Hair Follicle Anatomy

Introduction to skin and hair biology

The skin is the second largest organ in the body after the skeleton and of primary importance, integral to the survival of mammalian life, but often overlooked when examining the health and welfare of an individual. The heterogeneous skin structure is derived from the ectoderm and mesoderm of an embryo giving rise to the epidermis and dermis respectively. Within these generalized layers are specialized appendages also derived from the ectoderm and/or mesoderm including sensory nerves, sweat glands and hair follicles? The skin as a whole rests on subcutaneous tissue largely composed of a loose mesh of collagen fiber, fat cells and muscle tissue. The average adult has over 3,000 square inches of skin surface area and the fat-free skin accounts for at least 6 percent of an individual's total weight. In other words, the fat free skin of a 150 pound person weighs at least 9 pounds. The density of structures in the skin varies considerably depending on the body location, but on average one square centimetre of skin contains about 10 hair follicles and 15 sebaceous glands, 100 sweat glands, half a meter of blood vessels, 2 meters of nerves, with 3,000 sensory cells at the ends of nerve fibers, 200 nerve endings to record pain, 25 pressure receptors for the perception of tactile stimuli, 2 sensory receptors for cold, and 12 sensory receptors for heat.

With all these different properties the skin has a diverse range of functions:

• Support - The skin acts as a flexible physical support and covering for underlying tissues.
• Temperature - Through its extensive blood supply and sweat glands, the mammalian skin is able to maintain the constant temperature of a homoiotherm.
• Excretion - Waste materials such as salts and water are removed from the body via the skin's sweat glands.
• Vitamin formation - Photochemical action in skin produces vitamin D. The skin is our primary source.
• Sensory function - Through the extensive network of sensory receptors we have sensations of pressure, texture, temperature and pain.
• Pigmentation - Melanin pigments protect against the excesses of ultra violet light.
• Protection - The epidermis prevents desiccation of the internal organs and so provides the fundamental requirement for mammalian land colonization - freedom from water dependence. It prevents absorption of unwanted and potentially dangerous chemicals.
• Immunological defence - The epidermis, particularly the stratum corneum (the outer most keratinized skin layer), provides a passive defence against entry of opportunistic pathogenic organisms. Skin also performs an active role in immunity through immunological surveillance.

In short, the skin acts as a barrier and is the primary organ through which we interact with the external world. Significant in many of these properties is the hair follicle appendage. The key role of hair is to provide protection against heat loss. Hair traps air adjacent to the skin to provide an invisible, insulating layer. Several mammalian species produce special dense winter coats with added heat trapping properties. Otters have hair to trap a layer of air around their bodies keeping them warm and making them more buoyant while they swim. Many species go through malting cycles in tune with seasonal changes.

Hair can provide indications of sexual development through onset of secondary sexual characteristics from development of a mane on a male lion to beard development in humans. Hair may also be of importance in attracting mates and may be based on colour - such as silver back mountain gorillas - distribution or quality, all indicators of the general health and vitality of an individual. Alternatively, it may aid in camouflage for survival where mute tones or dappled colour blend with an animal's environment. Hair fiber also helps with protection forming a tough barrier helping protect the epidermis from minor abrasions and/or from ultra violet light. Specialized hair such as eyebrows and eyelashes protect the eyes by channelling or sweeping away fluids, dust and debris. Nasal hair plays an important role in trapping air borne foreign particles before reaching the lungs. Hair fiber may also increase the surface area for faster evaporation of sweat from neighbouring apocrine glands. Some hair follicles have a highly developed nerve network around them and provide sensory, tactile information about the environment. Consequently, the hair follicle is of great importance to the survival of mammals.

Although its importance for humans has diminished it is still significant - not just biologically, but also through cosmetic and commercial considerations. The secondary functions of hair are now of primary importance for humans. Hair styles are used to make a statement, to identify the individual with a particular faction of society, and/or to attract a mate. Hair is the foundation for a multi million dollar industry focused on presenting, augmenting, and preserving scalp hair plus removing unwanted body hair.

Hair follicle anatomy

The mature anagen hair may be divided into vertical or concentric (horizontal) compartments. The major vertical divisions of the hair include (from superficial to deep) are:

• The upper follicle consisting of the infundibulum and the isthmus
• The middle portion of the hair follicle comprising of the bulge
• The lower follicle consisting of the suprabulbar and the bulbar areas The upper and middle parts of the follicle are permanent, but the lower follicle regenerates with each turn through the hair follicle cycle. The major compartments of the hair from outermost to innermost include the:
• Connective tissue sheath
• Outer root sheath
• Inner root sheath
• Cuticle
• Hair shaft cortex
• Hair shaft medulla

Infundibulum

The infundibulum extends from the epidermis to the opening of the sebaceous gland duct. The epithelium of the infundibulum is continuous with the epidermis, thus its cells can regenerate the epidermis and replenish it after wounding or injury. The lumen (inner cavity) of the infundibulum normally contains the hair shaft, keratin material and the natural oil produced by the sebaceous glands of the follicles, sebum. The infundibulum includes the upper part of the hair canal.

The sebaceous gland

The sebaceous gland secretes sebum - a semi-fluid secretion consisting chiefly of fat, keratin, and cellular material. The cells at the periphery of the gland are small, nucleated, and devoid of lipid (fat). These cells divide and are displaced into the centre of the gland where they produce and accumulate lipid. They then lose their sub-cellular organelles, disintegrate and release their contents into the sebaceous duct. The cells are often divided into several lobes of the sebaceous gland connected together by a sebaceous duct. The duct has a single opening into the tube where the hair fiber sits. The sebaceous duct empties into the hair canal at the border of the isthmus and infundibulum. In addition to its role in producing sebum, the sebaceous gland may also be important in normal hair shaft outgrowth.

The isthmus

The isthmus extends from the opening of the sebaceous gland duct to the insertion of the arrector pili muscle. The outer root sheath of the isthmus, as compared to the epithelium of the infundibulum, has no granular layer and its cells possess pale cytoplasm due to an increased amount of glycogen.

Arrector pili muscle

The arrector pili muscle is connected to the epidermis at the other end. The contraction of the muscle pulls on both the hair to make it erect and pulls on the skin making a bumpy surface.

Bulge

The inferior most portion of the isthmus contains the bulge area of the hair follicle. The bulge region is believed to be the storage area for hair follicle stem cells. Hair follicles go through a cycle of growth, transition and rest. With each successive effort to produce hair fiber, the hair follicle must obtain a source of cells to form the matrix cell population that make hair fibers. The source of these cells is believed to be the bulge region. Although the bulge area is cylindrical and often difficult to identify in adult anagen (period of growth) hair follicles, it becomes quite prominent during telogen (period of inactivity).

The suprabulbar region

The suprabulbar area of the follicle is the region below the isthmus and above the bulb. This is comprised of four layers from outermost to innermost: dermal sheath, outer root sheath, inner root sheath, and hair shaft. In this region, the three inner root sheath layers viz. the Henle's layer, Huxley's layer, and the cuticle, completely keratinize and become impossible to differentiate from one another.

The perifollicular sheath

The perifollicular sheath (dermal sheath) envelops the epithelial components of the hair follicle and consists of an outer layer of cells called the connective tissue sheath and an inner basement membrane called the hyaline or vitreous (glassy) membrane. This basement membrane of the follicle is continuous with the interfollicular basement membrane and is most prominent around the outer root sheath at the bulb in anagen follicles. During catagen, the basement membrane in the lower portion of the follicle thickens and then disintegrates.

The outer root sheath

The outer root sheath forms a non-keratinizing region at the periphery of the follicle and is continuous with the epidermis. It extends all the way to the tip of the bulb, around which it is composed of two layers of greatly flattened cells. The outer root sheath contains many vacuoles, Golgi complexes (A Golgi complex is a complex of parallel, flattened vesicles and vacuoles that lies adjacent to the nucleus of a cell and is concerned with the formation of secretions within the cell), smooth and rough endoplasmic reticulum, mitochondria, (containing genetic material) and so on. The outer root sheath also contains great amounts of glycogen in the lower part of the follicle. Glycogen is the source of the energy for protein synthesis during hair growth, and the presence of glycogen in the outer root sheath suggests the existence of an energy-consuming activity in these cells.

The inner root sheath

The inner root sheath extends from the base of the bulb to the isthmus. The lower inner root sheath is characterized by large eosinophilic cytoplasmic inclusions called trichohyalin granules. Trichohyalin is a structural protein that is produced and retained in the cells of the inner root sheath and medulla of the hair follicle. Keratin fibers are also produced in the inner root sheath.

Based on structure, patterns of keratinisation and incorporation of trichohyalin, the inner root sheath actually consists of three different cell lineages; which are (from outermost to innermost)

• Henle's layer is one cell layer thick and is the first to develop trichohyalin granules and the first to cornify.
• Huxley's layer is two to four cell layers thick and cornifies above Henle's layer at the region known as Adamson’s fringe.
• The inner root sheath cuticle is one cell layer thick, develops a few trichohyalin granules, and begins to keratinize below Adamson's fringe. The cells of the inner root sheath cuticle overlap one another with their free edges oriented towards the deep portion of the follicle. These cells are in opposition to the cells of cuticle of the hair shaft that are oriented upwards, thereby anchoring the hair shaft in place. Thus, the fully cornified inner root sheath anchors and directs the growth of the emerging hair shaft.

The hair shaft

The hair shaft is the part of the hair that can be seen above the scalp. It consists mainly of dead cells that have turned into keratins and binding material, together with small amounts of water. The hair shaft is composed of three parts from outermost to innermost: the cuticle, the cortex, and the medulla (present in large terminal hairs).

• The cuticle is a single row of cells, which may be identified at the level of the lower bulb by its lack of trichohyalin granules (in contrast to cells of the inner root sheath) and melanin (in contrast to cells destined to become the cortex). These cells are cuboidal initially, then columnar as they extend up the bulb, and then flatten to form an overlapping pattern with their free edges oriented upwards. A healthy cuticle is more than just a protective layer, as the cuticle also represents the structure that controls the water content of the fiber. Much of the shine that makes healthy hair so attractive is due to the cuticle.
• The cortex makes up most of the hair shaft, and is the component that gives hair its special qualities such as elasticity and curl. The cortex is packed with strands of keratin, lying along the length of the hair. These keratin fibers are made of the low-sulphur keratins, and are compressed into bundles of larger fibers. The cortex also contains granules of the hair pigment melanin.
• In some of the terminal hairs, the cortex has a central hollow core, called the medulla. The medulla is easily identified as a pale-staining, sometimes discontinuous line of cuboidal cells. In some animals, the air within the medulla plays a role in the regulation of body temperature.

The hair follicle bulb

The bulb is the deep, bulbous portion of the follicle that surrounds the dermal papilla. The bulb contains the matrix cells, the living, actively proliferating group of cells, which differentiate and become keratinized to form the hair cortex. This rate of proliferation is one of the highest of any tissue in the body. As they grow and develop, these cells steadily push the previously formed cells upwards. When the cells reach the upper part of the bulb they begin to change and arrange themselves into six cylindrical layers, one inside the other. The inner three layers of cells become the actual hair. The outer three layers become the lining of the hair follicle - the inner root sheath. The cells of the future hair shaft are positioned at the apex of the dermal papilla and will form the medulla (in terminal hairs), cortex and hair shaft cuticle.

The dermal papilla

It is the dermal papilla, which directs and dictates the embryonic generation of a hair follicle. The Dermal papilla presents as a healthy "pear" shape in normal hair follicles, and consists of a highly active group of cells shown to be capable of inducing follicle development from the epidermis. It is comprised of spindle-shaped cells with cytoplasmic processes called fibroblasts, collagen bundles, stroma, nerve fibers and a single capillary loop. It is continuous with the perifollicular sheath (dermal sheath) of connective tissue, which envelops the lower follicle.