Saturday, 28 November 2015

The Islets of Langerhans

Living on an island gives me a small-small excuse to rabbit on about other islands as I have been doing all week. Argumentative SciFi writer Harlan Ellison wrote a rather challenging short story called "Adrift Just Off the Islets of Langerhans: Latitude 38°54'N, Longitude 77°00'13" W".  In which he pushed the imaginative idea that the Islets were a geographical feature. I'll save you a lot of trouble by noting that Washington DC is located at Latitude 38° 54' N, Longitude 77° 00' 13" W.  There is something romantic-sounding about the Islets of Langerhans, which I first met when studying high-school biology as a teenager. My mentor then - Mr Wilkinson - drew attention to this when we were learning about the pancreas, of which the Islets are an important part.
In the same course he also introduced us to the involucre of bracts, which is are the ring of green leaf-like things supporting the coloured part of daisy Bellis perennis and dandelion Taraxacum officinale flowers and other members of the Compositae Asteraceae. Actually he introduced us to the Involucre of Bracts, which/who might have been an obscure Highland prince like Lord of the Isles, Moncreiffe of that Ilk or the Laird of Glenbogle.  You should know that the inflorescence of Asteraceae is not a single flower with stigma, stamens, petals and sepals but a sweep of tiny separate flowers.  In the daisy there are two sorts: the yellow disc-florets in the middle and the peripheral ray-florets with one enormously exaggerated petal pointing outwards.  Have a close look next time you're making daisy-chains with your daughters.

The Islets are tiny blobs, barely visible to the naked eye (0.1mm-ish or about the size of a human egg) that look a little like warts on the edge of small blood-vessels in the pancreas.  Though they be but little, they are numerous and if all 3 million of them could be scraped into a tea-spoon, they would weigh about 2g.  Each one consists of a number of identical looking cells which have very different functions. As far as we can tell, each cell is specialised for producing one hormone but several different hormones are produced in each islet. You can use a technique called immune-histochemistry to attach different coloured markers to these hormones [L] which are all small proteins. That creates a false colour image - the islets are pinky-beige in real life - which is rather informative. Here [L] insulin has a green fluorescent dye attached while glucagon has a reddish tag.  You can see two things: one is that the insulin producing cells far out-number those making glucagon and also that the glucagon cells are located on the periphery of each islet.

If you're anything like me - expensively but inadequately educated - you have an excuse for never having heard of glucagon but you're reading the wrong blog if 'insulin' is a totally new word to you. In teaching Human Physiology for the last three years, I've learned a lot that I didn't know or about which I was wrong.  The theme in Hum.Phys is homeostasis, maintaining everything - core temperature, acidity, blood-pressure - in equilibrium.  Insulin and glucagon do this for blood-sugar levels: insulin reducing it and glucagon increasing it.  As I tried to explain about haemophilia and Factors VIII and IX, the normal body manages such things with exquisite subtlety 24/7 while pharmaceutical/medical surrogates are bludgeon-crude by comparison. It's like that with insulin therapy for diabetics: they inject far too much in one go and over the next several hours the concentration in the blood trails down to rather too low when the body receives another deluge by needle.

Mais revenons nous a nos îlots: there are two types of diabetes: type I occurring when the pancreas doesn't produce enough insulin; type-II when the pancreas is chuntering along nicely doing its job but the body fails to react appropriately to circulating insulin. It seems likely that type-I diabetes is an autoimmune disease, in which the immune system convinces itself that beta-cells in the Islets of Langerhans are 'foreign' and need to be taken out.  As the beta-cells produce the insulin on which glucose metabolism depends, this is response turns out to be a costly mistake. A lot of auto-immune disease occurs as a consequence of a viral or bacterial infection.  When the pathogenic insults has been beaten up and swept away, the immune cells are still spoiling for a fight and turn on some part of the body with a superficial resemblance to the ex-parrot-parasite. This is the best explanation for Guillain–Barré syndrome's appearance after a bout of the trots from Campylobacter food-poisoning.  It you wake up screaming tonight knowing the identity of the virus that causes type-I diabetes, and you are correct, you can sit back a wait for the Nobel Prize.

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