4 - Glands Flashcards
definition of a gland
an epithelial cell or aggregate of cells that are specialised for the secretion of a substance
how can glands be classified
basics - details on seperate cards
based on mode of secretion
- endocrine: products released into vascular (blood) or lymphatic system
- exocrine: products released into ducts that open to organ lumen or onto skin surface
exocrine can be classified further by mode of secretion
- apocrine
- holocrine
- merocrine
by their shape
- simple/compound (unbranched/branched)
- acinar or tubular
they can also be classified by unicellular/multicellular or their type of secretion
main endocrine glands are
- adrenal
- thyroid
- parathyroid
- pancreas
how can glands be classified
basics - details on seperate cards
based on mode of secretion
- endocrine: products released into vascular (blood) or lymphatic system
- exocrine: products released into ducts that open to organ lumen or onto skin surface
exocrine can be classified further
by mode of secretion
- apocrine
- holocrine
- merocrine
by their shape
- simple (unbranched)
- compound (branched duct system)
main endocrine glands
- adrenal
- parathyroid
- thyroid
- pancreas
what is a merocrine mode of secretion
form of exocytosis
has two pathways (regulated and constituative secretion, covered on next slide)
- membrane bounded compartment approaches cell surface
- it fuses with the plasma membrane
- its contents are in continuity with extracellular space and so dispersed by diffusion
- plasma membrane transiently larger
- membrane retrieved, stabilising cell surface area
merocrine secretion: regulated vs consituative secretion
merocrine secretion has two pathways
regulated
- active process - uses energy
- contents of vesicle can be anything within cell (eg organelle)
- secretory granules accumulate in large vesibles
- active secretion requires specific signal (Ca2+ ions)
- vesicle migrates to cell surface along microtubules
- in presence of Ca2+, membrane of vesicle fuses with plasmalemma
- cargo released to extracellular space
constituative
- the secretory product is not concentrated into granules
- the secretory product is instead packaged into small vesicles and continuously released to the cells surface
- used mainly to repopulate the plasma membrane with plasma proteins
- vesicles always moving (not waiting for signal)
note: product inside vesicle = cargo (secretion that’s released)
what is a apocrine mode of secretion
- non membrane bounded structure (eg lipid) approaches cell surface
- makes contact and pushes up apical membrane
- thin layer of apical cytoplasm drapes around droplet
- membrane surrounding droplet pinches off from cell
- plasma membrane transiently smaller (lose the apical surface of the cell)
- membrane added to regain original area
what is a holocrine method of secretion
- the secretory cell gradually fills up with secretory granules
- the cell organelles degenerate and the cells and nucleus die
- the plasma membrane breaks and the contents (secretum) empties
- dead cells are replaced by mitotic division of the basal cells
ie: disintergration of the cell and discharge of the whole cell into the extracellular space
note: the secretum in sebaceous glands = sebum
exocrine glands - unicellular vs multicellular
are either unicellular or multicellular
unicellular
the individual cells (eg goblet cells) of unicellular glands release their secretion (mucus) onto surface epithelium
multicellular
- have a duct system
- the glandular cells extend from the epithelial surface into underlying connective tissue
- they remain within the wall of the organ, or migrate to a distant area
- they discharge their secretory products (eg bile, digestive enzymes and saliva) into the lumen of organs via these ducts
what does mucus do
- lubricate the passage of materials (in the digestive tract)
- moistens the air (respiratory system)
- entraps inhaled dust and carbon particles (respiratory tract)
what are the different types of multicellular exocrine glands
- simple tubular eg large intestine. Secretory portion is straight tube formed by secretory goblet cells
- simple branched tubular eg gastric glands and mucus glands of the oseophagus, tongue and duodenum
- simple coiled tubular found in skin eg eccrine gland with secretory portion found deep in dermis. Eg merocrine sweat glands
- simple acinar secretory portion is an outpouching of the epithelial surface
- simple branched acinar duct is relatively short. found in stomach and also found as sebaceous (oil) glands
- compound tubular eg duodenum
- compound acinar alveolar shaped cells that are pyramid shaped and serous secreting eg pancreas
- compound tubuloacinar contain elements of both acinar and tubular glands, and can produce both serous and mucinous secretions eg submandibular salivary gland
tubular vs acinar vs alveolar
tubular
- longer, thinner shape
- ducts have a thin, tubular lumen
acinar
- round shaped gland
- thin, tubular shaped lumen
- pyramid shaped cells
alveolar
- round shaped gland
- large, round shaped lumen
note: acinar and alveolar glands look the same from the outside
types of hormone: steroid
synthesis and storage, transport in blood, location of receptor, response to receptor-ligand binding and examples
synthesis and storage
synthesised on demand from precursor cholesterol
transport in blood
bound to carrier proteins (lipid like)
location of receptor
cytoplasm or nucleus - some have membrane receptors too
response to receptor-ligand binding
when bind to nucleus… activation of genes for transcription and translation; may have non genomic actions
examples
oestrogens, androgens, progesterone, cortisol, glucocorticoids
types of hormone: catecholamines
synthesis and storage, transport in blood, location of receptor, response to receptor-ligand binding and examples
type of amino acid derived hormones, with thyroid hormones
synthesis and storage
made in advance by the adrenal glands; stored in secretory vesicles
transport in blood
dissolved in plasma
location of receptor
cell membrane
response to receptor-ligand binding
activation of second messenger systems
examples
adrenaline, noradrenaline and dopamine
types of hormone: glycoproteins and peptide hormones
synthesis and storage, transport in blood, location of receptor, response to receptor-ligand binding and examples
synthesis and storage
made in advance, stored in secretory vesicles
transport in blood
dissolved in plasma
location of receptor
cell membrane, regulated
response to receptor-ligand binding
activation of second messenger systems; may activate gene expression. Alter transcription + translation of gene.
examples
insulin, glucagon, prolactin, ACTH, PTH and gastrin
types of hormone: thyroid hormones
synthesis and storage, transport in blood, location of receptor, response to receptor-ligand binding and examples
type of amino acid-derived hormone, with catecholamines
synthesis and storage
made in advance, stored in secretory vesicles
transport in blood
bound to carrier proteins
location of receptor
nucleus
response to receptor-ligand binding
activation of genes for transcription and translation
examples
thyroxine (T4), triiodothyronine (T3) and reverse T3
types of hormone: lipid hormones
synthesis and storage, transport in blood, location of receptor, response to receptor-ligand binding and examples
synthesis and storage
synthesised on demand from precursors
transport in blood
dissolved in plsama and bound to carrier proteins (as they are lipids and are therefore hydrophobic)
location of receptor
cell membrane
response to receptor-ligand binding
activation of second messenger systems
examples
thromboxanes, prostaglandins, endocannabinoids
name the main endocrine glands in the body
location and secretions on seperate cards
- pituitary (anterior and posterior)
- thyroid gland
- parathyroid gland
- adrenal gland
- pancreas
endocrine gland: pituitary
location + secretions
location base of the brain
secretions
anterior
- TSH thyroid-stimulating hormone
- ACTH adrenocorticotropic hormone
- gonadotropins: FSH + LH ★
- GH growth hormone
- prolactin
posterior
- ADH antidiuretic hormone
- vasopressin
- oxytocin
★ FSH = follicle-stimulating hormone
LH = luteinizing hormone
endocrine gland: thyroid gland
location + secretions
location
anterior to the trachea (two lobes)
secretions
thyroxine (T4) and triodothyroinine (T3)
endocrine gland: parathyroid
location + secretions
location
lie on the dorsal surface of the thyroid gland (4 glands, 2 pairs)
secretions
parathromone (PTH)
endocrine gland: adrenal
location + secretions
location
top of each kidney (2 sections - medulla is inner and 2 cortex surrounds medulla)
secretions
cortex secretes corticosteriods (glucosteroids and minercoricoids)
small amounts of adrogen, oestrogen and progestin
endocrine gland: pancreas
location + secretions
location
left of and behind the stomach (both an exocrine and an endocrine gland)
secretions
- exocrine secretes digestive enzymes into the duodenum
- endocrine has cell clusters called islets of langerhans…
α islet cells produce glucagons
β cells secrete insulin
what is an example of a gland that is both endocrine and exocrine
the pancreas
what does the hypothalamus do
- located below the thalamus
- main regulator for everything and is central to homeostatis
- link between the nervous and endocrine systems
hypothalamus deals with
* thermoregulation, panting, sweating, shivering etc
* plasma osmolality via the osmoreceptors
* heart rate and blood pressure
* feeding, sateity, regulation of the GI tract
* circadian rhythms, sleep, hormones
* stimuli from the autonomic nervous system (both sympathetic and parasympathetic)
* emotion, sexual behaviour and mood
* lactation
what are some of the main differences between endocrine and exocrine glands
endocrine
- ductless
- secrete directly into blood flowing through them (allows the secretion to function at distant parts of the body
- secretions are called hormones
- all epithelial cells in the gland secrete the hormones
exocrine
- ducted
- secrete into a location of the body through a duct
- secretions are mostly enzymes or lubricants
- only cell at the apex of the duct secrete the products
describe the process of adenogenesis of exocrine glands
- growth signal recieved
- proliferation of daughter cells occurs and extracellular protein degradation enzymes produced
- epithelial cells invade space created
- central cells die off to produce duct aka canalicularisation
- link to mother cells remain
- significant amount of branching takes place
branching process on seperate card
describe the process of adenogenesis of endocrine glands
- Growth signal recieved
- proliferation of daughter cells occurs and extracellular protein degradation enzymes produced
- epithelial cells invade space created
- produce angiogenic factors to stimulate blood vessel growth in and around epithelial cells
- link to mother cells broken by apoptosis
- virtually no branching
how does branching occur
eg the lungs
- FGF10 (fibroblast growth factor) released by immatture fibroblasts (mesenchymal stem cells)
- epithelial cells move towards the signal
- two different fates…
- Tubule elongation (growth factor 1 active, GF2 inactive)
- Tubule branching (GF1 inactive, GF2 active)
elongation → branching → elongation → branching → elongation etc
elongation and branching stopped by Shh (Sonic the Hedgehog)
cells in exocrine glands
- epithelial cells lining the ducts
- epithelial cells that make the secretory products
some of the cell at the secretory ends of the ducts change morphology and class by turning into myoepithelial cells:
these are cells that have features of both an epithelial cell and a smooth muscle cell… these help to eject secretions from the duct, eg mucus that is sticky or in breasts to get the milk out
structure of the golgi apparatus
- stack of disc shaped cisternae
- one end of discs are flattened, the other concave
- discs have swellings at their edges
- distal swellings pinch off as migratory golgi vacuoles
golgi apparatus function
- transport through the sequential golgi apparatus cisternae
- packaging of sorted contents through condensation
- adding sugars to proteins and lipids glycosylation
golgi product destinations…
- majority extruded in secretory vesicles
- some retained for use in the cell (eg lysosomes )
- some enters the plasma membrane (glycocalyx)
glycosylation vs glycation
the covalent attachment of sugars by enzymes/without enzymes to proteins and lipids to form glycoproteins and glycolipids
glycosylation is with enzymes
glycation is without
what is the purpose of glycosylation (or glycation)
difference between two on different card
- to aid protein folding
- prevents protein digestion by intracellular proteases
- prevents lipid digestion by intracellular lipases
- cell recognition (blood groups)
- role on cell to extracellular matrix attachment
this is a critical function of the biosynthetic-secretory pathway of the endoplasmic reticulum and golgi apparatus
some disorders of glycosylation are known. all are rare and often lethal in utero
exocytosis vs endocytosis
exocytosis
secretion of molecules outside the cell via a vesicle fusing to a membrane
endocytosis
engulfing of molecules inside the cell via vesicle formation
phagocytosis vs pinocytosis
phagocytosis
the process by which cells (phagocytes) envelop or engulf other cells or particles. Mainly used by cells of the immune system.
pinocytosis
is the process in which lipid droplets are ingested by cells. Used by all cells, especially smooth muscle cells.
different types of transepithelial transport
aka transcytosis
- paracellular transport molecules may move (passive fuse and be filtered with water) through aqueous channels in the intercellular junction eg amino acids for hormone production
- …or through lipid cell membranes transcellular transport eg steroid hormones
- molecules with the appropriate characteristics may be transported by carrier proetins into or out of the cells (or by a counter-transport process) eg thyroxine transport across thyroid follicular cell
- those that are impermeable may also bind to cell surface receptors, be engulfed by the cell membrane (endocytosis) to be released inside cell, or expelled via vesicles outside of the cell and into extracellular space (exocytosis) eg cholesterol transport
types of glandular control
humoral
refers to the control of hormone release in response to changes in extracellular fluids (eg blood or the ion conc in blood)
eg a rise in blood glucose level triggers the pancreas to release insulin
hormonal
refers to the release of a hormone in response to another hormone
eg the hypothalamus produces hormones that stimulate the anterior portion of the pituitary gland, which in turn releases hormones that regulate hormone production by other endocrine glands
neural
in some cases, the nervous system directly stimulates the glands to release hormone
eg sympathetic NS directly stimulates adrenal medulla to release epinephrine and norepinephrine in response to stress
what is neurocrine communication
- neuron is a secreting cell
- electrical signal stimulates it to release hormones into the bloodstream
- the hormones travel through the bloodstream
- they reach a target cell with receptors for the hormone
eg in the hypothalmo-hypophyseal portal system… where trophic hormones synthesised by the neurones in the hypothalamus are carried to the anterior pituitary via the portal vessels
what are portal systems
- occurs when a capillary bed pools into another capillary bed through veins, without first going through the heart
- both capillary beds and the blood vessels that connect them are considered part of the portal sytem
- differ from the typical circulatory route of arteries → arteriole → capillary bed → venule → vein
portal system goes through 2 capillary beds before returning to the heart. Only 2 portal systems in human body:
- hypothalmo-hypophyseal portal system
- the hepatic portal system
the hypothalmo-hypophyseal portal system
- portal system of blood vessels that connect the hypothalamus with the anterior pituitary
- main function is to quickly transport and exchange hormones
- the capillaries in the portal system are fenestrated which allows a rapid exchange between the hypothalamus and the pituitary
development of the endocrine system
at 4 weeks gestation, no endocrine glands have formed, but from week 5 onwards…
- pituitary develops from neurohypophyseal bus and hypophyseal pouch
- thyroid develops from the floor of the pharynx (2nd pouch)
- parathyroid and thymus develop from 3rd and 4th pharyngeal pouches
- pancreas develops from the foregut
- adrenals develops from intermediate mesoderm and the neural crest
why is there always a small concentration of pituitary hormones in the blood?
short answer
eg TSH, GH, LH, ADH…
all of the pituitary hormones are subject to constituatice and regulated merocrine secretion
why is there always a small concentration of pituitary hormones in the blood?
short answer
eg TSH, GH, LH, ADH…
all of the pituitary hormones are subject to constituative and regulated merocrine secretion
this means that there are always some hormones being released via constituative secretion (continually released without stimulation) so there are always small concentrations of these hormones in the blood.
the pituitary gland
aka the hypophysis
- divided into two lobes (anterior and posterior)
- sits below the hypothalamus
- controls several of the other hormone glands
- endocrine gland
thyroid gland
- located at front of the neck
- has a left and right lobe (like two wings of a butterfly)
- made up of thousands of follicles that make T3 and T4
- in between follicular cells, there are parafollicular cells that produce peptide hormone calcitonin (independent of thyroid hormone synthesis)
calcitonin
- role is to monitor plasma concentrations and decreases Ca2+ levels (counteracts the fucntion of PTH)
- major effect: inibits osteoclast activity in bone (osteoclast breaks down bone, releasing calcium into bloodstream)
- minor effect: inhibits renal calcium and phosphate re-absorbtion in the tubular cells → more calcium and phsophate extreted, causing plasma calcium levels to decrease
note: high T3 or T4 = hyperthyroidism
low T3 or T4 = hypothyroidism
parathyroid glands
- on the back of each thyroid lobe, there are two parathyroid glands (one above and one below). There are 4 parathyroid in total
- constantly monitor the plasma calcium concentrations
- the chief cells make parathyroid hormone (PTH) which activates osteoclasts → more calcium in bloodstream
- PTH causes the bones to release calcium into the blood and prevents calcium loss by the kidneys; absorb more calcium from the GI tract
- negative feedback loop (calcium reaches set point and the parathyroid glands stop making PTH)
- fat cells are presesnt in parathyroid gland, as these provide energy
clinical consequences of parathyroid dysfunction
calcium is an important element for the nervous, muscular and skeletal system
allows nerves to conduct electricity and for our muscles to contract
hyperparathyroidism
- faily common
- too much PTH
- GI: loss of appetite, nausea, vomiting and constipation
- NS: fatigue, depression and confusion
- MSK: muscle weakness and bone/joint pain
- Urinary: kidney stones, increased thirst and urination
hypoparathyroidism
- rare condition and severe effects
- paraesthesia due to NS problems: tingling sensation
- muscle cramps
- fatigue
- coarse hair and brittle nails
- dry + rough skin
- twitching facial muscles
adrenal glands
medulla and cortex on different cards
- these are just above each kidney
- a mixed endocrine and neuroendocrine tissue
- each adrenal gland is ade up of an outer layer called the cortex, surrounding an area called the medulla
- the cortex can be further divided into three zones that produce hormones (more details on next cards)
the right AG: pyramidal shape
the left AG: crescent shape
adrenal glands are responsible for the stress response
adrenal medulla
- composed of parenchyma (functional part) of large, pale staining epitheliod cells called chromaffin cells
- blood vessels are present in medulla, not cortex
- the chromaffin cells are modified neurones
- chromaffin cells release catecholamines (adrenaline and noradrenaline) ★
- this area is slightly yellow due to the hormones they contain
- numerous myelinated, pre-synaptic sympathetic nerve fibres pass directly to the chromaffin cells which release their secretory products (adrenaline and noradrenaline)
- therefore, chromaffin cells are equivalent to post-synaptic neurones (an an example of neurocrine secretion)
★ adrenaline aka epinephrine
noradrenaline aka norepinephrine
other names for adrenaline and noradrenaline
epinephrine (adrenaline)
norepinephrine (noradrenaline)
both are catecholamines
adrenal cortex
THREE LAYERS
outer = zona glomerulosa
produces aldosterone, which regulates BP
middle = zona fasciculata
produces glucocorticoids (cortisone/cortisol), mobilises fats, proteins and carbohydrates
inner = zona reticularis
produces androgen precursors eg androstenedione ★, DHEA etc
★ goes onto make testosterone, progesterone etc
stress response
definition: maintenance of homeostatis in the prescence of stressors that require activation of a complex range of responses involving the endocrine, nervous and immune systems
stress: a state of real or percieved threat to homeostasis
activation of the stress response initiates a number of behavioural and physiological changes that improve an individual’s chance of survival when faced with homeostatic challenges
behavioural changes
- increased awareness
- improved cognition
- euphoria
- enhanced analgesia
physiological adaptations
- increased cardiovascular tone
- increased resp rate
- increased intermediate metabolism
decreased vegetative functions such as feeding, digestion, growth, reproduction etc
the principal effectors of the stress response are localised in the hypothalamus, the anterior lobe of pituitary and the adrenal
summary of adrenal gland responses to stress
adrenal medulla
SHORT TERM STRESS RESPONSE
- inc heart rate and bp
- liver converts glycogen → glucose released into blood
- dilation of bronchioles
- Δ in blood flow patterns leads to increased alterness
- inc metabolic rate
adrenal cortex
LONG TERM STRESS RESPONSE
mineralcorticoids stimulate
- retention of sodium and water by kidneys
- inc blood vol and pressure
glucocorticoids stimulate
- proteins and fats → glucose for energy
- inc blood sugar
- suppression of immune system
the pancreas
- sits in foregut, attached to curve in duodenum
- acinar glands grouped into lobules, and contain numerous zymogen granules
- connect through numerous intercalated ducts to the pancreatic duct
- pancreatic duct joins with bile duct to make common bile duct
- intercalated duct is lined with cuboidal epithelial cells
- consists of endocrine and exocrine components
endocrine components of pancreas
- surrounded by exocrine pancreas cells
- α cell secretes glucagon
- β cell secretes insulin
- δ cell secretes somatostatin (inhibitor of glucagon and insulin)
- islets of langerhans
other minor cells present
- PP cell: pancreatic polypeptide (inhibits bile, pancreatic enzyme and bicarbonate secretion)
- D-1 cell: vasoactive intestinal peptide (stimulates enzyme secretion and gut motility)
- EC cell: secretin (bicarb production), motilin (gastric and intestinal motility) and substance P (analgesia)
- E cell: ghrelin (increases feeding behaviour)
- G cells: gastrin (stimulates HCl production)
main exocrine functions of pancreas
produces
- trypsinogen which is converted to trypsin
- chymotrypsinogen which is converted to chymotrypsin
- lipase
- amylase
- ribonuclease
- deoxyribonuclease
- gelatinase
- elastase
