Lecture+6.1

Module 6 - Lecture 1: An Introduction to the Endocrine System and Hormones

Endocrinology: Definition and Etymology

  • The term "endocrinology" derives from Greek:

    • endon (within)

    • krino (to separate/distinguish)

    • logia (study)

  • The term "endocrine" indicates "secreting within" (Latinized Greek) + "ology" (branch of science).

  • Hormones originates from the Greek word “hormon” or “horman” meaning “that which sets in motion” or “impulse.”

  • The term was introduced in 1902 by Bayliss and Starling after discovering secretin.

  • Historical reference: In China, the use of human urine for purifying hormones dates back to 200 BC.

Hormonal Pathways

  • Hormones are secreted by hormone-producing cells and target cells that have specific receptors.

  • Vascular system plays a key role in hormone delivery.

  • Reference: Müller-Estert Biochemie, 2004.

Hormone Delivery Methods

  • Types of Hormone Delivery:

    • Endocrine: Hormones released into the bloodstream.

    • Neuroendocrine: Hormones released from neurons into the blood.

    • Paracrine: Hormones act on nearby cells.

    • Neurocrine: Hormones released by nerve cells acting on adjacent cells.

    • Autocrine: Hormones act on the cells that produce them.

    • Lumonal: Hormones delivered into the lumen of the gut.

  • Source: Hadley's and Levine's Endocrinology, Pearson.

Vertebrate Endocrine System: Key Glands and Functions

Glands of the Endocrine System

  • Hypothalamus: Produces hormones for the pituitary.

  • Pineal Gland: Releases melatonin, controlling sleep-wake cycles.

  • Thyroid Gland: Produces thyroxine, regulating metabolism.

  • Pituitary Gland: Central to hormone production for other endocrine glands.

  • Parathyroid Gland: Regulates calcium levels in the blood.

  • Pancreas: Controls glucose levels through insulin and glucagon production.

  • Thymus: Produces thymosin for T-cell development during childhood.

  • Adrenal Glands: Release stress-related hormones (epinephrine, norepinephrine).

  • Ovaries (female): Produce estrogen and progesterone necessary for reproductive activities.

  • Testis (male): Produces testosterone, crucial for sperm production and development of male characteristics.

Classification of Hormones

Structural Classification of Hormones

  • Lipid Derivatives:

    • Eicosanoids (e.g., leukotrienes, prostaglandins).

    • Steroid Hormones: Derivatives of cholesterol (e.g., androgens, estrogen, glucocorticoids).

  • Amino Acid Derivatives:

    • Small molecules (e.g., thyroid hormones, catecholamines like epinephrine).

  • Peptide Hormones:

    • Chains of amino acids, including polypeptides (e.g., insulin, glucagon).

  • Glycoproteins:

    • Hormones with sugar moieties (e.g., FSH, LH).

Steroid Hormone Synthesis

  • All steroid hormones arise from cholesterol.

  • Enzymes are crucial in the synthesis of steroid hormones.

Hormone Secretion Process: Protein and Polypeptide Hormones

  1. Transcription:

    • mRNA is synthesized from DNA.

  2. Translation:

    • mRNA binds to ribosomes, forming peptide chains.

  3. ER Processing:

    • Signal sequence directs the chain into the ER lumen and is modified to form prohormone.

  4. Golgi Apparatus:

    • Prohormone is packaged into secretory vesicles and processed into active hormones.

  5. Exocytosis:

    • Vesicals release hormones into the extracellular space.

  6. Circulation:

    • Hormones enter bloodstream for delivery to target cells.

Receptors for Hormones

  • Hormones act via specific receptors, triggering signal transduction pathways.

  • Types of Receptors:

    • Cell membrane receptors: Most peptide hormones use these, often acting through second messengers.

    • Nuclear receptors: Bind steroid and thyroid hormones, promoting transcription factors for action.

Ligand Binding Specificity

  • Receptors typically bind to one or a few closely related ligands.

  • Examples: Estradiol binds to estrogen receptors, while norepinephrine and epinephrine bind to distinct adrenoreceptors.

Agonists and Antagonists

  • Agonists: Mimic hormones, activating receptors (sometimes with greater potency).

  • Antagonists: Bind to receptors but do not activate them, blocking receptor activation by agonists.

Comparison of Agonists and Antagonists

  • Agonists: Activate receptors and produce biological responses.

  • Antagonists: Occupy receptors without activation, preventing agonists from exerting effects.