It may come as a surprise to nearly everyone to learn that except for mucus membranes, the palms of the hands and soles of the feet, both men and women, and all children, are completely covered in hair. How is that possible, you ask? It has to do with the differences in types of hair that exist on our bodies.
The fact is the “naked ape” is not naked at all. Per square centimeter, human skin has as many hair follicles as that of other great apes. The difference is not in the number, but in the fineness of the hair that grows from those follicles. These fine human hairs do not seem to be performing any of the functions of their counterparts in more hirsute species (insulation and, through coloration, either signaling or camouflage). So what are they for?
Some scientists feel that fine body hair may serve as an evolutionary alarm system, alerting their owner to creepy crawlies such as bed bugs and other biting insects or arachnids. Compared to our cave dwelling days, modern life has less need for such an alarm system. Nonetheless, we all freak when we feel something tiny and nearly weightless working their way across our skin during the night. The system works well.
Anatomy of hair – the hair follicle
The hair follicle is the mammalian skin organ that produces hair. Within the follicle, stem cells are responsible for hair production and the shape of the hair follicle has an effect on the hair shape and texture of the individual’s hair.
Hair follicle structure includes:
Papilla – a large structure at the base of the hair follicle.[The papilla is made up mainly of connective tissue and a capillary loop. Cell division in the papilla is either rare or non-existent.
Matrix – which surrounds the papilla.
Root sheath – composed of an external and internal root sheath. The external root sheath appears empty but is filled with cuboid cells. The internal root sheath is composed of three layers, Henle’s layer, Huxley’s layer and an internal cuticle that is continuous with
Bulge – located in the outer root sheath at the insertion point of the arrector pili muscle. It houses several types of stem cells, which supply the entire hair follicle with new cells, and take part in healing the epidermis after a wound.
Other structures associated with the hair follicle include the cup in which the follicle grows known as the infundibulum, the arrector pili muscles, the sebaceous glands, and the aprocrine sweat glands. Hair follicle receptors sense the position of the hair. The arrector pili muscle causes the follicle to become more perpendicular and protrude slightly above the surrounding ski. The phenomenon is the cause of “goose bumps”. The sebaceous gland produces the oily or waxy substance sebum. The higher the density of hair, the greater the number of sebaceous glands. During puberty, the production of sebum increases dramatically, sometimes clogging pores that becoming inflamed resulting in an acne pustule.
The structure of hair – the hair shaft
Hair fibers have a structure consisting of several layers, starting from the outside: the cuticle, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles; the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like. It is the source of mechanical strength and water uptake. It also contains melanin which gives hair its color.; the medulla, a disorganized and open area at the fiber’s center.
The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and other shaped fibers are generally wavier or curly. The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water. The diameter of human hair varies from .017 to .18 millimeters (0.00067
Three kinds of hair exist on the human body – lanugo, vellus, and terminal
Lanugo is very fine, soft, usually unpigmented, downy hair found on the body of the fetus and occasionally on newborn babies. It is the first hair produced by fetal hair follicles, and usually appears at about five months of gestation. It is normally shed before birth, around seven or eight months of gestation. It is sometimes present at birth but disappears on its own within a few days or weeks.
Lanugo hair is then replaced on the same surfaces by vellus hair which is finer and more difficult to see. Anyone who closely examines a skin surface where hair is not readily visible will see fine vellus hair protruding from every follicle.
The much more visible hair (e.g. on the head during childhood) that persists into adulthood is called terminal hair. It forms in specific areas and is hormone dependent.
Puberty dramatically increases the amount of terminal hair in both sexes, and from the same follicles that formerly produced vellus hair. The flood of hormones during our teen age years s the cause. Axillary and pubic terminal hair becomes visible as does hair on the extremities and the torso.
Males, because of substantially higher levels of testosterone, generally develop coarser, more visible hair, as well as facial hair. Ethic heritage, however, has great impact in how the terminal hair distribution pattern evolves. Some women are naturally much hairier than others, and there are men with surprisingly little body hair.
For women, excessive hirsutism can also indicate polycystic ovarian syndrome. The Ferriman-Gallwey scoring system (below) is used to evaluate the significance of hair patterns in women. Scores higher than eight are suggestive of abnormally high androgen (testosterone) levels and possible ovarian dysfunction. Notice that scores lower than eight are associated with considerable amounts of terminal hair that is widely distributed.
Phases of hair growth – anagen, catagen, and telogen
There are three distinct and concurrent phases of hair growth. All three occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase. Hair in different parts of the body also have specific characteristics that determine the length of hair.
Anagen phase – Anagen is the active growth phase of hair follicles during which the root of the hair is dividing rapidly, adding to the hair shaft. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for 2–7 years and is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
Catagen phase – The catagen phase is a short transition stage that occurs at the end of the anagen phase. It signals the end of the active growth of a hair. This phase lasts for about 2–3 weeks while the hair converts to a club hair. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2-week process, the hair follicle enters the telogen phase.
Telogen phase – The telogen phase is the resting phase of the hair follicle. When the body is subjected to extreme stress, as much as 70 percent of hair can prematurely enter a phase of rest, called the telogen phase. This hair begins to fall, causing a noticeable loss of hair. This condition is called telogen effluvium. The club hair is the final product of a hair follicle in the telogen stage, and is a dead, fully keratinized hair. Fifty to one-hundred club hairs are shed daily from an average adult.
Hair growth cycle times – affected by individual, hair color, and follicle shape Scalp:
- anagen phase, 2–8 years (occasionally much longer)
- catagen phase, 2–3 weeks
- telogen phase, around 3 months
- anagen phase, 4–7 months
- catagen phase, 3–4 weeks
- telogen phase, about 9 months
Hair exists in a variety of textures. Three main aspects of hair texture are the curl pattern, volume, and consistency. A chart of the types of textures of hair is below.
The derivations of hair texture are not fully understood. All mammalian hair is composed of keratin, so the make-up of hair follicles is not the source of varying hair patterns. There are a range of theories pertaining to the curl patterns of hair. Scientists have come to believe is that the shape of the hair shaft has an effect on the curliness of the individual’s hair. A very round shaft allows for less disulfide bonds to be present in the hair strand. This means the bonds present are directly in line with one another, resulting in straight hair. The flatter the hair shaft becomes, the curlier hair gets, because the shape allows more cysteines to become compacted together resulting in a bent shape that, with every additional disulfide bond, becomes curlier in form. As the hair follicle shape determines curl pattern, the hair follicle size determines thickness. While the circumference of the hair follicle expands, so does the thickness of the hair follicle. An individual’s hair volume, as a result, can be thin, normal, or thick. The consistency of hair can almost always be grouped into three categories: fine, medium, and coarse. This trait is determined by the hair follicle volume and the condition of the strand. Fine hair has a small circumference in relation to medium and coarse strands; coarse hair having the largest circumference. Coarse hair has a more open cuticle than thin or medium hair causing it to be the most porous.
Hello that is interesting subject matter. I was wondering your thoughts on PRP. Some people are using it for hair thinning or to improve skin health. Do you feel it is inflammatory or is there any science in terms of it providing any benefits – healing, collagen growth, vascular, etc. Also, on a separate note was wondering if one can micro needle and use your serum after instead of your microneedling solution. Is that ok if there are more ingredients in the serum or wound that pose more irritation potential? Wasnt sure if the serum can be applied in that way. Thank you!
DrGeorge will soon be adding to this series with his thoughts about current approaches to hair growth stimulation. I’ll give you a quick answer, and let him fill in the gaps. Some of the cytokines (not all) that we consider inflammatory may actually be helpful in reawakening hair follicles from a resting to an active growth state. The evidence shows that PRP is useful for stimulating hair regrowth. We are still discovering which growth factors in particular are most helpful. Keep in mind that PRP for hair growth is applied infrequently (e.g. at 2-4 week intervals) for a number of months. This is not the same as applying inflammatory cytokines to skin on a daily basis, as you might with a skin serum. With such infrequent application, the scalp has a chance to shift from an inflammatory to an anti-inflammatory milieu on its own. You avoid the chronic inflammatory stimulus. The same is true of face, although skin there is thinner and more likely to be sensitized, and so we strive for inflammation free rejuvenation, more like fetal development. In our own lab we have developed an approach to hair growth that involves driving msesenchymal stem cells down a certain pathway that changes the cocktail of cytokines and growth factors they produce, and which is known to stimulate hair growth. We then add to that some individual growth factors most important for hair regeneration. This can be used instead of PRP, and also in addition to PRP. For anything that disrupts the skin barrier (such as microneedling with PRP) we don’t want to put any molecule on the skin that is not native to humans. So we use only human growth factors, derived from human stem cells, and hyaluronic acid, which is also native to humans. We have also created a separate product for daily use that contains a plethora of other ingredients known to stimulate hair growth. This is all currently in clinical testing and we will report results soon.
PRP has been used with success in managing hair loss and as a topical for dermal needling – the so-called “Vampire Facial.” Not sure if this term is trademarked, but if you want a “Vampire Facelift” where PRP is injected in conjunction with fillers, there’s a doctor in Alabama who wants his royalty payment.
For those who may be unfamiliar with PRP (platelet rich plasma), let me first describe what it is and how it is obtained. The patient’s blood is drawn and centrifuged. The platelet rich layer is “activated” which allows its multiple growth factors and cytokines to be released.
As BFT readers know, platelets are chock full of growth factors and cytokines with the alpha granules being the main repository. In the granules are high concentrations of platelet-derived growth factor, endothelial growth factor and transforming growth factor (TGF), together with the anti-inflammatory and proinflammatory cytokines interleukin IL-1, IL-4, IL-6, IL-8, IL-13, IL-17, tumor necrosis factor (TNF)-α and interferon (IFN)-α. The overall pattern is highly pro-inflammatory. Notably, the powerful anti-inflammatory cytokines TGF-beta3 and IFN-gamma are not present.
The inflammatory response of tissues when platelets are activated is rapid onset of inflammation. The occurs regardless of the source of trauma. For an animal bite, inflammation is good as it will help destroy microbial contamination. For surgery with sterile instruments, or with significantly traumatic facial medical procedures, inflammation is associated with increased pain and tend to promote more fibrotic healing and abnormal pigmentation. Back to your questions.
1) There is good evidence PRP is helpful when injected into the scalp of patients with thinning hair. In that case the increase in blood flow associated with inflammation, and many of the bio-signals seem to be helpful. (Recall Rogaine enhances local blood flow.) PRP is also recognized as helpful is promoting bone growth, and other indications.
2) PRP with dermal needling does not make sense to BFT. If deep enough to cause bleeding, it is deep enough to cause local platelets the release there bio-signals, so why add more? Despite what many, many physician continue to believe. inflammation is NOT needed to initiate a healing response and my be counter productive as being pro-fibrotic and pro melanocyte stimulation. We much prefer our approach which is to utilize conditioned media from bone marrow stem cell cultures. Physiologic, the same way mother nature heals injury, and anti-inflammatory.
3) We do NOT recommend our AnteAGE or AnteAGE MD Serum (in the two part system we created) for at least half a day after microneedling. This is a very vulnerable time since the skin’s barrier protection has been breached and substances not normally a problem for intact skin may cause reaction. Our microneedling product contains physiologic substances with no added inactive ingredients. It is intended to be used during and immediately after dermal needling. Many scores of thousands of units have been used to date and we have yet to hear of a single adverse reaction. This product is to be used while the barrier is interrupted so more of the bio-signals in it can penetrate the skin.
I am interested in learning more about how stem cells control the follicle shape and therefore the fiber shape. I recently read a study Dr. Lance Setterfield posted on keratosis pilaris being caused by coiled hairs that rupture the follicular epithelium, leading to inflammation and abnormal follicular keratinization. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3681106/
This will give you an in depth summary. Bioengineering the Hair Follicle Stem cells resident in the bulge region of hair shafts secrete a number of chemokines, miRNA’s and cross talk growth factors that influence the architecture decisions.