177. The Hypophysis (pituitary Gland): Neural Primordium and Portal System

  1. The floor of the third ventricle, just behind the glandular primordium becomes depressed and produces the infundibulum, at about day 40 of gestation
    1. THE INFUNDIBULUM will give rise to the median eminence, the infundibular stem, and the pars nervosa, all of which make up the neurohypophysis
    2. THE DEPRESSION extends progressively toward the glandular primordium, and about day 45, its ventral end forms a diverticulum, which thickens. Its lumen also gradually fills. Thus, the pars nervosa or posterior lobe is formed and becomes attached to the posterior wall of Rathke's pouch
      1. The pars nervosa is attached to the diencephalon by a stalk, the infundibular stem
      2. The median eminence is the slightly prominent segment of the infundibulum of the hypothalamus, proximal to the infundibular stem, just below the third ventricl
  2. The neural lobe differentiates during month 4, and one can see specific neuroglial cells appearing, the pituicytes. These may be glandular, but this as yet has not been proven
  3. The neurohypophysis is then colonized by axons coming from the hypothalamus
    1. THESE AXONS form the hypothalamic-hypophyseal tract, the pathway for hypothalamic neurosecretions, which are rich in polypeptides and are elaborated by the hypothalamic nuclei from cells which appear to be both neural and glandular
      1. The paraventricular and supraoptic nuclei represent some of these nuclei
      2. The neurosecretory material is then conducted to the neurohypophysis by axons of these cells. This can be seen in the fetus at about month 4 of gestation
  4. Relationship between the neural and glandular hypophysis
    1. A PORTION OF THE NEUROSECRETION from the hypothalamus passes to the glandular hypophysis through the blood via a "portal" system. The latter involves
      1. The superior hypophyseal arteries originate from the internal carotid arteries and form a capillary bed in the proximal half of the neurohypophysis
        1. These capillaries collect the neurosecretory material
        2. Venules which follow them pass into the pars tuberalis and the anterior lobe of the gland, where they form a new capillary system which releases the neurosecretory material
        3. The above arrangement represents the portal system of the hypophysis, which constitutes the essential pathway of neuroadenohypophyseal relationships
      2. The inferior hypophyseal arteries also arise from the internal carotid arteries
        1. They irrigate the neural lobe and do not appear to be connected to the portal system
        2. The veins which follow carry away the hormones of the posterior lobe, which are modified neurosecretions, into the dural sinuses
    2. THERE ARE NO KNOWN NERVES to the anterior lobe of the pituitary gland except those following the blood vessels. The posterior lobe, however, has a direct connection, as previously mentioned, to the inferior surface of the brain, and this part of the gland is innervated by fibers from neuron cell bodies located in nuclei of the hypothalamus
      1. These neurons contain granules of neurosecretion in their cell bodies and axons, and it is thought that the hormones produced by the posterior lobe are actually produced in these neurons and stored in the posterior lobe
  5. Tumors of the neurohypophysis are very uncommon
    1. INFUNDIBULOMA: a rare tumor that appears to be of neurohypophyseal derivation and stimulates the structural pattern of the infundibulum
      1. Seen in children, it grows slowly in the floor of the third ventricle, causing pressure effects on neighboring structures
      2. The cells resemble pituicytes, and the tumor has a distinctive vascular pattern

the hypophysis (pituitary gland):  neural primordium and portal system: image #1