Cancer Cell Filopodia Characterized by Super-resolution Bright-field Optical Microscopy

Abstract

We use super-resolution bright-field optical microscopy of lateral resolution ~ 120 nm to measure the filopodium dynamics of lung cancer cells. The effects of epidermal growth factor on filopodium dynamics are characterized. OCIS codes: (100.6640) Superresolution, (170.3880) Medical and biological imagingCell migration plays a crucial role in the metastasis of cancers. At present, cell morphology is the most commonly used characteristic to identify high-motility cancer cells. For example, the filopodia, needle-like projections protruding from cell edges, are thought to be closely related to cancer cell motility [1,2]. Because the diameters of ordinary filopodia are smaller than the optical resolution limit, filopodium dynamics is difficult to quantify by using bright-field or phase-contrast optical microscopy. Although the fluorescence-protein labeled actin images of filopodia can help identify the presence and dynamics [3], the activities of cells transfected by fluorescence proteins could be different from those of normal cells. Moreover, in fluorescence microscopy the phototoxic effects on living cells could reduce cell viability or lead to cell death [4].In this work we use super-resolution bright-field optical microscopy to observe the dynamics of filopodia of living lung cancer cells. The contrast required by the super-resolution image restoration process is from the nanometer topographic sensitivity of non-interferometric widefield optical profilometry (NIWOP) [5], rather than fluorescence labeling. The NIWOP technique is achieved by applying the principle of differential confocal microscopy to widefield optical sectioning microscopy [6]. Because the axial response curve of the widefield optical sectioning microscopy is close to that of confocal microscopy, when the sample surface is placed approximately half a depth of focus away from the focal plane, the reflected signal intensity changes linearly as the surface height varies within a few micrometers. Therefore one can obtain nanometer depth resolution after the calibration of intensity along the optical axis [5]. We have also demonstrated the sub-diffraction-limit lateral resolution of NIWOP by using a maximum-likelihood estimation algorithm to restore the NIWOP topography [7]. In this work, the super-resolution image restoration is conducted by using commercial software with the iteration core employing the same algorithm (Huygens, Scientific Volume Imaging, Hilversum, The Netherlands) During the observation, the samples, CL1-0 human lung cancer cells, were placed in a 35-mm culture dish filled with culture medium (DMEM + 0.2% FBS + 1% PSA). No other treatment to the cells is required before the observation. In this paper, all the images are acquired by a water-immersion objective with a 1.2 numerical aperture (Nikon CFI Plan Apo VC 60× WI), and recorded with a 14-bit CCD camera cooled at -25 °C. The lateral resolution of this NIWOP system is about 250 nm, and the depth sensitivity is about 16 nm. The pixel size of images is 125 nm, and we re-sample th...