ENDOCRINE SYSTEM

• Endocrine organs


  • Secrete hormones through the blood (no ducts!)
  • Hormones travel elsewhere and have effects on different tissues
  • The classical endocrine system consists of all the organs
  • Male and female reproductive systems also have endocrine functions

  
• Pituitary gland


  • Located in the sella turcica (Turkish saddle) in the sphenoid bone
  • Connected to the hypothalamus
  • Two lobes: Anterior (adenohypophysis) and Posterior (neurohypophysis)
• Pituitary gland


  • Composed of cords of glandular epithelial cells separated by capillaries
  • Makes and secretes a bunch of hormones
  • Subdivisions: Pars distalis, Pars tuberalis, Pars intermedia

  
• Hormones of the anterior pituitary

  • Growth hormone (GH)
  • Prolactin (PL)
  • Follicle-stimulating hormone (FSH)
  • Luteinizing hormone (LH)
  • Adrenocorticotropic hormone (ACTH)
  • Thyroid stimulating hormone (TSH)
• Posterior pituitary (neurohypophysis)


  • Composed of neural tissue (mostly axons)
  • Subdivisions: Pars nervosa, Median eminence, Infundibulum and infundibular stalk
• Hormones of the posterior pituitary

  • Posterior pituitary doesn't make hormones! It secretes hormones made by the hypothalamus
  • Herring bodies are dilated portions of axons containing hormone-filled vesicles
  • Hormones: Antidiuretic hormone (ADH) and Oxytocin
• Control of the anterior pituitary

  • Hypothalamus produces and releases hormones that stimulate or inhibit hormone secretion from the anterior pituitary
  • Hormones produced by target organs exert negative feedback on both the hypothalamus and the anterior pituitary
• Thyroid gland


  • Composed of round follicles lined by simple squamous to cuboidal epithelium and filled with colloid
  • Follicular cells synthesize thyroid hormones (T3 and T4) and secrete them into the blood
  • Hypothalamus releases TRH, which makes pituitary release TSH, which makes thyroid release thyroid hormone
• Thyroid hormone


  • Increases growth and metabolism
  • Stimulates mitochondrial protein synthesis, increases absorption of carbohydrates, regulates fat metabolism, promotes cell growth
  • Increases basal metabolic rate and revs up most bodily functions
• Parafollicular cells (C cells)

  • Derived from neural crest ectoderm
  • Located between follicular cells and between follicles
  • Larger cells with clear cytoplasm and small secretory granules containing calcitonin
  • Calcitonin lowers blood calcium levels by inhibiting osteoclastic resorption
• Parathyroid glands


  • Four glands on posterior surface of thyroid
  • Main function is to secrete parathyroid hormone (PTH) to regulate calcium levels
  • PTH raises calcium levels in response to low serum calcium
  • Two main cell types: chief cells (secrete PTH) and oxyphils (function unknown)
• Zona glomerulosa


  • Outermost zone
  • Cells arranged in little clusters
  • Cells produce mineralocorticoids (mostly aldosterone)
  • Aldosterone stimulates sodium reabsorption by the kidney (leading to an increase in blood pressure)
• Zona fasciculata


  • Middle zone
  • Cells arranged in straight bundles
  • Cells make glucocorticoids (mostly cortisol), and a small amount of androgens
  • Cortisol is a long-term "stress" hormone that mobilizes resources and shuts down stuff you don't need
• Zona reticularis


  • Innermost zone
  • Cells form an irregular network
  • Cells make sex steroids (androgens)
  • Androgens have weak masculinizing characteristics
• Adrenal medulla


  • Contains chromaffin cells (modified sympathetic neurons) and a few sympathetic ganglion cells
  • Chromaffin cells produce catecholamines (mostly epinephrine and a little norepinephrine) when stimulated by preganglionic sympathetic neurons
  • Catecholamines are the "fight or flight" hormones that increase blood glucose, heart rate, blood flow to heart and skeletal muscle, and decrease blood to non-essential organs
• Pancreas


  • Has both exocrine (ducts) glands and endocrine (ductless) glands
  • Exocrine glands produce digestive enzymes
  • Endocrine glands produce hormones that regulate blood glucose: glucagon, insulin, and somatostatin
  • Also produces pancreatic polypeptide, which helps regulate the digestive system
• Main islet cells and hormones

  • A (alpha) cells secrete glucagon when blood glucose is low
  • B (beta) cells secrete insulin when blood glucose is high
  • D (delta) cells secrete somatostatin, which inhibits release of insulin and glucagon
  • F cells secrete pancreatic polypeptide, which inhibits release of digestive enzymes and causes relaxation of gallbladder and decreases secretion of bile
• Pineal gland


  • Located in brain near posterior third ventricle
  • Two main cell types: pinealocytes and glial cells
  • Pinealocytes produce melatonin (which helps induce sleep) in response to light
  • "Brain sand" (corpora arenacea) are globules of basophilic calcified material that can be seen radiologically