Contractile endothelial cells in terminal segments of certain hypophyseal anterior lobe veins.

A dual system of hypophyseal anterior lobe veins is suggested for bovine. One system is associated with the gland at large and utilizes the capsular plexus of veins, while the other, not utilizing the capsular plexus, is associated with the sinusoids of the hypophyseal portal veins. As they reach the capsule of the gland, those veins not utilizing the capsular plexus, are lined, at their terminal segment by endothelial cells with contractile and secretory elements. The contractile endothelial cells subserve a valve-like action on demand of glandular function.