Fresh (<3 hours old) ex vivo facial human skin removed as redundant tissue during reconstruction provided a clinically relevant in vitro model for laser resurfacing and its effects on dermal type I collagen; fresh (<12 hours old) bovine calcaneus tendon served as a concentrated array of essentially pure type I collagen for this study.18 Fresh specimens were wrapped in saline-moistened gauze and stored at 4°C before irradiation. Room temperature specimens were individually exposed to 1 (tendon and skin) or 2 (skin only) passes each with 3 different CO2 laser systems (UltraPulse 5000C, Coherent Medical, Palo Alto, Calif; TruPulse, Palomar Medical Technologies, Beverly, Mass; and SilkTouch, Sharplan, Allentown, NJ). The laser parameters that we used in this study are shown in Table 1. Skin specimens were irradiated perpendicular to the skin surface and cylindrical tendon specimens, once sectioned longitudinally to reveal a flat internal surface, were irradiated perpendicular to the open face so created. Two specimens each were irradiated with 1 or 2 passes using 0% (TruPulse and SilkTouch) or 10% (UltraPulse) pulse overlap and with interpass debridement using a saline-dampened cotton-tipped applicator. Each area was approximately 1.0 cm2. Specimens were fixed for LM in 10% buffered formalin, processed in paraffin, and stained with hematoxylin-eosin. Specimens were fixed for transmission electron microscopy (EM) in glutaraldehyde and prepared as previously described.19 Briefly, ultrathin (1-µm) sections were cut with a microtome (MT-7, RMC, Tucson, Ariz), placed on copper grids, and stained with uranyl acetate–lead citrate. The tissue was then examined with a transmission EM (EMU-4, RCA, Camden, NJ).