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Endocrine System and Intercellular Communication (Lecture Notes)

Mechanisms of Intercellular Communication

  • Direct communication: occurs between two cells in physical contact
  • Paracrine communication: chemical messengers transfer information from cell to neighboring cell within the same tissue
  • Autocrine communication: chemical messengers affect the same cells that secrete them
  • Endocrine communication: chemical messengers that travel in blood to reach distant target cells (hormones); receptors for the hormone (target cells)

Endocrine vs Nervous Communication

  • Endocrine vs Nervous
    • Endocrine: slower, longer lasting, hormones, mainly regulated by negative feedback, regulate homeostasis
    • Nervous: faster, short lived, neurotransmitters

Endocrine System (overview)

  • Endocrine system: endocrine cells, tissues, and organs that produce hormones
  • Endocrine cells: release secretions into the extracellular fluid (blood)
  • Exocrine cells: release secretions onto epithelial surfaces via ducts
  • Hormones regulate growth and development, reproduction, cell metabolism and energy balance, body water content and levels of electrolytes and nutrients, and mobilization of body defenses

Pituitary gland (hypophysis)

  • Lies within sella turcica, inferior to hypothalamus, connected by infundibulum
  • Anterior lobe (adenohypophysis): hormones control other endocrine glands
    • Pars tuberalis: wraps around infundibulum
    • Pars distalis: largest, most anterior region
    • Pars intermedia: narrow band bordering posterior lobe
  • Posterior lobe
    • Hypothalamus: part of the limbic system, inferior to thalamus, contains many nuclei that produce unique hormones, sexually dimorphic, regulates pituitary through hypophyseal portal system
  • Posterior pituitary regulation: synthesizes hormones released at posterior lobe (ADH and OXT)
  • Anterior pituitary regulation: secretes regulatory hormones that control activity of the anterior lobe
  • Classes of hypothalamic regulatory hormones
    • Releasing hormones (RH): stimulate secretion of hormones from anterior lobe
    • Inhibiting hormones (IH): prevent secretion of hormones from the anterior lobe
    • Rate of secretion controlled by negative feedback

Hormones of anterior lobe of pituitary

  • Thyroid-stimulating hormone (TSH/thyrotropin)
    • Release stimulated by thyrotropin-releasing hormone (TRH) from hypothalamus
    • Stimulates release of thyroid hormones from thyroid gland
  • Adrenocorticotropic hormone (ACTH)
    • Release stimulated by corticotropin-releasing hormone (CRH) from hypothalamus
    • Stimulates release of glucocorticoids (cortisol) from adrenal gland
  • Prolactin (PRL)
    • Release stimulated by prolactin-releasing hormone (PRH) from hypothalamus
    • Release inhibited by prolactin-inhibiting hormone (PIH) from hypothalamus
  • Growth hormone (GH/somatotropin)
    • Release stimulated by growth hormone-releasing hormone (GH-RH)
    • Release inhibited by growth hormone-inhibiting hormone (GH-IH)
    • Stimulates cell growth and division, especially in muscles and bones
  • Melanocyte-stimulating hormone (MSH)
    • Pars intermedia secretes MSH during fetal development, in young children, in pregnant females, and in the course of some diseases
    • Stimulates melanin production
    • Dopamine inhibits release of MSH
  • Gonadotropins
    • Release stimulated by gonadotropin-releasing hormone (GnRH) from hypothalamus
    • Follicle-stimulating hormone (FSH)
    • Follicle development and estrogen in females, sperm production in males
    • Luteinizing hormone (LH)
    • Ovulation, estrogen and progesterone in females, androgens in males

Hormones of posterior lobe of the pituitary

  • Oxytocin (OXT): stimulates the contraction of uterus during labor, the ejection of milk
  • Antidiuretic hormone (ADH) (vasopressin): stimulates the kidney to retain water, alcohol inhibits ADH release
  • Diabetes insipidus: diseases characterized by excessive thirst and urination due to low ADH levels

Thyroid gland

  • Lies inferior to the thyroid cartilage of the larynx, consists of two lobes connected by an isthmus, contains thyroid follicles (hollow spheres), follicle cavity contains a viscous colloid
  • Thyroxine (T3) and Triiodothyronine (T4)
    • Stimulated by thyroid-stimulating hormone (TSH)
    • Increase the rate of oxygen consumption and ATP production by mitochondria
    • Increase the metabolic rate of the cell by activating genes involved in energy use, glycolysis and ATP production
    • Cause and increase in blood pressure, heart rate and force of contraction
  • Calcitonin (CT): produced by C cells
    • Stimulates the excretion of calcium ions by kidneys
    • Causes a decrease in calcium ion absorption by digestive tracts
  • Follicular cells → release thyroxin (T3+T4; increase metabolism)
  • Parafollicular cells → release calcitonin (decrease blood calcium)

Parathyroid Glands

  • Four small glands embedded in the posterior surface of the thyroid gland
  • Parathyroid (principal) cells: secrete parathyroid hormone (PTH) to increase blood calcium

Adrenal Glands

  • Located superior to each kidney; outer adrenal cortex, inner adrenal medulla
  • Outer zona glomerulosa: mineralocorticoids (e.g., aldosterone – conserves Na+)
  • Middle zona fasciculata: glucocorticoids (e.g., cortisol – stimulated by ACTH)
  • Inner zona reticularis: androgens (main male sex hormones)
  • Adrenal medulla: releases catecholamines upon sympathetic stimulation
    • Epinephrine (E) and norepinephrine (NE)

Pineal Gland

  • Main part of the epithalamus
  • Pinealocytes: cells that use serotonin to synthesize the hormone melatonin
  • Influences circadian rhythm

Pancreas

  • Large gland that lies in the loop between the stomach and the small intestine, mostly retroperitoneal, contains exocrine and endocrine cells
  • Exocrine pancreas: clusters of cells called pancreatic acini
  • Endocrine pancreas: clusters of cells known as pancreatic islets
  • Pancreatic Hormones
    • Insulin: released by beta cells in response to high blood glucose levels, decreases blood glucose, moves glucose from blood to cells
    • Glucagon: released by alpha cells in response to low blood glucose levels, increases blood glucose, liver releases stored glucose
  • Diabetes mellitus: characterized by hyperglycemia, glycosuria, and polyuria
    • Hyperglycemia: abnormally high glucose levels in the blood
    • Glycosuria: presence of glucose in the urine
    • Polyuria: excessive urine volume, caused by inadequate or abnormal insulin or insulin receptors
    • Type 1 diabetes mellitus: inadequate insulin production by pancreatic beta cells, patients require daily injections or continuous infusion of insulin, usually develops in children and young adults
    • Type 2 diabetes mellitus: most common form, normal insulin but tissues have faulty receptors (insulin resistance), associated with obesity, treatment consists of weight loss and medication

Endocrine System (summary)

  • The endocrine system communicates via hormones distributed through the bloodstream to distant targets, contrasting with the fast, localized signaling of the nervous system
  • Major roles include maintaining homeostasis, regulating growth, metabolism, reproduction, and energy balance
  • Key regulatory principle: negative feedback loops connect hypothalamic releasing/inhibiting hormones, anterior pituitary hormones, and target endocrine glands
  • Common clinical concepts: thyroid disorders, parathyroid/calcium balance, adrenal hormone regulation, and pancreatic insulin/glucagon control