Ultrastructural changes in the capillary bed of human pituitary tumors.

The fine structure of the capillary bed of the anterior pituitary has been studied in 19 cases of pituitary tumor and 1 autopsy specimen. Tumor specimens were less well vascularized than the autopsy specimen. Endothelial cells within tumor specimens were often observed with swollen portions of cytoplasm, or cytoplasmic blebs, projecting into the capillary lumen. Blebbing, in many cases, nearly obstructed the capillary lumen and was most often associated with endothelial cells that had an electron-lucent cytoplasm, in contrast to endothelial cells that had an electron-dense cytoplasm, even within the same capillary profile. Compared with electron-lucent cells, electron-dense cells contained more endothelial filaments. Also observed were capillaries that had apparently broken apart releasing their contents into the pericapillary space and shrunken remnants of capillaries. A number of abnormal features were observed in the pericapillary spaces-ie, disruption of the parenchymal-pericapillary interface, disorganization of basal laminae, increased amounts of plasma proteins and cellular debris within the space and, ultimately, complete loss of the normal limits and characteristics of the space. Similar, though less pronounced, changes were observed in the autopsy specimen. The hypothesis is advanced that changes observed in the capillary bed are, in large part, a result of tumor growth, which increasingly disrupts tissue organization at the parenchymal-pericapillary interface and, because the tumor mass in the sella turcica can only enlarge upwards, compresses the pituitary stalk and portal veins. The result is ischemia and eventual necrosis. The observations are then in good agreement with recent reports on changes in capillary fine structure associated with ischemia.