Cell Migration Kinetics in the Stomach of Styela clava
(Urochordata: Asc id iacea)
THOMAS H. ERMAK’
Scripps Institution of Oceanography, La Jolla, California 92037
ABSTRACT By autoradiography with tritiated thymidine, the rate of cell
migration was studied for the groove population of epithelial cells lining the
stomach of Styela. After one hour, the-tritiated thymidine is incorporated into
undifferentiated, dividing cells. Labeled differentiated cells appear after three
days, and the migration of labeled cells to the extrusion zone takes about 16 days
( T = 1 . ) Stomach growth is accomplished by an increase in both number
and height of epithelial folds. The percentage rate of cell migration probably
remains fairly constant throughout adult life. However, it is decreased by starva-
tion and low temperatures.
In an earlier report (Ermak, ’75), cell the presumed zone of cell extrusion was
populations in the digestive tract of Styela taken as 100% (fig. 1). The leading edge
clava were described qualitatively with au- of labeled cells was considered to be the
toradiography. On a stomach fold, cells percentage of distance migrated from the
proliferate in restricted zones of pseudo- time of injection until sacrifice. Ten ran-
stratified cells, migrate into mature zones domly selected grooves were measured
of simple columnar cells, and are finally from the base of the groove to the junc-
extruded into the gut lumen. Cell renewal tion with the crest population. The median
on an ascidian stomach fold is quite simi- values from each individual were then
lar to that on a mammalian intestinal averaged and the median percentage mi-
villus (Leblond and Messier, ’58), which gration value for each time interval listed
has been extensively investigated. The in table 1. The median instead of the mean
present study defines quantitatively the was used for all studies since percentages
rate of cell migration in the stomach do not fit “t” distributions. Average transit
groove population, a renewal system where time was determined from the 100% inter-
absorptive and zymogen cells are replaced cept on the graph of percentage migration
by relatively undifferentiated cells. Growth as a function of time.
of the stomach folds as well as the influ- For a study of stomach growth, 35 speci-
ence of animal size, nutritional state, and mens 0.25 to 30 g in weight had the stom-
temperature upon the rate of cell migra- ach prepared for routine histology. The
tion are considered. number of folds was counted, and the
height of each fold was measured as in
MATERIALS AND METHODS the foregoing paragraph. Experiments on
All specimens of Styela clava were col- the influence of animal size, nutritional
lected at the Quivira Basin of Mission Bay, state, and temperature were conducted for
San Diego, California. The tritiated thymi- a period of ten days. This time was chosen
dine was administered and autoradiograms as an average time between the entrance
were prepared as previously described of labeled cells into the mature compart-
(Ermak , ’75). For the cell migration study, ment and the beginning of cell extrusion.
ten individuals were held at 19 1°C and For the temperature study, the Gabriel test
sacrificed at the following time intervals: was used to test the statistical difference
1 hour, 3, 10, 15, and 20 days after injec- in two or more medians. For the rest of the
tion. Cell migration was measured as fol-
lows: the base of the pseudostratified re- versity of California Department of Physiology, Uni-
1 Present address:
Medical Center, San Francisco,
gion was taken as 0% migration whereas California 94143.
J. ExP. ZOOL., 1 9 7 : 339446 339
340 THOMAS H. ERMAK
studies, the Mann-Whitney U test was used
to test the statistical difference between
median values (Dixon and Massey, '69).
To study the influence of animal size on
cell migration, ascidians were divided into
two groups of ten individuals each: sexu-
ally immature animals 1.0 to 2.5 g in
weight and sexually mature animals 11 to
Method of measuring cell migration on a stomach fold.
ature for one day before injection. The
aerated sea water was changed daily and
kept within 2 0.5"C of the experimental
temperature. No additional food, other
than that in the changes of water, was
Table 1 shows the percentage of stomach
20 g in weight. Both groups were injected
and then isolated for two days; for the re- TABLE 1
maining eight days, they were returned to Extent of cell migration i n groove population as
running sea water averaging 19°C. revealed by median (range, number of animals)
To examine the influence of nutritional percentage migration
state on cell migration, 20 animals 2.5 to ~ ~
Time interval % migration
4.5 g were collected. Half were starved in
filtered sea water for ten days prior to in- 1 hour 9%
jection and then starved thereafter. These ( 6-11%, 6)
ascidians lost an average of 0.5 g during 3 days 18%
the 2Oday period. The other animals were (12-22%, 10)
maintained in Mission Bay during the 10 days 45%
same time interval and gained about 1 g. (25-72%, 8)
All animals were kept at 13-15°C.
15 days 94%
For the temperature study, 40 animals (76-loo%, 8)
3 to 6 g in groups of ten were gradually
brought to 6, 13, 20, and 26°C over a one 20 days 100%
day period and maintained at that temper- (2
CELL MIGRATION KINETICS IN ASCIDIAN STOMACH 34 1
'D 25 0
I I I I I 1
5 10 15 20 25 30
body weight (gm)
Fig. 2 Number of stomach folds as a function of body weight. Each point represents the
number of folds i one animal.
groove height labeled at increasing time of 30 pm per day, is slower in the germinal
intervals after injection. About 9% of the than the mature compartment.
groove is labeled before any migration be- Histological sections show that the num-
gins and may be taken as the approximate ber of folds in the stomach lining increases
size of the germinal compartment. By three with increasing body size (fig. 2). Animals
days, most label is still confined to the weighing less than one g have 15 to 20
pseudostratified region (18%, 12-22% ). folds while those more than a gram have
Since this zone occupies about 17% (11- 20 to 30. Most folds appear to be added
2 4 % ) of the groove height, cell differ- before individuals reach a weight of about
entiation must be completed in the first 5 g; the number does not significantly in-
migrating labeled cells shortly after three crease after 10 g, Indeed, limited data sug-
days. At 10 days, the variability in cell gest a slight decrease in the largest ani-
migration rate is large, the percentage mals. These sections also show that smaller
labeled ranging from 25 to 72%. By ascidians have shorter folds (fig. 3 ) . The
graphic interpolation, cell migration is esti- average height of a fold varies from 0.58
mated to be completed by 16 (13-18) days. mm for a 3 g specimen to 1.65 mm for a
Migration, which occurs at an average rate 15 g animal. The height of a fold increases
342 THOMAS H. ERMAK
1.oo @ @
5 10 15 20 25 30
body weight (gm)
Fig. 3 Height of stomach folds as a function of body weight. Each point represents
the average height of ten folds from one animal.
until an animal weighs about 10 g; there- centage basis, remains fairly constant, the
after, the height remains constant with absolute rate of migration must be greater
growth. Johnson ('71) found that Styela in larger animals (about 50 pm per day)
clava reaches sexual maturity at about 8 than in smaller animals (about 30 pm per
cm. In the present investigation, 8 cm cor- day).
responds to a weight of about 8 grams. In starved and fed animals, cell migra-
Those ascidians smaller than 5 g are gen- tion is significantly greater in fed than
erally sexually immature. Thus, the most starved individuals (table 2). Ten days
rapid increase in fold length occurs dur- after the administration of tritiated thymi-
ing the period of growth before sexual dine, 60% and 36% of the groove heights
maturity. are labeled respectively in fed and starved
Autoradiograms of the stomach from ascidians ( p = 0,006). In addition, the
groups of small and large sized animals height of the grooves (428 -F- 45 pm vs
show no significant difference in the per- *
259 21 pm) and the height of the epithe-
centage of cell migration (table 2). The *
lial cells from base to apex (57 5 pm vs
constancy in percentage rate of cell migra- 31 f 3 pm) is likewise smaller in the
tion implies that the rate of cell renewal starved group, with both germinal and ma-
remains fairly constant throughout adult ture compartments affected. It is estimated
life. Although the migratory rate, on a per- that transit time through the mature com-
CELL MIGRATION KINETICS I N ASCIDIAN STOMACH 343
TABLE 2 phases. Before three days, cells are re-
Influence of animal size, nutritional state, and stricted to the pseudostratified region and
temperature upon cell migration giving the migration is slow. During this initial pe-
median (range, number of animals) riod, cells are involved in cell division and
percentage migration and
significance growth. A DNA-synthesizing cell has its
nucleus located in the basal portion of the
Condition % migration cell; before mitosis, the nucleus migrates
Small sized 51% (28-92%, 9) toward the cell apex (Ermak, '75). There-
Large sized 39% (29-62%,6) after, the nuclei of the daughter cells mi-
Significance N.S. grate basally. After three days, germinal
Starved 36% (21-70%, 9)
Fed 60% (27-82%,9) cells have differentiated into mature (ab-
Significance p = 0.006 sorptive and zymogen) cells. They are con-
(by a 1 tailed test) sidered to be differentiated by morphologi-
6" 21% (11-51%,6) cal criteria; biochemical, behavioral, and
13" 25% (19-33%, 8)
20" 46% (28-77%, 7) developmental criteria (as defined by Grob-
26" 77% (12-loo%, 6) stein, '59) were not applied. After differ-
6" 20",0.01<p<0.05 entiation, the cells grow in volume and
(by a 2 tailed test) 6"+26",0.01<p<0.05 their nuclei come to lie next to each other
13"+26";0.01<p<0.05 forming a simple columnar epithelium. Mi-
6"+ 13", N.S. 1 gration through the mature compartment
20"+26", N.S. is more rapid; the change in rate is due, at
1 N.S., not significant. least in large part, to proliferation of ger-
minal cells. Cell size and shape seem to
partment is lengthened about a week by affect migration rate in much the same
starvation. Qualitatively, labeled cells are way as changing tube diameter affects
still confined to the pseudostratified region water flow through a pipe. Water flow in-
i n starved animals, whereas most of the creases as water in a pipe of large diameter
labeled cells have migrated into the mature passes to one of small diameter.
zone in the fed condition. Starvation ap- Growth of the stomach is due to an in-
parently lengthens the turnover time of crease in both the number and height of
germinal cells. However, it is not presently epithelial folds. Most folds must be added
known which parts of the cell cycle are between metamorphosis and sexual matur-
most affected; moreover, it is possible that ity since the post-metamorphic ascidian
starvation could decrease the growth frac- juvenile has a smooth walled stomach, and
tion (defined by Cleaver, '67) of the cell folding occurs after metamorphosis (Scott,
population. '52; Trason, '57; Cloney, '61). In Styela, the
In the temperature study, significant stomach folds grow in height until an ani-
differences are observed between all me- mal reaches about 10 g; thereafter, they
dian temperatures except between 6 and stay about the same height. Both germinal
13°C and 20 and 26°C (table 2). At both and mature compartments increase in size,
low temperatures, labeling is almost iden- and adult ascidians exhibit no significant
tical. Ten days after the injection of tri- differences in the percentage rates of cell
tiated thymidine many pseudostratified but renewal during growth. In these respects,
few mature cells are labeled. Although Styela differs from the mouse, in which
there is no quantitative difference between the percentage rate of cell renewal on an
20 and 26"C, there is a distinct qualita- intestinal villus changes with animal size
tive difference. In most ascidians at 26"C, (Koldovsky et al., '66). O'Connor ('66)
only a few lightly labeled cells remain in states that villus growth in the fetal mouse
the pseudostratified region. At 20"C, how- intestine is not due to a higher mitotic
ever, many labeled cells are still confined rate but the result of an extrusion rate
to this region. below that of the adult. Such might also be
the case in Styela. The influence of body
size upon cell renewal is comparable to the
Cell migration in the stomch grooves of influence of cell population size in the
Styela clava may be divided into two same ascidian. Mucous cells in the esoph-
344 THOMAS H. ERMAK
agus and stomach (crests, raphe, and indirectly by influencing the rate of filter
bulb) are renewed at the same rate al- feeding and, thus, nutritional state. Nor
though cell population sizes differ in each did it consider the role of low temperature
region (Ermak, '75). isoenzymes (Hochachka and Somero, '73)
In Styela, starvation decreases the cell in the long term adaptation of ascidians to
population size, the compartment size, and winter temperatures. In the laboratory,
the rate of cell renewal. In mammals, star- however, small changes in temperature ap-
vation decreases the intestinal villus cell parently have little effect upon cell re-
population size (Hooper and Blair, '58; Deo newal.
and Ramalingaswami, '65). In the crypts, ACKNOWLEDGMENTS
the length of the cell cycle is also in- I am indebted to Dr. Nicholas D. Holland
creased, especially the time of DNA synthe- for his support, criticism, and advice. I
sis (Hooper et al., '68; Rose et al., '71). The thank Shirley Philibosian for her statistical
decrease in cell population size after star- assistance and Dr. David Epel for his sug-
vation is presumed to be due to a drop in gestions.
the rate of cell proliferation below the rate LITERATURE CITED
of cell loss (Hooper and Blair, '58). There- Berrill, N. J. 1929 Digestion in ascidians and
after, the rate of cell loss would probably the influence of temperaure. J. Exp. Biol., 6 :
come into equilibrium again with cell divi- 275-292.
sion. MacGinitie ('39) reports that food Cleaver, J. E. 1967 Thymidine Metabolism and
supplies of ascidians in Southern Cali- Cell Kinetics. John Wiley and Sons, Inc., New
fornia bays are enriched by dinoflagellates Cloney, R. A. 1961 Observations on the mech-
in summer. Such fluctuations in water anism of tail resorption in ascidians. Amer.
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in the field are probably somewhat shorter of cell population kinetics in the jejunum.
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tory (compare table 1 and 2 ) where food Dixon, W. J., and F. J. Massey 1969 Introduc-
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down in winter. In San Diego, sea surface mental temperature. Oikos, 26: 109-131.
temperatures range from about 12°C in Ermak, T.H. 1975 Cell Proliferation in the Di-
gestive Tract of Styela clava (Urochordata
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'72, M. Robinson, personal communica- with tritiated thymidine. J. Exp. Zool., 194:
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CELL MIGRATION KINETICS IN ASCIDIAN STOMACH 345
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