endocrine system

similarity between exocrine and endocrine glands

made of cells that are epithelial in origin

exocrine glands

synthesize substances that are released into ducts that are secreted onto the surface of the body (substances include oil, water, and ions

endocrine glands

synthesize hormones that are released into the bloodstream that will travel to and affect a target tissue

what must endocrine glands maintain?

a plethora of blood vessels from which they can monitor the blood and into which they can directly release hormones (made up of simple cuboidal cells arranged in small clusters or “cords” around fenestrated capillaries - clustering maximizes their contact with large numbers of capillaries)

what do cells release? (endocrine glands)

their hormones into the sinusoid (space) between the cells and capillaries (hormones will immediately enter the adjacent capillaries)

target cells

hormones influence only specific tissue cells, have receptor molecules to which a specific hormone can bind (once binding has occurred the target cell will respond in a preprogrammed way)

what do the effect of a hormone on a target cell depend on?

the preprogrammed response of the target cell, no the hormone itself (same hormones can have different effects on different targets because it is the target that determines the reaction, hormones arenkjm triggers to stimulate the target cell into action)

hormones are stimulated to make and secrete their hormones by three major types of stimuli:

humoral, neural, hormonal

humoral stimulus

hormones are produced and secreted in direct response to changing levels of ions or nutrients in the blood

neural stimulus

nerve fibers stimulate the release of hormones

hormonal stimulus

endocrine glands secrete hormones in response to hormonal stimuli received from other endocrine glands

regardless of how hormone secretion is stimulated…

it is always regulated by feedback loops, which can be positive or negative

negative feedback loop

maintains homeostasis, output of a process influences the activation of a hormone (stabilizing system)

negative feedback loop with essentially turn off the initial stimulus preventing what?

it from escalating (this ensures hormones stay within a normal desirable range, which maintains homeostasis in the body. ex: blood calcium levels drop, the parathyroid gland will secrete parathyroid hormone which increases calcium absorption from bones, kidneys, and intestine

positive feedback loop

increase leads to increase, blood concentrations of a hormone increases, the response of the effector organ stimulates further secretion, ex: the hormone that controls the progression of childbirth, oxytocin, stimulates the uterus to contract. contraction of the uterus stimulates further secretion of oxytocin

pituitary gland

(hypophysis) sits inferior to the brain in the hypophyseal fossa which is a depression in the sella turcica of the sphenoid bone, suspended from the hypothalamus via a stalk or infundibulum

anterior lobe (pituitary gland)

controlled by hormones secreted by the hypothalamus (hypothalamus secretes hormones which enter a venous plexus called the hypothalamic-hypophyseal portal system (HHPS) which contacts the anterior lobe

hypothalamic-hypophyseal portal system (HHPS)

hormones enter and travel directly to the anterior lobe where they then either stimulate or inhibit the production of hormones (hormones secreted then enter the venous capillaries that lead directly to general circulation where they will access targets elsewhere in the body)

what is the anterior lobe made of?

epithelial tissue and contains 5 different types of endocrine cells that produce 7 different hormones

thyroid stimulating hormone (TSH)

function: signals thyroid gland to secrete thyroid hormone, action: controls metabolic rate

adrenocorticotropic hormone (ACTH)

function: stimulates the adrenal cortex to secrete hormones, action: help the body cope with stress

follicle stimulating hormone (FSH) & luteinizing hormone (LH)

function: act on gonads, stimulating maturation of sex cells and inducing secretion of sex hormones, action: maturation of ovarian follicles, induces ovulation, maturation of sperm cells

prolactin (PRL)

function: targets milk producing glands in breast, action: simulates milk production

melanocyte stimulating hormone (MSH)

function: signals hypothalamus, and melanocyte production, action: appetite suppression, protection from UV rays

growth hormones (GH)

function: stimulates body to increase production of proteins and growth of the epiphyseal plates, action: stimulates growth of the entire body

posterior lobe (pituitary gland)

does not make hormones but only stores and releases hormones produced in the hypothalamus, made of neural tissue

hypothalamohypophyseal tract

connected to the hypothalamus, neuron cell bodies of the hypothalamus produce antidiuretic hormones and oxytocin that are transported from the hypothalamus to the posterior lobe of the pituitary gland via the hypothalamopypophyseal tract

where are hormones stored (posterior lobe)

neural tissue of the posterior lobe (neurons will fire to release the hormones into surrounding capillary beds for distribution throughout the body)

antidiuretic hormone (ADH)

function: stimulate the kidney to resorb water and return it to the blood, signals vasoconstriction, action: helps the body retain as much fluid as possible, increases blood pressure

oxytocin

function: contraction of smooth muscle of the reproductive tract, action: contracts uterus during childbirth, ejects breastmilk via muscular contraction, contracts the prostate gland. with ADH, causes the desire to cuddle, groom, and bond

histological identification of the pituitary gland is achieved by?

looking for the two distinct lobes of the gland, anterior and posterior lobe will stain differently using H&E

anterior lobe histology stain

stain dark with H&E due to the abundance of epithelial cells which make up the glandular tissue, gigantic

posterior lobe histology stain

made mostly of unmyelinated axons and dilated axon terminal which take up less dye making this region of the pituitary gland stain lighter

pituitary disorders

affect the release of growth hormone (GH): hypersecretion & hyposecretion

hypersecretion (disorder)

result in gigantism, children grow exceptionally fast, if this happens after the growth plates close results in acromegaly

hyposecretion (disorder)

results in pituitary dwarfism, bodies of normal proportions rarely reach 4 ft in height, can be given GH and reach a normal stature

hypothalamus

major controller of the endocrine system in 3 way

what does the hypothalamus produce?

hormones that are transported and stored in the posterior pituitary gland until they are needed, regulatory hormones which have an effect on the anterior pituitary gland (

what does the hypothalamus stimulate?

sympathetic neurons to the adrenal medulla during “fight or flight” which causes the adrenal medulla to release adrenaline

thyroid gland

produces thyroid hormone (TH) to increase basal metabolic rate/activity (slow v. fast metabolism)

gross anatomy of thyroid gland

largest endocrine gland located in the anterior neck, inferior to the larynx, two lateral lobes are connected by an isthmus, copious blood supply from two separate arterial sources

microscopic anatomy of thyroid gland

spherical follicles surrounded by numerous capillaries, walls of the follicles are made of simple cuboidal epithelium with a central lumen filled with a jelly-like substance called ‘colloid’, the colloid contains thyroglobulin which is a protein from which thyroid hormone is derived

what does the hypothalamus trigger? (thyroid gland)

release of thyroid stimulating hormone (TSH) from the pituitary gland when low amounts of TH are circulating in the blood, follicular cells make and secrete TH from the thyroglobulin in the colloid, TH then diffuses out of the follicular cells into the surrounding capillaries, Iodine is an essential nutrient for TH production

clinical condition: hyperthyroidism (thyroid gland)

overactive thyroid gland, many disorders can result in hyperthyroidism (autoimmune disease is a common etiology, immune system produces antibodies that stimulate thyroid hormone secretion)

parathyroid glands

regulation of blood calcium level, typically 4 glands are embedded in the posterior thyroid gland, not under control of hypothalamus

what do parathyroid gland cells produce?

parathyroid hormone (PTH) which regulates blood calcium levels (PTH is released into the bloodstream in response to low blood calcium levels which causes: osteoclasts to release calcium from bone, increase absorption of calcium from the tubular fluid in kidney, the small intestine to absorb more calcium)

adrenal gland

release epinephrine and norepinephrine, located on the superior border of each kidney also called suprarenal glands

microscopic anatomy of adrenal gland

has a centrally located adrenal medulla made of neural crest cells and an external adrenal cortex which surrounds the medulla (cortex has three zones or layers which each pick up different amounts of dye therefore can be differentiated from one another)

what is the adrenal medulla part of?

autonomic nervous system, made of chromaffin cells which are modified postsynaptic sympathetic neurons (release epinephrine and norepinephrine into bloodstream, short-term, systemic stress response)

adrenal medulla has 3 zones that secrete lipid based steroid hormones:

zona glomerulosa, zona fasciculata, zona reticularis

zona glomerulosa

secretes mineralocorticoids to regulate composition of electrolytes in the body, hormones increase water absorption in the kidney thus increasing blood volume

zona fasciculata

secretes glucocorticoids which helps regulate blood glucose levels during high stress situations, hormones keep blood glucose high to support brain activity while forcing other cells of the body to switch to fats and amino acids as an energy source

zona reticularis

secretes hormones that counteract stress, boost immunity, and improve mood

pancreas

secrete glucagon and insulin, located in the posterior wall of abdomen behind the stomach

microscopic anatomy of pancreas

made of both exocrine and endocrine cells, endocrine cells are contained in spherical bodies called pancreatic islets

main cells in islets - endocrine cells (pancreas)

alpha and beta cells

alpha cells (endocrine - pancreas)

secrete glucagon which cause cells in the liver to break down glycogen into glucose (glucose enters blood stream and raises blood sugar levels when they fall too low)

beta cells (endocrine - pancreas)

secrete insulin that signals cells of the body to take up glucose from the blood and store it (promotes the storage of glucose as glycogen in the liver, lowering excessive blood sugar levels)

thymus

t-cell maturity and immunocompetence, located in the superior mediastinum, atrophies during adolescense

microscopic anatomy of thymus

arranged in lobules that each have a cortex and a medulla, within medulla are epithelial reticular cells which have a rosy appearance and are useful for positive identification of the thymus

maturation and subsequent immunocompetence training is stimulated by what? (thymus)

thymic hormones - secreted by the epithelial reticular cells

gonads

secrete androgens, the testes and ovaries are the main source of steroid sex hormones

testes (gonads)

interstitial endocrine cells secrete androgens primarily testosterone (maintain reproductive organs and secondary sex characteristics, promote sperm formation)

ovaries (gonads)

androgens are secreted and converted into estrogen and progesterone by ovarian follicle cells (estrogen maintains the reproductive organs and secondary sex characteristics, progesterone signals the uterus to prepare for pregnancy