We are in the middle of a two-day vacation for the Italian and Milanese holidays of the Festa di Sant’Ambrogio and Immacolata, here at the International Medical School at the University of Milan. We have finished just over one month of studies, and within this time, learned some amazing concepts. Here are three examples.

my notes on the thymine dimer

1. The Thymine Dimer

People have been saying since my childhood (and before) that too much direct sunlight can cause skin cancer. However, I had never quite understood how it does. Why would light — the same stuff that comes out of lamps and campfires, the stuff we need to, well, see the world — possibly be harmful to our skin? After all, light is not a dangerous chemical that you can smell and taste, nor like a bacterial invader, which is living and finagles some way into our bodies. Light and the sun are both things that poets praise, metaphors for happiness and truth .

Alas, light, you may know, is made of photons. Photons are little packets of energy that, when they hit atoms and molecules, can alter them and the connections between them. It just so happens that ultraviolet light — which contains photons at a particular wavelength, and therefore, energy — is able to create a connection between two molecules in our DNA, either thymine or cytosine. (You may recall that DNA is made from four basic molecules, A, G, T, and C: adenosine, guanine, thymine, and cytosine. Thymine and cytosine are the two pyrimidines, which is why thymine dimers are sometimes referred to as part of a larger group of pyrimidine dimers.) When these connections form a dimer — di, two-part; mer, shape — the DNA breaks from its usual structure. Damaged DNA is at the origin of cancer. Sunlight contains ultraviolet light — you cannot see it, but it is there — and hence exposure to too much sunlight can cause cancer. In my notes, the vertical orange lines on the right are the connections betewen two hexagonal-shaped thymine rings, within the yellow highlights.

One of the satisfying things about medical school is that you learn explanations, for things you have been wondering about your whole life.

What's unusual about this picture?

What’s unusual about this picture?

2. People Whose Organs are a Mirror-Image of the Usual Orientation

Way, way early in your embryonic development, in fact, three weeks after you were conceived, you went through a process called gastrulation. During gastrulation, you developed, basically, three layers of different kinds of cells. Each of these layers would eventually go on to form a different set of organs, and these organs would, probably, end up in the proper places in your body, with the liver towards the right, heart towards the left, and so on. During gastrulation, an embryo becomes asymmetric, which is a good thing, because the human body, on the inside, is not symmetric. (Why we are symmetric on the outside, but not on the inside, is also a fascinating thing to ponder.)

One part of the process of becoming asymmetric involves a structure in the embryo called the primitive loop. It is thought that during gastrulation, the loop rotates in a certain clockwise pattern around the rest of the embryo. This is driven by the action of lots and lots of tiny cilia, which are hair-like projections on cells, which themselves beat clockwise. Now, very rarely, probably due to genetic reasons, the cilia beat in the opposite direction, or do not beat correctly, and the loop rotates counter-clockwise. The result is that from that very early third week in development, all the internal body parts end up reversed left-right.

And so, in fact, about one out of every 10,000 people has this condition, called situs inversus. Some of my classmates had heard about it, pointing out that NBA player Randy Foye is known for his situs inversus. Luckily, doctors say, Foye can still play basketball with no problems. That’s because people like Foye with situs inversus totalis, in which every organ is reversed (as opposed to only some), tend not to suffer major health problems as a result, precisely because all the organs are still oriented relative to each other, exactly where they should be.

It had never occured to me that there are people such that the insides of their bodies are laid out exactly in the opposite way as the vast majority of us. When our professor pointed this out, I gasped audibly.

the relative lengths of body segments, according to The Standard Human

the relative lengths of body segments of The Standard Human

3. The Standard Human

The Standard Human is not precisely something that we learned; it is more an idea, that comes from a textbook, namely Physics of the Human Body by Irving Herman, a professor at my alma mater, Columbia. Herman’s book is an elegant overview of biophysics, and has a lot to recommend it. It is concise, comprehensive, funny now and then, and beautifully typeset in LaTeX.

A point that Herman raises, at the beginning of the book, is this: for the next 12 chapters, we are going to be doing all kinds of quantitative studies of the human body (the kinds of analyses that we do in our biophysics class at IMS). How does air pressure vary in the lungs? What are the boundaries of our temperature regulation? How strong are the forces on your ankles when you are running?

Since we are going to be measuring and calculating and counting over and over and over again, Herman posits, it makes sense if we just define early on, in chapter one, exactly what the dimensions are, of the human who is going to be subjected to all this analysis. So, he comes up with The Standard Human. “We will often,” he writes, “but not always, model humans assuming numerical values for mass, height, etc., of a ‘standard’ human, a 70 kg man with parameters similar to those” in a whole bunch of tables which then follow in the text. The values “typify American males in the mid-1970s”, and so we end up with pages and pages of statistics related to our bodies, that probably most of us never even thought to measure. Herman does not just lay out the basics, like a heart rate of 65 beats/min or a lung capacity of 6.0 L. The Standard Human’s foot length is 15.2% of his height. His head and neck weigh 8.1% of his total mass, and, with a density of 1.11 g/cc, are slightly denser than the leg (1.06) and slightly less dense than the hand (1.16). The Standard Human can rotate his wrist forward (like a slap) 90°, but backward, 99°, and his hand’s center of mass is 50.6% from the proximal end (the wrist), thus 49.4% from the distal end (the fingertip). His bladder has a volume of 140 cc; his fingernails and toenails, 0.9 (all 20 together). He has a layer of fat, under his skin, of 5 mm.

The Standard Human is fascinating for a bunch of intertwined reasons. Have you ever stopped to consider the volume of your toenails? It is a bit strange — but not unhelpful — to think that there is a standard human at all. How lovely and oddly abstract, the idea that people, perpetually competing about superficial details like who is the strongest or the tallest or the most beautiful, can simply be averaged out into one standard who, somehow, represents all of us.

Except that he does not. The Standard Human is an American male from the 1970s. Anthropologically, it is intriguing that the author made this choice. Why male? Was this purely arbitrary (could he have equally chosen female; or an intersex standard)? Why American? Is it because these were the measurements that a particular set of anthropometrists were able to sample, way back when? And what has changed since the 1970s (probably fat!)? We can wonder about other characteristics of this textbook’s Standard Human, this American male from the 1970s. Does he speak English? Does he like peanut butter? What did he think of The Bad News Bears?

Let us be clear and generous toward Herman. He is not necessarily to be faulted for choosing this particular standard. The author explains very explicitly that these statistics apply to a particular biological sex, region, and period in time. When I look around my classroom at the University of Milan’s international English MD program — one of the, if not the, most diverse medical school classes in the world — The Standard Human seems a magnificent reminder of the elements that we all have in common. He also, however, does not typify us.

Erik Campano (Milan)

Erik Campano (Milan)

Blog Editor
Erik Campano (Milan)