Hair Transplantation : The Art of Micrografting and Minigrafting, Second Edition
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To accomplish natural, aesthetic results in hair transplantation, one must pay attention to many small details, starting with the important histologic features of hair follicles. If we longitudinally section a terminal hair follicle of the scalp, the follicle can be divided into three portions: 1 the infundibulum, which is the superior portion that extends from the follicular orifice to the entrance of the sebaceous duct, 2 the isthmus, which is the midsection of the follicle bounded superiorly by the sebaceous duct and inferiorly by the insertion of the erector pili muscle, and 3 the inferior segment, which is the section extending from the insertion of the erector pili muscle to the base of the follicle follicular bulb.
The follicular bulb is composed of the matrix cells and the dermal papilla. The infundibulum and the isthmus constitute the permanent portion of the hair fol- licle, because they remain intact throughout the entire hair cycle. In contrast, the inferior segment undergoes periods of regression and regeneration during the hair cycle.
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It is essential for the hair transplant surgeon to know at what depth the most important follicular compartments for hair regeneration are located. The average total length of a scalp terminal hair follicle or the distance between the scalp epidermal surface and the dermal papilla is about 4. FU, Follicular unit. Multiple follicular units of Closeup view various sizes.
Another key component of the follicle is the isthmus or midportion of the follicle. The isthmus is where the main niche of epithelial follicular stem cells bulge is located. A vertical section of a terminal hair immunostained with an antibody anticytokeratin CK15 is shown that labels bulge stem cells brown color. Note that CKpositive bulge stem cells extend all along the isthmus portion of the follicle. A microscopically dissected hair follicle is shown right , in which an oval indicates the CKpositive bulge region.
Epidermis, dermis, dermal appendages, and subcutaneous fatty tissue can be seen on vertical histologic sections of skin taken for skin biopsies or when checking tissue margins after excision of skin lesions. However, only transverse horizontal sections demonstrate that hair follicles grow in follicular units. This suggests that a unit constitutes, at least to some degree, a physiologic entity. This photo of occipital donor scalp shows how the hairs exit the skin in groupings follicular units. Most groupings in this area are two- and three-hair follicular units.
As a rule of thumb, the scalp hair density is about two to three times the FU density. Secondary germ cells Club Dermal papilla. The hair follicle has a unique capacity to reconstitute itself, and this property as well as its easy accessibility makes this a valuable and interesting organ for biolo- gists and stem cell researchers. Each hair follicle perpetually goes through consec- utive cyclical periods of growth anagen , involution catagen , and rest telogen. In humans, the follicular cycle is dysynchronous, which means that neighboring follicles can be at different stages of the cycle.
At the onset of a new growth cycle the start of anagen , the bulge stem cells are activated by the dermal papilla, emerging from multiple positive and negative dermal papilla signals.
Hair Transplantation : The Art Of Micrografting And Minigrafting, Second Edition
On activation, the stem cells exit the bulge and proliferate downward, creating a long linear trail of cells, the outer root sheath. Enveloping the dermal papilla at the hair follicle base, matrix cells divide rapidly before differ- entiating upward to generate the hair shaft and its channel inner root sheath. In mature hair follicles, the outer root sheath extends from the bulge to the matrix.
Hair follicles in different areas of the body produce hairs of different lengths, with the length proportional to the duration of the anagen cycle. For example, scalp hair follicles stay in anagen for 2 to 8 years and produce long hairs. The average rate of growth of scalp hair is approximately 0.
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Eye- brow hair follicles stay in anagen for only 2 to 3 months and produce short hairs. The catagen phase of scalp hairs lasts several weeks, and the telogen phase 2 to 3 months.
During telogen, hair growth ceases and the attachment at the base of the follicle becomes weaker until the hair is finally shed. After hair transplantation, the hair grafts enter into catagen and telogen phases. For this reason, significant growth of the hair grafts is not seen until these phases are over, approximately 2 to 4 months after the transplant. In addition, some of the native hairs often go into the catagen phase and then into the telogen phase from the trauma of the surgery telogen effluvium.
When the rate of hair loss exceeds the rate of growth, thinning and even- tually baldness develop. Follicular Stem Cells Stem cells are characterized by their multipotency and in vivo quiescence, and hair follicle stem cells are fundamental for the regeneration of the pilosebaceous unit. A hair follicle contains both epithelial and mesenchymal stem cells. The main niche of follicular epithelial stem cells is located at the bulge region. Mesenchymal stem cells have been found in the dermal papilla and dermal sheath. These cells have the capacity to differentiate into a range of cell types, making hair follicles a potential source of multipotent cells with therapeutic sig- nificance in regenerative medicine.
The presence of stem cells in hair follicles has opened a window for new treatment strategies in hair restoration. A number of research teams are working to isolate and culture follicular stem cells to inject or transplant them in a recipient balding scalp with the goal of forming new hair follicles, or activating and transforming dormant vellus hair follicles into terminal ones. Applied Anatomy: Alopecias Suitable for Hair Restoration Surgery Alopecia, a generic term for hair loss, results from a diminution of visible hair.
There are numerous types of alopecia; some are permanent and some are revers- ible.
In this chapter we will focus on androgenetic alopecia and the scarring alopecias, which are the reasons for most requests for hair restoration surgery. Androgenetic Alopecia Androgenetic alopecia, or common baldness, is characterized by the progressive, visible thinning of scalp hair in genetically susceptible men and in some women. The thinning is caused by the gradual miniaturization of the hair follicles.
Min- iaturization results in the conversion of large terminal hairs into small, barely visible, depigmented vellus hairs. At the cellular level, follicle miniaturization is thought to be caused by a reduction in dermal papilla volume as a consequence of a decrease in the number of cells per papilla. The clinical pattern of male androgenetic alopecia is well described in the Nor- wood classification system9 see Chapter 2. Most commonly, androgenetic alope- cia begins with bitemporal recession, followed by vertex baldness and midfrontal hair loss, with sparing of the occipital scalp, even in the most severe cases.
The pattern of androgenetic alopecia in women is characterized by diffuse central thinning over the midfrontal scalp, as described by Ludwig10 see Chapter 2. Sinclair photographic scale of female androgenetic alopecia. Hair loss in women: medical and cosmetic approaches to increase hair fullness. Br J Dermatol , These photos show the sequence of female androgenetic alopecia over time. In female androgenetic alopecia also known as female pattern hair loss , the anterior- most hairline is usually spared, and the thinning of the hair occurs in the parietal central scalp. These patients are candidates for hair transplantation if the donor scalp hair is thick and dense enough.
Women with androgenetic alopecia usually do not have abnormalities in circulating androgens, but they should be checked for iron deficiency serum iron and serum ferritin , and thyroid function tests TSH and free T4 should be performed to rule out other causes of diffuse hair loss. Women with bitemporal recessions, as seen in male pattern baldness, should also be screened for hyperandrogenism. Androgenetic alopecia in males and females, especially when it is severe and premature, may have significant psychosocial effects.
Pathophysiology of Androgenetic Alopecia The current scientific data support the thesis that male pattern androgenetic alo- pecia has a polygenic trait. Eight susceptibility loci have been described to date, including the androgen receptor gene on the X chromosome. It has been known for some time that androgens are important in the pathophysi- ology of androgenetic alopecia. Although testosterone is the major circulating androgen, to be maximally active in scalp hair follicles it must first be converted to dihydrotestosterone DHT by the enzyme 5-alpha-reductase.
The importance of DHT as an etiologic factor in male pattern hair loss is shown by the absence of this condition in men with a congenital deficiency of type II 5-alpha-reductase, and by varying amounts of hair regrowth in men treated with finasteride, a selec- tive type II 5-alpha-reductase inhibitor. In women, however, there is no consensus as to whether female pattern androgenetic alopecia is truly androgen dependent. Most affected women do not have biochemical hyperandrogenism, and women without circulating androgens may also develop female pattern androgenetic alo- pecia, suggesting a possible role for non-androgen-dependent mechanisms.
Recently it has been discovered that certain prostaglandins can also play an important role in the pathophysiology of androgenetic alopecia. Prostaglandin D2 reduces hair growth, and PGD2 levels are increased in the balding scalp of androgenetic alopecia.
Scarring Alopecias Scarring alopecia can be divided into primary and secondary types. In the primary type, the hair itself is the principal target for destruction. A patient with scarring alopecia should be clinically and histologically evaluated to specifically classify the condition.
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The resultant destruction of the stem cells inactivates further follicular cycling and leads to a permanent loss of the follicle. This patient has frontal fibrosing alopecia showing the typical recession of the frontotemporal hairline with loss of the sideburns and eyebrows.
Patients with this condition are normally not suitable candidates for hair transplantation. Fi- brosing alopecia should not be confused with the female pattern hair loss seen in the patient on p. These primary scarring alopecias have a tendency to be progressive and to recur intermittently over time.