Lecture 12: Pituitary and Adrenal (Suprarenal) Glands

• Pituitary and adrenal glands are endocrine organs

Pituitary Gland

• A combination of neural and glandular structures in the hypophyseal fossa (sella turcica) of the sphenoid bone.
• Major endocrine organ regulated by the Hypothalamus
• Connected by the Infundibulum and HypothalamoHypophyseal Portal System
• Influenced by other endocrine hormones
• Has 2 parts, the Anterior (Adenohypophysis) and Posterior Pituitary (Neurohypophysis).
• Adenohypophysis contains: pars distalis, intermedius, & tuberalis

Cell-Cell Signaling

• Hormonal secretion involves endocrine cells secreting hormones into the blood.
• Neurohormone secretion involves a neuron secreting neurohormones into the blood.

Pituitary Lobe Development

• Rathke's pouch develops into the Anterior lobe, made of glandular epithelial tissue (Adenohypophysis).
• Neuroectoderm develops into the Posterior lobe, made of neural secretory tissue (Neurohypophysis).

Hypothalamo-Hypophyseal Portal System

• Connects the hypothalamus capillary bed to the hypophyseal capillaries
• Regulating hormones travel from the Hypothalamus to stimulate or inhibit Adenohypophysis cells.
• Hormones from the Adenohypophysis enter the portal system to travel to the rest of the body.

Adenohypophysis

• Has 3 subregions, the pars distalis, pars intermedius, and pars tuberalis, and produces most endocrine hormones
• Has 3 cell types (based on secretory vesicle staining): chromophobes (50%), acidophils (40%), and basophils (10%)
• PAS+/- staining reveals functional types of cells from acidophils and basophils only.
• Produces hormones that are either small proteins or glycoproteins

Pars Distalis (PD)

• Contains Somatotropes (GH cells) that produce growth hormone (GH; somatotropin)
  • GH release is regulated by hormones from the hypothalamus and stomach
  • Stimulated by GHRH (hypothalamus) and ghrelin (stomach) and are the most prevalent cells (50%)
  • Inhibited by somatostatin (hypothalamus)
• Contains Lactotropes (PRL cells, mammotropes) that produce prolactin (PRL)
  • Stimulated by TRH and VIP (hypothalamus)
  • Inhibited by dopamine (hypothalamus)
• Contains Gonadotropes (FSH & LH cells) that produce luteinizing hormone (LH) & follicle-stimulating hormone (FSH)
  • Stimulated by gonadotropin-releasing hormone (GnRH; hypothalamus)
• Contains Corticotropes (ACTH cells) that produce proopiomelanocortin (POMC) as a precursor molecule Stimulated by corticotropin-releasing hormone (CRH; hypothalamus)
  • POMC is then cleaved within corticotropes into adrenocorticotropic hormone (ACTH), β-endorphin, enkephalin, β-lipotrophic hormone (β-LPH), melanocyte-stimulating hormone (MSH).
• Contains Thyrotropes (TSH cells) that produce thyroid-stimulating hormone (TSH)
  • Stimulated by TRH (hypothalamus)
• Folliculostellate cells
  • Do NOT produce hormones
  • May coordinate/regulate hormone release from other PD cells

Pars Intermedia (PI)

• Function is unclear, may be involved in α- or β-endorphin, MSH
• Contains basophils and chromophobes
• Defining characteristic is the presence of colloid follicles

Pars Tuberalis (PT)

• Highly vascular region with portal veins
• Cells arranged in clusters or cords
• Reactive/involved in ACTH, FSH, LH

Homeostasis

• Negative feedback slows down or diminishes a process.
• Positive feedback speeds up or amplifies a process.
• Negative and positive do not refer to the consequences as having a final effect but refer to the type of information (inhibition or stimulation) provided into the loop.

Adenomas

• 12% of primary brain tumors are adenomas (affecting the adenohypophysis)
• Different conditions can result based on excess or reduced hormonal output from affected cells.
• Examples: Prolactinoma, Acromegaly, Giganticism, Cushing disease

Neurohypophysis

• Composed of nonmyelinated axons extending from the hypothalamus and through the hypothalamohypophyseal tract.
  • Neurons originate in the supraoptic and paraventricular nuclei
• Has a neurosecretory function (NOT endocrine)
  • Products released into the fenestrated capillary network
  • Oxytocin promotes contraction of uterine muscle & breast myoepithelial cells
  • Anti-diuretic Hormone (ADH)/Vasopressin regulate water homeostasis & osmolarity of body fluids
• Pituicytes are specialized glial cells only in the pars nervosa
• Contains Herring bodies, which are large nerve terminal dilations

Hypothalamus

• Is the master control, located in the anterior diencephalon
• Endocrine (pituitary)
• Vegetative (visceral temperature regulation, autonomic, sympathetic & parasympathetic)
• Emotional (influences behaviors like sex, feeding, drinking, aggression, flight)
• Neurohypophysis via hypothalamohypophyseal tract (axons)
  • ADH (antidiuretic hormone, vasopressin)
  • Oxytocin
• Adenohypophysis via tuberoinfundibular tract (axons) to hypothalamohypophyseal portal system (capillaries).
  • Releasing hormones (stimulatory)
    • GHRH stimulates Somatotropes
    • TRH stimulates Thyrotropes
    • CRH stimulates Corticotropes
    • GnRH stimulates Gonadotropes
    • Prolactin-releasing factor stimulates Lactotropes
  • Inhibitory hormones
    • Somatostatin inhibits Somatotropes, Thyrotropes, and Lactotropes

Adrenal Glands

• Suprarenal; sit just superior to the kidneys (renals)
• Cortex forms the outer layer that develops from mesoderm
  • Produces steroid hormones: mineralocorticoids, glucocorticoids & gonadocorticoids
  • Has 3 zones: glomerulosa, fasiculata, reticularis
• Medulla forms the inner layer that develops from neural crest cells
  • Chromaffin cells produce catecholamines: epinepherine & norepinepherine

Adrenal Cortex

• Zona glomerulosa produces mineralocorticoids (aldosterone)
  • Regulates of Na, K homeostasis, H₂O balance
• Zona fasciculata produces glucocorticoids (corticosterone, cortisol) & gonadocorticoids
  • Regulates glucose synthesis, glycogen polymerization, & fatty acid metabolism
  • ACTH regulates glucocorticoid production and secretion
• Zona reticularis produces glucocorticoids (small amts) & gonadocorticords/androgens (dehydroepiandrosterone (DHEA))

Adrenal Medulla

• Chromaffin cells (medullary cells) produce epinephrine and norepinephrine
  • Secreted by two populations of chromaffin cells distinguished based on intracellular vesicles
• Has preganglionic sympathetic innervation