Hair scalp and skin are similar in structure. Hair scalp is the skin that covers the skull, and it contains more capillary vessels, nerves, oil, and sweat glands than the rest of the body skin. Hair scalp is composed of hair follicles. You can see a section of skin at the end of the page.
Its outer layer is keratin, and it is composed of dead cells. Under the keratin, there is Malpighi layer which is composed of living cells. The cells of this layer contain melanocyte pigment, and they give color to the skin. They protect the body from heat and cold.
It contains sweat glands, neural crests, capillary vessels, hair roots, tactile corpuscles. Its job is to provide heat, sweat and water balance of the body, to protect the body from external factors, to sense touch, pressure, and pain. Through nerves, it passes the information to the brain. With joy, fear,excitement and cold weather, skin nerves make hair upright by straining hair muscles.
Hair and Hair Structure
The approximate hairy skin surface for an adult is 1000 cm2. Every cm2 contains approximately 100 hair strands. Total hairy skin of a normal white race adult contains approximately 100.000 hair strands. For the yellow race, it is 140.000, and for the brown race, 110.000.
Actually, all of our skin is covered with hairs, except for our palms, foot soles, and lips. But many of them are unable to be seen by the naked eye since they are so thin and colorless. The hair on our body are 10 times more than the hairs on our scalps. Chest, shoulder, back, armpit and genitalia are the areas where hair grows stronger than the rest of the body. Hair is rich in carbohydrates, urea, uric acid, free amino acids, phospholipids, cholesterol, fatty acids, and most importantly, keratin. Hair’s main structure is keratin which is composed of 18 amino acids and keratinocytes which are in the base of dermal papilla
Keratin is a cross-linked, spiral structured, fibrous protein that is found in many areas of our skin. It gives strength to hair since it is a rare protein nearly insoluble in water.
Hydrogen: What makes hair elastic is the hydrogen bonds in keratin. Hydrogen ions give hair shine.
Sulfur: Polypeptide chains which form the keratin, get tied together with sulfur bridges. Hair is composed of keratin molecules which tie together through sulfur bridges.
- If the sulfur bonds are strong, the hair becomes resistant against physical and chemical factors. Disulfide bonds can be broken only with ultraviolet lights, high temperature, oxidant matters, strongly acidic and alkaline matters.
- Cysteine: Cross bonds which tied with disulfide bonds, make hair strong.
- Proteins: Proteins in hair are insoluble in water, and they resist against proteolytic enzymes.
The follicle is the structure that hair grows in. The porous structure that contains hair releases oil for the scalp and hair. Special areas outside of the porous structure contains special muscles.
Papilla is where the first signals for hair growth are sent. These signals connect with receptors in the related follicle cells. Cytokines are proteins that affect the growth in a positive of negative way, depending on the cell type.
Matrix cells cover the papillae and play an important part on hair growth by dividing fast.
Inner root sheath
Does not contain melanin., cannot be keratinized. Composed of Huxley’s layer and Henle’s layer.
Outer root sheath
Is composed of cells in a row. It wraps the follicle like a glove. In the early stages of anagen phase, it gets long by dividing. But for the next phases, the dividing stops.
There are 3 layers on a hair shaft: Medulla, cortex, cuticle.
Medulla: Medulla is the canal-shaped innermost part of the human hair. It has no known function and not all hair contains a medulla.
Korteks: The cortex is the center part of the hair and makes up the majority of the hair shaft. It gives the hair its special qualities such as elasticity and curl. The cortex is packed with strands of keratin that lie along the length of the hair.
Cuticle: The cuticle refers to the outside layer of the hair shaft. It is composed of transparent, overlapping and protective scale-like cells. The cuticle protects the inner structure of the hair. A healthy, flat cuticle layer promotes strength, shine and the overall appearance of the hair.
The Development of Hair
First hair follicles form around the mouth when we are still in our mother’s womb. Around the 22nd week, follicles finish their development. Hair follicles before birth are colorless, thin, and soft and called lanugo.
After birth, some of the hair turns into strongly pigmented hair, some remain colorless and soft, and some in between these two types. Developed follicles are different sized in different parts of the body. For example, 2-2,5 mm in the eyebrow, 3 mm in the arm, 3-3,5 mm in the arm pit, 3-4 mm in the genitalia, 3-3 mm in the leg. Below, hair development is described in graphics.
Hair development starts in the third month of the fetal life, and by time, all body gets covered by long lanugo hairs without the medulla. These fall off in the eighth month of fetal life and replaced by short lanugo hairs, and these are fall off in the third or the fourth month of life. Normally, lanugo hairs are no longer occurs after these phases. They are replaced by two types of hair: vellus and terminal.
Vellus hairs are one or two centimeters long, short hairs. They appear colorless because they contain very few pigments. They do not contain contiguous sebaceous gland and medulla. They do not affect physical appearance since they are thin and soft.
Normal hair characteristics are present in this type of hair. Usually, our hair and other body hair are counted as terminal hair. They grow out of the follicles which contain contiguous sebaceous glands. Terminal hairs are formed in dark pigmented hair fibers and medulla layer between them.
The color of hair is determined by melanocytes. Hairs get pigmented only in their active growth process. Because melanogenesis activity only happens the anagen phase of the hair cycle.
The Hair Growth Cycle
Active Growth Phase (Anagen)
Approximately %85 of human hair is in this phase. It can take 2 to 6 years.
Transition Phase (Catagen)
Catagen phase is followed by the resting phase and normally takes 1 to 4 months. Hair does not grow in this process. Approximately %15 of human hair is in this phase.
Resting Phase (Telogen)
Hair roots get to rest in this phase. First, the hair falls off, then, the follicle remains empty. After the resting ends, new healthy hair forms and therefore a new hair grow cycle begins.
Growth Rate of Hair
The daily growth rate of hair differs depending on the phase the hair is in.