The microvasculature of the corpus luteum in pregnant rabbit. A scanning electron microscopy study of corrosion casts

The vascular network of pregnant rabbit ovaries was studied by means of sca nning electron microscopy (SEM) of corrosion casts, in order to evaluate th e morphofunctional changes of the microcirculation of corpus luteum (CL). P regnant rabbit ovary showed an overwhelming vascularization. Ovarian hilus displayed an increase in the arterial spirallisation. The arterial spiral p attern was present along the entire vessel course, up to CL tissues. The CL of pregnancy was supplied by wide vascular plexuses (2-5 plexuses were fou nd in each pregnant ovary) whose major axis was about 2 mm. Luteal capillar ies showed a tortuous course and were arranged in a three-dimensional, wide and rounded-meshed network. Postcapillary venoconstrictions were present. The venous drainage appeared more developed then the arterial supply. Tight artero-venous contacts in hilar, juxtamedullar and medullar regions of the ovary were observed. These results clearly show that the morphofunctional expression of CL of pregnancy is greatly dependent on its hemodynamic contr ol. In particular, the increase of spirallisation exhibited by the arteries during pregnancy is likely to be considered a significant functional chang e. The spirallisation likely is a device for reducing the blood pressure th rough the CL. The artero-venous contacts, also previously described in hCG stimulated (pseudopregnant) ovaries, may support a counter-current like sys tem that may allow a veno-arterial exchange of small molecules through the wall of the facing vessels. In addition, in 10-day pregnant rabbit CL the c onsolidation of a well-developed capillary network was revealed, which is a sign that the CL of pregnancy reached the full morphofunctional maturation . Furthermore, the CL of 10-day pregnant rabbit did not present significant capillary permeabilization and dilation or angiogenic processes, aspects t hat were previously found in stimulated periovulatory ovaries. Indeed, chan ges of the arterial supply and Venous drainage of the CL of pregnancy were demonstrated. This suggests that the control of the blood flow through the CL of pregnancy may be transferred from the local capillary microcirculatio n to the regional artero/venous circulation. This may be probably related t o the significant increase of the ovarian blood flow necessary for the main tenance of CL endocrine functions during pregnancy.