DR. ALVIN

• The Endocrine System


  A system of the body which consists of ductless glands that release HORMONES into the blood streams
• The Endocrine System


  • Interacts closely with the nervous system
• Endocrine Organs


  • Pituitary gland
  • Pineal gland
  • Thyroid gland
  • Parathyroid gland
  • Adrenal gland
  • Pancreas
  • Thymus
  • Gonads
  • Hypothalamus
• Hormones


  A substance that is synthesized in one organ and transported by the circulatory system to act on another tissue
• Hormones


  They can also act on adjacent cells (paracrine action) and on the cell in which they were synthesized (autocrine action) without entering the systemic circulation
• The Target Cell Concept

  • Hormones affected a single cell type – or only a few kinds of cells – and that a hormone elicited a unique biochemical or physiologic action
  • A given hormone can affect several different cell types
  • More than one hormone can affect a given cell type
  • Hormones can exert many different effects in one cell or in different cells
• Hormone Receptors


  • Receptors discriminate precisely - they must distinguish not only between different hormones present in small amounts but also between a given hormone and the 10^6 – to 10^9 – fold excess of other similar molecules
• Functional Domains of Receptors

  • Recognition Domain - Binds the hormone ligand
  • Coupling Domain - Generates a signal that couples hormone recognition to some intracellular functions
• Comparison of Receptors with Transport Proteins


  • Insulin Receptor - A heterotetramer composed of two copies of 2 different protein subunits (α2β2) linked by multiple DISULFIDE BONDS
  • Growth Hormone and Prolactin Receptors - Do not contain intrinsic protein kinase activity, ligand binding results in the association and activation of a different protein kinase signaling pathway, the JakStat Pathway
  • Polypeptide Hormone and Catecholamine Receptors - Transduce signals by altering the rate of production of cAMP through G-proteins
• Classification of Hormones


  • Chemical composition
  • Nature of the signal used to mediate hormonal action within the cell
  • Solubility properties
  • Location of receptors
• Hormones and Organs they are Synthesized

  • Thyroid - Triiodothyronine, Calcitonin
  • Adrenal - Glucocorticoids, Mineralocorticoids
  • Ovaries - Estradiol, Progesterone
  • Testes - Testosterone
  • Pituitary - TSH, FSH, LH, Growth Hormone, Prolactin, ACTH
  • Kidney - Angiotensin II
  • Small Intestine - Glucagon-like peptide
  • Skin, Liver, Kidney - Calcitriol (1,25(OH)2-D3)
• Hormones and their Derivatives

  • Cholesterol - Synthesized in final form and secreted immediately
  • Catecholamines - Synthesized in final form and stored in the producing cells
  • Insulin - Synthesized from precursor molecules in the producing cell, processed and secreted upon a physiologic cue
  • T3 and DHT - Converted to active forms from precursor molecules in the periphery
• Adrenal Steroidogenesis


  1. Cholesterol - Synthesized in final form and secreted immediately

  2. Requires ACTH-dependent steroidogenic acute regulatory (StAR) protein for the transport of cholesterol to P450scc in the inner mitochondrial membrane

  3. Aldosterone - Mineralocorticoid Synthesis

  4. Cortisol - Glucocorticoid Synthesis

  5. Dehydroepiandrosterone (DHEA) - Adrenal androgen production
• Testicular Steroidogenesis


  1. Cholesterol - Immediate precursor of the gonadal steroids

  2. Rate limiting steps: Delivery of cholesterol to the inner membrane of the mitochondria by the transport protein StAR, 3 enzyme activities contained in 3 proteins
• Ovarian Steroidogenesis


  1. Theca Cells - Source of androstenedione and testosterone

  2. Converted by the aromatase enzyme in granulosa cells to estrone and estradiol

  3. Progesterone - Precursor for all steroid hormones, produced and secreted by the corpus luteum
• Calcitriol (1,25(OH)2-D3) Synthesis


  1. Precursors in the skin, liver, and kidney

  2. 25-hydroxylation in the liver

  3. 1-hydroxylation in the kidney mitochondria
• Steroidogenesis


  The process of converting progesterone to other steroid hormones
• Estrogenization


  Increased activity of aromatase may contribute to this
• Conditions that may increase aromatase activity

  • Cirrhosis of the liver
  • Hyperthyroidism
  • Aging
  • Obesity
• Tissues where aromatase is present


  • Adipose cells
  • Liver
  • Skin
  • Other tissues
• Precursors for 1,25(OH)2-D3


  Fish liver oil and egg yolk
• Production of 1,25(OH)2-D3 (calcitriol)

  Produced in the malphigian layer of the epidermis from 7-dehydrocholesterol in an ultraviolet light-mediated, nonenzymatic photolysis reaction