Follicular unit transplantation

# Overview
Follicular Unit Transplantation is a modern hair restoration technique where the hair is transplanted in groups of 1-4 hairs  exactly as it grows in nature.   It is a major advance over the older hair transplantation procedures that used larger grafts and often produced a pluggy, unnatural look.  In a properly performed follicular unit transplant, the results will mimic the way hair grows in nature and will be undetectable as a hair transplant.

# History
The first hair transplant in the United States was performed by Dr. Norman Orentreich in the late 1950s. He proposed the concept of donor dominance  the idea that grafts continue to show the characteristics of the donor site after they have been transplanted to a new site. This principle provides the basis for all hair transplant surgery.  Although donor dominance insured that transplanted hair will continue to grow, it did not insure that the results would look natural.
The initial hair transplants used grafts that measured 68 mm in diameter, about the size of pencil erasers.  These were obtained from the back of the scalp by literally punching out the hair bearing grafts with a sharp, round instrument and then placing them in holes made in the balding front-part of the scalp. The results of this punch-graft technique were often pluggy and unnatural and was referred to as a dolls head or corn-row look.
To improve upon the appearance of these earlier procedures, doctors developed the technique of mini-micrografting where the donor hair was obtained by removing multiple, thin strips of tissue from the back part of the scalp. This was subsequently cut into different size grafts. The larger pieces, containing from 4 to 12 or more hairs (mini-grafts), were used to create fullness. The smaller grafts of 1-3 hairs (micro-grafts) were used at the hairline. This procedure, called mini-micrografting, gained popularity in the 1980s and became the standard way hair transplants were performed through the mid-1990s.  Although much improved over the punch-graft techniques, the results were still not completely natural.
Follicular Units were first described in the medical literature by Headington in 1984.  
Follicular Unit Transplantation has its roots in the single-strip harvesting method and microscopic graft dissection technique developed by Dr. Bobby Limmer in the late 1980s and published in 1994. 
The conceptual framework for Follicular Unit Transplantation was introduced into the medical literature by Drs. Robert M. Bernstein and William Rassman in their 1995 paper "Follicular Transplantation" ,.  The procedure was further detailed in their paired articles, Follicular Transplantation: Patient Evaluation and Surgical Planning and The Aesthetics of Follicular Transplantation (1997).   The concept was further elaborated upon in the 1999 publication The Logic of Follicular Transplantation. 
By the year 2000, Follicular Unit Transplant (also referred to as FUT) was firmly established as the state-of-the-art due to its ability to produce totally natural results. However, because the procedure was more labor intensive and time consuming than mini-micrografting, it was adopted slowly by the medical community. A major impetus to the development of the procedure was consumer demand expressed on the internet and through consumer advocacy programs.

# Key Concepts
Follicular units are the naturally occurring groups of 1-4 hairs that are present in the human scalp.  Each group also contains sebaceous (oil) glands, nerves, a small muscle and occasional fine vellus hairs.  The essence of Follicular Unit Transplantation is to use these individual, naturally occurring follicular units exclusively in the hair transplant procedure.  This will accomplish a number of objectives critical to the hair restoration process. 
Maximizing Growth
Since the follicular unit is a distinct anatomic and physiologic entity, preserving it during the graft dissection is felt to maximize growth.  In the older mini-micrografting techniques, hair is harvested in multiple strips with the follicular units in each strip edge showing damage from the harvesting blades. The strips are then cut into smaller pieces, a process that further breaks up follicular units and risks additional damage to the follicles. 
In FUT, hair is removed from the back of the scalp in a single strip. Stereo-microscopic dissection then allows the individual follicular units to be removed intact from this strip without being damaged.
Follicular units can also be removed by direct extraction from the back and sides of the scalp with a small round, 1-mm punch-like instrument.  This procedure, called Follicular Unit Extraction or FUE, is gaining some popularity from its ability to avoid a linear donor scar.  However, it has the disadvantages of increased risk of follicular damage and causes greater wounding in the donor area than single-strip harvesting in terms of the total surface area of the scarring produced. 
Ensuring Naturalness
Another advantage of using naturally occurring follicular unit grafts of 1-4 hairs is that it ensures the naturalness of the transplant.  In mini-micrografting techniques, the graft size (ranging from 1 to 12 or more hair per graft) is determined by the doctor and does not necessarily correspond to the way hair grows in nature.  The advantage of follicular units over other grafting techniques in insuring a totally natural result, has been the main reason for its widespread acceptance.
The relatively constant density of follicular units at 1 unit per mm2 has been noted clinically  observed histologically by Headington .   This constancy of the follicular unit density makes it useful in a hair transplant in that the same number of follicular units should be used to cover a specific size bald area regardless of the actual hair density of the patient.  This will help to ensure that the distribution of grafts will be as natural as possible.  Transplanting a patient with low hair density, using the same number and spacing of follicular units as in a patient with high density, will help to ensure that there is proper conservation of donor hair for the long-term, even though the results will appear less full.
Minimizing Trauma to Scalp
Follicular units are relatively compact structures, but are surrounded by substantial amounts of non-hair bearing skin.  In stereo-microscopic dissection, this extra tissue can be removed without injuring the follicles, thus making the grafts smaller. Small grafts can then be placed into small incisions; minimizing damage to the scalps connective tissue and blood supply.
The larger wounds produced by mini-micrografting and plug transplants cause cosmetic problems that include: dimpling and pigment changes in the skin, depression or elevation of the grafts, and a thinned, shiny look on the scalp. The key to a natural appearing hair transplant is to have the hair emerge from perfectly normal skin.  The only way to ensure this is to keep the recipient wounds very small.
Another advantage of small wounds is creating a snug fit.  Unlike the punch and some mini-grafting techniques, which remove a small bit of tissue to make room for the new grafts, the small grafts used in follicular unit transplantation fit into a small, needle-made incision without the need for removing tissue.  This preserves the elasticity of the scalp and holds the tiny follicular unit graft snugly in place. After surgery, the snug fit facilitates wound healing and helps to ensure that the graft will get enough oxygen from the surrounding tissue to maximize their survival.
Making Large Hair Transplant Sessions Practical
There are several reasons to transplant large numbers of grafts in each session.  Large sessions: 1) allow the hair restoration to be completed quickly so that the patient has minimal interference with his/her lifestyle, 2) can compensate for Telogen effluvium, the shedding that frequently accompanies a hair transplant, 3) preserve the donor supply by reducing the number of times incisions are made in the donor area, 4) provide sufficient 1- and 2-hair grafts to create a soft frontal hairline and enough  3- and 4- hair grafts to give the patient the fullest possible look.