Thursday 6 November 2014

How many bones?

The expensive education which I am never tired of boasting about has brought me no little success in the mildly competitive world of PubQuizLand.  Cases of wine, boxes of chocolates and folding money have come back to teams of which I was part. Following an honorable tradition, we always return the cash to The Cause. PubQuizLand employs a restricted view of unqualified truth - there is one correct answer and it is considered bad form to quibble.  The poor organizers after all are providing an evening of entertainment and raid compilations of pub quiz questions PQQs without troubling to stop by Snopes or other debunking sites.  Now don't be annoying me by saying that elephants have four knees. It's bollix. No, I don't mean that elephants have four of them either. Bones? Segue!

PQQ. How many bones in the human body?
PQA. 206 . . . excepting when there aren't . . . and . . . accepting a bunch of conventions.

At The Institute, we're caught in a cross-fire of days off.  Monday Bank Holiday one week followed by Thursday and Friday off the following week for degree conferring. This throws classes into disarray if you have lab sections for the same class on different days. In Cell Biology we decided to intercalate an extra class for the classes that were being held: in two weeks we'll all be back on the same schedule.  What to do with those extra hours?  We've just finished an investigation of  a sheep's humerus coupled with a microscope investigation of bone structure. With skeletons on my mind, I thought we could usefully question the number of the bones = 206.  And it is delivers some interesting answers . . . and some less interesting because a bit more obvious.

Oscar "Trigger" Pistorius, for example, has a lot fewer bones than you and me partly because he was missing his fibula when he was born - medicos call this fibular hemimelia.  One solution is to amputate the dysfunctional limbs and design bionic legs, so Pistorius is 206 - 52. Similarly, some of the misfortunate people whose mothers had morning sickness in the 1960s, finished up missing several to many limb bones because their mums quelled the nausea with thalidomide.   But those tragedies can be binned as abnormal, partly because they are rare: fibular hemimelia has an incidence of 1-9:100,000 live births; there are maybe 6000 cases of thalidomide induced dysmelia. But also because they are obvious. All the shocking cases of landmine limb-loss are similar.

But what about polydactyly - the presence of extra digits?  I spent a decade looking at the distribution of this oddity among the cats of New England and the Canadian Maritimes - indeed I wrote a PhD thesis on the topic.  So I perk up a bit when the word hops into view. It turns out that polydactyly is surprisingly frequent - about 1:150 live births among black Africans; although much less common (1:1300) in Europeans.  The most common manifestation is extra fingers beyond the pinkie - called ulnar polydactyly and this is dealt with in the same way as we deal with the testicles of our rams lambs. With the lambs a thick rubber band is applied; with the unwanted digit it's a suture ligation.  A piece of string is tied so tight that blood-supply is halted and soon enough the finger drops off.  Better than than being called a freak in primary school and having your head flushed down the toilet.

If you're a Southern Baptist and went to Biblical Literalism School, you might hold that women have an extra rib because Genesis 2:22 says "And the rib, which the Lord God had taken from man, made he a woman, and brought her unto the man." Or man has one fewer . . . or the other way round. Empirically, from the data, I'm sorry to say that this is just not true.  But it is true to say that we have a variable numbers of ribs. Normally there are 12 pairs each attached to the thoracic vertebrae and most of them attached at the other end to the sternum.  AFAIK this is the definition of a thoracic vertebra.  Above those 12 thoracic vertebrae are 7 cervical ditto.  Occasionally (1:200 - 1:500 live births), the lowest of these (C7) has one or a pair of delicate little ribs . . . which clutter up the rather constrained space there, but are generally harmless. They turn out to be more common in women than men. Vertebrae? Segue!

The "normal" complement for the spine is: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 4 coccygeal vertebrae (N=33) but this complement is found in only about 20% of individuals studied.  4/5 of us have some sort of variant. "In one study of 748 vertebral columns, 717 had 17 thoracolumbar vertebrae, 26 had 18, and five had 16."  Lots of variation found in the sacrum and coccyx as well.  Weirdly the cervical vertebrae are  always seven in number, not only in humans but also in all 4,500 species of mammals including giraffes Giraffa camelopardalis and pygmy shrews Sorex minutus.  And of course "treasure your exceptions" this statement also has three known exceptions manatees Trichechus spp. and two genera of sloths Bradypus and Choleopus which have broken the rule and lived to be dissected by comparative anatomists.

Now here's an oddity.  The 206 bones in the human body doesn't include the teeth. Damned if I know what pedantic reason is put forward for this: probably that teeth aren't bones.  Any fule kno that humans have two sets of teeth: a deciduous or 'milk' set which comes gradually in before kids can walk and fall out one after the other, and are taken away by the tooth-fairy, sometime before puberty. This is variable: Julius Caesar is reputed to have been born with erupted teeth.  There are 20 of these baby teeth symmetrically arranged top/bottom and left/right: 2 incisors / 1 canine / 2 molars. Adult teeth are a) bigger and b) more numerous: 2 incisors / 1 canine / 2 pre-molars / 3 molars, to give a total of 32. That must be true ! It features in The Hobbit in the riddle-war between Bilbo and Gollum:

32 white horses upon a red hill

Now they tramp
Now they champ
Now they stand still.
What are they?
But this too is nonsense and I was able to demonstrate that with data in two Cell Biology classes. I asked everyone, including self, to count with somebody's tongue or finger how many teeth they had. In particular to count the number of erupted 'wisdom' teeth:
3rd molars
Nearly 2/3rds of us didn't have the full complement of adult teeth. The conventional explanation is that our common ancestor with the great apes had a sticky-out "prognathous" jaw and for a variety of reasons to do with social behaviour and sexual attractiveness, we humans have a much shorter jaw in which to insert the requisite number of teeth. What to do?  First delay the eruption of some of these fangs; second adopt a developmental pathway that stops the teeth developing at all at all. Come back in 10,000 generations and you'll find the normal adult dentition is closer to 7+7+7+7 or even fewer. I think that's way cool - to have first-year students doing some real "nobody's been here before" science and coming up with something unexpected. With great good fortune, these same students are doing a parallel course in "Anatomy" and the same week they had been told about variation in the presence of  the palmaris longus tendon in the wrist which is missing in 10-20% of normal adult humans.  You can diagnose yourself for presence/absence.  If it's missing in so many people without obvious handicap, you might conclude that this muscle is redundant.  It is often used (if present) for spare-parts reconstructive surgery.  That's cool too.

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