Physical principles of cellular motion

Cellular motion
Movement is a major characteristic of living organisms. All living cells are capable of movement in some respect. It can take the form either of movements of cells or of movements within cells themselves. The cytoskeleton forms the basis for most of the active movements exhibited by higher cells.

Swimming cells
Certain higher cells possess motile appendages - cilia or flagella - on their cell surfaces and use these either for swimming or for moving fluid over their surfaces. Cilia and flagella of higher organisms possess an internal axoneme comprised of a characteristic arrangement of nine peripheral doublet microtubules with a central pair of single microtubules and the basis of their movement is the relative sliding of the outer doublets driven by a pair of projecting arms consisting of the motor protein, dynein. Sliding is then converted into bending by additional links and spokes comprised of the large numbers of polypeptides identified in the organelle.

Cilia are about 0,25 mm in diameter, 2-10 mm long and occur on the cell surface in great number. They beats via a rapid sideways bending – the effective stroke – and this is followed by a slower recovery stroke in which a wave passes from its base to the tip to return the cilium to its original position. Cilia occur extensively on the epithelia, where they serve to move fluid over the cell surfaces, for example, in the lungs and airways, the eustachian tubes, the middle ear, the pharynx, the lining of the brain, as well as in the female reproductive tract.

Flagella have the same diameter as cilia but are much longer (10-200 mm) and a cell has only one flagellum or just a few. They normally propagate bending waves of constant amplitude from their bases to their tips. The beating of flagella results in the forward propulsion of the cell.

The flagellum is used by the motile male gamete, sperm, to swim through the fluids within the female genital tract.

Crawling cells = amoeboid movement
The crawling movement is the result of cytoplasmic streaming into cellular extensions called pseudopods. This movement strategy is not limited to the amoeboid cells, but is seen in many cells in the animal body, for example, lymphocytes during their role in the inflammatory response.

Internal movements = cytoplasmic streaming, vesicle transport
Cytoplasmic streaming is a periodic circular flow of cytoplasm within cells. This movement speeds the distribution of materials and organelles within the cell. Translocation based on microtubule motor proteins (kinesins and dyneins) is believed to occur in almost all higher cells and to involve the translocation of a wide range of membrane-bound organelles including nuclei, mitochondria, lysosomes and a wide range of vesicular traffic in secretory and endocytic pathways. In many cells a dual system for intracellular transport exists, and in such cases it is envisaged that microtubules provide the tracks for long distance travel whereas actin filaments provide those for local movement.

Dividing cells, contracting cells
Cell division does not merely consist of asymmetrical division of chromosome into daughter nuclei, but includes a physicial separation of the cytoplasm into two compartments as well. Movements of the chromosomes and chromatids at mitosis may result from the polymerization and depolymerization of the spindle microtubules. Microtubule motors transport the genetic material along the spindle microtubules and also affect the movement of spindle microtubules relative to each other. Once nuclear division has taken place, the cytoplasm divides by a process known as cytokinesis and is accomplished by a bundle of actin and myosin filaments forming a contractile ring in the cortical cytoplasm. The ring is an example of purpose-built temporary actin-myosin structures.

Movements in tissues
Cells from a wide variety of tissues are capable of movement. Typical of this are fibroblasts. Their movement is determined by their attachment to the substrate, whereby complicated structures are formed in the contact areas between the cell and its substrate. They are made up of special proteins in the cell membrane (integrins), which connect with other proteins on the inner side (actin filaments) and outer side of the cell (proteins of substrate).

Similar mechanism of movement can be found in nerve axons which contributes in development, or regeneration of the nervous system after injury. Adhesion molecules in the membrane of cancer cells may link with the cytoskeleton to maintain stable adhesive contacts, but alternatively may interact with the actin cytoskeleton and so enhance invasive migration, and metastasis.

Cell movements in development
Some of the most dramatic cell movements occur in embryos during early development. During gastrulation and also in later stages of development, starting with the formation of the central nervous system at neurulation, the repertoire of collective and individual cell movements involved is very varied. All movements involve cooperation of adhesive systems between the cells and their actin cytoskeletons to generate the forces involved.

Importance for medical field
The cellular movements are critical for normal embryogenesis, tissue formation, wound healing and defense against infection. It is also an important factor in diseases such as cancer metastasis, and birth defects. Hence, the cytoskeleton and cellular motion is an important topic in biomedical research.