USp(2k) Matrix Model: Schwinger-Dyson Equations and Closed-Open String   Interactions
  We derive the Schwinger-Dyson/loop equations for the USp(2k) matrix model which close among the closed and open Wilson loop variables. These loop equations exhibit a complete set of the joining and splitting interactions required for the nonorientable Type I superstrings. The open loops realize the SO(2n_f) Chan-Paton factor and their linearized loop equations derive the mixed Dirichlet/Neumann boundary conditions. 
H. Itoyama, A. Tsuchiya