Restoration of corpus luteum angiogenesis in immature hypothyroid rdw rats after thyroxine treatment: morphologic and molecular evidence

Abstract

Thyroxine (T4) plus gonadotropins might stimulate ovarian follicular angiogenesis in immature infertile hypothyroid rdw rats by upregulating mRNA expression of major angiogenic factors. Development of growing corpus luteum (CL) is strongly related to angiogenesis and to morphofunctional development of microcirculation. Our aim was to investigate if T4 is involved in CL angiogenesis and in the activation of capillary cells and angiogenic factors after ovulation in a spontaneous model of hypothyroidism, the rdw rat. Rdw rats were treated with T4 plus gonadotropins (equine chorionic gonadotropin plus human chorionic gonadotropin; eCG+hCG) or gonadotropins alone in order to evaluate the effects of T4 on early luteal angiogenesis, on microvascular cells and on expression of major growth factors which are involved in the regulation of angiogenesis. Wistar-Imamichi rats treated with gonadotropins were used as controls. The ovaries were collected 4 days after hCG administration and analyzed using morphologic and molecular approaches. Thyroxine plus gonadotropins stimulated the growth of CLs and follicles as in controls, differently from rdw rats treated only with gonadotropins, in which CLs were not found and only small follicles, often atretic, could be recognized. In T4 plus gonadotropin-treated rdw rats CLs showed increased microvasculature, numerous activated capillaries characterized by sprouting and other angiogenic figures, and associated pericytes. Quantitative analysis revealed that the number of pericytes in T4 plus gonadotropin-treated rdw rats was comparable with that found in control rats and was significantly higher than that found in gonadotropin-treated rdw rats. The mRNA expression of vascular endothelial growth factor and basic fibroblast growth factor was significantly higher in control rats and in T4 plus gonadotropin-treated rdw rats than in gonadotropin-treated rdw rats. mRNA expression of tumor necrosis factor α, transforming growth factor β, and epidermal growth factor did not show significant changes. Our data originally demonstrated that T4 promoted the growth of an active microcirculation in developing CLs of gonadotropin-primed hypothyroid rdw rats, mainly by inducing sprouting angiogenesis, pericyte recruitment, and upregulation of mRNA expression of vascular endothelial growth factor and basic fibroblast growth factor. In conclusion, we suggest that T4 plays a key role in restoring luteal angiogenesis in ovaries of immature hypothyroid rdw rats.