|Building Bodies from Slime
|Amoebae that form slime moulds
Some single celled organisms, such as the amoeba, look like
wandering masses of slime (most are microscopic but the largest are
slime sheets up to over one metre across) but some multicellular
creatures also build bodies out of slime. Indeed, this seems to be one
of the simplest multicellular body types and represents one of the most
ancient of Nature's experiments in multicellularity. Amoeba are not
bacteria, they belong to a group of protoctistans called the Protozoa
and resemble animal cells. However, even the much smaller and
genetically much simpler cells of bacteria can form multicellular slime
Click here for a more technical account of cellular slime moulds.
Most amoebae live as single-celled organisms in the water and soil, but
some amoebae can also form multicellular slime structures. Amoebae
are not bacteria, rather they are micro-organisms belonging to a group
called the Protoctista. Amoebal cells are typically 10-100 times the
diameter of bacterial cells and have the structure typical of animal
cells, but they are not animals because animals always form complex
multicellular bodies. Some amoebae (called myxamoebae), such as
Dictyostelium, will live in the soil as single cells that feed and reproduce
for many generations, but if these cells start to run out of food in their
neighbourhood, then they send chemical signals to one another and
the amoebae respond by streaming in long conveys to a common
rendezvous. When they arrive, these amoebae do something very
strange, they form a multicellular mound or aggregate that piles on new
coming cells, getting taller and taller. Eventually the cells at the tip of
the mound form a nipple-like protuberance and this takes charge as it
is designed to become the 'head' of our new organism. All this happens
on a small scale, these mounds are only a few millimetres in diameter.
|Above a mound of assembling Dictyostelium amoebae
on a glass slide, left, and a later stage with a tip, right.
Eventually, this tipped mound falls over and starts crawling around like a slug, with the tip raised up like a snout
behind which the rest of the body follows, leaving a trail of slime behind it as it does so. So our single celled
creatures have all come together to form a temporary multicellular body! The reason is, that this way they are
bigger and so can move faster and further. The job of the snout is to find a suitable place high up in the light
and air, from which to release spores into the wind or running rain-water. This slug-like creature is called a
grex, and is one to a few millimetres long, which is not bad for something that started out as amoebae one
hundredth of a millimetre in diameter! Each grex contains about 100 000 separate amoebae, all encased in
slime and working together as a single unit! The grex of Dictyostelium discoideum is white and translucent, but
experimenters frequently add colouring agents to make the different cells in the grex apparent.
Above: a slime mould grex (rendered with Pov Ray) crawling across a glass
slide, leaving a trail of slime behind it.
Left: when a grex finally finds a suitable place (or runs out of time) it will stop
moving, then form a mound which elongates into a relatively long stalk
(several millimetres long) with a rounded structure at the tip (the colour and
form varies tremendously depending upon species). This structure, called a
sporangium, will dry and break open, releasing amoebae in the form of
spores, into the wind or rain water to be carried off to new habitats. The
spores are dormant cells with tough walls to resist drying out. Hopefully some
of the spores will find a suitable place and germinate into single-celled
amoebae and live and reproduce happily, until they run out of food that is ...
then the cycle will start all over again!
Let's look at another creature that builds its body from slime, and may be over
one metre across!
Click the thumbnail left for bacterial
micrococlonies and biofilms.