An implant assembly for a host tissue is provided. The implant assembly comprises a pouch including wall means defining a chamber for holding a second member, the wall means including an outer vascularizing membrane having a conformation that results in growth of vascular structures by the host tissue close to an interface between the vascularizing membrane and host tissue. The assembly includes a second member that can be removably inserted in the chamber including an interior for receiving cells and wall means defining an immuno-isolating membrane that isolates the cells from the immune response of the host tissue.