Download Cell Movements in Hydra

AMER. ZOOL., 14:523-535
(1974).
Cell Movements in Hydra
RICHARD D. CAMPBELL
Department of Developmental and Cell Biology, University of
California, Irvine, California 92664
SYNOPSIS. All cells in hydra undergo continuous and systematic locomotion across the
polyp body. Different patterns and mechanisms of movement are exhibited by the
various cell types. Passive displacement is one of the most conspicuous forms of cell
movement: The epithelial cells of the body column and tentacles move centrifugally
due to the expansive growth of the tissues. Another factor in the passive displacement
of epithelial cells involves the propagation of morphological characters across the
tissue, rather than actual movement of cells or tissues per se. True cell locomotion,
where cells move relative to neighboring tissue by means of self-propulsion, is also
apparently exhibited by epithelial cells in certain situations, notably at sites where
morphogenetic changes are taking place (e.g., bud and tentacle bases) . Epithelial cells
always migrate as sheets of cells, never individually. In contrast, ncmatocytes and
interstitial cells undergo active migration individually. Nematocyte migration is strongly
polarized, very possibly by the mat of epithelial cell muscular processes along which
they move. Interstitial cell migration appears to occur under many conditions, but
not nearly as commonly as was classically supposed. Finally it appears that interstitial
cells and nematoblasts move in bulk through the intercellular spaces in some situations.
This movement is probably due to epithelial cell activities and is used to redistribute
cells located in the interstitial spaces.
The cells and tissues of hydra are in continuous flux. The polyp undergoes perpetual growth and tissue loss, coupled by
balanced cell renewal patterns involving
all cell types. During these processes some
cells migrate individually; others are
pulled in sheets. The intricacies of these
movements are becoming more and more
apparent as experimental and histological
studies progress. So far, however, the different types of cell movement occurring in
hydra have been studied by different investigators and have not been well correlated with one another. This paper summarizes our understanding of the various
forms of cell movements in hydra and
classifies and analyzes them in both mechanism and occurrence.
CELL AND TISSUE RENEWAL
The occurrence of cell locomotion is related to discrepancies between the patterns
of cell origin and destination. Cells arise
Supported by NIH Research Career Development Award 5-KO4-CM42595 and NSF Research
Grant GB 29284.
by either mitotic proliferation (epithelial
cells and interstitial cells) or by differentiation from interstitial cells (nematocytes
and nerves) (Campbell, 1967a; David and
Campbell, 1972; Davis, 1974). Gland cells
divide mitotically, but it is not known to
what extent, if any, they also arise by differentiation of interstitial cells. In all cell
types mitotic proliferation and differentiation are widely distributed throughout
the hydra body column (Fig. 1). There is
no local zone of growth or cell production
in the body; although it was once widely
held that growth was restricted to the subhypostomal region (Brien and ReniersDecoen, 1949; Burnett, 1961), this was
based on a false deduction and has now
been widely found to be untrue (Campbell, 1967&; Webster, 1971). The most
notable exceptions to the almost uniform
distribution of cell production are that
nerve cell production appears to be highest
in the vicinity of the hypostome, developing bud, or site or regeneration (Schaller,
1970) and that there is little or no cell
production in the tentacles, hypostome tip,
and region near the basal disc.
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524
RICHARD D.
CAMPBELL
Ectodermal epithelial cells
o
Endodermal epithelial cells
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X
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Ectodermal interstitial cells
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-o
o — '
•°"""O^No
2
Endodermal gland cells
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Position Along Hydra Column
FIG. 1. Distribution of cell proliferation along the
body column in Hydra littoralis. The ordinate
represents the mitotic index, averaged over measurements taken throughout the day and night, of
the four cell types considered: ectodermal epithelial cells, endodermal epithelial cells, ectodermal
interstitial cells (including all cells in the interstitial spaces of the ectoderm), and endodermal gland
and mucous cells. The abscissa indicates the body
region. No mitotic activity occurs in the tentacles.
(Modified from Campbell, 1967a) .
CELL MOVEMENTS IN HYDRA
Cell loss occurs primarily at the budding
region and at the body extremities (hypostome, tentacles, and basal disc) (Campbell, 19676). Budding involves segregation
of a large area of parent tissue with little
modification in its cellular composition
(Bode et al., 1973). Thus, all cell types are
lost in the proportions in which they
occur in column tissue. The polyp extremities continually wear by tissue atrophy (Brien and Reniers-Decoen, 1949),
and these regions are rather specialized in
cellular composition: most cells in the tentacles are nematocytes; as the distal fourth
of each tentacle is lost daily (Campbell,
19676), many thousands of nematocytes
(Bode et al., 1973) are shed. Hypostome
and basal disc tissues are comprised mainly
of epithelial, gland and nerve cells. Thus,
cell formation occurs in the body column,
and almost all cell loss occurs at the budding region or at the body extremities with
the nematocyte loss being particularly
rapid and restricted in tentacle tips.
CELL MOVEMENTS
The intricate, interweaving renewal patterns of the various cell types demand, or
result from, several types of cell locomotion. Some cell types may display several distinct types of movement. The forms
of cell translocation from one part of the
body to another may be classified in categories of Passive Displacement, Active
Movement (both cell and tissue), and
Passive Movement.
Passive displacement: The apparent
displacement of cells relative to the
morphology of hydra, but without
cells moving relative to neighboring
cells or structures
Hydra tissue grows continuously. This
expansion results in all tissue regions moving away from each other. Since cell and
tissue loss, localized at the top and bottom
of the body column, maintain constant
animal size, tissue continually moves from
the central body regions peripherally to
the sites of cell and tissue loss. This is one
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of the first types of tissue movement to be
mapped out in hydra (Tripp, 1928).
There is no a priori reason to assume
that any active type of cell migration
underlies these centrifugal column movements, although several instances of cell
locomotory activity have been found in
certain cases (see below). Rather, the tissue simply expands. Yet, there has been
the greatest confusion concerning the mechanics of these column displacements.
When the idea of a localized subhypostomal growth region was maintained (e.g.,
Brien and Reniers-Decoen, 1949; Burnett,
1961), one spoke of growth as forcing or
pushing tissue proximally and distally,
thus mechanically creating the column tissue movements. These descriptions are
conceptually unsound. Actually, there are
two components to the column tissue displacements and different mechanisms
underly them. One component is the
movement of tissue regions away from one
another. This is due to tissue expansion by
growth. The second component is the displacement of tissue regions relative to
morphological boundaries. This component should be viewed not as a tissue
movement phenomenon, but rather as migration of the morphology across the stationary (or expanding) tissue. The budding zone, situated near the base of the
column, continually migrates upwards into
the column by means of new buds always
arising above older buds (Brien and
Reniers-Decoen, 1949). The basal disc
migrates upward along the stalk since at
any time the cells at its upper edge are in
the process of becoming base cells. That is,
the cellular morphogenetic states associated
with being basal disc cells are continually
propagated upwards into naive stalk
tissue. This gives the false impression that
cells are moving downwards into the base;
actually the base is moving upwards across
the tissue. The result is similar in the two
cases, but the underlying mechanism is not.
A similar situation occurs at the tentacle
bases; the column cells adjacent to a tentacle base are always becoming new tentacle base tissue, thus elongating the tentacles. This process has the appearance of
526
RICHARD D. CAMPBELL
cells migrating outwards into tentacles.
Actually, tentacle organization is propagating inwards across the tissue. While both
interpretations yield equivalent patterns,
the concept of centripetal morphology migration is probably closer to the mechanisms involved.
Column tissue expansion and centripetal
morphology migration are generally balanced, leading to steady state development.
This balance is independent of growth
rate. Tentacle tissue turnover rates have
been measured and found to be relatively
independent of feeding rates (Dunne and
Campbell, unpublished). Basal disc cell
loss is probably also constant. Budding,
however, is facultative; it occurs only when
growth is in excess of peripheral tissue loss
and budding then occurs at a rate commensurate with the excess growth. Thus,
tissue growth and loss are almost always
balanced. They are unbalanced when
feeding rates are too low to balance the
constant peripheral tissue loss, and the
hydra diminishes in size (Tripp, 1928).
They are also unbalanced in the well-fed
individuals of mutant hydra which are unable to bud (Lenhoff, 1965; Moore and
Campbell, 1973a); in this case the hydra
continuously grow larger to giant proportions.
The combination of centrifugal tissue
expansion and counterbalancing centripetal movement of morphological regions
across this tissue give the appearance that
cells are migrating away from the central
column region. Thus, the most obvious
and well-mapped tissue displacements in
hydra need not involve any active cellular
migration at all.
Locomotion: Cells or tissues moving
relative to neighboring cells or tissue
structures, due to migratory behavior
of the cells involved
Active tissue (epithelial) migrations.
There are two prominent cases where
hydra epithelial cells apparently actively
migrate. These are (i) during certain col-
ECTODERM
ENDODERM
I
BUDDING
ZONE
l0
°
0
DAYS
FIG. 2. Ectodermal and cndodermal column tissue
displacements in Hydra viridis. The column positions of tissue markers (ordinate) are plotted as
a function of time (abscissa) . Colloidal carbon was
used to mark the ectoderm and symbiotic algae
were used to mark the endoderm. The tissue behavior of four (fl-d) animals is shown. In a-c, the
ectoderm is displaced distally relative to the endo-
derm; depending on the position of the markers
initially, the ectoderm and endoderm are displaced distally or proximally. In d, both tissues
move precisely in register proximally; this latter
behavior was always observed wihen the markers
were originally in the lower two-thirds of the
gastric column. (From Campbell, 1973.)
CELL MOVEMENTS IN HYDRA
527
umn expansions, where ectoderm and endoderm move relative to one another, and
(ii) during formation of buds and tentacles. In these cases, migration involves
the epithelia as sheets rather than single
cells. The locomotory structure is probably the muscular process of the epithelial
cell.
In column tissue displacements, the two
tissue layers may or may not move in register. In Hydra viridis the tissues move
together in lower column regions, but
within the upper quarter of the column
they move at different rates (Fig. 2). In
two other cases where ectoderm and endoderm column differential displacements
were looked for they were also found
(Shostak et al., 1965; Campbell, 1967c) so
probably they are of widespread occurrence. However, the particular patterns of
differential movement differ with species
and probably with different culture and
growth conditions.
Shostak et al. (1965) demonstrated the
ability of an isolated epithelial sheet to
migrate, using the mesoglea as a substratum. Interestingly, they found that
such migrations occurred in only one direction, as might be expected from tissues
of such a highly polarized organism. Their
experiments indicate that the mesoglea
probably acts as a substratum for this
locomotion.
The other situations which apparently
involve epithelial cell migration occur at
the sites of tentacle and bud formation.
FIG. 3. Orientation of muscular processes at the
margin o£ a developing bud of Hydra viridis. This
longitudinal section of a young hud extends from
the junction with the parent (at extreme left) to
about 1/6 of the length of the bud. The bud tip
is toward the right. The section is perpendicular
to the parent column axis. The ectoderm is at the
top and the mesolamella runs across the lower
portion of the illustration. At far left, the three
vertical arrows indicate three ectodermal epilheliomuscular cell processes which arc perpendicular
to the plane nf Ihe micrograph; Ihese are parallel
to the parent body axis, as ate all ecUxlerinal muscular processes in the parent. At the extreme right,
the muscular processes lie within the plane of the
micrograph. Thus, in this marginal region of the
bud the epithelial cells rotate as adjacent parent
tissue becomes incorporated into the bud. The
thickening of the mesolamella seen at the right,
where numerous cellular processes extend into and
through it, is typical of this site of cell rotation.
The three oblique arrows in the right center indicate three nearly mature nematoblasts (the
nematocyst is visible only in the lowest one) which
have moved from their site of differentiation
higher in the epillielinn to the muscular layer lining the inesolaniella. x 1,900.
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RICHARD D. CAMPBELL
Shostak and Kankel (1967) showed that
the position of a bud's base moves slightly
along the column in H. viridis as the bud
matures. This could well involve epithelial
cell migrations, although the pattern is
complex and its mechanism has not been
studied. Also, in both the tentacles and
buds, new epithelial cells continually add
on to the outgrowth's base all around it.
In the ectoderm the epithelio-muscular
cell processes are all oriented strictly axially
along the tentacles and body column
(Mueller, 1950), and in buds of all stages
(Campbell, unpublished). Therefore, the
epithelial cells entering tentacles or buds
from their sides must change their orientations by 90°. This change in orientation
occurs near the base of the outgrowth and
involves a gradual rotation. Probably it
is accomplished by the ectodermal cells
migrating slightly across the mesoglea;
those cells above and below the outgrowth
can migrate directly on without rotation;
those cells to the side migrate in a curved
path which results in the rotation. A
region of epithelial cell rotation at the
margin of a bud is shown in Figure 3.
Analogous migrations could occur in the
endoderm, although there the pattern of
muscular processes is much more complex
and has not been worked out in the
vicinity of bud and tentacle bases.
In epithelial cell migrations, one probably should consider the muscular process
as the organelle responsible for the movement. The muscular process has the appearance of a locomotory organelle. It also
exhibits the behavior necessary: it is contractile and highly mobile. During tissue
healing the muscular process is the first
cell extension which crosses and closes a
tissue gap (Bibb, 1971), and it may be
just as active in normal tissue. Epithelial
cell migration seems to use the mesoglea
as a substratum (Shostak et al., 1965)
(there is no other obvious substratum),
and the muscular process is the only portion of the cell or tissue contacting the
mesoglea. Another possible mechanism
(Campbell, 1968) of epithelial cell migrations is altered cell-cell contact relations
which are thought to cause morphogenetic
movements in embryos (Gustafson and
Wolpert, 1962; Burnside and Jacobson,
1968).
Active single cell migrations. Nematocytes: Nematocytes originate in the body
column of hydra and many subsequently
migrate to the tentacles where they become mounted for use. Nematocyte migration occurs entirely within the ectoderm
(Campbell, 1967c). The migration begins
by the nearly-mature nematoblasts settling
toward the base of the ectodermal epithelium (Fig. 2; 3 oblique arrows). This
migration occurs just at the time that their
synchronous developmental clusters, or nest,
disaggregates. Then they independently
migrate distally towards and into the tentacles. Although migrating nematocytes
are close to the mesoglea, they generally
do not touch it but instead glide along
the upper surface of the epithelial cell
muscular processes, which form a continuous, axially oriented corrugated mat.
It is reasonable to suspect that this highly
sculptured substratum guides and orients
nematocyte movements, much as tissue cultured cells may show "contact guidance"
along much finer surface features (Weiss,
1961). However, this question has not
been pursued experimentally, and different nematocyte types behave differently
(Herlands, 1972) indicating considerable
complexity in the control of nematocyte
migration.
The migrating desmoneme nematocyte
is a strongly polarized cell. The leading
edge consists of a pseudopod, behind which
is the cell nucleus. The nematocyst capsule trails at the back of the cell, with the
cnidocil end pointing backwards (Fig. 4).
This orientation appears to be usual for
nematocytes (Giinzl, 1971; Rahat and
Campbell, 1973). In the body column of
hydra the nematocytes migrate distally up
the column from their site of origin; this
is demonstrated by grafting isotopically
labeled tissue into unlabeled columns and
analyzing with radioautography the proximal and distal distributions of labeled
nematocytes after varying times (Fig. 5).
CELL MOVEMENTS IN HYDRA
FIG. 4. Nematocytes migrating at the base of the
ectodermal epithelium towards the tentacles in
Hydra attenuata. This is a surface view of a whole,
fixed hydra, at the focal level of the ectodermal
muscular layer. The faint vertical striations are epithelio-muscular cell processes. The direction of the
tentacles is towards the top of the photograph.
Desmoncmc nematocytes, indicated by double arrows, migrate strictly polarized with nucleus (n)
forward and nematocyst capsule (c) behind with
the cnidocil end trailing. A stenotele nematocyte
with similar orientation is at the lower right. These
nematocytes are migrating through interstitial celldepleted tissue, from a 4-hr-old implant of normal
tissue located off the lower side of the photograph.
The interstitial cells of the host were depleted by
the following, unpublished method: Hydra were
placed in a solution of 0.06% colcemid for 3 hr and
then maintained 20 days in "M" solution. Under
these conditions more than 99% of interstitial
cells, nematoblasts and nematocytes become depleted, mainly through phagocytosis and digestion
by epithelial cells, x 1,240.
In longitudinal and cross-sections of the
body column, migrating nematocytes are
seen in rather precise longitudinal or crosssection (Figs. 6, 7). This again implicates
the epithelial cell muscular process in guiding nematocytes, for there are no other
structures as uniformly oriented as the
nematocytes.
529
In the tentacles, nematocyte orientation
is not as constant. Nematocytes are found
pointing both distally and proximally and
many are oriented in other directions. This
suggests that nematocytes in the tentacles
do considerable wandering before being
incorporated into a battery.
Interstitial cells: Interstitial cells have
classically been considered as highly mobile
cells, easily recruited to sites of wounding
or budding. However, this view of the
interstitial cells arose at a time when it
was believed that they were the only cells
active in growth and tissue repair. Kanajew's (1930) important study represents
the first and still crucial demonstrations
that most morphogenesis and tissue architecturing is due to epithelial cells, not to
interstitial cells. This invalidates the major
historical reason for supposing that interstitial cells are migratory.
The available evidence for interstitial
cell migrations is mainly histological and
provides little information concerning the
extent of cell movements. The meager ex-
FIG. 5. Cell migration and displacement in Hydra
littoralis. These graphs indicate the distribution of
labeled cells in two hydra, one of which had been
labeled with tritiated thymidine, 3 days after exchanging hydranths and distal columns. In both
animals mature, migrating nematocytes are abundant in the distal column and in the tentacles
(not shown) . No labeled interstitial cells were
found outside regions where epithelial cells were
labeled, indicating no detectable interstitial cell
migration. (From Campbell, 1967c.)
530
RICHARD D. CAMPBELL
perimental data, in fact, suggest that interstitial cell migration is ordinarily rather
limited.
Some classical histological studies dealt
with interstitial cell migration by describing the appearances of these cells. These
studies have been generally interpreted to
be consistent with the notion that interstitial cells are amoeboid. However, it has
been cautioned (Campbell, 1967c) that
interstitial cell shapes, generally tear-drop
or lanceolate, do not actually resemble
those of most amoeboid cells. The cell
shape is due to compression within the
intercellular spaces. Interstitial cells do not
have filopodia, and electron microscopists
uniformly consider interstitial cell cytoplasm nearly devoid of organelles (Slautterback and Fawcett, 1969; Lentz, 1966)
FIG. 7. Nematocyte migration in Hydra attenuata
seen in longitudinal section (parallel to the axis of
the hydra) . This desmoneme nematocyte is migrating towards the lower left, in the direction of
the tentacles. The leading edge is occupied by a
pseudopod, which is typically smaller than the one
shown here. The nematocyst capsule trails with the
cnidocil end (extreme upper right) pointed backwards. The cell is separated from the mcsolamella
(in) by a muscular process of an epithelial cell.
Ectoderm is to the upper left, endoderm to the
lower right. X4,000.
FIG. 6. Nematocyte migration in Hydra attenuata
seen in cross section (perpendicular to the axis of
the hydra) . Two desmoneme nematocytes (arrows)
are migrating adjacent to each other, one (the
upper one, with nematocyst capsule visible) slightly
ahead of the other (lower one, with nucleus visible) . They are separated from Ihc mesolamella
(m) only by epithelio-muscular cell processes, although the lower nematocyte makes a minute contact with the mesolamella. Ectoderm is to the
right, endoderm to the left, x4,000.
in contrast to many active cells.
Some histological studies infer interstitial cell movements from changes in cell
distributions under experimental or unusual situations. For example, Tardent
(1954) found that during regeneration of
an hydranth, interstitial cells gradually
become depleted in a graded pattern along
the column; this was presented as evidence
that interstitial cells migrate distally during regeneration and there differentiate.
Similar statements have been made about
gonadogenesis, where interstitial cells become more abundant in the vicinity of
presumptive gonads and become depleted
elsewhere in the body.
The reason why no histological study
of this type provides evidence for inter-
CEIX MOVEMENTS IN HYDRA
stitial cell migration is that there are a
variety of activities in addition to migration which can affect interstitial cell differentiation, abundance, and distribution.
Interstitial cells proliferate and differentiate and their relative abundance is
altered by epithelial cell proliferation.
Unless one determines simultaneously the
rates of these various activities, it is not
possible to conclude that altered distributions of interstitial cells reflect migration.
Most experimental work on interstitial
cell migration has involved abnormal
hydra, generally X-irradiated or nitrogen
mustard-treated individuals. Results from
these experiments are spectacular and unusual and will be dealt with in the next
section because they do not deal with
normal hydra development.
Only three studies have been directed
specifically at experimentally detecting interstitial cell migration in normal hydra.
Both involved grafting together complementary pieces of hydra taken from unlabeled animals and from animals labeled
with the nuclear isotopic marker 3H-thymidine. Tardent and Morgenthaler (1966)
grafted basal, labeled hydra halves to
upper, unlabeled halves and at intervals
determined the total number of labeled
cells in the distal region. Cell migration
began within 3 hr of grafting and by 27
hr after grafting, a maximum of 137
labeled interstitial cells had advanced into
the distal portions. This must have represented several hundred migrating cells because not all of the proximal cells were
labeled. Tardent and Morgenthaler's experiments uphold the classic view that
interstitial cells are able to migrate. Their
data furthermore suggest that in a normal
hydra interstitial cell migration is somewhat limited. Although Tardent and Morgenthaler did not give data on the total
number of cells in these hydra, data from
Bode et al. (1973) for this strain of hydra
indicate that there are 10,000 to 25,000
interstitial cells and nematoblasts* in the
* Tardent and Morgen thaler's term "interstitial
cell" includes "large" and "small" interstitial cells
and probably many nematoblasts as defined by
David (1973) and Bode et al. (1973).
531
lower (labeled) half of these animals.
Therefore, either only one or several per
cent of the cells moved from the lower
half to the upper half during Tardent and
Morgenthaler's experiments, or else a substantial number of interstitial cells moved
but only 10 to 20 m^. The observations
do show that there are not ordinarily largescale or distant interstitial cell migrations
during normal hydra development.
Similar experiments were carried out for
1, 3, and 9 days by Campbell (1967c) using
H. littoralis. No migratory interstitial cells
were observed. According to the methods
used, this finding would set migratory cells
as less than 1% of the interstitial cells. Herlands (1974) furthermore showed that migration is primarily towards hypostomes.
Passive Movement: Translocation of cells
relative to adjacent cells or tissues
propelled by activities of other cells
Bulk movement of cells located in the
interstitial spaces. In some circumstances,
massive movements of cells take place
through the intercellular spaces. The beststudied examples occur when a piece of
normal hydra tissue is grafted onto a hydra
whose interstitial cells have been experimentally removed. Interstitial cell depletion can be produced by X-irradiation
(Zawarzin, 1929; Evlakhova, 1946; Brien
and Reniers-Decoen, 1955), or by treatment with nitrogen mustard (Diehl and
Burnett, 1964) or colchicine (Campbell,
unpublished). Histological examination of
chimeras consisting of normal and depleted
tissue reveals massive interstitial cell migration into the depleted tissue (Brien and
Reniers-Decoen, 1955) (Fig. 8). Migration occurs in heterografts (Diehl and Burnett, 1966) (Fig. 9) and into tissue of
opposite sex (Brien, 1953) as well as in
homografts. The rate of migration is impressive, although most investigators have
not counted the numbers of cells which
reinvade the depleted tissue. While Tardent and Morgenthaler (1966) found no
difference between the number of cells
migrating into X-irradiated and normal
hosts, figures and descriptions in the litera-
532
RICHARD D. CAMPBELL
depleted tissue is that all classes of interstitial cells and nematoblasts take part
(Diehl and Burnett, 1966; Tardent and
Morgenthaler, 1966) (Figs. 8, 9). These
include (after the terminology of David,
1973) the big interstitial cell, which presumably includes the indeterminate stem
cell; the little clustered interstitial cells,
which are probably determined to become
nematocytes but are not yet differentiated;
and nematoblasts. While large interstitial
cells sometimes have tear-dropped shapes
and could perhaps be considered as amoeboid, little interstitial cells and nematoblasts have no trace of appearance of
amoeboid cells and it is scarcely reasonable to suppose that they can actively
migrate.
FIG. 8. Interstitial cell migration from a small plug
of normal tissue (between large arrows) into a
host hydra whose interstitial cells had previously
been eliminated by using nitrogen mustard. Thirtysix hours after implantation of the normal tissue,
the animal was fixed and stained with toluidine
blue. The small arrows point to numerous interstitial cells, which have migrated outwards from
the implant in all directions. The two uppermost
arrows are near the most distal extent of migrating cells. The limits of the implant can be seen
by irregular black dots which represent carbon
particles (Campbell, 1973) in the implanted epithelial cells. The large round black dots throughout the host are stenotele nematocysts. Large and
small interstitial cells and nematoblasts have migrated. Both the donor and host are Hydra alienuata. x 120.
ture strongly suggest that depletion of
interstitial cells promotes interstitial cell
movements. In fact, all investigators who
have studied this problem state that this
migration is so extensive that the depeleted
tissue rapidly regains its normal histological aspect (see also legends of Figs. 8,9).
This high rate of movement distinguishes
this type of migration from that occurring
in normal hydra.
A second and crucial characteristic of
interstitial migration into interstitial cell-
FIG. 9. Migration of interstitial cells from normal
Pehnatohydra oligactis tissue (below large arrows)
into nitrogen mustard-treated Hydra attemiala tissue (above large arrows) . The P. oligactis tissue is
marked by carbon particles in the epithelial cells,
indicated by irregular dots at bottom of photograph. The large round dots throughout the photograph are stenotele nematocysts. The small arrows
point to interstitial cells which have migrated from
the P. oligactis tissue into the H. altenuala host
during 36 hr following the grafting. x!80.
CELL MOVEMENTS IN HYDRA
Another characteristic of bulk movement
of interstitial cells is that it proceeds very
rapidly for a day or two, and then slows
greatly or stops. Migration dependent upon
single, locomoting cells would not be expected to show this short term behavior.
The bulk movement of interstitially
located cells, including the presumably
non-migratory, nested nematoblasts, suggests that a distinct type of cell movement
operates in these situations and that
the moving cells themselves are not providing the propelling force. As yet the
mechanisms of this migration are unknown. The interstitial spaces in hydra
form one continuous network and bulk
migration seems to occur whenever normal
tissue is grafted onto tissue whose interstitial spaces are empty. It is possible that
pressure or "kneading" exerted by the epithelial cells is responsible for equalizing
interstitial cell distribution; grafting situations which produce a discontinuity in
interstitial space packing thus seem to result in massive movement of cells.
Another situation has been reported
where mass movement of cells might be
occurring. Moore (1971) and Moore and
Campbell (1973b) studied the ability of
normal tissue to induce budding in nonbudding strains of hydra. It was found
that interstitial cells, and perhaps nematoblasts, move from the normal tissue into
the regions of developing, induced buds in
great numbers. This could be a characteristic of these unusual grafting arrangements, but it is possible that in normal
hydra there is continuous bulk movement
of interstitial cells from the proximal tissue into devolping buds. This could help
explain the paucity of interstitial cells in
the stalk.
During oogenesis, large numbers of
oocytes fuse to form the egg cell which is
thus a huge complex mass spreading
through the intercellular spaces. The fusing oocyte may extend more than half-way
around the hydra column. Just before the
meiotic divisions, the oocyte becomes rapidly (in a few hours) consolidated into a
spherical cell. The force of this consolida-
533
tion is so strong that it ruptures the epithelial surface and the egg squeezes out
to the surface of the hydra. The egg has
been termed as amoeboid on the basis of
this retraction and its appearance, but its
amoeboid appearance is due to the spatial
pattern of oocyte fusion (Brien and
R.eniers-Decoen, 1950) and not due to the
cell's behavior. The histological evidence
is more compatible with the "retraction"
being due to a squeezing by peripheral
epithelial cells, forcing large oocyte mass
into a central mass (Moore, unpublished).
If so, egg consolidation might be related
in mechanism to bulk interstitial cell
migrations.
Mounted nematocytes and nerves. Other
types of movement which are probably
completely passive are exhibited by cells
which are intimately connected to epithelial cells: nerves and mounted nematocytes. These cells are presumably carried
with the displacing epithelial tissues. Although these passive cell movements have
not been observed directly, their existence
is in accord with cell production and turnover kinetics.
Germinal cells in tesles. In testes, there
is a graded germinal differentiation sequence with proliferating spermatogonia
near the mesolamella surface and mature
spermatozoids near the apical surface
(Brien and Reniers-Decoen, 1951; Schincariol and Habowsky, 1972). Thus, during
periods of spermatogenesis there is continual displacement of differentiating cells
in an apical direction. While this cell
movement is not long in range compared
to other migrations in hydra, it is exceptionally well ordered and probably critical to tissue functioning.
Cellular movements towards gonads during early stages of the sexual phase. It is
commonly stated that interstitial cells, or
their derivatives, migrate to sites of gonad
formation, thus resulting in the local
swelling of the gonads. However, this
migration has never been experimentally
documented and from kinetic and histological considerations might even be questioned. The extensive and long mitotic
activity of gonial cells preceding testis for-
534
RICHARD D. CAMPBELL
mation (Schincariol and Habowsky, 1972)
indicates that the primary source of these
cells is local proliferation. In some species
(e.g., Hydra pirardi and Pelmatohydra
oligactis) testes cover the entire body column, thus ruling out recruitment of
germinal cells from adjacent regions. In
ovaries, cell accumulation is due to final
consolidation of the egg cell (see above)
rather than interstitial cell migration.
Thus, although cell accumulation in gonads has been long described as due to
cell migration, there has never been experimental evidence for considering that
it is so.
CONCLUSIONS
All cells in a hydra are moving, in complete and continual tissue renewal patterns, with the various cell types showing
different, and often several simultaneous,
locomotory patterns. The mechanisms
range from active to passive and are intimately tied in with the developmental
pattern of the animal. Advances in cell
recognition, marking, and manipulation
appear now to enable us to determine
definitely and describe all the cell locomotory pathways in hydra. Recent experimental work indicates that the classical
histological analyses and concepts regarding cell movements in hydra were at times
misleading. Thus, the early considerations
that epithelial cells represented a fixed
framework of the adult hydra were completely reversed by Brien's demonstration
of continuous epithelial cell turnover. The
interstitial cell, long heralded as an errant
cell type, has now been found to function
largely in one place. Work in the near
future should provide greater understanding into the cellular mechanisms of hydra
cell locomotion and in what ways these
are related to cell differentiation and hydra
morphogenesis.
REFERENCES
Bibb, C. R. 1971. Tissue heaing, septate desmosome
formation, and graft rejection in hydra homografts and heterografts. Ph.D. Diss. University of
California, Irvine.
Bode, H., S. Berking, C. N. David, A. Gierer, H.
Schallcr, and E. Tienkncr. 1973. Quantitative
analysis of cell types during growth and morphogenesis in hydra. Wilhelm Roux' Arch. Entwicklungsmech. Organismen 171:269-285.
Brien, P. 1953. HomeogrelTes entre fragments
d'hydres normales et d'hydres irradiees. C. R.
Acad. Sci. Paris 237:938-940.
Brien, P., and M. Reniers-Decoen. 1949. La croissance, la blastogenese, l'ovogenese chez Hydra
fusca (Pallas) . Bull. Biol. Fr. Belg. 83:293-386.
Brien, P., and M. Reniers-Decoen. 1950. Etude
A'Hydra viridis (Linnaeus) (La blastogenese, la
speimatogenese, l'ovogentse) . Ann. Soc. Roy.
Zool. Belg. 81:33-110.
Brien, P., and M. Reniers-Decoen. 1951. La gametogenese et l'intersexualiti chez Hydra attenuata
(Pall). Ann. Soc. Roy. Zool. Belg. 82:285-327.
Brien, P., and M. Reniers-Decoen. 1955. La signification des cellules interstitielles des hydres d'eau
douce et le probleme de la r&erve embryonnaire.
Bull. Biol. Fr. Belg. 89:258-325.
Burnett, A. L. 1961. The growth process in hydra.
J. Exp. Zool. 146:21-83.
Burnside, M. B., and A. G. Jacobson. 1968. Analysis
of morphogenetic movements in the neural plate
of the newt Taricha torosa. Develop. Biol. 18:537552.
Campbell, R. D. 1967a. Tissue dynamics of steady
state growth in Hydra littoralis. I. Patterns of
cell division. Develop. Biol. 15:487-502.
Campbell, R. D. 1967ft. Tissue dynamics of steady
state growth in Hydra littoralis. II. Patterns of
tissue movement. J. Morphol. 121:19-28.
Campbell, R. D. 1967c. Tissue dynamics of steady
state growth in Hydra littoralis. III. Behavior of
specific cell types during tissue movements. J.
Exp. Zool. 164:379-391.
Campbell, R. D. 1968. Cell behavior and morphogenesis in hydroids. In Vitro 3:22-32.
Campbell, R. D. 1973. Vital marking of single cells
in developing tissues: india ink injection to trace
tissue movements in hydra. J. Cell Sci. 13:651-661.
David, C. N. 1973. A quantitative method for
maceration of hydra tissue. Wilhelm Roux' Arch.
Entwicklungsmech. Organismen 171:259-268.
David, C. N., and R. D. Campbell. 1972. Cell cycle
kinetics and development of Hydra attenuata. I.
Epithelial cells. J. Cell Sci. 11:557-568.
Davis, L. E. 1974. Ultrastructural studies of the development of nerves in Hydra. Amer. Zool. 14:
551-573.
Diehl, F. A., and A. L. Burnett. 1964. The role
of interstitial cells in the maintenance of hydra.
I. Specific destruction of interstitial cells in normal, asexual, and non-budding animals. J. Exp.
Zool. 155:253-259.
Diehl, F. A., and A. L. Burnett. 1966. The role of
interstitial cells in the maintenance of hydra. IV.
Migration of interstitial cells in homografts and
heterografts. J. .Exp. Zool. 163:125-139.
Evlakhova, V. F. 1946. Form-building migration of
regeneration material in hydra. C. R. Acad. Sci.
USSR 8:369-372.
CELL MOVEMENTS IN HYDRA
Giinzl, H. 1971. Dipurena reesi (Hydrozoa) . Wancterung der Cnidoblasten in den Rhizostolonin.
Gottingen: Institut fiir den Wissenschaftlicken
Film (Encyclopaedia cinematographica) , 15 p.
Gustafson, R., and L. Wolpert. 1962. Cellular mechanisms in the morphogenesis of the sea urchin
larva. Exp. Cell Res. 27:260-279.
Herlands, R. 1972. Studies on nematoqte movements in Hydra altemiata. Amer. Zool. 12:704705. (Abstr.)
Herlands, R. L. 1974. Studies on cell migration in
Hydra. Ph.D. Diss., Univ. California, Irvine.
Kanajew, J. 1930. Zur Frage der Bedeutung der
Interstitiellen Zellen bei Hydra. Wilhelm Roux'
Arch. Entwicklungsmech. Organismen 122:736-759.
Lenhoff, H. M. 1965. Cellular segregation and
hcterocytic dominance in hydra. Science 148:
1105-1107.
Lentz, T. L. 1966. The cell biology of hydra. North
Holland Publ. Co., Amsterdam. 199 p.
Moore, L. B. 1971. Non-budding hydra strains:
characterization and bud induction. Ph.D. Diss.,
University of California, Irvine.
Moore, L. B., and R. D. Campbell. 1973a. Nonbudding strains of hydra: isolation from sexual
crosses, and developmental regulation of form.
J. Exp. Zool. 185:73-82.
Moore, L. B., and R. D. Campbell. 19736. Bud
initiation in a nun-budding strain of hydra: role
of interstitial cells. J. Exp. Zool. 184:397-408.
Mueller, J. F. 1950. Some observations on the
structure of hydra, with particular reference to
the muscular system. Trans. Amer. Microscop.
Soc. 69:133-147.
Rahat, M., and R. D. Campbell. 1973. Nematocyte
migration in the polyp and single-celled tentacles
of the minute freshwater coelenterate, Calpasoma
dactyluptera. Trans. Amer. Microscop. Soc. (In
press)
Schaller, H. 1970. Isolierung und Charakterisierung
einer Substanz, die Kopfbildung bci Hydra in-
535
duziert. Ph.D. Diss. Eberhard-Karls Universitat,
Tubingen, Germany.
Schincariol, A. L., and J. E. J. Habowsky. 1972.
Germinal differentiation of the stem cell in
Hydra fusca: a model system. Can. J. Zool. 50:
5-12.
Shostak, S., and D. R. Kankel. 1967. Morphogenetic
movements during budding in Hydra. Develop.
Biol. 15:451-463.
Shostak, S., N. G. Patel, and A. L. Burnett. 1965.
The role of rnesoglea in mass cell movement in
hydra. Develop. Biol. 12:434-450.
Slautterback, D. B., and D. W. Fawcett. 1959. The
development of the cnidoblasts of hydra. An
electron microscope study of cell differentiation.
J. Biophys. Biochem. Cytol. 5:441-452.
Tardent, P. 1954. Axiale Verteilungs-Gradienten
der Interstitiellen Zellen bei Hydra and Tubularia und ihre Bedeutung fur die Regeneration.
Wilhelm Roux' Arch. Entwicklungsmech. Organismen 146:593-649.
Tardent, P., and U. Morgenthaler. 1966. Autoradiographische Untersuchungen zuin Problem der
Zellwanderungen bei Hydra attenuata Pall. Rev.
Suisse Zool. 73:468-480.
Tripp, K. 1928. Die Regenerationsfahigkeit von
Hydren in den verschiedenen Korperregionen.
Nach Regenerations- und Transplantationsversuchen. Z. Wiss. Zool. 132:476-525.
Webster, G. 1971. Morphogenesis and pattern formation in hydroids. Biol. Rev. 46:1-46.
Weiss, P. 1961. Guiding principles in cell locomotion and cell aggregation. Exp. Cell Res. Suppl.
8:260-281.
Zawarzin, A. A. 1929. Roentgcnologische Untersuchungen an Hydren I. Die Wirkung der
Roentgenstrahlen auf die Vermehrung und
Regeneration bei Pelmalohydra oligactis. Wilhelm Roux' Arch. Entwicklungsmech. Organismen 115:1-26.