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Planarians (Dugesia) are eukaryotic
flatworms that form the class Turbellarians in the Phylum Platyhelminthes (1).
They are related to other flatworms, e.g. tape and fluke however they differ by
one thing which is that they are non-parasitic. They are invertebrates and are one of the most basal
triploblastic organisms, with derivatives of all three germ layers; ectoderm,
mesoderm, and endoderm (3). They show bilateral symmetry and are one of the
simplest animals with mesodermic layers and their tissues are organised into
organ and organ systems (1).  As shown in diagram 1,
Planarians are formed various components that allow it to survive and work well
in its environment. The eyespots (ocelli) on their head are not actually
eyes but light sensitive receptors that direct the planarians away from the
light (negative phototaxis). They also use chemoreceptors on ciliated auricles, the
ear-like extensions of the head. They have two ventrolateral and many
transverse nerve cords that detect external stimuli (3).  They have a Pharynx located on the under part
of their body which is used to feed, however it is also used for excretion. Planarians
do not have a skeletal, circulatory or respiratory system and are known as
acoelomate as they do not have the principal body cavity that most animals have

Planarians are known as freshwater worms (as they are
sensitive to pollution), found at the bottom of ponds, streams or under rocks
(3). Planarians are predators and scavengers and
eat live or dead animals using their muscular retractable pharynx which can
extend out of the mouth opening on the ventral side up to half of their body
length (1).    

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Planarians contain cilia which enables them to glide across surfaces.
The cilia is located on the surface on the planarian and is attached to the
flame cell within the excretory canal.

Cilia are the oldest known cellular organelle, discovered
in 1675 by Anthony Van Leeuwenhoek (5). Cilia
is functionally and structurally similar to eukaryotic flagella as they both
contain a central bundle of microtubules called the axoneme which is nine outer
doublet microtubules surrounding a central pair of singlet microtubules in the
(7). Cilia can be divided into two groups,
primary and motile. Primary cilia are sensory organelles that contain a variety
of receptors. Cells typically sprout a single primary cilium with a
characteristic 9+0 cytoskeletal structure (axoneme). However, motile cilia can
occur on epithelial cells in the hundreds. They have a typical 9+2 axoneme, and
they play a mechanical role (9). The
‘9+2’ formation of microtubules can be seen through an electron microscope at
the cross section of axoneme. As seen in diagram 2, the surrounding doublets
contain two different tubules, A and B. Tubule A is a complete microtubule that
contains 13 protofilaments while tubule B only contains 10 protofilaments (7). The
axoneme is held together by three sets of protein cross-links. The central pair
of singlet microtubules are connected by periodic bridges surrounded by a
fibrous structure given the name inner sheath (7). The linkage between
microtubule doublets is by a protein called Nexin and the linkage between the
doublets and the singlets at the centre of cilia is done by a multi-unit
protein called Radial spoke. The inner and outer dynein arms are permanently
attached to tubule A and these reach out to the tubule B ahead of them. These
components are surround by the plasma membrane.

The point at which the axoneme attaches to the cell, it
connects with the basal body as shown in diagram 3. Basal bodies are like
centrioles as they have a cylindrical structure containing nine triplet
microtubules. Each triplet contains one complete 13-protofilament microtubule
(tubule A), which is fused to an incomplete tubule B, which is turn is fused to
an incomplete C tubule. Tubules A and B fill the whole axonemal shaft while C
stops within the transition zone, between the basal body (Kinetosome) and
axonemal shaft (7).

For bending to occur, dynein rows must be activated and
inactivated in a synchronous alternating pattern. Individual dynein arms have
been shown to behave as oscillators. In addition to the bending movements of
cilia, the ciliary membrane also undergoes an active longitudinal sliding
movement (9).

Mitogen-activated pathway kinases (MAPKs) are protein
Serine-Theronine kinases that convert extracellular stimuli into a wide range
of cellular responses. MAPKs
are among the most ancient signal transduction pathways and are widely used
throughout evolution in many physiological processes. All eukaryotic cells
possess multiple MAPK pathways, which coordinately regulate gene expression,
mitosis, metabolism, motility, survival, apoptosis, and differentiation (10).  The
mammalian MAPK family consists of extracellular signal-regulated kinase (ERK), p38, and c-Jun NH2-terminal
kinase (11). Each group of conventional MAPKs is made up of three, sequentially
acting kinases:  MAPK,  MAPK kinase (MAPKK), and MAPKK kinase
(MAPKKK). The MAPKKKs, which are protein Ser/Thr kinases, are often activated
by phosphorylation as a result of their binding with the small GTP-binding
protein  (Ras/Rho family) in response to
extracellular stimuli (10). MAPK
pathways are activated due to a series of binary interactions between the
kinase components or through the formation of a signalling complex containing
multiple kinases that is guided by a scaffold protein (11). Scaffolding
proteins also mediate MAPK cascade specificity by simultaneously binding several
components and organizing pathways in specific modules (10).

 P38 mitogen-activated protein kinase (p38 MAPK), is
an intracellular signal-transducing molecule, which plays an important role in
the regulation of a variety of inflammatory responses, including expression of
proinflammatory cytokines, leukocyte adhesion and chemotaxis (14). There are four p38 MAP kinases in mammals: ?, ?, ? and ?. Among all p38 MAPK isoforms, p38?
is the best characterised and is expressed in most cell types (13). The p38
MAPK subfamily can be divided into two distinct subsets, p38? and p38? ,
p38? and p38?. Their susceptibilities to inhibition at low concentrations by
the compounds SB203580 and SB202190 (13).

The experiment performed was to see how the inhibitor
SB203580, affected the locomotion of Planarians.

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