Contractile filopodia and in vivo cell movement in the tunic of the ascidian, Botryllus schlosseri

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Abstract
The in vivo movement of one class of cells in the tunic of the ascidian Botryllus schlosseri has been analysed using differential interference optics and time-lapse cinematography. Long (up to 200 μm), thin (0·35–0·5 pm diameter) filopodia radiate from the cell-body into the matrix of the tunic. Movement of the cell-body consists of a series of short, jerky displacements with frequent changes in direction between successive displacements. The net displacement of the cell may be extremely small when the displacements are short and frequently change direction, or considerable when successive displacements show a persistence of direction (up to 114 μm in 60 min). Deformation of the elastic cuticle covering the tunic at points of attachment of the filopodia has been used to record qualitatively changes in tension in the filopodia. Correlation of the changes in tension with changes in length of the filopodia and movement of the cell-body have permitted the following conclusions. Active contractions of filopodia (i.e. increase in tension during shortening) stretch and move the cell-body. These movements exert a force on trailing or opposing filopodia. Relaxations of filopodia (i.e. decrease in tension during lengthening) result in small movements of the cell-body due to the recoil of tension in the cell-body and opposing filopodia. The position of the cell-body in space at any one instant in time is therefore the resultant of the forces developed in all the filopodia. Movement results from unilateral modulation of the tension developed in the filopodia. Active contractions play a more significant role in movement than relaxations.