What are the seven endocrine glands?
7. Endocrine pancreas
(8. ovaries & testes)
What is the main structural difference between the endocrine system and exocrine systems that affects the ways in which their products are distributed around the body?
The endocrine system is comprised of DUCTLESS glands, made up of secretory cells that secrete hormones into the interstitial space, where they are rapidly absorbed into the bloodstream (vascular system).
Eg, In the pancreas, the different secretory cells of the Islets of Langerhans secrete hormones eg. insulin, glucagon, somatostatin & ghrelin into the blood stream.
The exocrine system, by contrast, is made up mainly of secretory cells that secrete their products, such as enzymes, into ducts, which enables them to secrete directly into the target area.
Eg. In pancreas, pancreatic acini secrete digestive enzumes eg. trypsinogen, chymotrypsinogen, carboxypeptidase, pancreatic amylase via pancreatic duct directly into duodenum.
What are the 11 tissues and organs that have endocrine functions?
What are the four main classes of hormones?
What is the structure of protein & peptide hormones?
- made from (>1) amino acids
- can be big or small
- water-soluble (unlike steroids)
- large hormones like insulin & GH can exhibit species differences
- linked by peptide bonds, with amino group at one end & carboxy at another
- ring structure linked by disulphide bridges present in some mature hormones
How are protein/peptide hormones transported in the bloodstream? Bound or unbound to plasma protein?
Unbound to plasma protein.
Is synthesis of peptide & protein hormones fast or slow?
Slow because they need to be transcribed, translated, modified then packaged.
What type of receptors do peptide hormones & protein hormones bind to?
Cell-surface, trans-membrane receptors
(N-terminus on cell surface, C-terminus inside cell):
1. G-protein coupled receptors (GPCR) - often interact with adenylate cyclase to produce cAMP from ATP
2. Receptor tyrosine kinase (RTK)
What are some protein/peptide hormones?
Thyroid-releasing hormone (TRH)
Vasopressin (aka AVP & ADH)
What is the basic structure of steroid hormones?
- three benzene rings + 1 five-membered ring
- common cholesterol precursor (27 carbons)
- lipid soluble
- large or small
Are steroid hormones typically transported bound or unbound to proteins?
Bound to binding proteins.
What distinguishes amine hormones from peptide hormones?
Amine hormones are made up of only ONE amino acid, while peptide hormones are made up of >1 amino acid.
Amine hormones are made up of either tryptophan (ideoleamines) or tyrosine (catecholamines, thyroid hormones).
Amine hormones are small. Peptide hormones can be large or small.
Amine hormones can be lipid insoluble (catecholeamines, idoleamines) OR lipid soluble (thyroid hormones). Peptide hormones are only LIPID INSOLUBLE/WATER SOLUBLE.
Amine hormones can bind to cell-membrane receptors (catecholeamines, idoleamines) OR intracellular receptors (thyroid hormones). Peptide hormones can ONLY bind to cell-membrane receptors.
What is the structure of catecholeamines?
Small - no species variation
One amino acid: tyrosine
Lipid insoluble - do not enter cells, thus bind to cell-membrane receptors such as GPCR
What are some examples of catecholamines?
What is the structure of idoleamines?
Small - no species variation
One amino acid: tryptophan
Lipid insoluble, so doesn't enter cells
Binds to cell-membrane receptors like GPCR
How do thyroid hormones differ from other amine hormones, such as catecholamines & idoleamines, and how are they similar?
Thyroid hormones, while categorised as amines because they are derived from tyrosine (like catecholamines), act very differently than the others in this class. In fact, they much more resemble steroid hormones in structure & function.
Thyroid hormones, like catecholeamines & idoleamines, are small, so there is no species variation. Nearly 100% travel bound to carrier proteins in the bloodstream.
But thyroid hormones are LIPID SOLUBLE, so they can enter cells, and bind to intracellular receptors, often inside the cell nucleus. Catecholeamines & idoleamines are lipid insoluble, so they can't enter the cell and thus bind to cell receptors in the plasma membrane.
What are some examples of idoleamines?
Seratonin (5-HT hydroxytryptophan)
What are some examples of thyroid hormones?
What are eicanosoid hormones made from?
Arachidonic acid (AA) in membrane phospholipids
How are eicanosoids synthesized from arachidonic acid (AA)?
membrane-bound enzyme phospholipase-A2 (PLA2), present in inactive form in lysosomes, releases AA → cyclooxygenase (COX) or 5-lipoxygenase (5-LOX) convert AA into prostaglandins (PG) or leukotrienes (LT), respectively → downstream enzymes change PG to different forms
Give some examples of eicanosoids.
PGE2 - softens cervix, causes uterine contraction, effectively stimulates progesterone secretion
PGF2α - promotes luteolysis, ie. degradation of corpus luteum, effectively reducing progesterone secretion & ↑ FSH secretion
Thromboxane (Tx) - clotting factor
Do eicanosoid hormones diffuse out of cells after they've been synthesized, or are they actively secreted?
They diffuse out of cells.
There is NO ACTIVE SECRETION.
They are transported UNBOUND to proteins in the circulation.
Contrast the synthesis of peptide hormones & steroid hormones.
Synthesized via “classical pathway” of mRNA transcription & translation:
translated in cytoplasm as long polypeptide pre-prohormone → “processed” in ER into prohormone ie., cleavage of N-terminal signal and/or glycosylation, which protects it from metabolism → packaged & stored in vesicles
Synthesized mainly in adrenal cortex, gonads, placenta
Cleavage of cholesterol at C21 & C22 result in core structure of 3 benzenes + 1 five-membered ring.
Contrast the secretion of peptide hormones & steroid hormones.
EXOCYTOSIS (regulated or constitutive)- secreted via exocytosis (need ↑[Ca2+]) → endopeptidase cleaves end of prohormone to facilitate folding into mature hormone just before entry into blood stream
What is synthesized diffuses out of cells
ie., NO DISTINCTION BETWEEN SYNTHESIS & SECRETION.
Do steroid hormones have rapid or slow onset of activation?
Slow. They have to enter the cell then bind to receptors in the cell nucleus.
What are examples of steroid hormones?
What is the role of the circulation in regulating the activity of hormones?
Endocrine (eg., insulin, aldosterone) & neuroendocrine hormones (eg. vasopressin, oxytocin) travel through the circulation from their site of synthesis to reach target cells in distal parts of the body.
Both negative feedback and positive feedback loops in endocrine signalling use the bloodstream.
Paracrine, autocrine & neurocrine hormones do not use the blood stream.
Paracrine hormones act locally (eg., insulin-like growth factors IGFs produced by liver, on adjacent or nearby cells within the same gland, diffusing through the extracellular space)
Autocrine hormones at on themselves, binding to receptors on their own surfaces (eg. prostaglandins)
Neurocrine hormones diffuse from a neurone into the synaptic cleft to affect an adjacent target cell (eg., Ach, norepinephrine, dopamine).
Describe the development of the hypothalamus.
In the early embryo, neuroectoderm of the forebrain (prosenecephalon), the primary brain vesicle, divides to form two secondary brain vesicles, telencephalon (endbrain, cortex) and diencephalon. From the diencephalon ventro-lateral wall, intermediate-zone proliferation generates the primordial hypothalamus.
The developed hypothalamus is located in the middle of the base of the brain, and encapsulates the ventral portion of the third ventricle.
Describe the development of the pituitary gland, aka hypophysis.
The hypophysis is an amalgam of two tissues. Ultimately, the two tissues grow into one another and become tightly apposed, but their structure remains distinctly different.
1. Anterior - Adeno - Upward Ectoderm
Early in gestation a finger of ectoderm grows upward from the roof of the mouth. This protrusion is called Rathke's pouch and will develop into the anterior pituitary or adenohypophysis.
2. Posterior - Neuroectoderm - Not upward
At the same time that Rathke's pouch is developing, another finger of ectodermal tissue evaginates ventrally from the diencephalon of the developing brain. This extension of the ventral brain will become the posterior pituitary or neurohypophysis.
What are the two ways peptide hormones, after being packaged by the Golgi apparatus into vesicles, are secreted, and which is the more common secretory mechanism?
The cell stores hormone in secretory granules and releases them in "bursts" when stimulated. This is the most commonly used pathway and allows cells to secrete a large amount of hormone over a short period of time.
The cell does not store hormone, but secretes it from secretory vesicles as it is synthesized.