What is a hair follicle? Well, a good way to understand is to think of hair like a field of grass. The hair follicles – like grass roots – grow in and under the skin or scalp of the head. These “roots” then grow the hair out of them and the hair is what you see. Just like grass, you can cut your hair, or pull it out*, but the follicle/root remains and will re-grow the hair.
To understand the scalp, you need to know that it contains three layers: the epidermis, the dermis, and a layer of fat known as the subcutaneous. The epidermis is the outer layer and is less than 1 mm thick. It’s mostly composed of dead cells that are regularly shedding off and being replaced with new cells. The epidermis contains both sebaceous (oil) glands and sweat glands. As the hair grows upward some of the waxy/oily material lubricates the hair shaft giving it that much admired sheen. In hot weather the sweat glands help cool the body temperature through evaporation of water on the scalp. The third stratum, the subcutaneous fat layer, contains large sensory nerve branches, as well as blood vessels, which nourish the scalp and are vital to maintain healthy hair.
Outer root casing (trichelemma): This is the outer cover of the follicle which forms in the dermis, then merges into the epidermis on the skin’s surface. At this place, a pore forms to provide an opening for the hair shaft to push through. Some people think of the sheath as a tunnel leading down to the spot where the hair can emerge.
Inner root casing: This is a second covering which is where the hair shaft grows within the outer sheath. It’s made up of three parts. First, the cuticle, which is the innermost layer of the follicle, which is in contact with the strand of hair as it grows. It becomes the surface of the hair shaft and is what you see as the hair appears. Second, the cortex, contains the greatest volume of the hair shaft, and is what gives hair its strength. Keratin, an organic protein which is also found in the horns of deer and rhinos, composes this part of the hair shaft. Third, the medullas, appears in the core of the hair shaft, but only in fully-grown hair as it prepares for its final phase—shedding.
Hair follicle bulbs function much like tulip bulbs, which lie below the surface of the ground and are located at the bottom portion of the plant or follicle. Inside the bulb, matrix cells form tiny spindles, which determine the width of the fully-grown hair.
* If the pulling is extensive and consistent enough to damage the actual hair follicle, then hair may be permanently damaged.