Overview of Endocrinology

• Where is the hypothalamus located and what hormones does it secrete?

  • Location: Base of the brain, below the thalamus, forming the floor of the third ventricle.
  • Hormones (mostly releasing/inhibiting):
  • TRH (Thyrotropin-releasing hormone)
  • CRH (Corticotropin-releasing hormone)
  • GnRH (Gonadotropin-releasing hormone)
  • GHRH (Growth hormone-releasing hormone)
  • Somatostatin (inhibits GH and TSH)
  • Dopamine (inhibits prolactin)
• Where is the pituitary gland located and what are its hormone secretions (anterior + posterior)?

  • Location: Within the sella turcica of the sphenoid bone, beneath the hypothalamus.
  • Anterior (adenohypophysis) hormones:
  • ACTH, TSH, LH, FSH, GH, Prolactin
  • Posterior (neurohypophysis) hormones (produced by hypothalamus, stored/released here):
  • ADH (vasopressin), Oxytocin
• Where is the pineal gland located and what hormone does it secrete?

  • Location: Near the centre of the brain, between the two hemispheres, in a groove between the superior colliculi.
  • Hormone: Melatonin (regulates circadian rhythm)
• What is the anatomical location of the thyroid gland and what hormones does it secrete?

  • Location: Anterior neck, just below the larynx, wrapping around the trachea.
  • Hormones:
  • T3 (triiodothyronine), T4 (thyroxine) – regulate metabolism
  • Calcitonin – lowers blood calcium levels
• Where are the parathyroid glands located and what hormone do they secrete?

  • Location: Usually 4 small glands on the posterior surface of the thyroid gland.
  • Hormone: Parathyroid hormone (PTH) – increases blood calcium levels
• Where are the adrenal glands located and what hormones are secreted by the cortex and medulla?

  • Location: On top of each kidney
  • Cortex hormones:
  • Glucocorticoids (cortisol)
  • Mineralocorticoids (aldosterone)
  • Androgens (DHEA)
  • Medulla hormones:
  • Catecholamines: Adrenaline (epinephrine), Noradrenaline (norepinephrine)
• Where is the pancreas located and what hormones does its endocrine portion secrete?

  • Location: Retroperitoneal, behind the stomach; extends from the duodenum to the spleen
  • Hormones (from Islets of Langerhans):
  • Insulin (β cells), Glucagon (α cells)
  • Somatostatin (δ cells), Pancreatic polypeptide (PP cells)
• Where are the gonads located and what hormones do they produce?

  • Testes (in scrotum): Testosterone (from Leydig cells)
  • Ovaries (in pelvic cavity): Oestrogen, Progesterone
• Where is the thymus located and what hormone does it secrete?

  • Location: In the mediastinum, behind the sternum, in front of the heart (prominent in children)
  • Hormone: Thymosin (involved in T-cell maturation)
• What is endocrine signalling?

  The release of hormones by endocrine glands into the bloodstream. These hormones travel long distances to act on target cells in distant organs or tissues that express specific receptors.

  📌 Example: Insulin released by the pancreas affecting glucose uptake in muscle and liver cells.
• What is neuroendocrine signalling?

  The process by which neurons release hormones into the blood rather than into a synapse. These hormones then act on distant target tissues.

  📌 Example: The hypothalamus releasing vasopressin or oxytocin via the posterior pituitary.
• What is paracrine signalling?

  The release of chemical messengers that act locally on nearby cells within the same tissue. These signals do not enter the bloodstream.📌 Example: Somatostatin released by pancreatic delta cells inhibits insulin release from nearby beta cells.
•  What is autocrine signalling?

  When a cell releases a chemical messenger that acts on receptors on the same cell that secreted it. This often regulates cell growth or immune responses.📌 Example: Activated T-cells producing and responding to IL-2 to promote their own proliferation.
•  What is intracrine signalling?

  Involves a hormone or chemical messenger that acts within the same cell where it was produced, without being secreted.📌 Example: Steroid hormones like oestrogen acting directly within the cell nucleus where they were synthesised.
• How do endocrine and paracrine signalling differ in range and delivery?

  • Endocrine: Long-range; hormone enters the bloodstream to reach distant targets.
  • Paracrine: Short-range; acts locally within the same tissue and does not enter the bloodstream.
• Compare autocrine and intracrine signalling

  • Autocrine: The secreted chemical acts on the cell surface receptors of the same cell.
  • Intracrine: The chemical remains inside the cell and acts intracellularly, often on nuclear or cytoplasmic receptors.
• What are the four major classes of hormones?

  1. Peptide and protein hormones
  2. Steroid hormones
  3. Amino acid-derived (amine) hormones
  4. Fatty acid-derived hormones (eicosanoids
• What are peptide and protein hormones made of?

  Peptide and protein hormones are composed of chains of amino acids.

  • Peptides: short chains (e.g. oxytocin)
  • Proteins: longer chains with complex structures (e.g. insulin)They are synthesised in the rough endoplasmic reticulum and secreted via exocytosis.
•  How do peptide and protein hormones exert their effects?

  They bind to cell surface receptors because they are hydrophilic and cannot cross the lipid bilayer.This activates second messenger systems (e.g. cAMP, IP₃) to trigger intracellular responses.
• Give two examples of peptide or protein hormones

  1. Insulin – regulates blood glucose
  2. ADH (antidiuretic hormone) – controls water balance in kidneys
• What are steroid hormones derived from?

  Steroid hormones are lipid-soluble molecules derived from cholesterol.
• How do steroid hormones act on target cells?

  Because they are lipophilic, they can diffuse through the plasma membrane and bind to intracellular receptors (cytoplasmic or nuclear).They directly regulate gene transcription.
•  Give two examples of steroid hormones

  1. Cortisol – stress response, metabolism
  2. Testosterone – male secondary sexual characteristics
• What are amine hormones derived from?

  Amine hormones are derived from the amino acids tyrosine or tryptophan.
•  How do amine hormones exert their effects?

  It depends on the specific hormone:

  • Catecholamines (e.g. adrenaline) are water-soluble and act on cell surface receptors.
  • Thyroid hormones (e.g. thyroxine/T4) are lipid-soluble and act on intracellular receptors.
• Give examples of amine hormones.

  1. Adrenaline (epinephrine) – fight-or-flight response
  2. Thyroxine (T4) – regulates metabolism
• What are fatty acid-derived hormones (eicosanoids)?

  Eicosanoids are hormones derived from arachidonic acid, a type of fatty acid. They act locally (paracrine or autocrine) and have short half-lives.
• Give two examples of fatty acid-derived hormones.

  1. Prostaglandins – involved in inflammation, pain, fever
  2. Leukotrienes – involved in allergic responses and asthma
• What is a neurosecretory cell?

  • A specialised neuron that synthesises and secretes hormones (rather than neurotransmitters) into the bloodstream in response to neural signals.
  • Bridge the nervous and endocrine systems by converting electrical signals into hormonal messages.
• Where are neurosecretory cells typically found in the human body?

  Primarily found in the hypothalamus. Key nuclei include the paraventricular and supraoptic nuclei, which project to the posterior pituitary gland (neurohypophysis).
• What hormones are secreted by hypothalamic neurosecretory cells into the posterior pituitary?

  • Oxytocin
  • Vasopressin (antidiuretic hormone, ADH)These hormones are stored and released from the posterior pituitary into the bloodstream.
• How do neurosecretory cells differ from typical neurons?

  Unlike typical neurons, which release neurotransmitters into synaptic clefts, neurosecretory cells release hormones into the blood. Their axon terminals end in capillaries, allowing direct entry of hormones into circulation.
• Why are neurosecretory cells crucial for integrating neural and endocrine functions?

  • Translate neural inputs (e.g., stress, sensory stimuli) into hormonal outputs
  • Allow the nervous system to regulate long-term physiological processes such as growth, metabolism, reproduction, and water balance
  • Form a key part of the hypothalamic-pituitary axis, a major neuroendocrine control system
• What is the role of neurosecretory cells in the hypothalamic-pituitary axis?

  They produce releasing and inhibiting hormones (e.g., CRH, TRH, GnRH) that travel via the hypophyseal portal system to the anterior pituitary, regulating secretion of tropic hormones like ACTH, TSH, and LH/FSH.
• How does the structure of a neurosecretory cell support its function?

  • A cell body in the hypothalamus (to receive and process neural signals)
  • Long axons projecting to the pituitary (posterior or median eminence)
  • Axon terminals near capillaries, enabling direct hormone release into blood
• What is the hypothalamic-pituitary axis (HPA)?

  A complex neuroendocrine system through which the hypothalamus and pituitary gland regulate the function of various peripheral endocrine glands by releasing specific trophic hormones.
• Which peripheral endocrine glands are controlled by the HPA?

  • Thyroid gland
  • Adrenal cortex
  • Gonads (testes in males, ovaries in females)
  • Mammary glands (via prolactin secretion, though not a classic endocrine gland)
  • Liver (indirectly, for growth hormone effects via IGF-1 production)
• Which hormone from the hypothalamus regulates the thyroid gland?

  Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to release thyroid-stimulating hormone (TSH), which then acts on the thyroid gland.
• How is the adrenal cortex regulated by the HPA?

  The hypothalamus secretes corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to release adrenocorticotropic hormone (ACTH). ACTH acts on the adrenal cortex to stimulate cortisol production.
• How does the HPA control the gonads?

  The hypothalamus releases gonadotropin-releasing hormone (GnRH), which triggers the anterior pituitary to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones act on the testes or ovaries to regulate gametogenesis and sex hormone production.