Time-resolved Acoustic Microscopy Dynamical Processes in Cell Biology Recent growth of interest in the mechanical properties of cells demands for the development of new tech- niques for the assessement of these atributes. Acoustic microscopy equipped with time-resolved signal analysis provides unique possibilities for the determination of dynamical processes in biological cells and al- lows probing biological specimen with submicrometre resolution. Local mechanical properties of cytoplasm control several cell mechanisms such as apoptosis and proliferation, cell shape, cell differentiation and mechanical sig- nal transduction. Mechanical stresses are assumed to alter the structural and functional properties of cells at the cellu- lar, molecular and genetic level. Increasing evidence is brought for- ward for a subtle interplay between cell shape, invasiveness of tumor cells, the cytoskeletal organization and gene ex- pression [1, 2]. The cytoskeleton’s vari- ous components – actin, microtubules and intermediate filaments – in concert with their accessory proteins facilitate these vital cellular functions [3, 4]. Con- sequently, changes to cellular function during differentiation or due to disease are mirrored in the cytoskeleton [5]. ity provides information about its state and may be viewed as a new biological marker. Among the existing methods for the determination of mechanical properties of living cells, acoustic microscopy is an im- portant and very promising technique. It depends on the elastic response of the material to acoustic waves and, therefore, provides information on local changes in mechanical properties without any physi- cal contact with the specimen. Acoustic microscopy provides some extraordinary advantages such as excel- lent spatial resolution, minimal invasive- ness due to sound frequencies in the GHz range, which do not cause any damage or disturbance of the cells [12], while at the same time is relatively fast. Pavlos Anastasiadis Malignant transformation of cells for instance where morphological changes caused by the cytoskeleton are, in fact, diagnostic for cancer. In general, malig- nant cells respond either less elastic or less viscous when stresses are applied [6, 7, 8]. Metastatic cancer cells have been found to display an even lower resistance to deformation [9, 10]. This stands to reason, since metastatic cancer cells must squeeze their way through the sur- rounding tissue matrix as they make their entrance into the circulatory sys- tem, where they travel to establish dis- tant settlements [11]. Taken together, these changes in cyto- skeletal content and structure are re- flected in the local mechanical properties of the cell. Thus, measuring a cell’s rigid- Fig. 1: Observation of cell division of HeLa cells with time-resolved acoustic microscopy. The time between the individual images is 10 min. Eike Christian Weiss Robert Lemor LIGHT MICROSCOPY G.I.T. Imaging & Microscopy 3/2007 • 65