Things Yoram knows: why is the forehead bald?

Eleanor asks: I have already come across several babies who are born with hair (downy) on the forehead area, why is it there and how did it disappear?  

In short: the hair is there because there is no reason for it to be absent: it is spread over the whole body in embryos. The hair on the forehead does not "disappear" even after birth, but its roots are small and do not respond to hormonal stimuli that cause hair roots in other places to produce thick hair.

The baby is usually born almost frozen, but the fetuses are quite hairy: already in the third month, the fetus is covered with a down called lanugo: this is hair that covers the whole body with the same density and its function is to help attach the layer of wax that protects the fetus to its skin. These hairs fall out about a month before birth into the amniotic fluid and part of them is swallowed by the fetus. This hair is one of the solid components in the fetal feces (myconium). The dense covering of body hair that has not had time to fall out is a characteristic sign of premature babies that sometimes gives them a "monkey" appearance.

Biologist Desmond Morris gave humans the nickname "naked ape" but we are not particularly "naked" or bald. Our body is covered with hair but most of it is short and thin.

Several types of hair grow on the skin: downy hair (vellus) that grows from small, narrow hair follicles that are not connected to sebaceous glands (skin fat) and large hair follicles that are built from a wide hair root and a sebaceous gland that covers the growing hair with a protective sheath. Such follicles are sensitive to hormones and are the ones that produce thick hair such as head or armpit hair. If you count the density of hair follicles in different parts of the body, it becomes clear that the forehead is not cold: the density of hair follicles in the forehead is 300-400 per square centimeter compared to only about 140 in the scalp. But these roots are small and the hormones that encourage hair follicles in other areas to grow do not affect them, the average hair diameter on the forehead is only about 15 microns compared to about 70 microns on the scalp. As if that were not enough, the fine hairs grow very slowly: the plume only grows by 0.03 millimeter a day compared to 0.45 on the scalp: it will take a whole month for the hair to grow a single millimeter and then it will be quite old: the duration of the growth of the plume is a few months and another 2-4 months will pass From the end of her hair growth until the follicle starts to grow a new hair. For comparison, a few centimeters north of the forehead, the hair on the head enjoys continuous growth for 4 to 8 years. In fetuses, the forehead hairs grow long enough to be visible, but soon after birth they adapt to the short life span and slow growth.

The larger the animal, the smaller the density of its hair

Thinness of hair is considered a human hallmark that distinguishes us from other mammals, but a comparison of hair density among mammals reveals a more complex picture. As a general rule, the larger the animal, the lower the density of hair on its body, so that in any case, humans should have been less hairy than mice or foxes. When you create a graph of hair density in relation to size, it becomes clear that not only we, but also chimpanzees are less hairy than predicted based on body size, which means that the evolutionary process that led to human baldness probably began even before our ancestors separated from the chimpanzee ancestors, and the hair density in humans and chimpanzees is close. The reason for human baldness is that a significant part of our hair is thin fluff (vellus hair) and does not develop into such thick hair on the head, armpits or beard. One of the reasons for the low density of hair on the limbs compared to the head is the body proportions of the fetus, which are different from our adults: when the follicles are formed in the third month, his head is large in relation to his body. This means that the limbs grow more during the last 2 thirds of pregnancy and during infancy than the head so that the hair follicles are spread over more skin and become less dense. The highest density of follicles is in the inner part of the nostrils where the skin houses no less than 1200 hair roots per square centimeter.

The atrophy of the forehead hairs distinguishes us from our relatives in the chimpanzees and gorillas whose head hair is uniform. During evolution the hair follicles on the forehead became smaller, lost the sebaceous glands and their ability to respond to hormones. Thus, when our ancestors moved from the shady forest to a hunter's life in open prairies exposed to sunlight, a smooth surface was created near the brain through which unnecessary heat can be effectively lost through sweating. Indeed, while we are similar to chimpanzees in terms of hair density, since the evolutionary separation we have increased the density of the sweat glands 10 times. The embryonic hair was not subjected to similar evolutionary pressure and indeed their hair is similar to the head hair of ape fetuses and adult monkeys. What would we look like if the hair follicles on the forehead kept their activity? It is possible that some people in the world carry, to their misfortune, the answer in their flesh. A rare genetic defect called Hyperthyroidism  hypertrichosis turns the hair on the head, including the forehead and sometimes on the whole body into thick and developed hair. In the past, people with this syndrome were presented in circuses and some of them, such as Annie Jones "the bearded lady" who lived in the 19th century in the USA, became a kind of celebrities whose appearance was also a source of their livelihood. 

Why can't the body hair be oxidized?  

The difference between the 2 types of hair is also the answer to Adi's question: "If you oxidize your head hair, after a while you will grow new hair, and the hair you oxidized will remain oxidized, but if you oxidize your body hair, we will not get similar results. Body hair remains oxidized for a limited period after two weeks By a month, the body hair returns to its original color. How is such a thing possible?

Hair color comes from the pigment melanin: a natural polymer that absorbs visible light. Melanin is black, meaning it absorbs all wavelengths of visible light thanks to its special molecular structure. In the melanin chain there is a sequence of alternating double and single bonds between carbon atoms (conjugated system). The electrons in these bonds can absorb light energy and move between a large number of dense energy levels. Hydrogen peroxide, as its name suggests, oxidizes, that is, robs the melanin of exactly these electrons and leaves the hair white. This oxidation is irreversible, which means that the hair cannot "return to its original color" because the hair we see is made of dead cells that do not regenerate and do not create new material.. When the operation is done on the hair of the head, the root of the hair continues to produce a fiber containing melanin that pushes the light hair up and creates the roots phenomenon The blacks that stand out in the oxidized scalp. The exact same thing happens with body hair, but since the life span of hair is much shorter, there is simply no time to see two-colored hairs until the oxidized hairs fall out and new, dark hair grows in their place.

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Thus, perhaps, we should have looked if the hair follicles on our foreheads had not degenerated.