A hair follicle can be described as a structure in the form of a long tube. It penetrates deep into the skin of the scalp. It is a structure around hair fiber and stretches from hair bulb deep in the skin to the skin surface opening.
The bulb of the hair follicle is the origin point of the hair. The estimated total number of hair follicles for a grown-up person is 5 million. Out of these 5 million hair follicles 1 million are on the head, and 100,000 on the scalp. There are only two external regions of the human skin devoid of hair follicles -- palms of the hands and soles of the feet.
A hair is basically composed of Keratin - a structural protein. Keratin is also a constituent of human nails and outer layer of the human skin. The hair follicle is a separate entity within the human skin. It is formed and maintained through the interaction of dermal and epidermal
The structure of the hair follicle is divided into three main parts. They are as follows -
(i) Dermal Papilla - The precursor of hair follicle, Dermal Papilla (DP) is a pear shaped structure found at the base of follicle. It consists of a highly active group of fibroblast cells derived from the mesoderm. These cells are held close to the base of the epidermal derived cells.
The epidermal derived cells produce the hair fiber and root sheaths. But there is a thin layer called the basement membrane or basement lamina or glassy membrane. It separates the DP cells from the hair fiber/sheath cells. It means that the basement membrane physically divides the cells descendant from embryonic ectoderm (epidermis) and embryonic mesoderm (dermis). This physical barrier plays a role in the human immunological protection. The DP cells are held in place by a capsule. It surrounds the DP cells in a cup and extends the sides of the hair follicle to the epidermis.
The entire structure of the hair follicle sits on a pad of fibrous tissues called Arao-Perkins body. Nerve fibers and blood vessels penetrate through small gaps in the hair capsule base and invade into the DP area. Bigger the size of the DP more is the number of cells. More cells mean thicker hair fiber produced by the hair follicle.
The DP cells are very active with lots of cytoplasm while the follicle produces a hair fiber. But these cells do not multiply. And they lose much of cytoplasm when the follicle is not producing a fiber. This turns them inactive.
(ii) Hair Fiber - The hair fiber is the core part of any hair follicle. Each hair fiber consists of three layers. These are medulla, the innermost core found only in mature thick hair; cortex, the middle layer, which provides color, texture and strength to hair; and cuticle, the outermost layer with a colorless appearance with the main function to protect the cortex.
Epidermal derived cells close to the DP remain undifferentiated cells called matrix cells. Their focus is on multiplication and proliferation to produce more cells. The cells produced in the center of the hair follicle are to become part of the hair fiber. They are the cortex (cortical) cells.
While the cells multiply the constant stream of production pushes the cells upwards towards the skin surface. While moving up the hair follicle they begin to differentiate into particular cell types. The cortex cells change from a round appearance to a flattened one. They are squeezed together into layers (lamella).
In case the follicle contains melanocyte cells, melanin pigment is incorporated into the cortex cells. The latter get keratinized and harden. Thus it becomes impossible for them to function properly and they die. They are then pushed away from the hair bulb region and upwards as new cells appear from behind. The cortex cells then become part of the dead keratinized fiber. Some large hair follicles contain a central stand of cells loosely organized and not packed together. This tube located in the very centre of the hair fiber is named medulla.
Cuticle is located around the outside of the hair fiber. It consists of more keratinized cells. But they arrange themselves in a slightly different way to cortex cells. While the cuticle cells are produced, they lay over the cortex cells and flatten into an overlapping roof tile fashion. They progressively flatten while getting older.
The cortex cells cannot function properly after getting keratinized and ultimately they die.
(iii) Root sheaths - They are the surrounding layers of the hair fiber. They are divided into Outer Root Sheath (ORS) and Inner Root Sheath (IRS). ORS forms the outermost layer of epithelium while IRS is below ORS and closest to the hair fiber. The arrector pili muscle is located in the "bulge" region of the ORS and this muscle is connected to the epidermis at the other end. This muscle makes your hair stand erect.
The sebaceous gland also extends from ORS. It comprises a few cells that produce oils (lipids). These cells are large with cytoplasm filled with vacuoles containing lipid. They are often divided into several lobes of the sebaceous gland connected together by a sebaceous duct. This duct has a single opening into the tube where the hair fiber sits.
The ORS surrounds the hair fiber and inner root sheath until deep into the dermis. The ORS tapers and ends just above the bulb region containing the DP. It comprises several layers of cells identified with unique ultrastructural properties.
IRS is the result of matrix cells located above the hair follicle. It can be subdivided into several layers. A single cell thick IRS cuticle layer is located adjacent to the hair fiber. It closely interdigitates with the hair fiber cuticle layer. The Huxley layer may comprise up to four cell layers. Then there is the single cell layer named IRS Henley layer. It runs adjacent to the ORS layer.
Density of hair follicles-
Human skin, having a heterogeneous structure, originates from the ectoderm and mesoderm of an embryo giving rise to the epidermis and dermis respectively. Within these generalized layers are specialized appendages. On an average, one square centimeter of skin contains 10 hair follicles.