Incidence and morphology of endometrial angiopathies in mares in relationship to age and parity

The morphology of endometrial blood vessels in uterine biopsy specimens from mares of varying age and reproductive status was examined by light (n = 117) and electron microscopy (n = 13), and additionally after elastase digestion (n = 86). Inflammatory vascular alterations were observed in 20.5% of the specimens. Smaller and larger arterial and venous vessels demonstrated mild to severe degenerative lesions. Unaltered vessels were detected only in maiden mares. Vessels in older maiden mares were frequently affected by angiosclerotic changes, characterized by mild to moderate perivascular and intimal sclerosis. The incidence and severity of angiosis increased with the number of previous pregnancies and with advancing age. Changes in multiparous mares resembled the so-called "pregnancy-sclerosis" of other species, with fraying and disruption of the membrana elastica interna, medial atrophy intimal, medial and adventitial elastosis and fibrosis, and calcification processes within the media. Ultrastructural studies revealed characteristic arterial changes in post-parturient mares, namely, disruption of the membrana elastica interna, as well as activated smooth muscle cells and immature elastic fibres within the intima and inner media, suggesting a pregnancy-induced pathogenesis. Haemodynamic and hormonal alterations during pregnancy and the puerperium possibly induce active vascular remodelling. Cycles of vascular growth during pregnancy and subsequent involution post partum are thought to result in progressive degenerative vascular changes, as seen in multiparous mares. Ageing processes, chronic inflammation and short foaling intervals have to be considered as additional pathogenetic factors. Furthermore, severe angiosis was frequently combined with phlebectasia and lymphangiectasia. This may indicate a reduced ability of the vessels to adapt to the varying demands of uterine circulation, with a decrease of uterine perfusion and lymph drainage. Angiosis in older, multiparous mares might therefore be intimately related to infertility.