In the human, the ascending uterine arteries give rise to approximately eight arcuate arteries that are embedded in the myometrium and form anastomoses with those emanating from the contralateral ascending uterine arteries [16]. These vessels then branch centripetally into radial arteries that penetrate the middle third of the myometrium and give rise to ~200 spiral

arteries [16]. The vascular pattern differs somewhat in the guinea pig or the rat. In these species, the arcuate (syn. mesometrial) arteries are located within the planar mesometrium and are therefore external to the uterus. Radial arteries emanate from the arcuates and are also external to the uterus. During pregnancy, these vessels may further ramify into those that supply a placenta SCH 900776 (pre-placental or spiral arteries) vs. myometrium (pre-myometrial or basal arteries). Although both types of radial arteries remodel GSK126 price during pregnancy, they may (rabbits [12]) or may not (rats [25]) do so to a different extent, depending upon species. Such interspecies variation in vascular anatomy presents an opportunity to dissect the potential contributions of placenta-specific vs. whole uterine (or horn-specific in the case of species with bicornuate uterus) influences on pregnancy vascular remodeling and its consequences. The time course of the proliferative responses

in the arcuate and radial arteries differs from that seen in the larger (main) uterine arteries, also with some variation occurring among species. In the guinea pig, DNA synthesis continues to rise until term in the radial artery, which is longer than seen in the main uterine artery [31]. Just the reverse occurs in the rat, as DNA synthesis peaks at mid-pregnancy in the radial artery

but later in pregnancy in the upstream main uterine artery (measured on day 20 of a 22 day gestation [13]). As discussed below, endothelial NO appears to be a key modulator of circumferential remodeling and can be stimulated by a variety of factors such as shear stress, estrogen, and VEGF [81, 55, 9, Dimethyl sulfoxide 28]. The literature on uterine veins is quite limited relative to that on arteries. Significant increases in venous diameter and length occur during pregnancy as well and comprise an important means for accommodating the ~40% rise in blood volume. The venous responses are associated with changes in connective tissue elements such as elastin and collagen; these, in turn, lead to altered biomechanical properties such as increased compliance [60]. In summary, uterine vascular remodeling in the upstream vessels begins earlier and is at least in part independent from downstream, placentation-related changes. In many respects, the changes in the uterine artery are anticipatory, enabling the maternal circulation to accommodate the exponential rise in fetal demand occurring near the end of gestation.