Hair vs. Hairs: Understanding Why One Grows Abundantly While the Other Stays Sparse

Human hair is born from the same biological structure: the hair follicle. However, their behavior differs radically. On the scalp, the hair grows continuously for years, while the body of the body reach a certain length before stopping to grow.

This difference between hair and hair is largely based on the organization of the famous hair follicle. According to the National Library of Medicine, this unit is made up of several segments, including infundibulum, isthmus and bulb, which plays a key role in hair growth. It is at this level that the cells of the follicle are divided quickly to form the rod of the hair or the hair.

In humans, the follicles producing hair penetrate deep into the dermis and sometimes in hypoderma, while those generating hair is limited to the superficial layers of the skin.

Another factor intervenes: keratin. This fibrous protein, main component of hair and hair, varies slightly according to the areas of the body. Evolutionary Anthropology stresses that human evolution has led to an alteration of genes coding certain hair keratins, which could explain the prolonged growth of the hair compared to the hair of the body.

A growth cycle that makes all the difference

Each follicle follows a growth cycle in several phases: Anagène (growth), catagen (transition), telogen (rest) and exogenous (fall of the hair). However, the duration of these phases varies according to the location of the follicle.

The hair remains in the anagène phase between two and six years old, which allows them to reach long lengths. On the other hand, the hair of the body stayed there only a few months before entering the rest phase, explaining why they never exceed a certain size. Popular Science highlights this difference by comparing the hair cycle to human growth: a teenager has a few years to reach his adult size, while a hair of the arm has only a few weeks to develop before being replaced.

The internal mechanisms controlling this cycle are extremely precise. The dermal papilla, located at the base of the follicle, sends signals to the surrounding cells to trigger cell division and produce new hairs. This interaction, regulated by growth factors and hormones, is essential for hair renewal.

An evolution specific to humans?

If most mammals keep a homogeneous coat, humans are distinguished by sparse hair and hair that can grow indefinitely. This characteristic intrigues researchers for a long time. Several evolutionary hypotheses try to explain why hair growth has lengthened while body hair remained short.

One of the advanced theories suggests that the gradual loss of hair on the body has been offset by more abundant hair, making it possible to protect the sun's skull while regulating body temperature. Another track evokes sexual selection: long hair could have constituted an attractive asset, signaling good health and genetic longevity.

This human singularity could also be linked to recent genetic mutations. Popular Science evokes hormonal regulation as a key element: testosterone, converted into dihydrotestosterone (DHT), directly influences the growth of hair and hair. For example, male baldness results from an increased sensitivity of the scalp follicles to the DHT, while the less affected body hairs continue to push normally.

Thus, the difference between hair and hair is not only due to the visible length, but is based on a complex interaction between biology, genetics and evolution. Understanding these mechanisms could one day pave the way for new solutions against hair loss or hormonal imbalances linked to hairiness.