Pituitary Gland & Endocrine System

Pituitary Gland Overview

• Located at the brain's base within the sella turcica
• Connected via a stalk to the hypothalamus
• Weighs about 0.5 g and measures 1 cm
• Consists of the anterior (adenohypophysis) and posterior lobes (neurohypophysis)

Anterior Lobe (Adenohypophysis)

• Produces trophic hormones stimulating hormone production in the thyroid, adrenal, and other glands
• Composed of epithelial cells derived embryologically from the oral cavity
• Contains acidophils (Somatotrophs & Mammotrophs), basophils (Corticotrophs, Gonadotrophs, Thyrotrophs), and chromophobe cells (degranulated acidophils/basophils)

Endocrine System Basics

• Glands orchestrate metabolic equilibrium among organs
• Signaling molecules classification includes autocrine, paracrine, and endocrine

Endocrine Signaling

• Secreted hormones act on distant target cells
• Effects occur on organs DISTAL from the synthesis site

Hormones

• Trigger biochemical signals when interacting with cell surface receptors
• Peptide hormones are the primary type
• Diffuse across the plasma membrane and interact with intracellular receptors

Negative Feedback

• Endocrine hormone inhibits trophic factor production, maintaining hormone levels
• Target tissue secretes factors down-regulating gland activity

Endocrine Diseases

• Processes that disturb normal endocrine system activity include:
  • Impaired synthesis or hormone release
  • Abnormal hormone-target tissue interactions
  • Abnormal target organ responses

General Disease Classification

• Under/Overproduction of hormones
• Diseases linked to mass lesions

Posterior Lobe (Neurohypophysis)

• Contains pituicytes (modified glial cells secreting ADH & oxytocin)
• Axonal processes extend from the hypothalamus via the pituitary stalk
• Releases two peptide hormones: oxytocin and antidiuretic hormone (ADH)

Oxytocin Production

• Cervix dilation leads to oxytocin release and uterine smooth muscle contraction during parturition
• Released upon nipple stimulation, causing lactation
• Synthetic oxytocin induces labor

Antidiuretic Hormone (ADH)

• Conserves water during dehydration and hypovolemia, restricting diuresis
• Decreased BP sensed in cardiac atria and carotids stimulates ADH release
• Increased plasma osmotic pressure stimulates ADH release
• Hypervolemia and distention inhibit ADH secretion

Hormone Release Regulation

• Anterior and posterior pituitary are connected by a stalk and have feedback inhibition
• Hypothalamus secretes releasing hormones, while the anterior pituitary secretes the rest
• Stimulatory factors secreted include:
  • TRH (thyrotropin-releasing hormone)
  • CRH (corticotropin-releasing hormone)
  • GHRH (growth hormone-releasing hormone)
  • GnRH (gonadotropin-releasing hormone)
• Inhibitory hypothalamic influences:
  • PIF (prolactin inhibitory factor or dopamine)
  • GH-RIH (growth hormone-release inhibiting hormone or somatostatin)

Pituitary Diseases

• Increased or decreased secretion of ADH

Hyperpituitarism

• Arises from excess trophic hormone secretion
• Causes include hyperplasia, adenomas, carcinomas of the anterior pituitary, nonpituitary tumors secreting hormones, and hypothalamic disorders

Hypopituitarism

• Results from deficiency of trophic hormones
• Caused by Ischemic injury, Surgery or Radiation, Inflammatory disorders, or Mass effects of non-functional pituitary adenomas

Local Mass Effects

• Proximity of the pituitary to optic nerves and chiasm can cause compression of decussating fibers in the optic chiasm -> visual field defects like bitemporal hemianopsia
• Expanding adenomas can cause headache, nausea, and vomiting and acute hemorrhage (pituitary apoplexy)
• Effects include sella expansion, bone erosion, disruption of diaphragmatic sella visual defects increase ICP and Pituitary apoplexy

Pituitary Adenomas & Hyperpituitarism

• Adenomas in the anterior lobe are the common cause of hyperpituitarism
  • In endocrine gland pathology, it is more common to have BENIGN lesions/masses/tumors
  • GH and prolactin are the most common combination secreted
• Carcinomas of the Anterior pituitary, Extrapituitary tumors secreting hormones, and Hypothalamic disorders are less common
• Large pituitary adenomas (non-functioning) cause hypopituitarism by destroying adjacent normal anterior pituitary parenchyma

Pituitary Adenoma Classification

• Pituitary adenomas are usually found in adults 35-60 y/o
  • Microadenoma: <1 cm
  • Macroadenoma: >1 cm
  • Functional Adenoma: excess hormones
  • Nonfunctional Adenoma: No signs or symptoms so caught later.
• Hormone-Negative -> No hormone
• Single Cell Type with a single predominant hormone

Genetic Abnormalities of Pituitary Adenomas

• Usually Monoclonal
• G-Protein mutations are common - associated with MEN syndrome
• Approximately 5% by germline loss-of-function mutations

Pituitary Adenoma Morphology

• Soft and well-circumscribed -> small and confined to turcica
• Can extend superiorly into the suprasellar region and compress optic chiasm leading to cellular reponses and distortions
• Monomorphic and Composed of uniform, polygonal cells
• Has sparse reticulin framework
• 30% are invasive: "metastasis"

Treatment for Pituitary Gland Adenomas

• Excision Surgery or Bromocriptine or restore GH or radiotherapy