- Rests in the sella turcica, the saddle-shaped depression in the sphenoid bone.
Hypothalamic Input:
- The hypothalamus collects information from throughout the body and uses it to regulate pituitary hormone secretion.
- Hypothalamic neuroendocrine cell axons terminate in the median eminence and posterior pituitary, where they secrete various neurohormones.
– 5 hypothalamic hormones act on the anterior pituitary lobe.
– 2 hypothalamic hormones are released by the posterior pituitary lobe.
Anterior & Posterior Pituitary Lobes:
- The anterior and posterior lobes originate from different embryologic tissues, which determines their cellular makeup and functions.
- The anterior and posterior lobes are separated by the pars intermedia.
Anterior Pituitary:
- Anterior lobe is derived from epithelial tissue that grows out of the primitive oral cavity (Rathke's pouch).
Pars distalis
is the "lobe" part; the pars tuberalis* wraps around the pituitary stalk (be aware of intertextual variation regarding the pituitary stalk, infundibular process, and infundibular stalk).
Hypothalamic-Hypophyseal Portal Circulation*
– Primary plexus in median eminence
– Secondary plexus in anterior lobe
– These plexuses are connected via portal veins
Histology:*
The anterior lobe is sometimes referred to as the
adenohypophysis because of its gland-like components; it is highly vascularized with various "-troph" cells that receive inhibitory and/or releasing signals from the hypothalamus via the hypothalamic-hypophyseal portal
system.
– Cords of epithelial cells are in close contact with vascular sinusoids.
–
Acidophilic cells include:
Somatotrophs (~50% of anterior lobe), which release
Growth hormone
Lactotrophs (~15-20%) which release Prolactin
–
Basophilic cells include:
Corticotrophs (~20%), which release Adrenocorticotropin
Thyrotrophs (~5%), which release Thyroid-Stimulating Hormone
Gonadotrophs (~10%), which release Follicle-Stimulating Hormone and Luteinizing Hormone.
Posterior Pituitary:
Aka, neurohypophysis.
- The posterior lobe is derived from nervous tissue of the hypothalamus.
Maintains direct connection to the hypothalamus via the pituitary stalk*.
– If pituitary stalk is cut superior to the pituitary gland but hypothalamus is still intact, the posterior pituitary hormones will still be secreted.
The pars nervosa
is the "lobe" part; the infundibulum* is a funnel-shaped connection to the hypothalamus.
- The posterior pituitary lobe receives arterial blood.
The posterior lobe is sometimes referred to as the neurohypophysis* because it comprises nervous tissue.
- It releases 2 peptide hormones that are synthesized in large-bodied neurons with cell bodies in the hypothalamus:
– Antidiuretic hormone (aka, arginine vasopressin/vasopressin) is produced primarily in cell bodies of the supraoptic nucleus.
– Oxytocin is produced primarily in cell bodies of the paraventricular nucleus.
Histology:*
–
Unmyelinated axons of supraoptic and paraventricular nuclei form the hypothalamic-hypophyseal tract.
–
Herring bodies are temporary dilations in the axons where ADH or oxytocin accumulate.
–
Pituitcytes have cytoplasmic processes that surround and support the axons (look like astrocytes) (pituicytes are the majority cell type in the posterior pituitary).
- Released hormone enter fenestrated capillaries.
Histopathology of Anterior Pituitary Adenomas
- Pituitary adenomas are classified according to size, cell type, functional vs nonfunctional, genetics, etc.
- "Benign" lesions are not without complications:
– For example, growth hormone-secreting adenomas cause
acromegaly and gigantism.
- Growth hormone-secreting adenoma (image is with HE stain).
– Microscopic features: granular cytoplasm, round nuclei with fine chromatin.
– Reticular stains will show decreased reticulation in tumors.
– May be diffuse, densely, or mixed diffuse and densely granulated (dense is more common).
– Be aware that neoplastic ganglion cells can be present (rare, but associated with acromegaly)
– Macroscopically, these tumors are often tan or gray.
- Prolactin-secreting adenomas are the most common anterior pituitary hormone-secreting tumors.
– Can be sparsely or densely granulated.
– Microscopic features: small acidophilic or chromophlic cells arranged in sheets.
- THS-secreting adenomas are rare.
– Usually in people 50+ years old.
– Functional:
goiter and hyperthyroidism.
– Microscopic features: chromophobic, elongated or angular cells in sheets with fibrosis.
- ACTH-secreting adenomas = Cushing's Disease
– Microscopic features: Basophilic cells, round nuclei surrounded by granular cytoplasm.
– Usually in women 40-50+ years old (prepubertal tumors have equal male/female distribution)
– Hyopercortisolaemia can cause Crooke's hyaline change in non-neoplastic corticotropic cells, which is characterized by rings of cytokeratin accumulation.
- GNRH-secreting adenomas
- Often present as "non-functional" macroadenomas that compress the optic chiasm or invade the cavernous sinus.
– Functional turmors can cause ovarian hyperstimulation syndrome.
– Microscopic features: Perivascular rosettes.
– Cannot be traced to a specific cell subtype (improved typing techniques are helping to reduce this diagnosis).
– Microscopic features: Chromophobe.
View MRI of pituitary adenomas with macro vs. micro, and additional details.
