Why does human hair have a grain?

Most human hair emerges from the skin at an angle, not straight out. What’s more, individual hairs tend to emerge at the same angle as their neighbors. This happens on the head, on men’s faces, and on arms and legs. Why does this happen? Why don’t hairs emerge normal to the skin or at random angles?

In many cases, hair is adapted to lay flat against the skin, but can be made to stand up and provide better insulation from cold by trapping a layer of air.

We see the same trait in almost all mammals. And in feathers and scales for that matter. Maybe the challenge is to find something that doesn’t have a grain.

Anyway, I have to think that, for mammals, it plays a role in heat retention. You want the hairs to create an insulated layer against the skin, which means 1) a controlled size of gaps between hairs and 2) ability to control the thickness of the layer as needed to warm up or cool off. Some mammals even have an outer layer of coarse hair and under layer of fine layer.

I was kind of hoping for more of a cellular/biochemical explanation, but the adaptability/evolutionary angle is probably a lot more to the point and almost certainly a lot more understandable.

Hairy ball theorem doesn’t really apply here, except very slightly, but I wanted to mention it, if only because it’s called Hairy Ball Theorem


I’m not sure that we know enough about how cellular system organize themselves to answer this one specifically (but if someone on the Internet knows, I’m sure they hang around here).

What we do know in a general sense is that much of tissue development is driven by chemical transmitters and receptors. As you get farther away from a transmitter, the signal gets weaker and then eventually there is a response. Human fingers are a bit like that; we normally stop at five because that’s where the chemical signal strength has become weak enough that it no longer triggers new fingers. We discovered this, in part, by studies of chicken embryos - move a piece of wing tissue from one place to another in the embryo, and you can get extra wing bones or even extra wings because of how you changed the transmitter strength in the surrounding tissue.

So when you’re talking something like hair, it’s almost certain that each follicle is signalling, in some way, to other nearby follicles in a way that controls both their orientation and density. (If not the follicles, then perhaps something in the skin tissue itself.)

The cellular/biochemical search term you’re looking for is “planar cell polarity”, which describes the phenomenon of cells in a flat tissue (such as the skin) all orienting themselves in the same way, so that they all point hairs in the same direction. There is a great deal known about the genes that control it and maintain it, but IIRC there is still some gaps in understanding how it is initially set up in the embryo.

As an aside, “why” questions in biology are usually quite simple: A trait is the way it is because organisms with that trait tended to have more descendants than those without. But “how” questions, like what you’re really wondering about, tend to be monstrously complicated, such that we usually only have the barest glimmer of understanding. This makes an interesting contrast to most other sciences, where “how” question can be tackled, but “why” questions are generally meaningless.

Oh, and I’m reporting this for forum change.

A very good Question, worthy of GQ. Moved to General Questions.

samclem, moderator.