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