Overview showing well circumscribed HBL nodule partly surrounded by a crescent-shaped mantle of peritumoral pituitary parenchyma. (A) Optical contrast between the faint eosinophilic hue of the HBL nidus and bright red granular quality of adjacent somatotrophs. (B, C) Adjacent section planes treated with immunohistochemistry, showing segregation of adenohypophyseal neuroendocrine cells (B) and mesenchymal-like immunophenotype (C) of the HBL nodule. (D) Detail view of boxed area in (A) shows the HBL to be comprised of an irregular reticular meshwork of tortuous, thin-walled capillaries that tend to be interspersed with pale stromal cells. (E) Gomori's reticulin stain highlighting the brisk transition from the acinar outline of native adenohypophyseal follicles (upper third) to the vascular-dominated basement membrane pattern of HBL. (F) High-power view of HBL showing polygonal contours and cytoplasmic vacuolation of stromal cells encased by capillaries. Some nuclear pleomorphism, as also evident in this microscopic field, is of no prognostic significance. (G) A minority of stromal cells were stained for epithelial membrane antigen. (H) Scattered mast cells are a characteristic complement of HBL. If not labeled otherwise, microphotographs have been made using hematoxylin and eosin stain. Original magnifications: (A-C) × 30; (D, E, H) × 100; (F, G) × 400.