See also chemistry of hair explained
Introduction: Biology of the Hair:
Hair shaft consists of an outer cuticle surrounding a central cortex which may contain a central medulla. (Folin, Forensic Science International 83 (1996) 191-199).
Hair consists of : (1) an outer cuticle wrapping and protecting the hair fiber: (2) a cortex (the hair fiber itself): The cortex is composed mainly of 3 different types of proteins: fibrous keratins which constitute filaments, non-fibrous matrix proteins and cell membrane peripheral proteins. (3) a central medulla.
Hair follicle is a dynamic tissue that cycles between an on-state (anagen) and off-state (telogen). There are three major compartments, the hair shaft (medulla, cortex and cuticle), the inner root sheath (cuticle, Husley’s lyaer and Henle’s layer) and most peripheral, the outer root sheath. At the common base of these compartments are the pluripotential matrix cells that give rise to the hair shaft and inner root sheath through differentiation. Beneath these matrix cells are a group of mesenchymal-derived cells that form the dermal papilla which play an important role in inducing hair follicle neogenesis. (Bertolino, “Differential Expression of Type I Hair Keratins” J Invest Dertmatol 94: 297-303, 1990).
The part of the hair base embedded in the skin is called the hair follicle and consists of plural different tissues. It is involved in the differentiation and growth of hairs. HFs regenerate by repetitive hair growth cycles (see below) and produce hair fibers during postnatal life.
–Cuticle:
The cuticle consists of a thin overlapping layer of cells, like a system of scales, and provides protection for the inner core of the hair, known as the cortex. Cuticle cells are mechanically robust and are resistant to many environmental factors, including the penetration of foreign molecules, due to the high degree of isodipeptide and disulfide bonds present in its various lamellar layers. The cuticle represents roughly 15% of the weight of a strand of hair.
In the hair follicle the cuticle develops as a thin layer of cells between the hair shaft cortex and the inner root sheath. Once the cuticle cells begin to differentiate they accumulate cysteine rich, glycine rich keratins in their cytosol (Jenkins, J Invest Dermatol, 1994, 103(3), 310-7).
–Cortex:
The cortex is the middle region of the hair and represents about 80% of the weight of a strand of hair. The cortex is composed of hierarchal structures of elongated, fibrous alpha-helical keratin extending along the fiber axis embedded in an amorphous protein matrix. It is the cortex with its system of intertwined protein fibers and supporting matrix that gives hair its tensile properties (strength, elasticity) of hair.
The cortex of animals hairs, which represents about 85% of the fiber weight, is made up of microfibrils embedded in a matrix of high-sulphur and high-glycine and tyrosine proteins. Microfibrils are composed of closely packed intermediate filaments (IF) arising from the interaction of type I and Type II keratins. Comparisons of the amino acid sequences of keratins show that the central rod domains, containing about 310 amino acid reisudes, are highly conserved while the N- and C-terminal end domains of Type II deratins exhibit appreciable species specific differences, which can be exploited to produce monoclonal antibodies that react exclusively with the hair fibers from a given animal. For example, Paluzzi (“anti-keratin monoclonal antiboides for identifying animal hair fibers” Textile Research Journal 2004, 74: 458) dislose large difference in the immunoreaction with keratins from cashmere compared with those from wool.
Adipocytes and Epithelial HF stem cells (eHFSCs): In the skin, hair follicles (HFs) grow cyclincally, alternating between resting (telogen), actively growing (anagen), and regressing (catagen) phases. Transition into anagen is critically dependent on transient proliferative activation of epithelial HF stem cells (eHFSCs) located in its bulge and the hair germ compartment. Despite residing beneath the skin surface, eHFSCs demonstrate responsiveness to external regenerative cues via neuronal circuits. Although sever skin injuries can lead to scarring and loss of HFs, evidence suggests the eHFSCs can respond to local inflammation, promoting hair regeneration following mild injuries or infections. For instance, superficial skin injuries, like repeated friction, habitual biting, insect bites, and orthopedic case irritation can lead to excessive hair regrowth. Deeper injuries, such as burns, vaccinations, and bone fractures, also induce local hair regrowth. During tissue infection, adipose tissue can rapidly produce antimicrobial peptides to suppress bacterial growth. Given its integral presence in various organs and proximity to tissue resident stem cells, adiopose tissues likely function as a reactive signaling niche, capable of stimulating tissue resident stem cells on demand, such as upon injury. Adipocytes sotre large quatities of lipids for future metabolic needs and rapidly release them into the environment to meet urgen regenerative deamin. Upon completion of this task, adipocytes are refilled with lipids and restore their primary physiological function as energy depots. In the skin, close proximity of dermal white adipose tissue (dWAT) cells to eHFSCs facilitates reciprocal regulation between them. Skin inflammation has been shown in a mouse model to rapidly and reversible induce dWAT adipocyte lipolysis through macrophage to adipocyte crosstalk, whereas macrophages induced production of serum amyloid A (SAA)3, a lipolysis inducer in adipoctyes. Released monounsaturated fatty activated quiescent eHFSCs by promoting mitochondrial biogenesis and fatty acid (FA) oxidation (FAO) through peroxisome proliferator activated receptor gamma coactivator 1-alpha (Pgc1-alpha) signaling. Ht all FAs were euqally potent stem cell activators. Aalysis of dWAT derived lipids showed that major constituent FAs were long chain unsaturated fAs (oleic acid, linoleic acid, palmitoleic acid, palmitic acid). Monounsaturated FAs preferentially activate eHFSCs by pormoting FAO for energy production. In summary, adipocytes serve as facultative niche cells for eHFSCs in injured skin. Responding to an upstream immune trigger, adipoctyes udner rapid lipolysis preceding proliferative activaiton of eHFSC. (Tai, “Adipocyte lipolysis activates epithelial stem cells for hair regeneration through fatty acid metabolic signlaing” Cell Metabolism 37, 22-2-2219, 2025).
Dermal papilla (DP): is enveloped by the dermal sheath in the hair bulb of a hair folicle (HF). The DP consists of specialized mesenchymal Dermal papilla cells (DPC) which control the development of hair folicles via-cell-cell interactions and extracellular molecules. The DP controls the hair growth cycle and directs hair bulb matrix epithelial cells to differentiate to produce hair fiber. Secretion factors of DPs also participate in controlling the HF morphogenesis and regulate the phase transition of hair growth cycle, such as fibroblast growth factor.
surface of hair: is composed of the protein Keratin, containing both acidic and basic amino acids. The amino acid composition of keratin is such that at natural pH values, the hair possesses a net negative charge. (US2,067,499).
Hair Growth Cycle:
Anagen: The first cycle, known as anagen, the follicle is generated and new hair grows. When a new hair cycle is initiated, it is thought that a signal from the dermal papilla stimulates the stem cells, which are thought to reside in the permanent portion of the follicle, to undergo a phase of downward proflieration and genesis of a new bulbous base contianing matrix cells which then surround the dermal papilla. As a new anagen state progresses, these hair matrix cells produce a new hair. Each follicle appears to be under completely asynchronous control, resulting in a continuum of follicles in anagen, catagen, and telegen phases, leaidng to a relatively homeogenous hair distribution.
Catagen: During the second stage, the follicle elongation ceases and the follical regresses because the matrix cells stop proliferating. At this stage, the lower, transient half of the follicle is eliminated due to terminal differentiation and keratinization, and programmed cell death. Also during catagen, although the dermanl papilla remains intact, it undergoes several remodeling events, including degradation of the extracellular matrix that is deposited during analgen. At the close of catagen, the hair is only loosely anchored in a matrix of keratin, with the dermal papilla located just below. The catagen stage occurs at a genetically predetermined time, which is specific for each hair type in a species.
Telogen: is characterized by the follicle entering a quiescent phase, during which the hair is usually shed.
Hair Care:
Protect your hair by shampooing every other day instead of daily. Shampoos and excess shampooing can strip hair of moisture. Use warm water and a mild shampoo with sunscreen. Apply extra conditioner to keep your hair hydrated, shiny, and soft. Don’t overstyle with the blow dryer or flat iron. And protect your hair from the elements by wearing a hat.