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Stages of Development and Hair

Our hair  is  located in one of three stages called anagen, catagen and telogen.
Most of our hairs, approximately 84% are in anagen phase. This phase is the growth phase and it continues for approximately 2-6 years. Other phases are shorter than this one  (2-5 weeks). 


The hair strands fall and re-grow in line with these phases. The percentage of the hair in catagen phase is 2%, and the percentage of the hair in telogen phase is 14%. The growth rates of the hair strands are significantly different in different parts of the body. The fastest growth rate belongs to the hair on the scalp and it is 0.5 mm daily in average.


And compared to the males, the hair growth rate is higher in females. Especially malnutrition in respect of protein has a negative effect on hair growth. Various factors such as vitamin deficiencies, genetic factors, hormonal disorders, hormonal deficiencies or excessive hormones, deficiency of 5 alpha reductase enzyme are very effective in hair growth. Since the percentage of water that the hairy skin includes is very little (20%), the role of the liquids on the growth and development of hair is open to discussion. The relationship between 5 alpha reductase and hair growth, hair loss, hair split is known for many years, and various studies are conducted to examine this relationship. With 5 alpha reductase enzyme, testosterone is converted to Dihydrotestosterone (DHT). DHT is more potent.



The strength of genital region and arm pit hair strands, which are one of the secondary sexual characteristics, is ensured by testosterone. However it also increases hair loss which is the primary indicator of androgenic alopecia. The testosterone increases when the amount of 5 alpha reductase enzyme decreases, and this makes this problem more apparent. Similar to testosterone, estrogen also has a significant effect on hair development and it delays the start of anagen phase.


Besides the little amount of water it contains, hair is rich in respect of carbohydrates, urea, uric acid, free amino acids, phospholipids, cholesterol, fatty acids and most importantly keratin. Since keratin, which is rich in respect of methionine and cysteine amino acids (building blocks of proteins), is one of the rare proteins that don’t dissolve in water, it adds strength to the hair.

In the womb, the first hair follicles are formed around the mouth. Approximately around 22nd Week, the development of all surface follicles are completed. The hair follicles before birth are colorless, thin and soft. They are named as lanugo. While some of the hair transform into hair with strong pigments after birth (terminal hair), some of them remain again as colorless and soft (vellus) and the rest are in between these two types (intermediate). 


Before puberty, there is no terminal hair in any other place than scalp, eyebrows and eyelashes. However, hair starts to grow on arm pits, genital region, shoulders, back, chest, arms and legs. In respect of the surface it covers, the maximum number of hair follicles is on the head. The number of follicles on the head is one million, the number of hair follicles on the scalp is one hundred thousand and the total number of follicles on the entire body is five million. The follicle density on the cheeks and forehead are significantly larger than any other region of the body.

Like nails, hair is a hard extension including keratin as its primary material. Its innermost layer is called medulla, the layer above that is called cortex, and the outermost layer is composed of a real protector called cuticle. These layers that cannot be perceived by the naked eye can only be seen with the help of a microscope. In the following figure, it can be seen that hair follicles, which are perceived as simple structures, are in fact complex structures composed of millions of cells.
The cells that determine the color of the hair are inside the hairy skin and they are located inside a live tissue called hair bulb. And hair bulb is also located inside a shell and it is the basic unit that enables hair growth by cell division since it wraps a section called papilla which is rich in terms of blood vessels.  This section is alive since it is nourished with vessels. This whole alive unit is called “follicle”. For a follicle to stay alive, it should be nourished by the vessels, stimulated by the nerves, directed and supported by the muscles. Even though there is no close relation with the live tissue, sweat glands are required to throw out the sweat accumulated by the body. 
Follicles are arranged in an order as units. As these units may include single follicle (front hairline and the regions above the ears generally have singular follicular units); they may also include two, three, four and even more follicles. For example the upper region of the back of the neck, in other words the area between two ears, is a region that mainly includes units with multiple follicles.


In an adult, the surface covered with hair is around 1000 cm2. There are approximately 100 hair strands in each cm2. In the hairy skin of a normal adult, the total number of hair strands is around 100 thousand. This number is for Caucasians. In yellow race, this number is around 140 thousand. An in dark-skinned race this number is around 110 thousand.

In fact, our entire body (except palms, soles and lips) is covered with hair. However since most of them don't contain melanin, they are colorless and small. So they cannot be perceived by the naked eye. The number of hair strands in our body is 10 times more than the number of hair strands on our scalp. Chest, shoulders, back, arm pits and genital regions are the areas that the hair strands are stronger. In the following cross-section, you can see the location of hair follicles between the skin layers.


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