Endocrine I Flashcards
What is the key difference between endocrine and exocrine glands?
Exocrine: release secretory products into ducts
Endocrine: release secretory products into extracellular space which then passes to bloodstream to target organ
Where is the hypothalamus located?
In the encephalon of the brain
What does the hypothalamus do?
- Coordinates endocrine functions
- Integrates endocrine and ANS functions
What are the two populations of cells within hypothalamus glands?
1) neurons that produce and release neurotransmitters
2) special neurons that produce and release hypothalamic neurosecretory hormones
What is the master endocrine gland?
Pituitary gland
What is the role of the hypothalmus, generally speaking?
1) coordination of endocrine functions
2) integration of endocrine and autonomic functions
Where are hypothalamic neurosecretory hormones syntesized?
neurons of the hypothalamus
Where are hypothalamic neurosecretory hormones stored?
axon terminals
Where do these axon terminals terminate?
median eminence
Where do the hormones go after being released from the axon terminals?
they pass into the primary capillary plexus [the fenestrated capillaries located in the median eminence]
After the primary capillary plexus the hormones drain…
into the hypophyseal portal veins (located in the infundibulum)
After the hypophyseal portal veins, the hormones pass into …
the secondary capillary plexus [the sinusoidal capillaries with fenestrated epithelium located in the anterior lobe of the pituitary]
Where do the releasing or inhibitory hormones pass into to influence secretory cells?
The releasing of inhibiting hormones/factors pass into the parenchyma of the anterior lobe of the pituitary to influence secretory cells there
What does Growth Hormone-Releasing Hormone GHRH stimulate?
GHRH stimulates the secretion of somatotropin (aka growth hormone GH)
What does Prolactin-Releasing Hormone PRH stimulate?
PRH stimulates the secretion of prolactin
What does Prolactin-Inhibitory Factor PIH inhibit?
PIH inhibits the secretion of prolactin
What does Corticotropin-Releasing Hormone (CRH) stimulate?
CRH stimulates the secretion of adrenocorticotropin (ACTH)
What does thyroid-stimulating hormone-releasing hormone (TSH-RH) stimulate?
[aka Thyrotropin-releasing hormone TRH]
TSH-RH stimulates the secretion of thyroid stimulating hormone
What does Gonadotropin-Releasing Hormone (GnRH) stimulate?
GnRH stimulates the secretion of Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH)
Where are the cell bodies of neurosecretory cells located?
In the supraoptic and paraventricular nuclei of the hypothalamus
What do supraoptic and paraventricular nuclei of the hypothalamus synthesize?
They synthesize vasopressin (antidiuretic hormone ADH) and oxytocin.
They also synthesize neurophysin (a carrier protein) which binds to the above 2 hormones.
What forms the hypothalamohypophyseal tract?
neurosecretory cells give rise to unmyelinated axons which form the hypothalamohyopophyseal tract
What hormones does the hypothalamohypophyseal tract carry?
vasopressin and oxytocin
Where does the hypothalamohypophyseal tract terminate?
it descends to terminate near the capillaries in the posterior lobe of the pituitary gland
Where are the hypothalamic hormones (ADH and oxytocin) released into circulation?
capillaries in the posterior lobe of the pituitary gland that drain into the general circulation
What are other names for the anterior pituitary?
The anterior pituitary is also referred to as the pars distalis, pars anterior, and anterior lobe
Describe the anterior pituitary gland (3 things)
1) Covered by a fibrous CT capsule
2) Consists of cords of cells (glandular epithelial tissue) and reticular fibers
3) Contains fenestrated sinusoidal capillaries (of secondary capillary plexus)
What is the purpose of the fenestrated endothelial lining?
Permits the diffusion of releasing factors into gland and permits gland secretory products to pass into capillaries
What are the secretory cells of the anterior pituitary gland ?
Chromophils (acidophils and basophils) and Chromophobes
What does “chromophil” mean?
color friendly
these cells contain secretory granules that have an affinity for dyes
What are the most common cells within the anterior pituitary?
acidophils
What are the two different types of acidophils?
somatotropes and lactotropes
What stimulates somatotropes?
GHRH
What inhibits somatotropes?
Somatostatin
What do somatotropes secrete?
somatotropin (GH)
What is the function of somatotropin?
increases cellular metabolic rates and influences long bone growth
What does excessive GH cause?
gigantism in children and acromegaly in adults
What stimulates lactotropes (mammotropes)?
PRH (Prolactin-Releasing Hormone)
What inhibits lactotropes ?
PIF (Prolactin Inhibitory Factor)
What is the function of lactotropes?
Produces prolactin which promotes mammary gland growth during pregnancy and lactation following birth
What are the three types of basophilic cells found in the anterior pituitary gland?
corticotropes
thyrotropes
gonadotropes
What stimulates corticotropes?
CRH (Corticotropin-Releasing Hormone)
What is the function of corticotropes?
Secrete ACTH which stimulates the adrenal cortex
What stimulates thyrotropes?
stimulated by TRH
What inhibits thyrotropes?
T3 and T4 in the blood
What is the function of thyrotropes?
secrete TSH (thyrotropin)
What stimulates gonadotropes?
stimulated by GnRH
What is the function of gonadotropes?
secrete FSH and LH
Functions in male and female reproduction
What does “chromophobe” mean?
color fearing
these cells have no affinity for dyes
What is the most common type of pituitary adenoma?
prolactinoma
What are chromophobes?
Science is not entirely sure
They may be degranulated chromophils
Where is the infundibulum located? what is it continuous with?
Located in the posterior pituitary
The infundibulum is continuous with the median eminence of the hypothalamus
What is another name for the posterior pituitary?
Pars nervosa
Is the pars nervosa an endocrine gland?
NO
it is neural secretory tissue
What is the function of the pars nervosa?
It receives unmyelinated hypothalamohypophyseal tract erminals containing ADH and oxytocin THEREFORE it serves as a storage depot for neurosecretions produced by neurons of the supraoptic and paraventricular nuclei of the hypothalamus
What are accumulations of of neurosecretory granules in the axons and axon terminals of the hypothalamohypophyseal tract called?
Herring bodies
What causes the herring bodies to release their contents? where do they release their contents?
nervous stimulation cause these granules to release their contents near the fenestrated capillary plexus
What is the function of ADH/vasopressin?
lower urine volume (resorb water)
What is the target of ADH/vasopressin?
distal tubules and collecting ducts of the kidney
What is the target of oxytocin?
uterine myometrium and myoepithelial cells of mammary gland
What is the function of oxytocin?
stimulates smooth muscle contraction of uterus (parturition) and mammary gland (lactation)
What does a Herring body contain?
either ADH or oxytocin
What are pituicytes?
glia-like, local cells that cover and support axons and their terminals
Are pituicytes found in the posterior pituitary gland?
YES
What is a pituitary adenoma?
common, benign tumor of the anterior pituitary
- it may affect the secretory activity of other cells in the anterior pituitary and may compress and wear down surrounding neural tissue and bone
Damage to the hypothalamus or pars nervosa may cause what disease?
Diabetes insipidus
–> Reduction of ADH leads to insufficient water resorption by the kidney and polyurea (excess urination) and dehydration
What tissue is the capsule of the thyroid gland derived from?
deep cervical fascia
-consists of dense irregular collagenous tissuewhich gives rise to trabeculae/septa that partition the gland into lobules
What is the function of trabeculae in the thyroid gland?
trabeculae carry blood vessels, lymphatic vessels, and nerve fibers deep into gland
Where are parathyroid glands located?
within the capsule covering the posterior surface of the thyroid gland
What hormones does the thyroid gland synthesize?
T3 (triiodothyronine)
T4 (tetraiodothyronin, thyroxine)
calcitonin (thyrocalcitonin)
What is the purpose of T3 and T4?
These hormone regulate cell and tissue metabolism and heat production
What is the purpose of calcitonin?
It is involved in the regulation of blood calcium levels
What is the basic structural and functional unit of the thyroid?
Thyroid follicle
Describe a thyroid follicle.
Spherical, variable diameter
Surrounded by a basal lamina, reticular fibers, and then a fenestrated capillary plexus
What is the purpose of a thyroid follicle?
To store glandular secretory product in follicular lumen (extracellulary)
Is the follicle enclosing a lumen unique to the thyroid gland?
YES
In contrast, other endocrine glands accumulate their secretory products in the parenchyma (cellular part of the gland) and therefore the secretory product is stored within the secretory cells
What is the activity level of a simple squamous follicular epithelium?
low level of activity
What is the activity level of a simple cuboidal follicular epithelium?
normal level of activity
What is the activity level of a simple columnar follicular epithelium?
highly active
Which surface of the follicular cells comes in contact with the colloid?
the luminal surface of the cell membrane
What are parafollicular cells derived from?
neural crest
Where are parafollicular cells located?
Wedged between follicles (not exposed to colloid!), lie within basal lamina surrounding follicles and are near many capillaries
How do parafollicular cells appear histologically?
light-staining with a round nucleus
What do the secretory granules of parafollicular cells contain?
calcitonin
What is calcitonin?
peptide hormone
When is calcitionin released?
calcitonin is released by parafollicular cells when blood calcium levels are high
How does calcitonin lower blood calcium levels to normal?
1) inhibits bone breakdown by osteoclasts
2) promotes calcium deposition in bone (osteoid calcification)
What does the follicular lumen contain?
colloid
What does colloid consist of?
thyroglobulin (a glycoprotein) with T3 and T4 bound to it
What cells synthesize thyroglobulin?
follicular cells
How many tyrosine residues does each thyroglobulin have?
120
Where is iodide oxidized?
on the apical cell membrane facing the colloid
it is oxidized into iodine and then remains in follicular lumen
What secretes thyroid stimulating hormone?
Basophils (thyrotropes)
Where does TSH bind?
TSH binds to TSH receptors on basal membrane of thyroid follicular cells
What is the site of thyroglobulin synthesis? glycosylation?
synthesis: rER
glycosylation: rER and golgi apparatus
What role does the golgi apparatus play in synthesis of thyroid hormones?
- Site of glycosylation
- Site of formation of vesicles containing thyroglobulin
How do the thyroglobulin containing vesicles release their contents?
The vesicles adhere to the luminal aspect of the cell membrane and release contents into lumen via exocytosis
Where is iodine reduced to iodide?
GI tract
iodine is from the diet
How does iodide enter the follicular cells?
Via active transport with sodium/iodide symporters at the basal membrane of the follicular cells
Where does oxidation of iodide take place?
oxidation of iodide takes place in the active sites of thyroid peroxidase (membrane-bound) on the follicular APICAL cell membrane facing the colloid
- Oxidation occurs in the colloid
- H202 is needed for oxidation of iodide
What catalyzes the iodination of thyroglobulin’s tyrosine residues?
thyroid peroxidase
produces MIT and DIT
Where does the iodination of thyroglobulin’s tyrosine residues occur?
where the luminal aspect of follicular cell contacts the colloid
MIT = ?
monoiodotyrosine
DIT = ?
diiodotyrosine
Where is iodinated thyroglobulin stored?
follicular lumen
Neighboring MIT and DIT undergo a coupling reaction to form ?
triiodinated tyrosine
Neighboring DIT and DIT undergo a coupling reaction to form?
tetraiodinated tyrosine
Do all MIT and DIT undergo coupling reactions?
No, many remain uncoupled
Is thyroglobulin a hormone?
NO
it is the inactive storage form of the thyroid hormones
What stimulates the release of thyroid hormones?
TSH
What causes filopodia to form at luminal surface of follicular cells?
TSH adhering to the TSH receptors on basal membrane of follicular cells
What occurs after the formation of filopodia?
Endocytosis of colloid
Where do vesicles enclosing colloid unite with endosomes?
in cytoplasm
What splits the iodinated tyrosine residues from thyroglobulin?
proteases within the endosomes
How are iodinated tyrosine residues released into the cytoplasm after splitting from thyroglobulin?
as MIT, DIT, T3 and T4
When is triiodinated tyrosine called T3 (triiodothyronine)?
MIT + DIT = triiodinated tyrosine
when triiodinated tyrosine is cleaved from the thyroglobulin, it is called triiodothyronine ( T3 )
When is tetraiodinated tyrosine called T4 (tetraiodothyronine or thyroxine)?
DIT + DIT = tetraiodinated tyrosine
when tetraiodinated tyrosine is cleaved from the thyroglobulin, it is called tetraiodothyronine ( T4 )
True or False: T3 is produced only by the thyroid follicular cells.
FALSE
T4 is produced only by the thyroid follicular cells.
Most of T3 is produced via conversion from T4 by the kidney, liver and heart.
What are the physiological effects of T3 and T4? (5)
1) increased cellular metabolism
2) increased growth rate
3) increased mental activity
4) decreased formation of phospholipids and triglycerides (enhance the production of endogenous cholesterol)
5) decreased formation of fatty acids
Via stimulation of endocrine gland functions and carbohydrate metabolism
An increase in the synthesis of thyroid hormones results in …
1) decreased body weight
2) increased heart rate, metabolism, respiration, muscle function, and appetite
What do very high levels of thyroid hormone cause in men and women?
Muscle tremor, fatigue, impotence (men), abnormal menstrual bleeding (women)
