There is a simple, if slightly eccentric, experiment that shows this. In 1999, trading on a shared devotion to each other and to science, a married pair of scientists used each other as guinea pigs. They excised a piece of dermis from his scalp and then transplanted it to the hairless region underneath her arm. It may seem surprising that she didn’t reject (in the immunological sense) her spouse’s tissue, but it appears that hair follicles are somehow protected from immune-system surveillance. In the event, shortly after the wound healed she started growing long scalp hairs in the area that had received the transplant. The experiment showed that the dermis has a voice, one that tells the epidermis: ‘make follicles here’. Indeed, the change that spreads like a rash across the foetus as it develops hair follicles is the dermal cells acquiring that voice in succession – a volubility that spreads to dermal cells everywhere bar those in the fingertips, palms, soles, lips and genitals, which for some reason remain silent.
If, in the conversation of the skin, the dermis’s instructions are the opening gambit, it is one to which the epidermis has immediate right of reply. As dermal cells spring to life, urging the epidermis to make follicles, it must, with regularity and firmness, reply ‘no’. Were it not to do so, the foetus’s skin would become a single giant hair follicle, or perhaps a tumorous mass of malformed follicles and hairs. The way in which the epidermis counters the dermis is what gives hair follicles their precise spacing. Each newly formed hair follicle issues instructions that prevent the epidermal cells around it from also becoming hair follicles. Not only does each newly formed follicle prevent surrounding cells from hearing the dermis’s insistent demands, it probably shuts them off at source.
The words in this conversation seem to be signalling molecules of the sort that we have come across before. Bone morphogenetic proteins are good candidates for the epidermal inhibitor. Bird feathers are distantly homologous to mammal hair, and if a bead soaked with BMP is placed on a chicken embryo’s skin, the infected patch will not form feathers. If the same experiment is done with fibroblast growth factor, extra (albeit weirdly distorted) feathers will form – perhaps it is the original follicle-inducing signal. These molecules are thought to work in the same way in our hair follicles. But the signals around the developing follicle are so various, abundant and dynamic that it is difficult to know what they all do. We do know that mice engineered with defective hair-follicle signals are often bald.
GRASSLESS FIELDS
The one thing that many of us would dearly like to know about hair is why we lose it. Just how many men suffer from ‘androgenetic alopecia’ or ‘male pattern balding’ is a matter of definition, but claims that it can be detected in 20 per cent of American men in their twenties, 50 per cent of thirty-to-fifty-year-olds, and 80 per cent of seventy-to-eighty-year-olds seem about right. Balding is truly a white man’s burden: Africans, East Asians and Amerindians (Native Americans) all have lifetime probabilities of balding lower than 25 per cent. Medically innocuous, it is a dispiriting disorder. When Ovid wrote in Ars amatoria: ‘A field without grass is an eyesore/so is a tree without leaves/so is a head without hair,’ he spoke for legions. For at least a century Americans have shown a marked aversion to electing bald men to their nation’s highest office. Excluding Gerald Ford (1974–77), who was bald but not elected, the last bald president was Dwight D. Eisenhower (1953–61). Europeans have been more sympathetic to the bare-headed politico (Churchill, Papandreou, Simitis, Giscard d’Estaing, Mitterrand, Chirac, Craxi, Mussolini), but even they lagged behind the Soviets, who inexplicably installed, if not exactly elected, bald and hirsute leaders in strict alternation: Lenin (bald), Stalin (hairy), Khrushchev (bald), Brezhnev (hairy), Andropov (bald), Chernenko (hairy), Gorbachev (bald) – a tradition that has been maintained in the Russian Republic with Yeltsin (hairy) and Putin (comb-over).
What causes balding? Samuel Johnson’s views on the matter – ‘The cause of baldness in man is dryness of the brain, and its shrinking from the skull’ – may be safely discounted, as can the theory, popular around 1900, that it was due to the wearing of hats. But dermatologists are hard pressed to offer more convincing explanations. Baldness obviously runs in families, but claims that it is due to a single recessive mutation or else ‘inherited from the mother’s side’ (recessive X-linked) are wrong. Male pattern balding is caused by several genes, none of which has been yet identified. Whatever they are, they must affect the life-cycle of the hair follicle.
Hair follicles have the peculiar habit of periodically destroying and then reconstructing themselves. Most of the time they simply produce hair. A single scalp follicle can work on lengthening a hair for anywhere between two and eight years; the longer it does so, the longer the hair becomes. Mouse follicles work on a given hair for only two weeks, which explains why their fur is so short. When the follicle comes to the end of its growth period it begins to retreat within the skin and die, and the hair falls out. Halfway down the follicle, however, there is a bulge of epidermal cells – ‘stem cells’ – that have two remarkable properties: they are immortal, and they can become all the other types of epidermal cells of which the follicle is made. They are the stuff from which the follicle rebuilds itself.
But not in bald men. Instead of rejuvenating into a fully productive follicle, all that is produced is a pale and feeble imitation of the real thing; a follicular epigone capable only of making tiny hairs. Why this happens remains a mystery. One fact is, however, known: to go bald you need testosterone, and plenty of it. In the passage of Historia animalium in which Aristotle tells us that eunuchs are tall, he also says that they do not go bald, an observation confirmed in 1913 by a study of the last of the Ottoman eunuchs. The first rigorous demonstration that testosterone, rather than any other testicular hormone such as estrogen, is the culprit came from a 1942 study by the American physician James Hamilton. Some of the fifty-four eunuchs he studied were born without testes; some had been castrated as boys out of medical necessity (inguinal hernias, for example). Hamilton does not reveal where he found the rest of his experimental subjects, but one of his later papers suggests that they were mentally retarded men who had been castrated as boys in Kansas mental institutions, a legacy of eugenic programmes that ran in the United States until the 1960s (and even later elsewhere). Consistent with Aristotle’s claim, none of the men who had been castrated before their late teens developed any sort of baldness, not even the relatively high foreheads that nearly all mature men have. This wasn’t because they all happened to come from families with good hair – several had balding male relatives. Proof that the eunuchs’ boyish hairlines were due to their lack of testosterone came when Hamilton gave them male hormone supplements and some of them began to lose their hair. When he stopped the treatment, it promptly grew back.
The need for balding men to have their testicles is the likely origin of the idea that prematurely bald men are unusually virile. It is a claim that has the ring of wistful propaganda about it. (Even Julius Caesar, it is said, rejoiced in the title ‘the bald adulterer’.) To be sure, there is a sad irony in the fact that the very hormone that gives men their beards in puberty denudes their scalps a few years later, but there is no evidence that prematurely bald men either have more testosterone than their hairier contemporaries or father more children. On the other hand, it is probably a lack of testosterone that prevents women from going bald. Women who acquire, for whatever reason, abnormally high levels of testosterone not only grow beards but tend to go bald as their baldness genes, hitherto silent, manifest themselves.