Lec 35 - Adrenal, thyroid and parathyroid glands

• The endocrine system us a complex network of glands and organs that release hormones directly into the bloodstream
• Two types of endocrine hormones: peptide hormones and steroid hormones
• Peptide hormones are freely soluble in the blood. Synthesized as a protein. It will interact with a transmembrane receptor on a target cell. The peptide will interact on the outside and then have a secondary messenger system on the inside of the cell
• Steroid hormones are lipid or cholesterol derived. Typically have a carrier protein within the blood because not soluble within the blood. Then can cross the cell membrane to go directly in cell and effect transcription and translation. (membrane diffusible)
• Organs with endocrine cells

  • pancreatic islets
  • ovaries
  • testes
• Endocrine organs

  • pituitary gland
  • pineal gland
  • thyroid gland
  • parathyroid gland
  • adrenal gland
• The adrenal gland is an encapsulated organ. It's blood supply from the suprarenal arteries (superior, middle, inferior renal arteries). The arterial supply comes from the outside in (cortex to medulla). The capsule is made of DICT. The cortex is broken into 3 layers. The medulla is completely surrounded by cortex
• The five categories of hormones released by the adrenal gland

  • mineralocorticoid hormones
  • glucocorticoid hormones
  • gonadocorticoid androgens
  • catecholamine epinephrine
  • catecholamine norepinephrine
• The hormones produced by the cortex of the adrenal gland are

  • mineralocorticoid
  • glucocorticoid
  • gonadocorticoid androgens
• Epinephrine and norepinephrine are released from the adrenal medulla. It is a sympathetic gland and is specialized neuronal tissue and derived from neuronal crest cells
• Mineralocorticoids control salts - ex. aldosterone
• Glucocorticoids control glucose regulation
• The three layers of the adrenal cortex

  • zona glomerulosa
  • zone fasciculata
  • zona reticularis
• Adrenal gland contains a portal system - capillary -> through adreno-cortical sinusoids -> capillary bed -> central vein - this is because some of the cortex hormones act on the medulla. Also have another blood route of oxygen rich blood to the medulla via the medullary arteries
• The zona glomerulosa contains predominately cholesterol based hormones so they have to have large stores of lipids in the cells to produce these hormones. Also contain mitochondria and smooth ER. Secretes aldosterone in response to angiotensin II. Increases absorption of sodium in renal tubules. Part of the hormone synthesis and modification is occurring in mitochondria
• What layer of cortex is this - zona glomerulosa

  
• The zona fasciculata contains lipid droplets, mitochondria with tubular cisterna (to increase surface area). Secretes cortisol in response to adrenocorticotropic hormone (ACTH). Cortisol promotes glucose secretion from hepatocytes. Cortisol suppresses inflammatory responses. Cells arranged into cords with sinusoids between them. Mitochondria will appear speckled because of the tubular cisterna
• What layer of the cortex is this - zona fasciculata

  
• The zona reticularis contains lipid droplets, mitochondria, smooth ER. Cells secrete androgens in response to adrenocorticotropic hormone (ACTH). Major source of male sex hormones (testosterone) in biological females. Cells are irregular in size, shape, and lumen
• What layer of the cortex is this - zona reticularis

  
• Adrenal medulla contains chromaffin cells which contain abundant membrane-bound granules. Do not contain lipid vacules like the cells of the cortex. Still contain sinusoids
• What layer of the cortex is this - medulla

  
• 20% of chromaffin cells secrete norepinephrine and 80% secrete epinephrine
• Chromaffin cells secrete norepinephrine or epinephrine following nervous input (sympathetic)
• Glucocorticoids required for methylation of norepinephrine to form epinephrine
• Tyrosine converts to DOPA -> dopamine -> norepinephrine
• To make epinephrine, enzymatic activity induced by glucocorticoids (from the cortex) -> PNMT -> coverts norepinephrine into epinephrine. Carried out of the cell by carrier protein chromogranin
• The thyroid gland sits beneath the larynx and oversits the trachea
• Thyroid gland releases

  • thyroxine (T3 & T4)
  • calcitonin
• Label where the thyroid and parathyroid is

  A) parathyroid

  B) thyroid

  
• The cells of the thyroid are called follicular epithelium and are simple cuboidal with a central large lumen filled with colloid. The cells + the colloid is called thyroid follicle. Between the follicles will be capillaries and parafollicular cells.
• In the thyroid, follicular epithelial cells secrete thyroglobulin stored as colloid in the thyroid follicles. Thyroglobulin is the precursor to the thyroid hormone - secrete it into lumen as colloid to store it
• Label the thyroid follicle

  A) colloid

  B) follicular epithelium

  C) capillaries

  
• Production of thyroid hormone

  • pumps transport iodide across basal membrane and iodide concentrates within colloid
  • secretion of thyroglobulin and thyroid peroxidase (works on the iodine - takes away an e-) into colloid
  • iodination of thyroglobulin at the cell membrane by thyroid peroxidase
  • Binding of iodide and thyroglobulin into iodothyroglobulin after reuptake of colloid droplets and fusion with lysosomes
  • release T3 and T4 into circulation
• When there is a signal to release T3 and T4 - iodothyroglobulin will phagocytose colloid droplet and fuse with a lysosome which will process the proteins to mature proteins and then it'll secrete mature T3 and T4 into the blood stream
• Thyroid hormone production is regulated by thyroid stimulating hormone (TSH) from the pituitary gland. The hypothalamus sense T3 and T4 levels and communicates with the pituitary
• Parafollicular cells release calcitonin which lowers blood calcium levels

  
• Parafollicular cells secrete calcitonin which binds to receptors on osteoclast and inhibits bone reabsorption. Calcitonin binds to osteoclasts to turn them off and lowers levels of calcium in our blood
• The number of osteoclasts actively resorbing bone and releasing calcium into the circulation is regulated by calcitonin and parathyroid hormone
• We cannot live without our parathyroid because it is very important for maintaining blood calcium levels