environments, so this method of comparing groups of identical and non-identical twins enables you relatively easily to quantify the heritability of a trait. So, for example, in the case of a height study, if it shows that the identical twins are considerably closer in height than the non-identical twins then this strongly indicates that genes play an important role. The actual analysis that can be applied to a study like this, both statistically and genetically, is far reaching and complex but the principle remains the same – the greater the similarity between identical twins compared to non-identical twins, the greater the heritability of the trait.
One of the most recent of these large studies conducted by Peter Visscher of the Queensland Institute of Medical Research in Australia looked at 3,375 pairs of Australian twins and siblings and found that the heritability of height is around 80 per cent. Other studies have come up with similar findings, including one that looked at 8,798 pairs of Finnish twins, in which the heritability was found to be 78 per cent for men and 75 per cent for women. Interestingly, similar studies in Asia and Africa have found the per cent heritability to be around 65 per cent or lower, because these regions tend to have populations that are less mobile and so more ethnically and genetically defined compared to the greater genetic homogeneity we see in the west.
Regardless of the height you reach as an adult, the journey to get there is not steady, and only now are we beginning to truly understand the extraordinary process behind this rapid growth, a process that is dependent on an intricate interplay between your genes, brain, a cascade of chemicals and every bone in your body.
GROWING PAINS
In the first six months of your life, you grew more than at any other time since. It’s a growth spurt unlike anything else our bodies experience, with most of us growing a massive 30 cm in that first year. As a new parent this is particularly evident. It seems some days as if my daughter is growing in front of me. If we continued to grow at this rate, we would be 10 feet (~3 m) tall by the time we were 10 years old, but by the end of that first year that frenzied growth rate has slowed down and will continue at a far more subtle speed until the madness begins again at puberty.
The secrets behind the process of growth reveal how the body works as an integrated system, not just separate organs and limbs functioning in isolation. Starting at the business end of the process, the bones that really define your growth are the long bones of your body, in particular the femurs in your thighs, the fibulas and tibias in your lower legs, and the humerus, ulna and radius in your arms. These are the site of the major longitudinal growth during that first year of development. These bones don’t just uniformly increase in size as they lengthen; the growth is focused around a particular part of the bone called the metaphysis found at the end of each long bone. If you looked at an X-ray of the metaphysis region of the fibula and tibia of a 10-year-old, you might conclude that the child has a broken leg. But what you are actually seeing in the ‘fracture’ across the bone is the location of growth, a line that is called the epiphyseal plate or growth plate. This is a soft disc made of hyaline cartilage (the same cartilage that you can feel in your nose) and it’s here that cells called chondrocytes divide throughout the first 15 or so years of life and the rate of division increases furiously during a growth spurt. As the chondrocytes divide they secrete cartilage, a protein matrix that forms the template for bone, and the continuous division pushes the older cells towards the shaft. These gradually die and become ‘mineralised’. The chondrocytes die, and cells called osteoblasts move in and secrete bone tissue into the cartilage. It’s this process that results in the elongation of the bone – this is how we grow.
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
