam- individually during a growth and degrowth
DEGROWTH AND CELL DIFFERENTIATION IN
schedule, two experiments were conducted
with marked animals to determine variability of growth. Medium-sized medusae
were divided into two groups (Fig. 7),
Aurelia
843
one of which was fed and the other starved,
It is clear from the results shown in Figure
7 that the range of variability seen in prior
experiments (for example Fig. 6) utilizing
u
20
40
TIME
60
80
100
120
IN DAYS
FIG. 7. Twelve medium-sized, individually marked
medusae, randomly divided into two containers and
either starved or fed. Animals were usually measured every 4 days.
844
W.
M. HAMNER AND R.
M. JENSSEN
o
UJ
2
20
40
60
8O
TIME
100
IN
120
140
160
180
DAYS
FIG. 8. Individual medusae starved, fed, and starved on prescribed schedules.
populations can be reduced by following
the growth of marked individuals.
The ease of manipulating growth pattern is emphasized by the results of a second experiment where the patterns of
growth were pre-programmed and individual medusae were regressed, grown, and regressed on schedule (Fig. 8). These data
show that the growth of individual animals can be followed with precision. The
lag in regrowth following starvation, seen
most clearly in Figure 8, also shows that
experiments on the growth of individual
animals should span at least 10 days. Regression is a more rapid response for larger
animals than is growth for small animals;
reliable regression data can be obtained
easily in 4 days. In both cases, however,
the growth and regression responses are
predictable and the growth pattern of
Aurelia is easily modified in the laboratory.
These experiments on the pattern of
whole animal growth and degrowth indicate that:
1) Growth and degrowth are not limited
by stage of initial development except for
very small medusae.
2) Degrowth of various tissues is allometric. For example, gonads regress in 5 to
8 days, and the bell decreases more rapidly
during the first several weeks than do the
oral arms. Thereafter regression appears
identical to, but reversed from, normal
growth.
3) Recovery from degrowth appears
morphologically identical to normal growth
sequences.
4) Natural onset of reproductive activity
in the field does not necessarily alter subsequent capacity for degrowth or return to
sexual maturity.
5) Reproductive and somatic tissues do
not degenerate after spawning in the laboratory. Gonadal tissue can be renewed and
maintained in a ripe condition apparently
indefinitely.
6) Sexual maturation is a size-dependent
phenomenon, not an age-specific develop-
DEGROWTH AND CELL DIFFERENTIATION IN
mental event.
7) Growth patterns of individual medusae can be measured precisely and
manipulated according to prearranged
schedules in the laboratory.
Cellular differentiation
These experiments were designed to see
how whole animal degrowth affects differentiation of spermatogonial cells. The spermatogonia-sperm sequence was chosen for
these experiments because (i) the gonad
of mature males regresses rapidly, usually
within 5 to 8 days after onset of starvation; (ii) the gonads of at least one
hydromedusan have been shown to incorporate 3H-thymidine (Roosen-Runge, personal communication) and Aurelia is likewise favorable for such an experiment;
(iii) 3H-thymidine labeled spermatogonial
cells can be followed in marked individual
medusae by serially biopsying the gonad in
this large animal, and their fate determined via autoradiographic techniques;
and (iv) the prior history or age of any
given animal has no effect on gonadal
maturation or regression.
Of the two attempts to label the spermatogonial cells only the second experiment was successful. Apparently Aurelia
does not circulate enough sea water through
the gonadal-gastric cavities and radial
canals to label the spermatogonial tissues
when the label is administered in the
ambient sea water while the medusa is
feeding. When "hot" thymidine was introduced directly into the gonadal-gastric
pouch, good labeling resulted. Animals
were then starved. By day 2 some of the
spermatogonial cells were labeled, although
deposition of silver grains was light. Many
follicles were not labeled at all, but this
probably reflects the fact that the spermatogonial cells within a follicle undergo
synchronous development, whereas adjacent follicles develop independently. By
day 8 spermatozoa were labeled. These
sperm were concentrated in the now highly
compressed follicles, and this accentuated
the depositions of silver grains above the
Aurelia
845
follicles. It is interesting to note that because of the pulse-labeling nature of the
experiment, the follicles in the testes of
Aurelia contained only unlabeled primary
spermatogonial cells and labeled spermatozoa after 8 days. Other cell types in the
differentiation sequence of the spermatogonial cells were absent, although many
non-germinal cells, presumably phagocytes,
were obvious and heavily labeled also. In
some animals the gastric filaments were also
strongly labeled. The differentiation sequence for the spermatogonial cells appears
to be similar to that reported for Hydra,
but in Aurelia spermatogenesis takes only
8 days at 17 C, as opposed to the 20 days
reported by Schincariol and Habowsky
(1972) for Hydra usually maintained at
Each gonad of the experimental Aurelia
regressed entirely during the 8 days of the
experiment, becoming so small that the
tissue became difficult to biopsy. Furthermore, the entire animals degrew rapidly
also, with an average loss of 1.0 cm bell
diameter in 6 days from adults that originally ranged in size from 13.4 cm to 15.8
cm. Thus, the whole animal regresses and
the gonad within that animal regresses,
but spermatogenesis, once initiated, proceeds irrespective of outside events.
DISCUSSION
Many of the techniques that are used
to study animal growth are ingenious, but
they are generally somewhat contrived.
For example, one of the most widely used
techniques to study animal growth exploits the phenomenon of compensatory
hypertrophy. The literature associated with
this technique is enormous but the hypertrophic response is seldom very dramatic.
Further, it provides data on the homeostasis of tissue response in an unbalanced
and odd situation. It is difficult to ascertain if data collected with this technique
really provide insight into the mechanisms
controlling growth in an intact and healthy
animal. Controlled whole animal degrowth
and recrudescence of the medusa Aurelia,
846
W. M. HAMNER AND R. M. JENSSEN
however, is easy and the processes are
neither abnormal nor an artifact of injury
or damage. Aurelia is particularly suitable
for studies of the control of growth because
it is relatively large, and while Hydra and
Planaria degrow similarly, they are small
animals which require special techniques
and delicate hands. Aurelia is often the
size of a dinner plate, and it can sustain
repeated biopsy.
Although growth in Aurelia is normally
facultative, regulatory constraints do exist
for small medusae. Animals below 1.5 cm
bell diameter apparently cannot regress
further; this may reflect a true metamorphosis between the ephyra and the juvenile
medusa stages. The metamorphic events
from ephyra to medusa are not dramatic,
and no compelling reason had been given
previously to consider the ephyra stage as
a qualitatively different developmental
stage from the young medusa. We believe
the ephyra of Aurelia is indeed dissimilar
because medusae cannot degrow and become ephyrae again. Apparently development of the medusae from the scyphistoma
is a directional metamorphosis involving
irreversible transformations. Growth patterns of the medusa after this metamorphosis remain facultatively reversible, and
provide the researcher with an unusual
tool.
Degrowth is probably the term of choice
for this phenomenon because it connotes
a reversed but nonetheless active growth
pattern. Degrowth does not, for example,
seem to be a simple cessation of mitotic
activity. Starvation in Hydra causes a
marked reduction in number of mitotic
figures, but complete cessation of mitosis
in any portion of the animal does not
occur (Campbell, 1965). Aurelia undoubtedly degrow by reducing mitotic activity
also, but Aurelia does not degrow via
simple cellular attrition. If this were the
case, entire specific tissues would be lost
differentially since different cell lineages
invariably have characteristically different
division rates and longevities. The gonad
does regress rapidly, reaching an immature
stage of development in the first week of
starvation, but further reduction of the
remaining spermatogonial cell population
is slow. The bell is the next portion of
these still large animals to show disproportionate regression; the pendant oral
arms remain elongate, hanging below the
reduced bell, and accentuate this allometry,
but within several weeks this allometry
disappears and degrowth of the animal
proceeds uniformly and is morphologically
identical to that seen during normal
growth sequences both in the field and
laboratory. Damaged tissue is also rapidly
repaired during regression, thus anabolic
activities cannot have ceased entirely.
Degrowth of other selected tissues, which
we did not examine in detail, is not easily
explained by an hypothesis of cellular
attrition. Nervous tissue does not tend to
divide once an animal has become adult.
If the behavior of an animal is somehow
coded in the fine structural relationships
of neurons, simple cell loss during degrowth would put the entire animal at a
disadvantage. Degrowth without loss of
behavioral repertoire might require that
the axons be shortened, not lost. Thus, in
the nervous system, degrowth may also be
an active and homeostatically controlled
response.
Other terms have been used to describe
degrowth also. For example Child (1911)
used the terms "senescence and rejuvenescence" for degrowth in planaria; cleBeer
and Huxley (1924) referred to "dedifferentiation and reduction" in Aurelia; and
Beck and Bharadwaj (1972) used the
terms "retrogression" and "regrowth" for
a beetle larva. These terms are mostly inappropriate and have either mechanistic
connotations (i.e., dedifferentiation) or
imply a reversion to a more youthful condition (i.e., rejuvenescence and retrogression) . It may well be that some types of
degrowth are accompanied also by reversed
morphogenesis (as claimed by Child and
by deBeer and Huxley), but, since there
are already separate terms to distinguish
the phenomenon of growth from that of
morphogenesis during a normal developmental sequence, it is perhaps best to use
DEGROWTH AND CELL DIFFERENTIATION IN
a vocabulary of opposites for reversed sequences. Thus, degrowth and regrowth
connote subsequent changes in size. Dedifferentiation is the appropriate opposite
of differentiation (but only at the cellular
level). The opposite of morphogenesis is
morphoretrogression (the term "morphodegeneration" would result in the formation of degeneromorphs, and is thus
unacceptable).
It is not unreasonable to suspect that,
like nervous tissue or germinal tissue, all
of the other cell lineages within the animal
also have discrete adaptive strategies for
growth and degrowth, if only because there
are so many different ways to regulate the
size of any given tissue. For example, the
total number of cells could be changed by
changes in cell death rate (intrinsically or
phagocytotically), mitotic rate, or selective
cell movement or differentiation. Maintenance or regulation of tissue size is probably similar to problems of population
regulation, but almost nothing is known
about this phenomenon at the cellular
level because it has been so difficult to
manipulate the number of cells in most
laboratory animals.
We chose to examine the growth patterns of populations of germ cells in
Aurelia in greater detail because of the
rapid and selective regression of the gonad
during starvation, because sexual maturation is apparently a size-dependent, not an
age-specific, developmental event, and because populations of germ cells can be controlled experimentally as easily as can size
of the whole animal.2 Spermatogonial cells
are particularly easy to investigate because
of their density, because they readily incorporate tritiated thymidine, and because
2
Reproductive development may not be strictly
size-dependent. The experiments which regulated
size (see especially Fig. 6) did so by alterations in
feeding schedule, so perhaps reproductive development is limited by nutrients also. If animals could
be maintained in the "medium" 8 to 10 cm bell
diameter range perhaps by constraining them in
small aquaria, while still being fed heavily it is
possible that excess food would stimulate precocious
reproductive maturation, even though growth was
curtailed by container size.
Aurelia
847
they subsequently exhibit synchronous differentiation. Thus, given populations of
cells can be marked and followed. When
the germ cells of Aurelia were labeled
and the medusae starved, we found that
population regulation in the cell lineages
of the testes is a relatively complex phenomenon. For example, once spermaiogenesis has begun, germ cells continue to
differentiate for 8 days even though both
the tissue containing those cells and the
animal containing that tissue have begun
to regress. Clearly one mechanism of cellular population regulation in the normal
testicular tissues is via selective differentiation into different cell types, i.e., spermatogonial cells to sperm. The sperm which are
produced move into the center of the follicle and normally would be released during spawning, thus eliminating these cells
from the tissue and effecting a reduction
in size of the gonad as the number of cells
decrease via selective cell movement and
via cell differentiation. During starvation,
however, the cells do not appear to be
released, but are probably ingested by
phagocytes (which appear progressively
"hotter" in autoradiograms of tissue biopsied from animals starved for longer periods of time). Whatever the means of
eventual elimination, the initial mechanism for control of spermatogonial cell
number in the testis is that of programmed
and independent cellular differentiation,
followed by selective cellular movement.
Gonadal degrowth is regulated also apparently by selective changes in rates of
mitosis and meiosis. Testes which have regressed for 8 days contain only spermatogonial cells and labeled sperm. Mitosis must
have ceased, because no secondary spermatogonial cells are present in these follicles, and meiosis must have continued
because there are no unlabeled spermatocytes. Hence control of cell populations in
these testes involves selective cell movement, cell differentiation, and mitosis. It
also involves the ability of the cells to
respond to the size of the whole animal
as well, since sexual maturation is most
likely size dependent. Clearly, cellular pop-
848
W. M. HAMNER AND R. M. JENSSEN
ulation control in the gonad of Aurelia is
quite complex.
In Aurelia there are thus at least four
different phenomena that contribute to
cellular regulation of gonadal tissue components—selective cell differentiation, selective changes in mitotic rate, selective
cell movement, and differential phagocytosis. It is certainly possible that other species
of animals also regulate cell population
structure in testicular tissues via similar
devices, but until comparative investigations are available we cannot assess the
extent of this generalization. Cell biologists
have believed for too long that "The Cell"
is a unit of structure with only minor
variations, but this attitude is both typological and simplistic, and it has inhibited
the formation of inductive generalizations.
We must remember that the spermatogonial cells of Aurelia bear only a phenotypic resemblance to the spermatogonial
cells of man; spermatogonial cells of men
and jellyfish are as different genetically as
are men and medusae. The category "Spermatogonial Cell," as a special "type" of
cell variant, is a category based on phenotypic resemblance. Since most cell biologists pride themselves on their knowledge
of recent advances in DNA research, it may
come as a surprise to realize that most
of their generalizations are organized by
a phenotypic typology.
One can avoid this phenotypic typology
by beginning with the premise that each
cell lineage in each species of organism
has its own adaptive developmental control system which regulates cell number,
cell position, and phenotype. This control
system may be appropriate for only one
particular cell lineage in one particular
species of organism and may have no general applicability. Alternatively, the control system may be of general significance
to a wide variety of cell lineages in an
assortment of varied taxa. Similarities,
should they exist, may be either the result
of limited cellular solutions to similar
problems (Pantin's "homoplasty") or the
result of common ancestry (homology),
but in either case the extent of generaliza-
tions about developmental mechanisms can
be assessed only by comparative investigations. Those who would advocate simple
explanations for "Growth" or submit one
hypothesis for "Differentiation" presume
too much (see especially Sonneborn, 1970).
Our experiments have stimulated another line of thought also. We have been
impressed by the observation that during
degrowth in Aurelia there is an apparent
decoupling of spermatogenesis from the
normal developmental control mechanisms
that regulate cellular phenomena in the
rest of the animal. Thus, once spermatogenesis begins, spermatogonial cells differentiate irrevocably into sperm. This directional developmental sequence not only is
independent of the growth pattern of the
testis, but also is independent of the growth
pattern of the whole animal. This developmental independence of cell lineages is
reminiscent of other developmental phenomena that have been discussed elsewhere, by Lewontin (1970) with regard
to cancerous tissues, by Mintz (1970) for
cell clone selection in mice, and by Lenhoff (1965) to explain heterocytic phenomena in hydra. The idea is analogous
to concepts discussed by Sonneborn (1970)
regarding the genetic independence of cell
organelles and cortical inheritance patterns in Paramecium. Each of these examples may be a "special case," but it is
possible that cellular independence is
manifested only under unusual circumstances. Circadian relationships must be
examined under unusual experimental circumstances because strong coupling devices
normally obscure their expression. Perhaps
we will also learn more about cellular
spatial relationships when we "decouple"
their control systems from those that integrate the entire organism. Perhaps unusual
organisms such as Aurelia will help in this
analysis.
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