Measuring Cell Adhesion Forces

A micromanipulation technique was used to measure adhesion forces between cells as described previously [33 (link)]. Briefly, forces were measured on the stage of an inverted epifluorescence microscope (Leica) equipped with objectives of 63× (PL FLUOTAR; NA/0.7; C PLAN NA/0.75) and with a cooled CCD C5985 (Hamamatsu, Massy, France) or Coolpix 5000 camera (Nikon). Cells were manipulated at 37 °C with two micropipettes, each held by one micromanipulator connected to a combined hydraulic/pneumatic system and a pressure sensor making it possible to control and measure the aspiration applied to the cells. Micropipettes were pulled (model P-2000; Sutter Instrument, Novato, CA, USA), cut and fire-polished with a homemade microforge, such that their i.d. was 4.0–5.5 µm. The aspiration applied to the left pipette was measured using a pressure sensor (model DP103-38; Validyne, Northridge, CA, USA). Aspiration was monitored continuously during the separation process and the values recorded for each of the last two cycles in the series (P_n−1 and P_n) were used to calculate the separation force (SF) for each doublet using the equation: SF = π(d/2)² (P_n−1 × P_n)/2, where d is the i.d. of the left pipette. Results for around 20 measurements were used to obtain the mean force of separation for a specific contact time.