Secrets of the Human Body. Andrew Cohen

      The long bones of the human skeleton.

      It’s only once you reach adulthood that the activity in this area stops due to a process of programmed cell death (oddly controlled by oestrogen, the female hormone, in both boys and girls) and the growth plate closes and stops growing. The old growth plate becomes visible on X-rays as an epiphyseal line, a faint scar notched into your bones that you will carry for the rest of your life. At this point, bones can no longer elongate, growing any taller is now impossible.

      BRAIN–BODY INTERFACE

      The full story of your miraculously extending bones starts far away from your skeleton. Nestled deep inside the centre of your brain, just behind your eyes, is a structure no bigger than the size of an almond, called the hypothalamus. It is from here that growth is controlled.

      Your brain facilitates your conscious desires by sending signals to your muscles. This is your ‘somatic’ nervous system, the one that allows you to consciously move about, to speak, to look at things. But, in parallel, you have another subconscious, or autonomic, nervous system governed largely by the hypothalamus. It integrates more data than it’s possible to calculate, from all your sense organs, your memory and experience, your cerebral cortex and amygdala, and it uses this data to control functions of your body that you likely take for granted. Digestion, heart rate, sweating, the size of your pupils and also growing. The hypothalamus is the link between the brain and the body.

      Part of this regulation is control of the body’s hormones or endocrine system. The hypothalamus secretes hormones itself which include vasopressin (which controls thirst and water reabsorption by the kidneys) and oxytocin (the ‘love’ hormone, which has a range of effects including stimulation of milk secretion and uterine contractions).

      But most of your endocrine or hormone system is located around your body in specialist endocrine organs, like your thyroid, gonads or adrenal glands. The hypothalamus controls these organs remotely through a cascade of hormone signals sent first to the pituitary. The pituitary gland is around the size of a pea and dangles beneath the hypothalamus from the underside of your brain, on a stalk. It sits behind and between your eyes resting in a little bowl of bone in the base of your skull called the sella turcica or Turkish seat. Via this tiny organ your hypothalamus controls your reproduction, sex drive, lactation, metabolism and of course your growth.

      It’s not a simple process. The hypothalamus secretes the unimaginatively named growth hormone releasing hormone (GHRH) or growth hormone release inhibiting hormone (GHRIH). These in turn signal to the pituitary to release or stop releasing growth hormone. Growth hormone then directly acts on the cells of your body, instructing them to divide, and it stimulates the liver to produce insulin-like growth factor 1 (IGF-1) which also makes you grow. It takes the brakes off cell division and causes the growth of almost every cell in the body.

      Levels of IGF-1 and growth hormone can be affected by a huge range of processes which feed into the hypothalamus: insulin levels, disease, protein intake, stress, genes, physical fitness and sex hormones.

      We see this kind of signalling cascade with almost all biological processes, whether it’s the immune signalling pathways inside cells that Chris studied in his PhD, or the whole body cascades of chemical signals from organ to organ. They allow for delicate control of biological processes at multiple levels, with each organ feeding back information to regulate the process. They are also remnants of our evolutionary past. As organisms became more complex, it was easier for evolution to add another layer of control than to redesign from scratch. As we’ve seen in other chapters, we still have ancient systems in our modern bodies but with extra lines of code to allow for more regulation.

      We all produce growth hormone (and thus IGF-1), every day throughout our lives. In adulthood the average healthy individual produces about 400 micrograms a day (a scarcely visible amount), and it plays a crucial role in the maintenance and renewal of our bodies as well as controlling a host of other bodily functions. In children and teenagers, the levels of growth hormone are much higher, reaching 700 micrograms a day in the midst of our most rapid periods of growth, and it’s these levels that drive the process in the growth plates of young bones. In this way, through the cascade of hormones from the brain, which travel through the blood vessels of your body to command the cells in the growth plates of your long bones to divide and push those bones a little longer, millimetre by millimetre, you grow.

      As is often the case in medicine, we have understood how body systems function in healthy people by studying those for whom these systems have gone wrong. Dwarfism occurs when IGF-1 is not produced or when the receptors on the cells’ surfaces that should detect it are absent or defective.

      Conversely, tumours of the pituitary may secrete excess growth hormone, and if this happens in childhood prior to fusion of the epiphyseal plates, then gigantism results. Although these tumours are extremely rare in childhood, they have produced two extremely well-known actors, including Richard Kiel (the infamous ‘Jaws’ villain from two James Bond movies) and Andre the Giant, a wrestler and actor from The Princess Bride.

      As well as being giants over 7 ft (213 cm) tall, both of these stars exhibited the other effects of excess growth hormone secretion, a condition called acromegaly. If growth hormone secretion occurs after the long bones have fused, then you can’t grow any taller, but bones and other tissues continue to grow. The brow ridge and jaw thicken, the tongue and hands become vast and thick, and the voice deepens. In athletes using growth hormone as an illegal performance-enhancing drug, jaw changes often necessitate orthodontic braces to realign teeth – a subtle tell-tale sign for doping.

      It’s a beautiful, complex cascade that we have been able to understand in greater and greater detail through the revolution in molecular biology and genetics over the last 50 years, but one particularly strange thing about our growth through childhood and adolescence has remained a mystery. Unlike any another primate, we have a very odd pattern of growth through to adulthood. As we’ve already seen in this chapter, the first six months of life witness the most rapid period of growth, but then this slows dramatically through the next 10 years – a time we humans call childhood. Unlike any of our nearest relatives, including chimpanzees and bonobos, we grow at a fraction of the maximal rate through this period. It’s as if the race to adulthood is on hold, until suddenly we burst into activity again around the age of 10 as we experience the growth spurt of puberty. The mystery is why. Why do we all follow this oddly stunted pattern of growth? In the last few years an intriguing hypothesis has emerged to explain the biological oddity we call childhood.

      GOOD THINGS COME TO THOSE WHO GROW

      In June 1765 Daines Barrington, the British lawyer, naturalist and distinguished fellow of the Royal Society, made his way the one mile from his home in King’s Bench Walk in the heart of legal London, to a rather less respectable address on the east side of Soho. The reason for his journey into this more unsavoury area of London was to visit the temporary occupants of 21 Frith Street – an Austrian man named Leopold and his two children 14-year-old Nanneri and 8-year-old Wolfgang.

      Under his arm Barrington carried a clutch of documents and papers, but most importantly a newly composed music manuscript written in a ‘challenging, contemporary Italian style’. The purpose of bringing the manuscript was to place it in front of the young boy Wolfgang so that Barrington could check for himself whether the rumours that had spread across London regarding this boy’s precocious musical talent were really true. The boy was, of course, Wolfgang Amadeus Mozart and Barrington’s test would be an easy trial for him to pass. Just by sight, the young Mozart played the piece effortlessly and perfectly, at the very first time of