Stress Hair Loss - Introduction
The entire scalp contains around 100,000 pigmented, terminal hair follicles. Blonds tend to have more, at 120,000, and redheads fewer, at 80,000 hair follicles . There is ethnic variation, with an average density of 250–310 hairs/cm2 in people of European decent approximately 150 hairs/cm2 in African-American patients ,and 120 hairs/ cm2 in Asian patients In Europeans, hair shafts usually show an oval cross section; diameters can range from approximately 50 to 120 μm.
Very fine hair with diameters less than 50 μm is most frequently seen in the Scandinavian population and northwestern Europe [8,9]. Hair of people originating from east Asia (China, Korea, and Japan) is usually referred to as oriental or Asian hair. It generally shows the
greatest diameter, ranging from 100 to 130 μm . Asian hair shafts are straight with no or very few twists along
the shaft and with a round cross section. The hair of people from sub-Saharan Africa is highly characteristic in shape. African hair is considerably
flattened and grooved and frequently varies in diameter along one single shaft.
Every hair follicle undergoes an individual recurring cycle with growing and resting periods. The growing period (anagen) persists for 2–8 years, and the hair grows approximately 1 cm/month or 0.35 mm/day during this time. During the hair cycle, the middle and upper portions
of the hair follicle are the permanent segment, whereas the lower portion is nonpermanent . The root (bulb area) of an anagen terminal follicle reaches deep into subcutaneous fat tissue. The anagen phase is followed by a transition period (catagen) of 2 weeks, during which the hair follicle undergoes programmed apoptosis. This transitional state is followed by a resting period (telogen) that lasts around 3 months. During telogen, the hair does not grow longer; the shaft is anchored in the mid-deep dermis.
Unlike most fur-bearing animals, where the hair cycle is synchronous, on the human scalp there is an asynchronous mixture of hairs actively growing and resting. In a healthy scalp, 80%–90% of hair follicles are in anagen. A normal anagen to telogen ratio for the scalp hair is 9:1, although seasonal variations can be found. The scalp sheds around 100 telogen hairs per day. Normal scalp skin also shows a variable number
of small vellus hair follicles. Vellus hair follicles cover our entire body, except for palms and soles, with variable densities in different body sites. Vellus hairs are anchored in the mid-upper dermis; the hair shaft is free of color and less than 30 μm thick .
During the hair cycle, the middle and upper portions of the hair follicle are the permanent segments of the hair follicle, whereas the lower portion is nonpermanent. (a) The growing or anagen hairs are anchored deeply within the subcutaneous fat and cannot be pulled out easily. The telogen hairs are located higher up in the dermis and can be pulled out relatively easily. The scalp consists of almost 90% hairs in anagen, 1% in catagen, and 10% in telogen. Anagen may last up to 2–6 years, telogen 3 months, and catagen 3 weeks. This ratio is usually
uniformly distributed over the entire scalp. The dermal papilla (dp) is pulled upward with each cycle and during telogen is closely associated with the stem cells of the bulge area. Communication signals between dp and stem cells of the bulge probably determine the length of anagen
and the matrix girth of the next hair cycle. (b) The newly formed anagen hair pushes out the previous telogen hair.
Vellus-like hairs are less than 0.03 mm in diameter and rarely grow more than 1–2 mm. Terminal hairs are coarse, over 0.06 mm in diameter, and can grow up to 1 m (3 ft).
The Anatomy of Trichology
To appreciate an organized protocol for alopecia, it is important to review the basics of hair anatomy. Hair follicles are skin appendages that are formed in week 16 of the early fetal period. They can be divided into four parts :
1.Infundibulum, extending from the follicular orifice to the sebaceous gland
2.Isthmus, extending from the sebaceous gland to insertion of the arrector pili muscle
3.Suprabulbar area, insertion of the arrector pili muscle to matrix
4.Bulb, consisting of the dermal papilla and matrix intermixed with melanocytes
The lower portion of the follicle consists of five major portions: (1) the dermal papilla; (2) the matrix; (3) the hair shaft, from the inside to the outside of medulla, cortex, and cuticle; (4) the inner root sheath (IRS), consisting of the IRS cuticle, Huxley's layer on the inside
and Henle's layer on the outside; and (5) the outer root sheath (ORS) (Figures 1.6 and 1.7). The base of the follicle is invaginated by the dermal papilla, which contains highly vascularized connective tissue.
Dermal papilla fibroblasts are inherently different from nonfollicular fibroblasts. There is a large amount of acidmucopolysaccharides
within the dermal papilla, staining positively for Alcian Blue and metachromatically for toluidine blue. The ground substance consists of not only
non-sulfated polysaccharides such as hyaluronic acid, but also sulfated mucopolysaccharides such as chondroitin sulfate. Increased activity of alkaline phosphatase can be found in the anagen phase.
The hair matrix has large vesicular nuclei and deeply basophilic cytoplasm. DOPA- positive melanocytes are interspersed between the basal cells of the matrix. Melanocytes are dendritic neural crest–derived cells that migrate into the epidermis in the first trimester. Melanin
is a complex quinone-/indole–quinone-derived mixture of biopolymers produced in melanocytes from tyrosine.
Melanin is incorporated into the future cells of the hair shaft through phagocytosis of the distal portion of the dendritic melanocyte. Melanosomes of the hair follicle are larger than those of the epidermis. They lie singly or within groups not within lysosomes. They are
located usually in the interfibrillary matrix, within the cells and only rarely in the intercellular space in the hair cortex. Two different types of melanin can be distinguished: eumelanin is brown or black, and pheomelanin, which results from the incorporation of
cysteine, is yellow or red]. In eumelanin-containing follicles, melanocytes contain ellipsoidal melanosomes with lamellar internal structure
(eumelano-somes). Pheomelanogenesis is associated with melanocyte-containing spherical melano-somes, which have a less well-defined internal structure containing granules and vesicles. Eumelaninogenic and phaeomelanogenic melanosomes can coexist in the same melanocyte but are produced in different pathways
A preponderance of eumelanin is associated with brown or black hair and a preponderance of phaeomelanin with red or blond hair. The absence or relative absence of both melanin types results in white hair The hair follicle is divided into four parts: bulb, suprabulbar area, isthmus, and infundibulum.
Cells of the hair matrix differentiate into six different types of cells, each of which keratinizes at a different level. The outer layer, Henle's layer, of the IRS keratinizes first, establishing a firm coat around the soft central portion of the follicle. The two opposed cuticles covering the hair shaft and the inside portion of the IRS keratinize next, followed by the Huxley's layer. The hair
cortex then follows, and the medulla is last. The hair medulla appears amorphous because of its only partial keratinization. It may not always be present. The hair cortex during upward growth from the matrix cells keratinizes gradually by losing its nuclei and becoming filled with keratin fibrils.
No-keratohyalin granules (i.e., keratinizing epidermis) or trichohyalin granules (i.e., keratinizing IRS) are formed during the keratinization of the cortex. Keratin of the cortex is hard keratin, in contrast to the IRS or epidermis, which is soft keratin. The hair cuticle is the outermost layer of the hair shaft. It consists of overlapping cells arranged like shingles and pointing upward with their peripheral portion. Up to the isthmus, the cells of the hair shaft cuticle are tightly interlocked with the cells of the IRS cuticle, resulting in the firm attachment of the hair to its
IRS. The hair shaft and the IRS move upward in unison.
The different layers of the hair follicle. Melanosomes, either eumelanin or pheomelanin, during anagen are transferred from melanocytes to matrical cortex cells via dendritic ends. The IRS is composed of three layers). None these layers contains melanin, and all keratinize with
trichohyalin granule formation. These granules stain eosinophilic, in contrast to the basophilic keratohyalin granules of the epidermis. The cuticle of the IRS consists of one layer of flattened overlapping cells that point downward in the direction of the hair bulb. Because the cuticle cells of the hair shaft point upward, these two types of cells interlock tightly. Trichohyalin granules are few in the IRS cuticle. Huxley's layer is two cell layers thick and develops numerous trichohyalin granules.
Henle's layer is only one cell layer thick and already shows numerous trichohyalin granules as it emerges from the matrix. Just before the isthmus, the IRS becomes fully keratinized. However, at the level of the isthmus the IRS disintegrates and loses its tight connection to the hair
shaft cuticle. The IRS cuticle cells therefore do not contribute to emerging hair but serve as a hard moldingscaffold up to the arrector pili muscle.
The ORS extends from the matrix cells to the entrance of the sebaceous duct, where it changes into the infundibular epidermis. The ORS is thinnest at the level of the hair bulb, gradually increasing in thickness, and is thickest in the middle portion of the hair follicle, the isthmus. In the lower portion below the isthmus, it is covered by IRS and does not undergo keratinization. The ORS is rich in vacuolated cytoplasm owing to its plentiful glycogen.
The point of insertion of the arrector pili muscle is referred to as the bulge area. The bulge area is where hair follicle stem cells are located. Stem cells from the bulge area migrate to other portions of the follicle and differentiate into different specialized layers. The isthmus is the segment that extends from the arrector pili muscle to the sebaceous gland duct entrance.
No more IRS can be found here. The ORS undergoes trichilemmal keratinization, producing large homogeneous keratinized cells without the formation of keratohyalin granules. The upper portion of the follicle above the entrance of the sebaceous duct is the infundibulum. It can be regarded as a funnel, filled with scaled-off keratinocytes and sebum. The sebum flow and the movement of the hair shaft push the dead keratinocytes toward the skin surface. The infundibular epidermis undergoes keratinization with the formation of keratohyalin granules similar to the interfollicular epidermis.
The glassy or vitreous membrane, which forms a homogeneous eosinophilic zone peripheral to the ORS, is periodic acid Schiff (PAS) positive and diastase resistant.
t differs from the interfollicular basement membrane
zone by being much thicker. It is thickest around the
lower third of the hair follicle in the suprabulbar area. The
fibrous root sheath is composed of thick collagen bundles.