Abstract: A process for encapsulating and immobilising mammalian stem cells, ovarian follicular cells, gametes, ovarian follicles or mammalian embryos which are able to auto-organise into three-dimensional structures in vitro, and express biological functions in a manner similar to that which is observed in the organism in vivo is described. The capsules are constituted by: a nucleus containing stem cells, ovarian follicular cells, gametes ovarian follicles or mammalian embryos and/or a biocompatible and/or biodegradable polymer; a semi-permeable membrane constituted by a divalent or trivalent metal ion salt of alginic acid, optionally cross-linked on the inner and/or outer surface and/or on both surfaces, optionally vehicularising a second or more cellular species.