Different hair fiber types and their significance

Types of hair fiber

Barring the areas like palms of the hands, soles of the feet, foreskin, and lip vermillion, hair and hair follicles can be found in a all over the human body in wide diversity. There are different types of hair follicle depending on the structure and shape of the hair fiber, gross size, and time of appearance on the body. Generally, lanugo hair-producing hair follicles, vellus hair follicles, intermediate hair follicles and terminal hair producing follicles can be identified. A hair follicle may move from one category to another as it changes size and hair fiber production over time.

Lanugo hair. Lanugo hair is fine, soft, poorly pigmented and devoid of central medulla. It is the first hair growth produced in utero and by developing hair follicles. Lanugo hair may also be observed in adults with various forms of hypertrichoses.

Vellus hair. Vellus hair, is non-medullated, fine and poorly pigmented. Vellus hair continues to grow throughout life and they may constitute 7-25% of the hair present. They are found even in areas usually considered to have only terminal hairs, such as the scalp.

Intermediate hair. Intermediate hair is first observed postnatally as the scalp hair growth subsequent to the initial lanugo hair growth. Intermediate hair is characterized by a relatively rough cuticle, sparse pigmentation and a fragmented or absent medulla. Intermediate hair can also be seen in adults when they develop pattern baldness. As the scalp hair follicles get smaller they may stop producing terminal hair and produce intermediate hair until they fully miniaturize into vellus hair follicles.

Terminal hair that grows for a significant length has a larger cross section diameter as compared to other hair fiber types. It is pigmented, with the exception of those hairs affected by canities. Terminal scalp hair, that serves a key role as protection from UV light and as an insulator against heat and cold, may vary in size and shape, with body location and potential function. Eyebrows and eyelashes are the examples of specialized terminal hair; they protect the eyes by draining or sweeping away fluids and dust. Eyelashes have the highest diameter of all body hair, strong pigmentation, and they have a relatively short active growth phase. Nasal hair may play a role in preventing insects and other airborne material from entering the nasal cavity.

The hair follicles can be further classified on the basis of morphological characteristics of the hair follicles themselves. The primary anatomical distinction of human hair follicles depends largely on gross size to define vellus hair producing follicles and terminal hair producing follicles. Vellus hair follicles are small and penetrate the reticular dermis but not the subcutaneous fat layer. As a morphological characteristic, vellus hair follicles frequently display skirt-like epithelial structures or perifollicular connective tissue capsules with a striking clear space between the outer root sheath and such skirt-like structures, or the outer root sheath and perifollicular connective tissue capsule. Mucinous substance, rich in perifollicular nerve endings fill up this space and, among other cell types, contains elongated fibroblasts and mast cells. The typical dermal papilla in a vellus hair follicle contains just 10-30 cells. Compared to vellus hair follicles, a typical terminal scalp hair follicle dermal papilla contains 200-400 cells and correspondingly the rest of a terminal hair follicle is much larger. Terminal hair follicles reach into the deep dermis and their bulbs are located in the subcutaneous fat.

Further sub classification can be made on the basis of morphology of hair follicles. For example, sebaceous hair follicles in seborrhoeic regions of the skin display relatively large, well developed sebaceous glands associated with small, fragile hair follicles. Hair follicle types may also be differentiated on the basis of their functional characteristics such as different responsiveness to androgens in scalp hair. All hair follicles display more or less the same basic developmental and structural characteristics; but they significantly vary in morphological presentation. However, of the hair follicles located in the different body regions, hair follicles of one predominating subtype form rather homogenous groups with only moderate intra- and inter-individual variability.