Beyond the surface: A deep dive into hair anatomy and growth
The human body is covered almost entirely with hair, most of which are tiny, colorless vellus hairs. As mentioned above, the few places devoid of hair include the palms, soles, and mucosal regions of lips and external genitalia. Mammalian hair has many functions, including protection against external elements, thermoregulation, producing pheromones, apocrine sweat, and sebum, and playing a role in social interactions.
Whether you’re struggling with hair loss, thinning hair, or simply curious about how your hair grows, understanding its anatomy and growth cycle is key to maintaining healthy hair.
The Building Blocks: Understanding Hair Anatomy
Each hair has a hair shaft and a hair root.
- Hair Shaft: Visible Strength
The shaft is the visible part of the hair that sticks out of the skin.
The hair shaft comprises:
- Cortex
- Surrounding cuticle cells
- Central medulla found in thicker hair
The bulk of this hair fiber belongs to the cortical layer, which plays a vital role in determining the physical and mechanical properties of the hair, such as strength, texture, and color.
The shaft is composed predominantly of macrofibrils, which are rods of microfibrils meshed together in a matrix.
- Hair Root: Where Hair Growth Begins
The hair root is in the skin and extends down to the deeper layers of the skin. It is surrounded by the hair follicle (a sheath of skin and connective tissue). The follicle is the primary structure from which hair can grow. The histological arrangement of the follicle is divided into outer and inner root sheaths.
- The outer root sheath (ORS) has been recognized as a ready supply of multipotent stem cells that differentiate into several cell types, including melanocytes and keratinocytes. More specifically, these stem cells are thought to reside in a distinct bulge area located between the insertion of the arrector pili muscle and the ductal opening of the sebaceous gland.
- The inner root sheath (IRS) consists of the Henle layer, the Huxley layer, and the cuticle layer, which also helps affix the growing hair shaft to the follicle, a task bolstered by the production of keratins and trichohyalin by the IRS cells.
The cuticle comprises flat overlapping cells covering the hair shaft from the root until it exits from the epidermis. The cuticle is of considerable cosmetic importance as it gives the hair an untangled appearance and shape.
- Hair bulb: At the base of the hair, the hair root widens to a round hair bulb. The hair bulb is the region of the follicle that actively produces hair. The bulb extends into the dermal layer of the skin and surrounds the dermal papilla, an important structure derived from mesenchyme, made of rich stroma, associated nerve fibers, and a loop of the capillary that supplies nutrients.
- Hair papilla: The papilla is believed to be a primary orchestrator in the hair growth process, conducting the precise signals that determine the size and color of the hair shaft via a complex mix of essential growth factors, including insulin-like growth factor, stem cell factor, keratinocyte growth factor, and bone morphogenetic protein.
- Arrector pili muscle: Each hair follicle is attached to a tiny muscle (arrector pili). Autonomic nervous innervation primarily provides control of the arrector pili muscle. Contraction of these tiny muscles makes the hair “stand on end.” This is likely a vestigial function related to fur; erecting the shafts served to trap air, conserve heat in cold climates, and cut a larger silhouette to intimidate rivals or would-be predators.
- Nerve: Nerve supply to the hair follicles is similar to that of the surrounding network of dermal nerves in that it is composed of both sensory afferents and autonomic sympathetic nerves. Sensory information from hair stimulation enhances tactile ability. These nerves sense hair movement and are sensitive to even the slightest draft.
- Blood vessels: Vascular supply is provided by small arterioles originating in the subcutaneous fat. The vessels nourish the hair follicle by delivering oxygen and nutrients, eliminating waste, and promoting growth. Subtle hair loss on the lower extremities can sometimes hint at the underlying peripheral arterial disease.
Hair growth Cycle
Hair growth is regulated by vascular, endocrine, and neural stimuli in addition to age and nutritional habits. Hair from the scalp is shed at approximately 100 to 200 follicles/day and grows at a rate of about 0.35 mm/day, 1 cm/month, or 15 cm/year.
Like skin, hair forms by rapid division and differentiation of stem cells, forming keratinocytes that migrate, flatten, and die, forming keratinized cells. The final hair product exposed on the skin’s surface will be composed entirely of keratin. The growth of the hair follicle is cyclical. Stages of rapid growth and elongation of the hair shaft alternate with periods of quiescence and regression driven by apoptotic signals. This cycle can be divided into three phases:
anagen (growth), catagen (transition), telogen (rest), and Exogen (shedding).
Anagen: Anagen growth is the active phase in which the hair follicle takes on its onion-like shape and works to produce the hair fiber. The anagen phase can be further broken down into proanagen and metanagen phases.
- Proanagen sees the follicle proliferating hair progenitor cells and begins the differentiation process.
- The new hair shaft appears on the skin’s surface to mark the metanagen phase.
The anagen phase as a whole can last for several years.
Catagen: The catagen phase begins with the end of the anagen phase and is characterized by a transition into quiescence. During this phase, which can last a few weeks, the hair follicle undergoes apoptosis-driven regression and loses about one-sixth of its standard diameter. The formation of a club hair, an important prognostic indicator in assessing hair pathology, also occurs at this time.
If many hairs form club hair at once and are subsequently shed, it can give the appearance of thinning. Some conditions this may occur in include, but are not limited to, hypothyroidism, hyperthyroidism, stress, vitamin deficiencies, and post-childbirth.
Telogen: Next is the telogen or resting phase of the hair cycle, in which the hair follicle is dormant, and growth of the hair shaft does not occur. About 10 to 15% of all hairs on the body are in this resting phase at any given time and can remain in this state for a variable amount of time, depending on the location of the hair – from a few weeks for eyelashes to nearly one year for scalp hair. The exact mechanism that controls passage from one phase into the next is not fully known.
Exogen: The exogen phase is essentially an extension of the telogen stage of hair growth. During the exogen phase, hair is shed from the scalp, often helped along by washing and brushing. Losing 50 to 100 hairs per day during the exogen phase is normal.
During the exogen phase, which can last several months, new hairs are growing in the follicles as old hairs fall away.
Conclusion
In conclusion, understanding hair anatomy and the growth cycle is crucial for maintaining healthy, strong hair. The hair shaft, hair follicle, and hair root all play distinct roles in the process of hair growth, while factors like nutrition, hormonal balance, and stress can influence the health of your hair. By learning about the various phases of hair growth, you can better appreciate the intricate processes that determine hair health and growth. Proper hair care, nutrition, and the right hair growth supplements can help ensure your hair remains healthy throughout its growth cycle. Whether you’re dealing with thinning hair or looking to improve hair quality, understanding hair growth at a deeper level is the first step to achieving your hair goals.
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References
- https://www.ncbi.nlm.nih.gov/books/NBK499948/
- https://www.ncbi.nlm.nih.gov/books/NBK546248/
- https://www.healthline.com/health/stages-of-hair-growth