Hair is a threadlike, keratinized outgrowth piercing the epidermis but actually roots from the dermis.
It is a characteristic feature of the mammalian class and forms the animal's outer covering or pelage.
Hair grows in varying degrees among the species of the mammalian class.
Most mammals have abundant hair that covers almost the entire body while humans, on the other hand, are among the hairless mammals.
Structure Hair is made up of the hair shaft, which is exposed on the skin and the hair root, which is found deep within the dermis in a structure called the hair follicle.
Hair is typically composed of dead cells, keratin and other proteins.
Only the actively dividing cells at the root, which give rise to the whole hair structure, are alive.
The hair follicle, which is the portion of the hair found within the dermis of the skin, is made up of four distinct layers.
The innermost layer of cells undergoes keratinization to form the medulla.
The medulla is the core of the hair shaft.
Following the medulla is the layer called the cortex.
The cortex forms most of the internal structure of the hair.
This layer undergoes keratinization more than the medulla.
The third layer of the hair is the cuticle.
Overlapping keratin plates form this layer.
This arrangement of the cuticle prevents matting of the hair follicle.
The hair shaft is composed only of these three layers.
It does not contain the fourth and fifth layers.
The fourth layer of the hair is the internal root sheath.
This layer is almost unkeratinized and disappears at the level of sebaceous glands.
This leaves a space through which the glands secrete sebum and coat the hair shaft.
The fifth and the outermost layer is the external root sheath.
This layer is not involved in the formation of the hair.
Types Lanugo is the first hair type that develops in humans.
It is made up of soft, slender hair that appears on the third to fourth months of fetal life.
These hairs usually shed off after birth.
Vellus hair is the term used to identify the short hairs found all over the body except the palms, soles, and the underside of the fingers.
Terminal hair develops from vellus hair during puberty.
Terminal hairs are coarser, longer and more pigmented than vellus hair.
They usually develop on the armpits, genital areas, faces of males, and in some parts of the trunk and legs.
The most obvious of the terminal hairs is the head hair.
Functions The most important function of hair is heat conservation in order to insulate the body from the cold temperature.
In other mammals, different hair color and color patterns serve as defense mechanisms by acting as camouflage.
Modified hairs called quills like those of the porcupine serve as physical defense mechanisms.
Phases of Hair Growth Anagen phase is the growth phase of the hair cycle.
85% of the hair is in the growth phase which lasts for about 2 to 6 years.
Catagen phase is the transitional phase between the two.
This usually takes about 1-2 weeks.
During this phase, the hair follicle shrinks and the hair root diminishes.
Telogen phase is the resting phase of hair growth.
It lasts for about five to six weeks.
About 10-15% of the entire hair is in this phase of the cycle.
At this phase, 25-100 hair normally falls off.