Morphological evidence for the presence of arteries in human prolactinomas

This study was undertaken to clarify the vascular anatomy of human prolactinomas and specifically to determine whether arteries were present. Sixteen prolactinomas were studied by electron microscopy. The presurgical diagnosis of the tumors as prolactinomas was based on the findings of hyperprolactinemia and radiographic abnormalities, and was confirmed by the electron microscopic features of the specimens as well as by immunocytochemical staining for PRL. In addition to the presence of fenestrated endothelial cells, which are characteristic of the normal capillaries of the anterior pituitary, 13 of the 16 prolactinomas contained arteries. These arteries ranged from well formed vessels with multiple layers of smooth muscle cells to abnormal terminal arterioles, i.e. vessels with fenestrated endothelium surrounded by a variable number of smooth muscle cells. Arteries were not found in anterior pituitaries from 8 patients with no pituitary disease. In the prolactinomas, smooth muscle cells also were found, either isolated in the pericapillary connective tissue space or in small cords some distance from the vessel lumen. The results suggest that vascular changes, including arteriogenesis, occur in prolactinomas (and possibly other types of pituitary tumor). The arteries entering the anterior pituitary directly could be congenital or develop during formation of the tumor. An arterial blood supply to a region of the anterior pituitary could result in the escape of that area from hypothalamic regulation, since systemic blood contains negligible levels of hypothalamic hormones. In the case of PRL-secreting cells, which are tonically inhibited by the hypothalamic hormone dopamine, this would result in hypertrophy, hyperplasia, and possibly tumorigenesis.