Directional Pituitary Blood Flow: A Microcinephotographic Study*

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Abstract
Anatomical studies have demonstrated vascular routes that could allow the pituitary gland to secrete directly to the brain. To assess the direction of pituitary blood flow, a bolus of Lissamine Green was injected into the cervical carotid arteries of 26 female pigs. The direction of pituitary blood flow was observed through the operating microscope and was recorded by microcinephotography. Flow patterns were correlated with porcine pituitary angioarchitecture. We observed blood to enter the neurohypophyseal capillary bed caudally (at the neural lobe) just before entering it rostrally at the median eminence. Within the neurohypophysis, blood flowing retrograde from the neural lobe met blood flowing anterograde from the median eminence at a variable boundary zone within the infundibular stem. Blood left the rostral region of the neurohypophysis (the median eminence) via portal routes and passed to the pars distalis. The remainder of the neurohypophysis drained into the cavernous sinus via the neurohypophyseal limbs of Y-shaped pituitary veins lying at the junction of the pars distalis with the neural lobe and lower infundibular stem. On occasion, blood did pass from the neural lobe to the adjacent pars distalis by capillary routes, but the adenohypophyseal territory supplied in this fashion was small. The pars distalis drained into the adenohypophyseal limbs of Y-shaped pituitary veins. The pattern of blood flow was not greatly affected by hypo- or hypertension. Stalk section, however, enlarged the vascular territory supplied from the neural lobe. These observations of directional blood flow in vivo suggest that the division of the neurohypophysis into rigid vascular territories is not warranted and that the dorso-medial zone of the adenohypophysis can receive blood from the adjacent neural lobe and can be influenced by neural lobe secretions. Under the conditions of this experiment, blood flow from the pars distalis to the neural lobe to the median eminence was not observed. It is concluded that pituitary secretion directly to the brain by this route does not occur, at least under these experimental conditions.

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