WEBVTT
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Welcome to the chapter on the cell types.
We have learned that all sponges are built
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from two basic epithelial types, the
choanoderm and the pinacoderm,
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and that between them is a mesohyl layer.
In this chapter we are going to learn a bit
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more about the cell types that are building
those epithelia, and the cell types that are
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present in the mesohyl.
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Again, we're going to look at this juvenile
of Sycon ciliatum, because this is the
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simplest sponge body plan you can think
of, and it allows us to look very carefully
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at the different cell types that
are present in this sponge.
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You can see the spicules on the surface
of this living specimen, and this is
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a histological section in which the spicules
have been removed, and this is a resin
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section, very thin, approximately 5
micron-thick section that has been
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stained with hematoxylin and eosin,
which allows us with great precision
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to see the cells that are
building this little juvenile.
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And those cells are rather tiny, this
scale bar is ten microns, so you
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can think of those cells as being certainly
less than 10 microns in diameter.
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So what cell types can we recognize here?
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We can see the mesohyl cell that is
sandwiched between the two layers,
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the choanoderm built of choanocytes
and the pinacoderm built of pinacocytes.
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And you can also see that between
those layers there are different cell types
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this is called a porocyte, and it's the
cell type that is forming openings,
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little tunnels between the layer of
choanoderm and the layer of pinacoderm.
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Those cells called porocytes are shaped
like a well, like a little ring, and their
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opening allows the water to flow from
the outside to the inside and then it
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can be expelled by the osculum.
So if we look into details of those cells,
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you can think about the mesohyl cells as
often amoeboid, almost always amoeboid
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although not always. Many of them are
proliferative and they perform many
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functions including synthesis of the
skeleton, but also storage of nutrients,
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sometimes they harbor bacteria and
sometimes they are on the way of
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differentiating into gametes.
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The porocytes, as I said before, are
connecting the choanoderm with the
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pinacoderm. They have a very interesting
feature that they are contractile so they
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are able to close or open, and that way they
regulate the water flow through the sponge.
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On the very outside of the juvenile
or all sponges are pinacocytes.
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They are epithelial cells and they are not
only on the outside of the sponges, but
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they are also at the base of the sponges
when they're attaching to the substrate.
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These are called basopinacocytes.
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And they can also be on the inside of the
sponges where they are lining canals.
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They are called endopinacocytes.
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Now the cells, the pinacocytes that are
on the outside of the sponges are called
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exopinacocytes, simply meaning that
they are pinacocytes on the outside.
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There is another cell type or a couple of
sub-cell types that are present in sponges
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which are very similar to pinacocytes
and these are the cells that are building
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the sphincter of the osculum. Again,
they are contractile and they are able
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to regulate the water flow by either
opening or closing the sphincter.
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Perhaps the most exciting cell in sponges
are the choanocytes, so they are epithelial
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as mentioned before, they are proliferative,
they're one of the major proliferative cell
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types in sponges. They form chambers
either lining the entire inner part of the
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sponge or forming little choanocyte
chambers as in the leucanoid body plan,
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and they have a number of critical features
that allow us to define them as choanocytes
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and they are necessary for their function.
They have a flagellum that is beating,
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the beating of the flagellum causes
the water to flow through the body.
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Now the flagellum is surrounded
by a collar that is giving the name
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to the choanocytes, "choano" meaning
collar and this collar is built of actin-based
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microvilli, and this collar is responsible
for the capture of food that is brought
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by water, and the food in sponges
is mainly bacteria and micro algae,
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although we also know
that sponges are capable
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of capturing many smaller particles than
bacteria and micro algae, including the
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soft organic matter and some sponges
are also able to capture larger prey.
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Now I said that choanocytes are very ex-
citing and they are for a variety of reasons.
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So you can see here in isolated clusters
of choanocytes that they are still alive,
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the flagella were beating as
I was taking this photo.
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So you can see this very long flagellum
and the collar, this green element here
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are microalgae that have been captured by
the sponge before it has been dissociated
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into cell clusters.
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What has been noticed already in the 19th
century is that those clusters of choanocytes
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or individual choanocytes are strikingly
similar to protists, and those protists are
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called choanoflagellates, again because
"choano" means collar and they also have flagella.
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This is a single choanoflagellate form of
Saplingoeca rosetta, and this is a cluster
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that looks strikingly like the
cluster of isolated choanocytes.
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And from the 19th century people have
been wondering if there is importance
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to this similarity. Is that just an effect of
convergence or does it tell us something
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very important about evolution of sponges
and of animals? And we're going to talk about
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that in the next chapter when we talk about
phylogeny of sponges and phylogeny of animals.