To achieve satisfactory results in hair transplants, how hair originates and develops must be taking into consideration. This article describes the structure and distribution of hair follicles on the scalp.
Human hair follicles are divided into three parts:
The infundibulum extends from the follicular orifice and to the entry of the sebaceous duct.
The isthmus extends from the entry of sebaceous duct to the attachment of the arrector pili muscle.
The lower segment of the hair follicle extends from the insertion of the arrector pili muscle to the keratogenous zone. This segment is divided into two areas: Suprabulbar and bulb.
In the follicle, the sebaceous duct entry and the arrector pili muscle insertion are fixed structures that are maintained throughout the different stages of the follicular cycle. The follicle lower segment participates intensely in the modifications produced on the follicle throughout the different phases, entering into regressive cell death at the catagen stage and absent in the telogen stage.
The bulge is located in the arrector pili muscle insertion area and is the main site of hair follicle stem cell accumulation.
Human hair does not grow in an isolated manner. Human hair growth in small clusters on the scalp can clearly be observed under the microscope. These are known as follicular units and are typically composed of 1-2-3-4-5 follicles surrounded by a strip of collagen.
Apparently a more refined technique in the manipulation of the transplanted hair follicles, respecting these follicular units can have considerable importance in the survival rate of the grafted hair. It has been observed that the follicles that have been isolated from their group have less vitality and a lower survival rate.
It is important to differentiate the concepts of hair and follicular unit density. Hair density refers to the total number of hairs emerging from the scalp in a specific area. Follicular unit density is the number of follicular clusters (follicular units) in the same area.
The number of follicular units per square centimetre in the occipital region of the scalp is 65 to 85 per cm2. Whereas follicle and hair density in this area is 124 to 200 hairs/cm2, which tends to be higher if the total hair density of the patient is greater.
The tendency in every patient is for 2-3 follicles to cluster in the follicular units. This increases when total hair density is greater, decreasing to units with just 1 follicle if the hair density is low.
The average distance between follicular units is 1 mm in patients with high hair density and 1.4 mm in patients with low density (less than 120 hairs/cm2).
It seems that the total number of follicular units per surface unit is relatively constant, approximately one follicular unit per mm2. This proportion is multiplied in the distribution of follicular units throughout the scalp. The number of follicles in each follicular unit is established by the patient's hair density.
A simple rule of thumb is to consider that the number of follicular units tends to be approximately half the density of hair/cm2.
There are a great deal of variables in average follicular and hair density due to racial influences in Caucasians, Asians and Africans. Caucasians have a higher density with 200 hairs/cm2, while Asians and Africans have a hair density of approximately 165 hairs/cm2.
Most of the hair on the scalp emerges in follicular units composed of two hairs and less commonly follicular units with 3 hairs or more can arise.
Progressive hair loss associated with pattern male baldness is probably due to a decrease in active follicles within each follicular unit. The total number of follicular units is stable while baldness progresses due to the descent in hairs that make up the total in each follicular unit.
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