176. The Hypophysis (pituitary Gland): Glandular Primordium

  1. Introduction: the hypophysis or pituitary gland is an unpaired gland located in the bony sella turcica of the sphenoid bone of the skull. It is found in all vertebrates and consists of 2 major parts of different embryonic origin
    1. THE GLANDULAR PART arises from an evagination (outpocketing) of the ectodermal epithelium covering the roof of the stomodeum, and during embryonic development, the glandular primordium becomes located anterior to the neural primordium
    2. THE NEURAL OR DIENCEPHALIC PART arises from an evagination of the floor of the third ventricle (from neuroectoderm)
    3. THE GLANDULAR PRIMORDIUM is induced first by the anterior end of the notochordal system (or the prochordal plate). This system next induces the neural primordium or infundibulum. From this time on, each primordium affects the development of the other by means of reciprocal induction
  2. Development of the glandular primordium
    1. THE CELLS OF THE STOMODEAL SURFACE ECTODERM just ahead of the buccopharyngeal membrane become thicker than the rest, at about day 21 of gestation (7 somite stage). This placodal primordium is very near the wall of the diencephalon and just in front of the notochord
    2. THE FLAT PRIMORDIUM then invaginates and penetrates the mesenchyme in the direction of the diencephalon as the diverticulum of Rathke and forms Rathke's pouch, which grows toward the brain
      1. Just behind the buccopharyngeal membrane, the entodermal epithelium also forms another pouch, the pouch of Seessel, which is involved in the formation of the glandular hypophysis in lower vertebrates, but which usually is not present in humans; however, it may be seen and related to certain tumors, such as a craniopharyngioma
      2. By week 5, Rathke's pouch has elongated and become constricted at its attachment to the oral epithelium by the pharyngohypophyseal stalk, which regresses and eventually disappears during week A remnant of this stalk may persist and give rise to a pharyngeal hypophysis in the pharyngeal roof
        1. Rarely, accessory masses of anterior lobe tissue may occur outside the capsule of the gland, but within the sella turcica, or in the substance of the bone
    3. DURING SUBSEQUENT DEVELOPMENT, cells of the anterior wall of Rathke's pouch proliferate actively and give rise to the pars distalis of the pituitary gland
      1. Later, a small extension of the pars distalis, the pars tuberalis, extends up and around the infundibular stem
    4. THE EXTENSIVE PROLIFERATION of the anterior wall of Rathke's pouch reduces the lumen to a narrow residual cleft
      1. This cleft generally is not recognizable in the human adult gland and usually is represented by a zone of cysts
    5. IN HUMANS, CELLS OFTHE POSTERIOR WALL OF RATHKE'S POUCH do not proliferate but give rise to the thin, poorly defined pars intermedia
    6. THUS, THE ADENOHYPOPHYSIS or glandular portion of the gland consists of the pars distalis and tuberalis, which make up the anterior lobe, and the pars intermedia, all arising from oral ectoderm from the roof of the stomodeum
  3. Histogenesis begins at about month 4
    1. THE SURROUNDING MESENCHYME appears to induce the glandular character of the cellular differentiation which takes place from the same cellular layer. The cells also reflect the bilateral symmetry seen during the gland's development
      1. The basophil (PAS+) cells are much more numerous in the anteromedial portion of the gland than in its posterolateral portions, whereas the acidophils (PAS-) show the opposite arrangement
      2. The chromophobe cells are spread almost uniformly through the gland

the hypophysis (pituitary gland): glandular primordium: image #1