The Hair
Hair is composed of a scleroprotein substance called keratin and lacks blood vessels and nerves. It usually contains pigments (except in albinos) and sometimes also contains interstitial air bubbles that give the hair a silver color. The structure of the hair consists of modified epithelial cells arranged in layers around a central medulla (or core) and covered with thin, flat scales. The root of each hair is located in an invagination of the epidermis called the hair follicle. Hair grows from the base of the follicle and is nourished by blood vessels present in a papilla located within the follicle, which extends a bit into the hair root. A small muscle, the arrector pili or hair erector, is attached to each hair follicle. The muscle contracts under the control of the sympathetic nervous system, causing the hair to stand up (“goosebumps”). Most mammals have tactile hairs that grow, in many cases, on the upper lip and eyebrows, with the roots situated over highly innervated erectile tissue.
Hair is produced from hair follicles located on the surface of the skin. Originally, it develops in the early stages of fetal life and falls out in a regular cycle throughout life. The hair cycle is not synchronized in humans. Typically, the scalp has a majority of hairs in the growth phase, and a minority in the transitional and resting phases of the cycle. The growth phase of the hair follicles on the scalp has a longer duration than the other phases, and this duration is related to the final length of the hair. (see hair growth cycle)
Hair is influenced by the entire body, especially hormonal influences, stress, fatigue, etc. These influences act at the level of “the hair factory,” the dermal papilla, located at the hair root. This papilla is rich in cells, especially containing numerous capillaries, and their number and activity are essential.
STRUCTURE AND CHEMICAL COMPOSITION OF HAIR
Hair is an incredible material, biologically lifeless yet possessing a vitality of its own. Understanding its complexity requires delving into its intricate structure. Hair resides in a skin depression known as the hair follicle, extending through the dermis and eventually reaching the subcutaneous layer, enveloped by a sheath of connective tissue. The follicle, with an oblique orientation, concludes in a thickening called the hair bulb, housing an ovoid cavity filled with loose connective tissue—the dermal papilla. This papilla, linked to dermal tissue through a small opening, serves as the nutritional source for the hair. In an active follicle, the lower half of the papilla is surrounded by mitotically active bulb cells. Rapid hair growth occurs when blood circulation is active, contrasting with slower growth in less active states.
– External reticular sheath
– Internal reticular sheath: This, in turn, is formed by three thin layers: 1. Huxley’s layer, 2. Henle’s layer, 3. Cuticle of the internal reticular sheath.
– Connective tissue sheath
The longitudinal organization of the hair follicle can be divided into seven regions with anatomical boundaries.
The permanent portion of the follicle begins in the hair canal region, extending from the skin surface to the dermoepidermal junction. Below it extends the intraepidermal part, the ‘infundibular unit.’
The infundibulum extends down to below the opening of the sebaceous gland. It continues with the isthmus, starting below the opening of the sebaceous gland and ending in the promontory area, where the erector pili muscle attaches.
The transitional portion of the follicle begins at this point and extends to its depth.
The lower follicle includes the keratinized zone and extends from the promontory area to the apex of the hair bulb.
The hair bulb is the deepest portion of the follicular structure and surrounds the follicular papilla. The bulb is onion-shaped; if a line is drawn at the level of its greater diameter, dividing the bulb into two regions, upper and lower, this line is known as the “Critical Level of Auber” and separates a lower ‘active’ or germinative region from an upper keratinized region.
The matrix is formed by the part of the bulb below the critical level of Auber, consisting of undifferentiated cells that divide rapidly.
Another important anatomical feature of the follicle is that it is inclined in relation to the skin surface. The place where it forms an acute angle with the skin surface is the anterior zone of the follicle; the portion forming an oblique angle is the posterior.
The innervation of the hair follicle is similar to that in the entire skin area. The nerves surrounding it are sensory fibers, extending from the base of the bulb to its junction with the epidermis.
Some myelinated nerves run parallel to the follicle, and finer ones form a network around it. The arrangement of nerves in the follicle allows it to be considered a ‘follicular tactile organ.’
From the dermal plexuses or muscle-cutaneous arteries, dense plexuses of arterioles and capillaries originate, surrounding the follicle in the anagen phase. Blood vessels extend parallel to the direction of the follicle, and there are communications between them, forming a mesh around the lower third of the follicle; in the middle third, there are few interconnections, but these increase around the sebaceous gland. Some arterioles penetrate the dermal papilla, forming coils inside.
There is a direct relationship between the size of the follicle and vascularization, with smaller ones surrounded by fewer vessels. During the catagen period, the vascular system remains practically intact. Only when the lower third of the follicle reduces to a thin row of cells do some capillaries in the network degenerate.
In the telogen phase, the vessels that used to surround its lower part form a collapsed loop that takes the form of a bouquet. When the follicle becomes active again, the new bulb advances through the collapsed vessels below the dermal papilla, forming a new vascular network.
uralmente, está formado por:
– Root: It is the fixed part of the hair located within the hair follicle and extending to the hair bulb, where the papilla that nourishes the hair is situated. The root undergoes significant metabolic and mitotic (cell division) activity.
– Shaft: It is the free and main part of the hair, also called the shaft, and is composed of three concentric parts: cuticle, cortex, and medulla. The diameter of the shaft ranges from 40 to 100 microns, and the tensile strength of the hair is approximately 40 to 60 grams.
– Cuticle: It occupies 9% of the total thickness of the hair and is a protective layer resistant to physical and chemical forces that could quickly damage the hair fibers as they emerge from the skin. Its integrity provides shine to the hair and ease of sliding when brushed.
The cuticle consists of very thin, flattened cells, arranged on the surface of the hair like “tiles” with their free edge facing upward. The free edge of these cells interlocks with the edges of similar cells covering the outer root sheath, and their free edges point downward, making it difficult to pull out a hair, at least part of the inner root sheath. (a phenomenon of medical-legal interest)
– Cortex: It constitutes the middle layer of the hair shaft, occupying 70% of the total thickness of the hair, and is responsible for the mechanical properties of the hair. The cortex consists of several layers of cubic cells that gradually flatten and, after keratinization, transform into spindle-shaped cells, densely grouped with retracted nuclei. Pigment granules are observed in the cells and between them.
– Medulla: It represents 21% of the hair’s thickness. It is located in the central part and is composed of rounded, non-nucleated (if present, nuclei are very rudimentary), and lightly pigmented corneal cells, partly separated by air spaces. The keratin in these cells is soft, with a low sulfur content.
Hair, in cross-section, has a different appearance depending on the race:
- In Chinese, Eskimo, and Native American populations, it appears round.
- In individuals of African descent, it is elliptical.
- Wavy hair, present in individuals of various nationalities, including Caucasians, appears oval.
Chemically, hair is composed of the following substances: 28% proteins, 2% lipids, and 70% water. The most abundant protein is keratin, which constitutes between 85 and 90 percent of the total weight of the hair. Keratin is a protein composed of polypeptide chains rich in cysteine. Its main elements include carbon, hydrogen, oxygen, nitrogen, and sulfur. In smaller amounts, it contains calcium, copper, cadmium, mercury, zinc, lead, iron, arsenic, silicon, magnesium, uranium, vanadium, sodium, and potassium.
Types of Hair
Hair shape is one of the most precise and hereditary characteristics. The almost black hair of Papuans, Melanesians, and Africans grows from a curved follicle that continues in a spiral with a flat cross-section. The hair of Chinese, Japanese, and Native American individuals is straight, thick, long, and usually black. It grows from a straight follicle, with a circular cross-section, and has a easily distinguishable medulla. Ainu, European, Indian, and Semitic hair is wavy, growing from a straight follicle with a tendency to curl; the cross-section is oval, and the color varies greatly from individuals, ranging from light blonde to black.
Hair color comes from melanin, a pigment that gives hair and skin its color. There are two types of melanin, one responsible for dark tones (eumelanin) and the other for light tones, such as blonde and red (pheomelanin). The mixture and amounts of these pigments determine the natural color of your hair. In some way, it’s also true that the lighter the hair, the less melanin it has. A person with black hair has much more melanin than someone with blonde or red hair.
In humans, four types of hair are generally discussed, characterized mainly by their texture and length: lanugo, vellus, intermediate, and terminal.
Lanugo is fine, soft, and lightly pigmented, lacking a central medulla. There are two layers of lanugo, one covering the fetus and falling out just before birth, and another that grows after birth and falls out during the third or fourth month of life.
Vellus generally reaches a length of less than a centimeter. It grows in young children after the lanugo falls out and is fine, lightly pigmented, and medullated. Vellus continues to grow throughout life, usually accounting for between 6% and 25% of scalp hair.
Intermediate hair has approximately a centimeter in length and forms on the scalp of children between 3 and 7 months old. It can last up to 2 years.
Terminal hair grows much more than a centimeter and is denser and thicker than other types of hair. It grows on the scalp, eyebrows, and eyelashes before puberty. Secondary sexual hair, like pubic hair, armpits, or beard hair in males, is also terminal hair.
APPROXIMATE DATA AND FIGURES RELATED TO HAIR ON THE SCALP
- Number: 100,000 to 150,000.
- Density: 300 to 400 per cm2.
- Diameter: 40 to 100 microns (thousandths of a millimeter).
- Length: Up to 50 to 100 cm.
- Growth rate: 1 cm per month.
- Growth duration: 2 to 6 years (average of 3 years).
- Normal shedding: 50 to 100 per day.
HAIR GROWTH CYCLE
Hair development in humans begins in the embryo, and by the sixth month, the fetus appears covered in very fine hair called lanugo. In the early months of life, lanugo falls out and is replaced by hair, which is thicker on the head and eyebrows, while the rest of the body has fine, fuzzy hair. During puberty, thick hair appears in the armpits and pubic area in both sexes, and in males, it starts to grow on the upper lip and chin, giving rise to the beard and mustache. The growth rate varies with a person’s age and the length of the hair. When short, it grows between 1 cm and 1.5 cm per month, but the growth rate is halved when the hair is longer. The greatest growth occurs in women between 16 and 24 years of age.
Hair grows thanks to an active and constant mechanism, at a rate of 0.4 millimeters per day. Hairs are in different phases of growth and rest, as each follicle has its own cycle, marked by a different “biological clock” from those around it. Every day, new hairs grow on the head while others fall out, according to a cycle that is continuously renewed.
Each cycle consists of different phases: growth, regression, and rest. During a normal hair cycle, the hair grows, rests, and falls out to make way for new hair. In the scalp, the daily loss of between 50 and 100 hairs is considered normal, with excess shedding possibly indicating a disturbance in the cycle. The balance in the rhythm and duration of the hair growth cycle determines the stability of the hair mass. Growth phases are much shorter in body hair, explaining its limited length. The duration of the hair cycle varies depending on age and body region, as well as the length and thickness of the hair or fur, respectively.
Follicles can be in the growth phase or “anagen,” in the transition phase or “catagen,” or in the resting phase or “telogen.” In humans, the growth cycle is not synchronized, so every day, some hairs fall out, and others begin to grow.
It’s worth noting that cutting hair does not influence its growth.
– Growth or anagen phase
The anagen phase is the growth phase of the hair follicle. Approximately 80% of the hair on the scalp is in this phase at any given time. These follicles are metabolically very active and, as a result, highly sensitive to nutritional changes and chemical damage. For example, during chemotherapy for cancer treatment, many patients lose most of their hair.
The hair on the scalp has a relatively long anagen phase, which can last between 2 and 5 years (an average of 3 years). Hair cycles on any other part of the body are generally shorter, and the anagen phase lasts only months, not years.
There is a direct relationship between hair length and the duration of the anagen phase. In other words, the longer the anagen phase, the more the hair grows. An exaggerated example of this is what happens in Merino sheep, where it is believed that the anagen phase lasts a lifetime, and the wool never falls out, continuously growing. Obviously, the growth rate is also an important factor, typically around 0.34 mm per day (1 cm per month).
– Transition or catagen phase
This is the shortest phase of the hair cycle, with an average duration of 14 days. The number of hair follicles in this phase at any given time is very small. During this phase, mitosis (cell division) stops in the germinal matrix, and the already formed cells complete their differentiation. The deepest part of the follicle shortens and shrinks. The cells that have left the matrix undergo rapid keratinization without advancing upward. In this way, the follicle transforms into an ovoid mass attached to the hair. This mass is surrounded by an epithelial covering in the form of a sac of undifferentiated epidermal cells originating in the outer root sheath. This sac extends under the sheath in a filament connecting it to the now atrophic dermal papilla, while the mass is released and ascends.
– Resting or telogen phase
Telogen is the resting phase of the hair cycle, and under normal conditions, less than 20% of the hair is in this phase at the same time. Telogen lasts approximately 3 months. Upon transitioning from the transitional phase to telogen, the hair temporarily stops growing.
Telogenic hair has a club-shaped end inside the follicle, and usually, once keratinized, it falls out during this phase or in the next anagen cycle. Occasionally, telogenic hair does not fall out, and another anagen hair may grow next to it in the same follicle. During this 3-month period, a new matrix gradually forms from the stem cells of the basal layer of the sheath, returning to the anagen phase. This cycle repeats about 25 times throughout life. The dermal papilla regulates the hair growth cycle.
Certain conditions can increase the ratio of hair in telogen and temporarily lead to significant density loss. It is crucial to diagnose this form of alopecia correctly and differentiate it from others, such as diffuse androgenetic alopecia, to avoid mistakes in treatment approach.
NORWOOD SCALE
In 1975, a dermatologist and hair transplant surgeon, Dr. O’Tar Norwood, building on the previous work of Hamilton, developed a classification model for male pattern hair loss that is widely used today.
He divided androgenetic hair loss in men into two common patterns:
- The ordinary type, characterized by hair loss that starts in two different areas (the temples and the crown) and gradually merges, creating an extensive balding area.
- Type A (less frequent), characterized by “front-to-back” recession. Hair loss that is not of genetic origin and generally follows a different pattern.
Some women also lose hair according to the pattern described by Norwood, but more commonly, they experience diffuse hair loss rather than specific patterns like men.
Hair loss tends to progress with age, although the rate of loss can vary considerably from person to person and change significantly over time.
Although hair density in a given loss pattern tends to decrease with age, there is no way to predict which pattern a young man with early male pattern baldness will eventually assume. Generally, those who start losing hair in their third decade (20s) are the ones who will experience more severe loss. However, individuals who end up being extensively bald do not always show significant hair loss in their 20s. In some men, the onset of male pattern hair loss may be delayed until the late fourth to fifth decades of their lives.
These characteristics define the cyclical, variable, and unpredictable nature of Androgenetic Alopecia.
LUDWIG SCALE
It classifies female androgenetic alopecia into three evolutionary degrees:
(Grade I) Minimum.
The first sign is a thinning of the hair, generally at the front and top of the head. Progressively, you’ll notice more hair in your comb, reduced volume, and slower growth.
(Grade II) Moderate.
Volume loss becomes more pronounced, creating a kind of transparency, allowing the outline of the head to be visible through the hair. The hairs become very fine, fragile, brittle, and lighter.
(Grade III) Severe.
At this stage, baldness is already established. In the frontal line, the aromatase enzyme helps protect the hair from hormonal action. At least 60-70% of the hair on the top of your head has already thinned, and covering the baldness becomes very challenging from any angle.
