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In the previous chapters we have learned that
sponges have very diverse larval types and
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body types and in this chapter we're going
to focus on Sycon ciliatum because, as I
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mentioned in the previous chapter that
larvae of Calcaronean sponges from the
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genus Sycon are one of the simplest and
they are also one of the best-understood
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when it comes to development. So in this
chapter we are going to go into detail of
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embryonic development of Sycon ciliatum.
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This is the adult. As in many sponges,
but not in all, there are no specific areas
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of the sponge where the embryos develop,
but the embryos develop throughout the body.
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So during the reproductive season, if
you section transversely Sycon ciliatum
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you will find a lot of little elements,
which upon higher magnification,
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appear to be embryos.
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Those embryos give rise to larvae that
we talked about are called amphiblastula,
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they swim with this ciliated
part in the anterior part,
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and then the trailing end is
made of the non-ciliated part.
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You can see the cilia, very thin, very
long cilia, surrounding half of the larva.
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The embryos develop between the
choanoderm and the pinacoderm layer
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where the oocytes are found and where
they are fertilised in a way that we are
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not really convinced about where the
sperm comes from, but we know that
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the oocytes are fertilised as
they are in the mesohyl layer.
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I have made a number of those little Plastiline
models because the development, while simple
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I think it benefits from having a good look
at how those cells are changing position
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and how they are differentiating.
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Alright, so the first cell
division is very simple,
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the cleavage gives rise to two identical
cells and here on this histological section
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you can see that those two blastomeres
are connected by a number of cytoplasmic
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bridges and those bridges will remain in
place throughout the cleavage stages.
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In the next stage, there is
another equal cell division
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so it ultimately gives rise to four identical
cells that are positioned in this rhomboid
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fashion, flat between the
pinacoderm and the choanoderm.
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The next division is a bit different because
it results in the formation of two different
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layers, the blue ones here indicate
cells that are closer to choanoderm
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and the yellow with orange dots
are the cells that are closer
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to the pinacoderm layer.
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Why are there orange dots here?
Well, this is because very early
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on, perhaps already at the 4-cell
stage, but definitely at 8-cell stage
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we can find a different type of cytoplasm in
the corner of those bottom layer of cells,
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and this cytoplasm will become cleaved
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during the cleavage until
it forms only four cells.
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Those four cells form a cross if you
look at a section of the embryo
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or a larva, and they are
therefore called cross cells.
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We do not know what their role is,
but we have a suspicion they might
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have sensory function and you are going
soon to see how they give this very unique
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tetraradial symmetry to the
forming embryos and larvae.
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So these are four large
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cross cell nuclei visible
here with DAPI staining
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that is allowing us to see
DNA and in particular nuclei
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and they are present around the equator,
this is how the section is made here
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between micromeres, which
are the ciliated cells of the embryo.
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Alright, so the third cell division resulted
in the formation of two different layers
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and those two different layers
will give rise to macromeres
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close to the choanoderm here and
micromeres and cross cells close to
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the pinacoderm and the cross
cells are on the equator.
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This is a different histological section,
you can see that the macromeres are
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indeed larger than the micromeres.
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As the cells continue to divide,
they form a cup-shaped embryo,
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sometimes in the literature called
stomoblastula, and this cup-shaped
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embryo is formed without any major
cell movement between the cells.
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The cells remain connected to each other
for quite a while by those cytoplasmic
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bridges we mentioned before, and they
form an epithelial cup-shaped structure.
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As the division continues, there is this
opening that is communicating with the
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choanocytes and we can see the cross
cells that remain more and more visible,
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more and more different than other cells around
the equator and between the micromeres.
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As soon as the cleavage is completed,
the cells start to differentiate.
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The macromeres remain epithelial cells
that are surrounding the opening of the
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embryo, and the micromeres start to
differentiate by forming flagella at the
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at one of their surfaces. Now, perhaps
surprisingly, the flagella that you can see
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here, very clearly visible, are
pointing inside of the embryo.
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Now you remember from looking at the
larvae that the larvae have cilia or flagella
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on the outside of the larva, and of course
this is necessary for the larva to swim.
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So, how are those flagella or cilia,
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how are they going to
start pointing outside?
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We are looking here at the embryo
from the top through the opening.
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This is a stage that we call the pre-inversion
and the reason why we call it pre-inversion
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is because it's going to invert.
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So first the innermost part starts
to come through the opening,
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the opening is between the macromeres,
but also between the choanocytes and
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then as if you were taking a sock inside
out or as if you were looking at embryonic
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development of a Volvox alga, the embryo
inverts -or everts- completely so that the
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cilia start pointing outwards.
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After the cilia start pointing outwards,
you have an embryo that is again cup-
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shaped, but now
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while the macromeres remain in
communication with the choanocytes,
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the micromeres have their
cilia pointing outwards.
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And there is another rather interesting
thing that is going to happen that is
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a few cells of maternal origin are
going to migrate from the mesohyl
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to the formed embryonic cavity, and
you can see them very clearly here
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on the section they are those larger cells
that do not form a continuous epithelium
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of the macromeres and the micromeres.
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The cross cells are still present, there is
four of them, you can see three of them
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here, but we haven't got
one on this section.
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Now in the next step, the larva or the
late stage embryo that is flat, kind of
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doughnut perhaps doughnut-
or flat coin-shaped
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is going to change by changing individual
cell shapes, so the cells that are forming
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the embryo, the micromeres and the
macromeres, stop being flattish like they
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were here and they start being wedge-
shaped or you can think of it as
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carrot-shaped in a way.
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It first happens to the micromeres
and the macromeres remain
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flat, so they are kind of stretched here.
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And then it also happens to the macromeres
and in the end we have a larva that is bullet-
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shaped, very good for swimming, swimming
with the nose the anterior part in front
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and inside of the larva there is pigment
that is present in the innermost part of
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the micromeres and there are also squeezed
between macromeres and squeezed between
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the larval cavity, there are maternal cells
that are taken for the ride by the larva
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as it is leaving the parent sponge.
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So the larva swims for a while before it
metamorphoses into adult. How does it do it?
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It makes a contact with the substrate
- kelp, rocks, some other algae -
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with the micromeres, and then
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in a very quick process,
while the macromeres
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maintain their epithelial character and
they stretch all around the former larva
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what we now call the post-larva,
the micromeres become amoeboid
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so they undergo epithelial to mesenchymal
transition and they start being amoeboid,
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rather quickly-moving cells
within the post-larva.
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Very importantly, both the maternal cells
and the cross cells they generate and
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they do not form the adult body
plan. So those cells are transient,
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we do not know their roles, cross cells
perhaps sensory, and we have honestly
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no idea why the maternal cells are taken
with the larva as they leave the mother.
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Within a couple of days there is
a lot of differentiation happening.
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Those yellow cells that were micromeres,
ciliated epithelial cells and then they were
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amoeboid for a couple of days,
they undergo another transition
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becoming epithelial again, forming the
first chamber, and they are going soon
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to differentiate into choanocytes.
The former macromeres maintain
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their epithelial character and they become
exopinacocytes and basopinacocytes.
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Some of the cells of the inner cell mass,
those cells derived from the micromeres
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remain amoeboid, they move around in
the post-larva and some of them become
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sclerocytes, which are cells that are forming
the spicules and you can see in this image
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of a living post-larva you can see those
spicules being formed by sclerocytes,
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cells that are secreting them, and again
those spicules are built from calcium
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carbonate in calcareous sponges.
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Within the next couple of days, the
development progresses so that the
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osculum opens at the apical end, a lot of
spicules are being formed, some of those
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spicules are not so simple diactines
anymore, but some of them become
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a bit more complex, having three or even
four rays, and quite importantly, porocytes
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differentiate, most likely from pinacocytes,
and those porocytes form connections
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between the choanocytes
and the pinacocytes.
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And in that way the juvenile sponge
becomes a very simple filter-feeder unit.
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The water is drawn through the porocytes,
the water movement is due to the beating
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of the flagella, and then the water
is expelled through the osculum,
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a single apical osculum, but the bacteria
and microalgae and some other tiny particles
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are captured by the collar
as food for the sponges.
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This very simple juvenile asconoid level,
because there are so many larvae produced
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by Sycon ciliatum in the Norwegian fjords,
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we can then look at the algae, at the kelp
and at the other algae growing on kelp
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and three to six weeks after settlement
we see masses of tiny juveniles present
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everywhere in the kelp forest.
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Within the next couple of weeks,
those juveniles stop being asconoid,
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they become syconoid, and as we can see
on the magnification here, they are built
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from asconoid-like units
surrounding the central atrium.
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Those sponges, Sycon ciliatum, are an
annual species and they grow through
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the year and then they release the larvae in
the spring and majority of those individuals
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die, but there are many sponges that can
live much longer - they can live in the
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deep, cold water, they can llive for
hundreds or even thousands of years.