Anatomy of the hair follicle

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Hair follicle anatomy

The hair follicle is nothing but a complex mini-organ. A single hair follicle is composed of multiple mesenchymal and epithelial cell layers - all of them comprise more than twenty different cell populations. Along with the sebaceous gland and the arrrector pili muscle, the hair follicle becomes the part of the so-called “pilosebaceous unit”. However, different hair follicle types may contain these components in varying proportions.

The basic hair follicle structure can be divided into different regions on the basis of 1. Morphology - whether the structure is ectoderm- or mesoderm-derived - and 2. the hair cycling characteristics of each structural unit. At the most basic level, the difference is made between the permanent, superficial structure and the transient cycling component of the hair follicle which includes the hair bulb. The physical dividing line between these two components lies a little below the bulge region and the insertion of the arrector pili muscle. You can again subdivide the permanent portion of the hair follicle structure. The area starting from the skin surface reaching to the point of the sebaceous gland duct opening to the hair canal is marked as the infundibulum. The superficial part of the hair follicle infundibulum is called the acro-infundibulum. It is lined by epidermis including a well developed stratum corneum and a stratum granulosum layer. The lower part of the infundibulum, called the infrainfundibulum, may experience a continuous loss of epidermal differentiation occuring towards this isthmus.

The isthmus is the part that lies between the sebaceous gland duct opening and the bulge. The isthmus provides a border zone that is relatively devoid of distinctive features. Though the thick vitreous membrane between ectoderm and mesoderm visible elsewhere in anagen stage hair follicles becomes noticeably thinner here. The wall of hair follicle isthmus consists of two or three rows of flattened cells. The angle of orientation of these cells can be observed to change along the length of the isthmus as the outer root sheath merges with the skin epithelium. There is a bulge region below the isthmus - it provides the insertion site of the arrector pili muscle. Additionally, it harbours the so called bulge region, which is a specialized compartment of the outer root sheath. It in turn forms a niche for epithelial and neurectodermal stem cells as well as various immature cell populations including immature Langerhans cells and mast cells. The part that extends from the skin surface down to lower end of the bulge region forms the permanent portion of the hair follicle; no significant cyclic changes are observed in this area. However, the presence of hair follicles, and also their size and their density, affects the enlargement of the skin surface to a considerable extent. This phenomenon particularly influences the percutaneous penetration processes.

The inferior unit, on the other hand, extends from the bulge to the base of the hair follicle bulb; it can be further subdivided into the bulb and suprabulbar region. The epithelial compartments of the hair follicle include the outer root sheath and the inner root sheath. Both of them are composed of different cell sublayers, and the matrix cells. The outer root sheath that starts from the matrix cells in the hair bulb extends up to the entry level of the sebaceous duct. Outer root sheath cells contain clear vacuolated cytoplasm - they are filled with large amounts of glycogen. The outer root sheath below the isthmus does not contain keratins. The inner root sheath disintegrates at the level of the isthmus and it is here that the outer root sheath keratinizes without forming granules. Outer root sheath cells are characterized by the presence of a large diversity of mediators, hormones and receptors. All of them are keratinized. Because it hardens before the presumptive hair fiber does, hence, it is believed to control the cross sectional and longitudinal shape of the hair produced. A vitreous or basal membrane separates the mesenchymal sheath from the epithelial root sheaths. A dense vascular network surrounds this whole complex. A cuff is formed with free nerve endings; it provides the basis for intensive piloneural interactions.

Proliferative activity of matrix keratinocytes gives rise to the hair shaft and the inner root sheath - the hair growth is the result of this entire process. The hair shaft and the inner root sheath are localized in the bulb where they sit on top of the dermal papilla. The dermal papillas are clusters of specialized mesenchymal cells with important hair growth inductive properties. The fact that surgical removal of the dermal papilla and the lower dermal sheath prevents hair growth indicates the importance of these specialized mesenchymal cells - they act as the key signalling centre in hair follicles. A blood capillary loop is located within the dermal papilla of terminal hair follicles - the nutrition to the papilla and the overlying matrix cells is supplied through this. Dermal papillae at the root of vellus hair follicles typically do not have capillaries – their nutrition diffuses in from blood vessels around the outside of the vellus hair follicles.

Hair follicle anatomy diagram
t = hair shaft
0 = hair follicle opening
mu = arrector pili muscle
s = sebaceous gland
r = radicular portion of hair shaft
f = connective tissue sheath of hair follicle
ex = outer root sheath
i = inner root sheath
bc = bulb of hair follicle
p = dermal papilla
g = adipose tissue
a = arteriole in cross section
n = nerve
gs = coils of sweat gland
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