Vascular changes in the corpus luteum during early pregnancy in the cow.

The present study determined vascular changes in the bovine corpus luteum (CL) at Day 16 (early maternal recognition period) and Day 40 in early pregnancy and compared them to the CL from Day 12 and Day 16 of the estrous cycle. The CLs were analyzed in the central and peripheral regions, where site-depending features of vessels and angiogenic factors are evident. The same protein level of the endothelial cell marker von Willebrand factor was retained in the CL from Day 16 of the estrous cycle to Day 40 of early pregnancy. The protein level of pericytes and smooth muscle cells was determined using smooth muscle alpha-actin; the level decreased at Day 40 of early pregnancy in both regions of the CL. No significant change in the expressions of vascular endothelial growth factors VEGF(164) and VEGF(120) mRNA occurred from Day 16 of the estrous cycle until Day 40 of early pregnancy. Angiopoietin (ANGPT)-2 / ANGPT-1 mRNA ratio (an index of instability of vasculature) increased in the periphery at Day 16 of the estrous cycle and then decreased until Day 40 of early pregnancy. The results suggest that there is no difference in vascular structure between non-pregnant and pregnant luteal tissue during the early maternal recognition period (Day 16). Also, luteal rescue by early pregnancy may be not associated with further blood vessel formation but rather may be related to the decrease of blood vessels per unit of area and blood vessel stabilization in the bovine CL.