Experimental models for ovarian tissue and immature follicles.

Primordial and growing follicles are abundant within the ovaries of healthy young female mammals. While our understanding of follicular dynamics is based mainly on studies of normal ovaries in intact animals, techniques such as ovarian grafting and in vitro culture, in particular when used in combination with cryopreservation, have provided both significant additional insights and a source of mature oocytes. Primordial follicles are small, quiescent, and most commonly located within the collagen-rich outer portion of the ovary. Providing that appropriate collection, handling, freezing, and thawing methods are used, they can tolerate cryopreservation very well irrespective of whether they are frozen within a whole ovary (small species), as ovarian pieces, or as individual, isolated, follicles. Animal studies show that grafts of such fresh or frozen materials can, providing that they contain viable follicles, form mature fertilizable oocytes, produce hormones, and support pregnancies to term. Grafts can be returned to the original donor (autograft), but grafting between histocompatible individuals of the same species and between species (xenografts) is also possible. Cryopreservation and grafting are therefore useful both as practical and as experimental tools. Clinically, ovarian tissue has started to be collected and frozen for patients who are at risk of ovarian failure. Very recent case reports show that such frozen ovarian tissue autografts can support the return of menses and antral follicle formation in patients, although as yet no pregnancies have been established.