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Pituitary autotransplantation eliminates direct vascular contact between the hypothalamus and the adenohypophysis, and enables us to study the role of the hypothalamus in regulating adenohypophysial endocrine activity. The aim of this study was to investigate vascularization of the pituitary autografts. Three-month-old male Wistar rats were hypophysectomized, and their adenohypophyses were autotransplanted under the renal capsule. The animals were killed 3 weeks after autotransplantation. The grafts were removed and studied by using histology, immunohistochemistry and transmission electron microscopy. In the central portion of the grafts, organizing necrosis was apparent. The peripheral portion of the graft contained all adenohypophysial cell types, with a predominance of lactotrophs. Vascular endothelial growth factor and hypoxia-inducible factor were expressed in the graft mainly in the perinecrotic areas. Several capillaries inside the grafts were lined by continuous unfenestrated epithelium, while others were lined by fenestrated endothelium, suggesting that neovascularization is the result of two processes: ingrowths of capillaries from the renal capsule to the graft, and neoformation of capillaries from pre-existing adenohypophysial vessels. In conclusion, hypoxia seems to be an important factor in the vascularization of pituitary autografts. Mediated via hypoxia-inducible factor, hypoxia stimulates vascular endothelial growth factor secretion, which plays a crucial role in angiogenesis.