Portal veins are veins interposed between two capillary networks. In this case they are interposed between the capillaries of the median eminence of the ventral hypothalamus and the sinusoids of the anterior pituitary.
The hypophyseal portal venous system carries releasing hormones from pep-tidergic neurons in the hypothalamus directly to target cells in the anterior pituitary cells, without dilution in the systemic circulation. It is formed by superior and inferior hypophyseal branches of the internal carotid artery, which form a complex capillary plexus in the median eminence of the hypothalamus. Hormones reach the median eminence via axoplasmic transport through axons of the tubero-infundibular tract.
Six to ten long portal vessels descend along the anterior surface of the pituitary stalk, opening into sinusoids between secretory cells in the anterior pituitary. Eighty-five per cent of the blood supply for the anterior pituitary is carried by the hypophyseal portal tract, the remaining 15% from the superior hypophyseal artery.
The median eminence is a specialised region of the floor of the third ventricle, which gives rise to the pituitary stalk. It is characterised by: High vascularity; Fenestrated endothelium; Lack of neuronal perikarya;
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