Cells show a great variation in their shape. THey may be spherical, oval, rounded, elongated, cuboidal, cylindrical, tubular, discoidal or irreglar.
It can be said by:
.Nerve cells are long to conduct impulse from distant parts of the body to the brain and vice-versa.
.Muscle cells are long and contractile to help in the movement of bones.
.Xylem vessels are long and elongated to help in the conduction of water and minerals.
.Guard cells in the leaves of a plant are kidney-shaped to open and close the stomata to allow exchange of gases.
Therefore, we conclude that, shape of the cells is often related to the different functions they perform.
hope it helps...^_^