Anatomy of the Pituitary Gland

DEVELOPMENT OF THE PITUITARY GLAND

• Two Major Components
  1. Adenohypophysis (Anterior Lobe): Derived from oral ectoderm.
  2. Neurohypophysis (Posterior Lobe): Derived from neural ectoderm of the forebrain floor.
• Formation of Rathke Pouch (Weeks 4–5 of Gestation)
  • A pouch-like recess in the ectodermal lining of the stomodeum (primitive mouth).
  • Grows upward to meet the undersurface of the forebrain.
  • The connection between Rathke pouch and the stomodeum (craniopharyngeal canal) is usually obliterated.
  • A persistent remnant can become a “pharyngeal pituitary,” which may later give rise to ectopic hormone-secreting adenomas.
• Infundibular (Neural) Outgrowth
  • Develops from the floor of the third ventricle (neural ectoderm).
  • Forms the infundibular process, which becomes solid except at its upper end (the infundibular recess of the third ventricle).
• Fusion and Further Differentiation
  • Rathke pouch fuses with the infundibular process.
  • Lumen of Rathke pouch typically obliterates, but a Rathke cleft may persist.
  • Anterior Lobe (Pars Distalis) arises from Rathke pouch; Posterior Lobe (Pars Nervosa) arises from the infundibular process.
  • The neurohypophysis (posterior lobe) contains axons (about 100,000 fibers) from the supraoptic and paraventricular nuclei of the hypothalamus.
  • Remnants of Rathke pouch may persist at the boundary, forming small colloid cysts.
• Pars Tuberalis, Pars Intermedia
  • The pars tuberalis extends from the ventral wall of the anterior lobe and encircles the pituitary stalk.
  • The pars intermedia is a thin region at the interface between the anterior and posterior lobes; in humans, it often becomes interspersed with anterior lobe cells but can produce POMC and ACTH.
  • The median eminence is the part of the tuber cinereum lying immediately above the pars tuberalis.

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DIVISIONS OF THE PITUITARY GLAND AND RELATIONSHIP TO THE HYPOTHALAMUS

• Adenohypophysis (Anterior Lobe)
  1. Pars Tuberalis: Thin strip of tissue around the median eminence and upper neural stalk.
  2. Pars Intermedia: Thin zone posterior to any persisting cleft, in contact with the posterior lobe.
  3. Pars Distalis (Pars Glandularis): Main secretory portion.
• Neurohypophysis (Posterior Lobe)
  1. Infundibular Process (Neural Lobe)
  2. Infundibular Stem (Neural Stalk)
  3. Median Eminence (expanded upper end of the stalk/tuber cinereum)
• Pituitary Stalk
  • Formed by the infundibular stem plus surrounding parts of the adenohypophysis (pars tuberalis).
  • Approximately 15% of the neurohypophysis extends up the stalk into the median eminence.
• Nerve Supply (Hypothalamohypophysial Tract)
  • Main functional/anatomical supply of the neurohypophysis.
  • Supraopticohypophysial Tract: Arises in the supraoptic and paraventricular nuclei; runs in the anterior (ventral) wall of the stalk.
  • Tuberohypophysial Tract: Arises in the dorsal/posterior hypothalamic regions, including paraventricular nucleus and tuberal/mamillary areas.
  • These fibers terminate mainly in the neurohypophysis.
  • Damage above the median eminence affects ~85% of the supraoptic/paraventricular neuron cell bodies; damage at the level of the diaphragma sellae affects ~70% of these cells.
• Hypothalamic Boundaries
  • Anteroinferiorly bounded by the optic chiasm/tracts, posteriorly by the posterior perforated substance and cerebral peduncles, and separated from the thalamus by the hypothalamic sulcus.
  • Merges anteriorly with preoptic septal region and posteriorly with the midbrain tegmentum.
• Connective Tissue Trabecula
  • Separates the anterior and posterior lobes internally for a variable distance, forming part of the vascular bed.
  • The embryonic cleft (Rathke pouch remnant) may persist in this trabecula, more easily seen in newborns and often disappears with age.

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BLOOD SUPPLY OF THE PITUITARY GLAND

• Arterial Supply
  • Superior Hypophysial Arteries (right and left) arise from the ophthalmic or internal carotid artery segment.
    • Each divides into anterior and posterior branches to the pituitary stalk.
    • Artery of the Trabecula: A branch supplying the connective tissue trabecula in the anterior lobe.
  • Inferior Hypophysial Arteries (right and left) branch from the intracavernous segment of each internal carotid artery.
    • Surround the infundibular process with an arterial ring.
    • Supply the posterior lobe and lower infundibular stem.
• Hypophysial Portal System
  • Primary Capillary Network in the median eminence and infundibular stem receives hypothalamic releasing and inhibiting hormones from nerve endings.
  • Long and Short Hypophysial Portal Veins:
    • Convey blood (and hypothalamic hormones) from the primary capillary network to the secondary plexus of sinusoids in the anterior lobe (pars distalis).
    • The anterior lobe has no direct arterial supply; it relies on these portal vessels.
  • Venous Drainage flows from the secondary plexus into dural sinuses around the pituitary, then into general circulation.
  • Occlusion of these portal vessels can lead to ischemic necrosis of the anterior pituitary.

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ANATOMY AND RELATIONSHIPS OF THE PITUITARY GLAND

• Size and Position
  • Reddish-gray, ovoid organ measuring ~12 mm (transverse), 8 mm (AP), 6 mm (vertical).
  • Weighs ~500 mg in men and ~600 mg in women.
  • Sits in the hypophysial fossa of the sphenoid bone, beneath the diaphragma sellae (a dura mater fold).
  • The floor of the hypophysial fossa is part of the roof of the sphenoid sinus.
• Diaphragma Sellae
  • Pierced by a small aperture for the pituitary stalk.
  • Separates much of the pituitary gland from the optic chiasm.
  • The subarachnoid space may extend partially into the sella (“partially empty sella”) and can allow blood or CSF to enter this space.
• Hypothalamus
  • Lies superiorly, forming the floor and lateral walls of the third ventricle.
  • Mamillary bodies mark the posterior limits of the hypothalamus.
  • The optic chiasm lies just anterior/superior to the pituitary stalk and diaphragma sellae.
• Optic Chiasm
  • A flat, quadrilateral bundle of optic nerve fibers at the junction of the anterior wall of the third ventricle with its floor.
  • Rests on the diaphragma sellae behind the optic groove of the sphenoid bone.
• Pineal Gland
  • A small, conical gland lying below the splenium of the corpus callosum.
  • Rarely, ectopic pineal tissue can be found near the floor of the third ventricle.

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RELATIONSHIP OF THE PITUITARY GLAND TO THE CAVERNOUS SINUS

• Cavernous Sinuses
  • Dural venous channels flanking the pituitary gland on each side.
  • Reticulated Structure: Contains interlacing filaments around the internal carotid artery.
  • Medial Wall: Internal carotid artery, abducens nerve (VI).
  • Lateral Wall: Oculomotor (III), Trochlear (IV), and the ophthalmic and maxillary divisions of the trigeminal (V).
  • The two cavernous sinuses communicate via anterior and posterior intercavernous sinuses, forming a circular sinus around the pituitary.
• Superior and Inferior Petrosal Sinuses
  • Superior Petrosal Sinus: Connects cavernous sinus to the transverse sinus; runs in the attached margin of the tentorium cerebelli.
  • Inferior Petrosal Sinus: Also connects cavernous sinus to other posterior dural sinuses.
• Internal Carotid Artery (Intercavernous Portion)
  • Curves upward, forward, and then upward again through the cavernous sinus.
  • Perforates the dura of the sinus roof near the anterior clinoid process.
  • Surrounded by sympathetic fibers.
  • Gives off hypophysial arteries to supply posterior lobe and median eminence (portal system).

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RELATIONSHIPS OF THE SELLA TURCICA

• Location and Dimensions
  • A deep depression in the body of the sphenoid bone.
  • Normal adult dimensions:
    • AP length <14 mm
    • Height <12 mm (floor to a line from tuberculum sellae to posterior clinoid tip)
• Sphenoid Bone Overview
  • Composed of a median body and two pairs of wings (greater and lesser), plus two pterygoid processes.
  • Sphenoidal Air Sinuses: Large cavities within the body, separated by a septum (often oblique).
  • Superior surface articulates with the ethmoid and frontal bones; houses grooves for the olfactory bulbs, and the chiasmatic sulcus for the optic chiasm.
• Key Features
  • Tuberculum Sellae: A raised area behind the chiasmatic sulcus.
  • Dorsum Sellae: Posterior boundary of the sella turcica, ending in the posterior clinoid processes.
  • Hypophysial Fossa: Deepest part, housing the pituitary gland.
  • Clivus: A shallow depression behind the dorsum sellae, leading to the basilar part of the occipital bone.
  • Lateral aspects relate to the cavernous sinus and the foramen lacerum region.
• Nasal Relations
  • The crest of the sphenoid bone and the perpendicular plate of the ethmoid.
• Surgical Considerations
  • Modern pituitary approaches (microscopic or endoscopic) aim to avoid major vascular structures and the optic pathways.
  • Transsphenoidal surgery exploits the sphenoidal sinus route to access the sella turcica.