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THE establishment of a vascular supply is a critical requirement for cellular inflow of nutrients, outflow of waste products, and gas exchange in most tissues and organs (1). In endocrine glands, the vascularization not only serves such needs but also provides a pathway for the specific secretory products (2, 3). Furthermore, in the anterior pituitary (4–6) and in the adrenal medulla (7, 8), an unusual angioarchitecture, where a portal capillary plexus delivers venous blood originating from an adjacent gland, is intimately involved in the control of the secretory activity. In the adrenal medulla, this vascular design may even determine the ultimate secretory product (8).

Not surprisingly, the cardiovascular system is the first organ system to develop and reach a functional state in an embryo (9–12). In the human, primitive blood vessels appear as early as day 15, and a circulation with a …