The Endocrine System

1. The Endocrine System

2. Endocrine system: characteristics • Endocrine system is mostly controlled by the nervous system • Endocrine system controls most of the processes occurring in the body • Divided into cranial & extracranial endocrine glands • Cranial = hypothalamus, pituitary & pineal glands • Extracranial = thyroid, parathyroid, thymus, pancreas, adrenals, gonads, GI tract & placenta

3. Cranial endocrine glands • Hypothalamus: • Many “nuclei” within this region to control various aspects of homeostasis • Exhibits control over pituitary gland • Blood supply / blood flow pattern is important for control • “portal system”: capillaries in hypothalamus drain into “portal venules” (veins) that connect to capillaries in the pituitary gland (capillary-vein-capillary) • Hypothalamic neurons will release “releasing hormones” or “inhibiting hormones” into these capillaries/portal veins to eventually target the pituitary gland

5. Pituitary gland - Hypophysis • Known as “the conductor” or “master gland” but is itself under control of the hypothalamus • Inferior region of the brain, nestled within the sellaturnica of the spenoid bone • 2 regions (visibly different) • Adenohypophysis (anterior lobe) • Most of the hormones are produced here • Neurohypophysis (posterior lobe) • More “neural” area, 2 hormones

6. Pituitary gland (hypophysis) • Anterior lobe (adenohypophysis) • Derived from ectodermal glandular tissue • Linked to hypothalamus via the infundibulum which has the hypophysial portal vascular system (vascular communication) • Body = anterior pituitary proper – secretes 5 different hormones, prolactin (PL), growth hormone (GH), thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), Follicle stimulating hormone (FSH) and luteinizing hormone (LH) • Posterior lobe (neurohypophysis) • Derived from nervous tissue • “extension of the brain” • Releases 2 hormones (which are synthesized in the brain but stored in the posterior pituitary), oxytocin and vasopressin (or antiduiretic hormone (ADH)) • Intermediate lobe: • Thin, sandwiched between the 2 above layers • secretes a minor hormone (melanin-stimulating hormone=MSH)

8. Pineal gland • roof of the 3rd ventricle) • Larger in children than adults • Secretes melatonin: involved in circadian rhythm

9. Extracranial endocrine glandsThyroid gland • Thyroid gland • Below larynx, bi-lobed lateral to the trachea • Largest endocrine gland • Bloodflow via external carotid & subclavian arteries • Simple cuboidal epithelia (heavily reliant on iodine) • Follicular cells secrete thyroxine (T4), Tri-iodo-thyronine (T3) , parafollicular cell or C cells secrete calcitonin

10. Parathyroid glands • Posterior to the thyroid gland (para = around) • 4 distinct glandular formations • Secretes parathyroid hormone (= parathormone = PTH)

11. Pancreas • Has BOTH endocrine & exocrine functions • Endocrine = Endocrine cells grouped into the “Islets” (Islets of Langerhans). • 3 types of cells: • Alpha cells: secrete glucagon • Beta cells secrete insulin • Delta cells secrete somatostatin

12. Minor gastrointestinal endocrine glands • GI tract (gastrointestinal tract) • Numerous endocrine cells distributed throughout GI tract (often single cells) • Additional endocrine glands: • Salivary glands • Brunner’s glands (duodenum)

13. Adrenal glands • Located on the top of the kidneys • Composed of 2 major parts: • Adrenal cortex: 3 layers • Zonaglomerulosa secretes mineralocorticoids • Zonafasciculata secretes glucocorticoids • Zonareticularis secretes sex hormones • Adrenal medulla: is an extension of the sympathetic autonomic nervous system (specialized 2nd motor neurons which secrete adrenaline)

14. Gonads • Testicles: Contains the seminiferous tubules where sperm is made. In between the tubules, the Leydig cells secretes testosterone • Ovaries: The developing follicle secretes estrogen and progesterone

15. Placenta • Transfers waste from fetus to mother and nutrients from mother to fetus • Secretes estrogen & progesterone • Also secretes human chorionic gonadotrophin (hCG)…what’s used to change color of pregnancy test sticks

16. Pituitary pathophysiology • Panhypopituitarism: reduced pituitary activity or total loss of pituitary function • Abnormal growth hormone: • Inadequate during childhood = pituitary dwarfism • Inadequate during adulthood = Simmond’s disease • Premature aging • Oversecretion during childhood = gigantism • Oversecretion during adulthood = acromegaly • Bones thicken, soft tissues grow inappropriately

17. Acromegaly Gigantism Occurs during adulthood Begins during childhood WHY are patients who suffer gigantism taller than those who suffer acromegaly?

18. Thyroid & parathyroid pathophysiology • Hypothyroidism • During childhood = cretinism (“cretins”) • During adulthood = myxedema • Goiter (abnormal thyroid growth) • Endemic = inadequate iodine intake • Grave’s disease

19. Pancreatic pathophysiology • Diabetes mellitus • Type I diabetes: insulin dependent due to autoimmune destruction of pancreatic beta cells (loss of insulin production) • Type II diabetes: insulin insensitive due to reduced responsiveness to insulin (metabolic obesity)

20. Adrenal pathophysiology • Pheochromatocytomas: chromaffin cell tumor • Excessive norepinephrine secretion = resembles ANS overstimulation • Addison’s disease: decreased mineralcorticoid & glucocorticoid secretion • Constant hypoglycemia, electrolyte imbalances • Cushing’s syndrome: increased glucocorticoid secretion (Zona fasciculata) • Altered metabolism and physical changes indicative of edema

21. Pediatric Cushing’s syndrome