The Effects of Plasminogen Deficiency on the Healing of Tympanic Membrane Perforations

The healing of tympanic membrane (TM) perforations is a complex wound healing process including inflammation, migration of keratinocytes and tissue remodelling. Most TM perforations in human heal spontaneously, however some perforations become chronic, and the reason why is still largely unknown. In cutaneous wound healing plasminogen (plg) has been shown to play an important role. Plg is converted into the protease plasmin regulated by two plasminogen activators (PA), urokinase type PA (uPA) and tissue-type PA (tPA). The aim of the present thesis was to evaluate the role of plg in healing of TM perforations, both in vivo and in vitro. The main objectives were to determine the healing capacity of the TM, the involvement of keratinocytes, fibrin(ogen) and inflammatory cells in the healing process. The studies were performed in plg deficient and uPA deficient mice, with littermate wild type (wt) mice as controls It was shown that myringotomies of the TMs in plg deficient mice still remained open 143 days following a perforation. The wound area was characterized by an abundant recruitment and accumulation of inflammatory cells; mainly macrophages and neutrophils, an arrested keratinocyte migration and a fibrin deposition covering the surface of the TM. The TM perforations in the wt mice all healed within 11 days. Interestingly, the myringotomies of the plg deficient mice closed by reconstitution with systemic injections of plg, whereas injections of PBS had no affect on the healing. To characterize mechanisms involved in the development of persistent TM perforations in plg deficient mice after a myringotomy the early inflammatory response during the first 48 hours was studied. The recruitment and accumulation of inflammatory cells in the perforated TMs were found to be similar between the plg deficient and the wt mice. Myringotomized TMs in uPA deficient mice healed similar to perforations of wt controls. Neither did the keratinocyte migration nor the occurrence of inflammatory cells differ between these genotypes. In the in vitro experiments TMs from plg deficient and wt mice, were dissected out, perforated and cultured in absence or surplus of plg. A decrease in perforation size was seen in all groups regardless of genotype or amount of plg in the medium. 7 In conclusion, the present studies show: • Plg is essential for the healing of TM perforations in mice. • The altered healing process after a myringotomy in plg deficient mice involves a disturbed keratinocyte migration, a massive deposition of fibrin and …