Apart from during tumour growth and wound healing, the adult vascular system is comparatively quiescent. However, in the ovary, angiogenesis takes place during follicular development and with formation of the corpus luteum. During the monthly cycle, angiogenesis plays a crucial role during the process of follicular development and formation of the corpus luteum after ovulation. Yet these newly formed blood vessels undergo regression as most of the developing follicles become atretic. Furthermore, the corpus luteum has a functional lifespan of around 2 weeks before it too regresses, unless it is rescued during a fertile cycle in which case the vasculature must be maintained. Thus the ovary represents an exceptional tissue in which to study the regulation of blood vessel growth and regression. Using the marmoset monkey, we have established a model in which putative angiogenic factors in the ovary may be identified and the effects of their manipulation determined. The aim of these experiments, carried out under the Animals (Scientific Procedures) Act (1986) is to allow the prospects for manipulation of angiogenic factors in disorders of the ovary and uterus to be evaluated. The studies are based upon quantifying blood vessel area and endothelial cell proliferation cells on ovarian tissue sections, monitoring changes in expression patterns of putative angiogenic factors using in situ hybridisation and targeting these factors by specific antagonists in vivo at selected stages of the ovulatory cycle.

After treatment with vehicle or angiogenesis inhibitor, marmosets were humanely killed following an I.V. injection of sodium pentobarbitone. Quantification of endothelial cell proliferation shows that angiogenesis commences in the pre-antral follicle, increases during follicular development and shows an intense peak in the early corpus luteum. Changes in expression patterns of angiogenic factors allow the construction of a working model of ovarian angiogenesis. As an example, it is clear that the hormone-producing cells of the follicle and corpus luteum synthesise vascular endothelial growth factor (VEGF), one of the principal angiogenic factors, and its receptors, Flt and KDR are present on the endothelial cells in the thecal layer of the follicle and in the corpus luteum. Treatment with VEGF inhibitors results in a marked decrease in endothelial cell proliferation in the follicle and is accompanied by a decline in granulosa cell proliferation. Inhibition of VEGF during the early or mid-luteal phase results in a marked suppression in luteal angiogenesis, failure of development of the microvascular tree and an inhibition of luteal function as judged by low plasma progesterone concentrations. The treatment increases expression of VEGF while decreasing expression of its receptors, shedding light on the regulation of ligand and receptor.

These results illustrate how angiogenic factors may be studied with respect to an ovarian model and suggest that manipulation of angiogenesis should be a powerful approach to either promoting or inhibiting the normal processes of folliculogenesis, ovulation and corpus luteum function.We thank our collaborators Dr R. Bicknell (ICRF, Oxford) and Dr S.J. Wiegand and Dr J. Rudge (Regeneron, Tarrytown) for their contributions.

Fraser, H.M. & Lunn, S.F. (2001). Reproduction 121, 355-362.