['In long-term peritoneal dialysis, myofibroblast-like cells found in the interstitium of the peritoneum are assumed to be a transformed type of mesothelial cell-epithelial-mesenchymal transition-positive [EMT(+)] human peritoneal mesothelial cells (HPMCs)-because they express a mesothelial marker, cytokeratin. However, no direct evidence about how these cells are able to invade from the mesothelium has yet been obtained.', 'In this study, we aimed to verify whether EMT(+) HPMCs would, in vitro, invade three-dimensionally along certain chemotactic factors.', 'We used reverse-transcriptase polymerase chain reaction to measure expression of Snail, E-cadherin, α(5)-integrin, and matrix metalloproteinase 2 (MMP2) messenger RNA (mRNA) in HPMCs exposed to 10 ng/mL transforming growth factor β1 (TGFβ1) and how that expression corresponds to cell motility, as represented by a video movie. We used the Transwell (12 μm pore diameter: Sigma-Aldrich, Tokyo, Japan) to construct a three-dimensional (3D) cell migration chamber. In the lower chamber, a concentration gradient of fibronectin (FN) or albumin(Alb) was formed in 0.1% type I collagen by diffusion (C(0)=22 nmol/L; concentration gradient: C/C(0)=0.7). All cells beneath the membrane were counted 72 hours after 5×10(4) EMT(+) HPMCs (HPMCs after a 48-hour exposure to 10 ng/mL TGFβ1) had been spread in the upper chamber.', 'After 72 hours, the increased motility of HPMCs resulting from their exposure to 10 ng/mL TGFβ1 had returned to baseline, but they retained an elongated morphology. Expression of Snail and MMP2 mRNA reached maximum at 24 hours. Expression of E-cadherin declined, and expression of α(5)-integrin increased continuously. In the 3D invasion study, significantly enhanced invasion by EMT(+) but not EMT(-) HPMCs was clearly seen in the presence of a FN concentration gradient (p<0.01), although invasion by EMT(+) and EMT(-) HPMCs in the absence of a FN concentration gradient was not statistically significantly different. Compared with the EMT(+) control (no concentration gradient), invasion by EMT(+) HPMCs was 2.1 ± 0.5 times (p<0.05) and 1.4 ± 0.4 times (p=nonsignificant) higher along the FN and Alb concentration gradients respectively. Increased invasion along the FN concentration gradient was significantly inhibited (p<0.05) when the HPMCs were pre-incubated with 5 μg/mL RGDS (a blocker for α(5)-integrin to FN).'] Is three-dimensional invasion of epithelial-mesenchymal transition-positive human peritoneal mesothelial cells into collagen gel promoted by the concentration gradient of fibronectin?