Changes in endometrial blood vessels in the endometrium of women with hormone replacement therapy-related irregular bleeding

Abstract

Background: Irregular bleeding affects up to 60% of hormone replacement therapy (HRT) users. The mechanism of this bleeding is not understood. Reduced endometrial microvascular integrity appears to underlie breakthrough bleeding in pre-menopausal women and the aim of this study was to establish whether similar changes are seen in HRT users and hence to elucidate a possible mechanism of irregular bleeding. Methods: Endometrium from 34 HRT users with amenorrhoea, irregular bleeding or regular bleeding was assessed for endometrial endothelial cell density (anti-CD34), number of blood vessels per mm2, vascular basal lamina components (laminin, collagen IV and heparan sulphate proteoglycan) and in 32 subjects and 23 controls for perivascular smooth muscle α (SMA). Findings were compared with a control population of 29 post-menopausal women not using HRT, other sex steroids or tamoxifen and with no vaginal bleeding. Staining intensity was assessed in a blinded fashion in all immunohistochemical studies. Results: Four significant differences in endometrial blood vessels were observed between HRT users and controls: (i) a significantly lower density of endometrial endothelial cells (EC staining for CD34) per mm2 was present in HRT users compared with controls (P < 0.001); (ii) endothelial cells (EC) were predominantly organized within blood vessels (83%) in controls but in HRT users EC were dispersed in the tissues with only 29% in organized vessels (P <0.001); (iii) supportive perivascular cell SMA was significantly reduced in 23 post-menopausal HRT users compared with 23 post-menopausal controls (n = 29, P = 0.013) and (iv) an atrophic or inactive histological pattern of endometrium was more frequently seen in the controls (P < 0.001). Conclusions: These findings support the hypothesis that exposure to HRT profoundly alters endometrial blood vessels, reducing structural integrity thereby predisposing to irregular bleeding in HRT users.