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The Endocrine System
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
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
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 sella turnica 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
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)
Pineal gland
• roof of the 3rd ventricle)• Larger in children than adults• Secretes melatonin: involved in
circadian rhythm
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
Parathyroid glands
• Posterior to the thyroid gland (para = around)– 4 distinct glandular
formations• Secretes parathyroid hormone (=
parathormone = PTH)
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
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)
Adrenal glands• Located on the top of the
kidneys• Composed of 2 major parts:
– Adrenal cortex: 3 layers• Zona glomerulosa secretes
mineralocorticoids• Zona fasciculata secretes
glucocorticoids• Zona reticularis secretes sex
hormones– Adrenal medulla: is an
extension of the sympathetic autonomic nervous system (specialized 2nd motor neurons which secrete adrenaline)
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
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
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
Acromegaly Gigantism
Occurs during adulthood Begins during childhood
WHY are patients who suffer gigantism taller than those who suffer acromegaly?
Thyroid & parathyroid pathophysiology
– Hypothyroidism• During childhood = cretinism (“cretins”)• During adulthood = myxedema
– Goiter (abnormal thyroid growth)• Endemic = inadequate iodine intake• Grave’s disease
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)
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
Pediatric Cushing’s syndrome