Amoeba (plural amoebas/amoebae) is a genus that
belongs to Kingdom protozoa. Generally, the term is used to describe
single celled organisms that move in a primitive crawling manner (by using
temporary "false feet" known as pseudopods).
Amoebas are eukaryotes,
which means that their genetic material are well organized and enclosed within
a membrane (nuclear membrane)
* The word amoeba comes from the Greek word
"amoibe" which means to change
Amoebas are simply single celled organisms. As
such, they can only be viewed using a microscope. There are methods that can be
used to observe these organisms.
The first and simplest methods involves viewing amoebas under the microscope without staining. This is a simple method
that allows students to view them live as they move around. The second
method involves fixing and staining to get a better view of the
structure and organelles of the organism.
For this technique, the student may either
observe a sample of pond water directly to identify the organism or
conduct a simple culture to grow and increase the number of amoebas.
culture is a simple exercise that involves the following few steps:
Place a few pondweeds in a petri-dish
and add some water to cover the weed
Leave the culture is a dark
room for a few days and wait until a brown scum forms on the surface
Procedure for Microscopy
Using a dropper, place a
few drops of the sample on a microscope glass slide (a sample of pond water or
a small sample from the culture)
Gently cover the sample
with a cover slip and mount on the microscope stage for viewing
Start with low power and
increase gradually to observe the specimen
When viewed, amoebas will appear like a colorless
(transparent) jelly moving across the field very slowly as they change
shape. As it changes its shape, it will be seen protruding long, finger
like projections (drawn and withdrawn).
For this technique, the specimen (amoebae) is
first cultured using such culture and saline agar slopes. Following the
culture, a sample of the specimen is further concentrated using a centrifuge
(at 3 Krpm for about 10 minutes).
This is continued by the following few steps:
Wash the pellet using 75 percent seawater,
Neff's saline or any other appropriate solution
Allow the specimen to settle on a cover slip in
a moist chamber until the amoebae adopt a normal locomotionary morphology (the cover
slip can be treated with sodium hydroxide depending on the type of amoeba)
While settling on the cover
slip, pipette freshly prepared Nissenbaum’s Fixative on the sample and allow to
stand for 5 minutes
Wash the sample with
acidified HgCl2 for about 7 minutes
Wash the sample with 50%,
35%, 15% ethanol for about 5 minutes
Wash the sample again with
distilled water for about 5 minutes
Some of the other fixatives that can be used
Staining the amoebae is aimed at enhancing
visibility of the mitotic figures.
Some of the stains that can be used to stain
the amoeba cells include:
Kernechtrot (Nuclear Red)
Modified Field’s Stain
Klein’s Silver relief stain
For Heidenhain’s iron Haematoxylin, staining
involves the following steps:
Inoculation of the sample
in 2 percent ammonium ferric sulphate for about 1 and a half hour
Rinsing the sample in distilled
Inoculation of the sample
in 0.5% haematoxylin and 2% ammonium ferric sulphate for about one and a half
Wash the sample with tap
After staining, mount the slide on the
microscope stage to observe.
When viewed under the microscope, students will
notice tiny dark spots in the cytoplasm of the organism while the cytoplasm is
* Direct observation of the organism (without staining) has a great
advantage in that the amoebae are still alive and motile when being viewed
under the microscope.
This allows the students to see the finger like
projections (pseudopods) elongate and shorten as the organism moves about or
engulfs given substrates. However, this technique does not allow students to
view the cell’s organelles.
Fixing and staining on the other hand kills the
amoeba, which means that students will not get to see the organism moving in
the field of view but staining
increases contrast, allowing students to get a better view of the organelles in
Essentially, Pseudopodia are temporary
projections of the cytoplasm that make it possible for amoebae to move.
Pseudopods are some of the most distinguishable features of amoebae and their
formation is based on the flow of the protoplasm.
The organism contracts
in a manner that pushes the cytoplasm to fill and expand a pseudopod while pulling at adhesions at the back of the cell.
According to studies, the process
involves the following steps:
With pressure from ectoplasm, which is an
exterior gel, endoplasm (interior fluid) is forced to flow forwards in the cell.
On reaching the tip of the membrane, the
pressure causes the endoplasm to form a pseudopod.
The endoplasm is then forced back towards the
ectoplasm and turns to gel (this causes the new pseudopod to disappear) The new
gel is again converted to endoplasm and again under pressure moves to the
membrane to form a new pseudopod.
Apart from using pseudopod to move around,
amoebae also use them to engulf food particles. Here, the pseudopod surround
the particle while an opening on the membrane allows the particle to move into
the cell and into a food vacuole where it is digested by enzymes.
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