Lecture 37 Flashcards
Blood supplies in endocrine and exocrine pancreas
Each Islet of Langerhans receives blood through an INSULOACINAR PORTAL SYSTEM forming a network of fenestrated capillaries
The exocrine pancreas receives blood:
- from an independent acinar vascular system
- from the islet capillaries
- Each pancreatic acini has its own blood supply via surrounding capillaries. Ep cells in the acini retrieve important components from
capillaries to make enzymes. The enzymes leave through duct system through the apical lumen at centre of pancreatic acini. This is called ACINAR VASCULAR SYSTEM. - Pancreatic acinar cells also receive blood from the insuloacinar portal system. An
arterial forms a capillary network within an islet of langerhans and the venules then form capillaries that join the vessels surrounding
the pancreatic acini. Some components made by cells in the islet of langerhans are delivered to the pancreatic acini.
2 major cell types in islet of Langerhans
Alpha cell: 15-20%, glucagon, increase blood glucose
- if we are hungry, glucagon is released to increases blood glucose levels
- mostly at periphery of islet
Beta cell: 60-70%, insulin, decrease blood glucose
- sugar-rich meal causes insulin to be released, and causes the sugar to be stored in the form of glycogen
- mostly at centre of islet
Alpha and beta cells are not segregated by any CT. Alpha and beta cells have junctions bw one another and can sit next to each
other.
Insulin and glucose processing in Beta cells
Beta cells have transporter called GLUT-2 that puts glucose into cell cytoplasm
If glucose rises in the blood stream and enters beta cells, it triggers release of mature insulin that is stored in the vesicles. Insulin then floats in systemic regulation and counteracts the rise in glucose. If this is sufficient to lower the glucose levels, then
nothing else happens
But if there is continuous influx of glucose into cell, then it triggers the synthesis of further insulin molecules:
- insulin is polypeptide made in RER, and has a signal sequence, A chain, C peptide and B chain
- mature insulin only has A chain and B chain
- C peptide will be removed from molecule after disulfide bonds are formed bw alpha and beta chains, which occurs in golgi
apparatus - then the c peptide is eliminated by proteases within the Golgi apparatus. As a result, we have vesicles that contain the
mature insulin molecule, plus the C peptide section. Both will be released - Insulin is metabolically very active and has short half life. After it does its activity, it is destroyed
- C peptide is more stable. If you want to find out how efficient your islet of langerhans make insulin, it is better to look at how much C peptide is made bc it hangs around longer, rather than looking at insulin conc which breaks down quicker
- no metabolic function for the C peptide
Pineal gland - general and in animals
Important for mammals that have circadian
rhythms that are different from human
Human circadian rhythm is controlled by hormones too but not to the extent where it is in other animals.
Polar bears hibernate. It is related to food supply.
Grazing sheep reproduce only at times when there is abundant grass growing. Not only is food supply linked to their daily activity, but also to their reproductive activity. This is related to the changes in light and dark cycles and through the production of a melatonin hormone made in pineal gland.
Pineal gland is an outgrowth of the ep that surrounds the third ventricle and is part of the diencephalon.
Pineal gland - development
- A dorsal neuroepithelial evagination of the diencephalon forms the pineal gland
- cells migrate outwards and form folded structure - The neuroepithelial tissue folds and form compacted gland that is surrounded by capsule
- Cells differentiate into Pinealocytes (make melatonin) and interstitial glial-like cells (holds things together)
H and E: pineal gland
Pineal gland is cone-shaped. Pineal gland looks homogenous. All cell types are fairly closely packed
Brain sand (deposits of calcified carrier proteins). Often hormones are transported through carriers through the body; Some of them congregate or aggregate and are called brain sand. Brain sand has no metabolic function. Can have brain sand in brain and still make good enough hormone
Pinealocyte nuclei are large round with lots of euchromatin
Interstitial nuclei are black, highly condensed, full of heterochromatin.
Brain sand landmark
EM: white deposit allows the person who examines the brain to know exactly where the centre of the brain. Pineal gland is
centre of brain
Melatonin regulates circadian rhythms
Melatonin regulates circadian rhythm, reproductive activity of many mammals, wake/sleep cycles. Human reproductive activity does not depend on melatonin. But circadian rhythm important with melatonin; people take melatonin to regulate jet lag
- Pinealocytes synthesize melatonin from tryptophan.
- Melatonin is transported along cytoplasmic processes which terminate along capillaries. Melatonin is released into capillaries
- Axons from sympathetic nerve fibers originating in the superior cervical ganglia innervate pinealocytes and regulate melatonin production. This is along the spinal cord
How does pineal gland “see” light?
- Light signals are transmitted to the Suprachiasmatic nucleus via the Retinohypothalamic tract
- Signals are sent to the thoracic spinal cord by the Hypothalamospinal tract
- Signals are sent to the Superior Cervical Ganglia
- Postganglionic sympathetic fibers alongside the carotid artery convey signals to the Pineal Gland
- Darkness stimulates melatonin production and light suppresses it
Galactorrhea
inability to produce milk
Amenorrhea
inability to have menstrual cycle
