BIOL 242 Ch. 17: The Endocrine System

BIOL 242 Chapter 17: The Endocrine System

Function of the Endocrine System

  • Main Role: The endocrine system coordinates and integrates the activities of body cells in conjunction with the nervous system.

Anatomy and Location of Endocrine Organs

  • Organs Involved:
      - Pituitary gland
      - Thyroid gland
      - Parathyroid gland
      - Adrenal glands
      - Pancreas
      - Gonads (testes and ovaries)
      - Pineal gland
      - Thymus

  • Neuroendocrine Organ: Hypothalamus

  • Organs With Both Endocrine and Exocrine Glands: Pancreas and gonads

  • Tissue Composition: Glandular epithelia
      - Exocrine Glands: Use ducts to transport secretions
      - Endocrine Glands: Secrete hormones directly into the bloodstream via blood vessels

Hormones

  • Definition: Hormones are chemical substances, typically proteins or steroids, secreted by endocrine cells that regulate metabolic functions at target cells slowly.

  • Composition: Most hormones are composed of amino acids:
      - Small modified amino acids
      - Polypeptides
      - Proteins

  • Classification by Structure:
      - Amino acids
      - Steroids (cholesterol derivatives)
      - Eicosanoids
        - Types: Leukotrienes (mediate inflammation/allergies) and prostaglandins (multiple targets/effects)
        - Typically act on short distance (autocrine/paracrine)

Molecular Biology of Hormone Action

  • Hormones can be either:
      - Lipid Soluble: Steroids and thyroid hormones
      - Water Soluble: Amino-acid based hormones, excluding thyroid hormones

  • Factors Affecting Target Cell Response:
      1. Blood levels of hormones
      2. Number/sensitivity of receptors (up/down regulation)
      3. Affinity of hormone-receptor complex

  • Half-Life of Hormones: Affects hormone levels in the blood.
      - Water-soluble hormones can’t pass through the plasma membrane without receptor binding.
      - Lipid-soluble hormones can pass through the plasma membrane and bind to receptors inside the cell.

  • Mechanisms of Action:
      - Water-Soluble:
        - Requires receptor binding and involves G protein signal transduction
        - Triggers second messenger cascades (e.g., cAMP or PIP2)
        - Response is target cell dependent (can inhibit or stimulate various responses)
      - Lipid-Soluble:
        - Forms activated receptor-hormone complex
    environments and directly affects gene transcription.
        - Responses may include secretion, apoptosis, or growth.

Hormone Release Mechanisms

  • Hormone synthesis and release are typically controlled by negative feedback systems for regulation.

  • Stimulation Types:
      - Humoral Stimulation: Hormone secretion due to direct changes in blood levels of ions/nutrients (e.g. PTH secretion, insulin, aldosterone).
      - Neural Stimulation: Hormone secretion in response to neuronal impulses (e.g., adrenal medulla release catecholamines during stress).
      - Hormonal Stimulation: Secretion due to regulatory hormones from other endocrine glands (e.g., most anterior pituitary hormones).

Anatomy of the Pancreas

  • Function:
      - Exocrine: Acinar cells assist in digestion.
      - Endocrine: Islets of Langerhans (1-2% composition of the pancreas) produce:
        - Alpha cells: secrete glucagon
        - Beta cells: secrete insulin
        - Delta cells: secrete somatostatin
        - PP cells: secrete pancreatic polypeptide
        - Epsilon cells: secrete ghrelin

  • Fasted vs Fed State:
      - Fasted State:
        - Glucagon is active; insulin secretion inhibited
        - Liver conducts glycogen breakdown (glycogenolysis);
        - Muscle/fat decreases glucose uptake.
      - Fed State:
        - Insulin is secreted; glucagon secretion inhibited
        - Liver conducts glycogen synthesis (glycogenesis);
        - Muscle/fat increases glucose uptake for glycolysis.

  • Glucose Homeostasis:
      - Can arise from dietary glucose or liver glycogen stores.
      - Insulin stimulates glycogenesis and glucose uptake by muscles/adipose tissue.
      - High blood glucose (fed state) leads to insulin secretion from the pancreas.
      - Low blood glucose (fasted state) leads to glucagon secretion to promote glycogen breakdown, increasing blood glucose levels.
      - Insulin inhibits glucagon secretion and acts alongside cortisol (from the adrenal gland) to stimulate gluconeogenesis, raising blood sugar levels.

Diabetes Overview

  • Future Statistics: Expected 32 million people with diabetes by 2030.

  • Definition: Diabetes mellitus is characterized by hyperglycemia and osmotic diuresis.

  • Diagnosis: Fasting plasma glucose >140 mg/dl.

  • Symptoms: Excessive thirst, urination, hunger.

  • Types of Diabetes:
      - Type I Diabetes:
        - Autoimmune disease
        - Typically diagnosed in individuals under 20
        - Requires insulin injections.
      - Type II Diabetes:
        - Accounts for 85-90% of cases
        - Often linked with obesity but not exclusively
        - May be managed with or without insulin.

  • Pathophysiology of Type II:
      - Involves abnormal insulin secretion, loss of glucose responsiveness in islets, resulting in decreased insulin production and insulin resistance in target tissues.

  • Normal Blood Sugar Levels: 90-110 mg/dl

Anatomy of the Adrenal Glands

  • Location: Two pairs of adrenal glands situated on the superior portion of the kidneys, enclosed by a fibrous capsule and fat cushion.

  • Structure:
      - Adrenal Medulla: Composed of neural tissue; part of the sympathetic nervous system.
      - Adrenal Cortex: The larger portion, responsible for producing corticosteroid hormones that help cope with stress.
        - Components:
          - Zona Glomerulosa: Produces mineralocorticoids, which regulate minerals/water in the blood.
          - Zona Fasciculata: Produces glucocorticoids and some gonadocorticoids.
          - Zona Reticularis: Produces adrenal sex hormones (gonadocorticoids) and some glucocorticoids.

Summary of Adrenal Hormones

  • Mineralocorticoids (chiefly aldosterone):
      - Regulation: Stimulated by the renin-angiotensin mechanism, elevated K+, low Na+ blood levels, and ACTH.
      - Effects on Target Organ: Kidneys increase Na+ levels while decreasing K+, leading to increased water reabsorption, thus rising blood volume and pressure.

  • Glucocorticoids (chiefly cortisol):
      - Regulation: Stimulated by ACTH.
      - Effects: Promote gluconeogenesis, increase blood sugar, mobilize fats for energy, stimulate protein catabolism, assist in stress response, and depress inflammatory/immune responses.
      - Hypersecretion Effects: Cushing’s syndrome.
      - Hyposecretion Effects: Addison’s disease.

  • Gonadocorticoids (chiefly androgens):
      - Regulation, effects, and hyper/hyposecretion implications discussed.

  • Catecholamines (epinephrine and norepinephrine):
      - Released upon stimulation by preganglionic fibers of the sympathetic nervous system, mimicking sympathetic nervous system activation.

Anatomy of the Thyroid Gland

  • Overview: Largest exclusively endocrine gland,
      - Highly vascularized, located in the anterior neck region, inferior to the larynx.

  • Cell Types:
      - Follicles produce thyroglobulin, which gives rise to T3 and T4; the lumen of follicles stores colloid (thyroglobulin + iodine).
      - Parafollicular cells produce calcitonin.

Parathyroid Gland

  • Structure: Usually consists of four tiny yellow-brown glands.

  • Cell Types:
      - Chief cells produce parathyroid hormone (PTH).
      - Oxyphil cells whose function is currently unknown.

Functions of the Thyroid and Parathyroid Glands

  • Calcium Homeostasis:
      - Calcitonin: Important in childhood for calcium regulation.
      - Thyroid Hormone: Includes Thyroxine (T4) and Triiodothyronine (T3), affecting nearly all cells except the adult brain, spleen, uterus, and thyroid gland. Increases basal metabolic rates and body heat production.

  • Conditions:
      - Hyperthyroidism (Graves’ disease): Symptoms include weight loss, goiter, and increased appetite due to excessive TH production.
      - Hypothyroidism: Symptoms include weight gain and fatigue due to decreased TH or TSH production.

Synthesis of Thyroid Hormones

  1. Thyroglobulin synthesis and modification occur in the lumen of the follicle.

  2. Iodide (I-) trapping is done via facilitated diffusion from blood into cells.

  3. Iodide is oxidized to iodine (I2) and iodination occurs at the thyroglobulin colloid tyrosine residues, forming T1 and T2.

  4. Coupling of T2 and T1 occurs, resulting in T4 (two linked T2s) or T3 (T2 + T1).

  5. Colloid endocytosis to lysosomes facilitates the recycling of thyroglobulin; T4 and T3 remain in the colloid.

  6. Cleavage of hormones for secretion occurs in lysosomes to diffuse hormones into the bloodstream.
       - TSH stimulates TH secretion, and TH synthesis is enhanced post-secretion.
       - T4 is a prohormone of the active hormone, T3, which is converted at target cells.

Gonads, Thymus, and Pineal Glands

  • Gonads:
      - Male: Testes (produce testosterone)
      - Female: Ovaries (produce estrogen and progesterone)
      - Function: Involved in sexual maturation.

  • Thymus:
      - Located in thorax deep to the sternum, large in children, shrinks with age.
      - Produces thymic hormones (thymopoietins, thymic factor, thymosin); essential for developing T lymphocytes and immune response.

  • Pineal Gland:
      - Situated on the third ventricle roof of the diencephalon and contains calcium salts in adults.
      - Major secretion: Melatonin, which plays a role in sleep cycles, with higher levels leading to drowsiness.

Anatomy of the Pituitary Gland

  • Also known as hypophysis or “pea-on-a-stalk”.

  • Consists of:
      - Stalk (infundibulum)
      - Anterior lobe (glandular)
      - Posterior lobe (neural tissue)

  • Posterior Lobe: Part of the brain, derived from hypothalamic tissue.

  • Anterior Lobe: Connected to hypothalamus via the hypophyseal portal system.

Hypophyseal Portal System

  • Function: Circulates regulatory hormones from the ventral hypothalamus to the anterior pituitary, controlling the release or inhibition of anterior pituitary hormone secretion.

  • Composition:
      - Primary capillary plexus (in infundibulum)
      - Hypophyseal portal veins
      - Secondary capillary plexus (in anterior pituitary lobe)

Pituitary Hormones Summary

  • Anterior Pituitary Hormones:
      - Growth Hormone (GH): Stimulated by GHRH, inhibited by GH and IGFs, affects liver, muscle, bone, cartilage, promotes growth, mobilizes fats, spares glucose.
      - Thyroid-stimulating Hormone (TSH): Stimulated by TRH, inhibits feedback from thyroid hormones, stimulates the thyroid gland to release thyroid hormone.
      - Adrenocorticotropic hormone (ACTH): Stimulated by CRH, inhibits feedback from glucocorticoids, stimulates the adrenal cortex for glucocorticoid release.
      - Follicle-stimulating Hormone (FSH):
      - Luteinizing Hormone (LH):
      - Prolactin (PRL):

Big Picture: Interrelationships Between Systems

  • Endocrine System Interaction:
      - Skeletal System: PTH and calcitonin regulate calcium levels; GH, T3, T4, and sex hormones support skeletal development.
      - Muscular System: GH influences muscle development; muscle activity triggers catecholamine release.
      - Nervous System: Hormones influence development; the hypothalamus controls the anterior pituitary and produces hormones.
      - Integumentary System: Androgens activate sebaceous glands, and estrogen hydrates skin; vitamin D synthesis occurs.
      - Cardiovascular System: Hormones affect blood volume, pressure, and heart contractility; erythropoietin stimulates RBC production.
      - Lymphatic System/Immunity: Hormonal responses affect immune response/inflammation, thymic hormones contribute to lymphocyte generation.
      - Respiratory System: Hormones influence ventilation and provide oxygen for hormonal functions.
      - Digestive System: GI hormones assist in digestion; catecholamines affect digestion; nutrient availability is essential.
      - Urinary System: Aldosterone/ADH regulate renal function, controlling RBC formation via erythropoietin; vitamin D precursors are converted.
      - Reproductive Systems: Hormones from hypothalamus, anterior pituitary, and gonads mediate sexual development; oxytocin and prolactin are involved in childbirth and lactation.