Human Hair Anatomy & Physiology
Biological and structural analysis of hair has been conducted and decoded by scientists to a significant extent.
Hair is a complicated structure that comprises of the root (or follicle) and the shaft (the visible part of hair). The root is enclosed in the hair follicle, submerges into the skin in inclination and ends down to the bulb.
At the base of hair follicles lie dermal papilla cells, biological structures that are very important to the follicle, as they bear capillary vessels which send nutritive elements from blood to cells.
In dermal papilla also lie the androgens receptors and the fibroblasts, which have an active role in the hair growth cycle.
Shaft is the visible dead part of hair and consists of 3 layers, created by the matrix cells of the bulb:
- The cuticle, which consists of colorless, flat overlapping cells
- The cortex, which contains pigment (melanin) that defines hair color. The cortex defines hair texture (straight, curly etc), includes keratin and is responsible for the shaft growth
- Τhe medulla, the ‘’heart’’ of the shaft, usually found in hairs of big diameter
The different layers of the visible part of hair (shaft) are due to the alterations in the morphology and structure of matrix cells of the bulb. In the matrix cells takes place protein synthesis -especially Keratin synthesis- which contributes to the strength and endurance of the hair shaft and to nail configuration as well. Keratin also lies in the skin. Keratin is a group of proteins that contain sulphur and is being produced in the keratogen zone of the root. Hair consists of proteins (65% – 95%), lipids (1% – 9%), trace elements,polysaccharides and water.
Hair growth types
Ι) Lanugo hairs
In the 3rd month of pregnancy, very smooth, soft and colorless hairs cover all the embryo’s body and remains till few weeks before birth.
II) Vellus hairs
Vellus hairs are fine, soft and barely pigmented hairs, usually 1 or 2 cm. Due to the small quantity of pigment, they are nearly invisible. They emerge from hair follicles and usually have no sebaceous glands, so they don΄t get oily.
ΙΙΙ) Terminal hairs
They are the distinguishable hairs of body and head, bigger in diameter and length than vellus hairs. They are responsive to hormonal influence and the hair follicles they come from have sebaceous glands. In men who experience androgenetic alopecia, the most common type of male hair loss, a percentage of the terminal hairs progressively weakens and converts into vellus hairs.
THE HAIR CYCLE
The normal life span for each hair is from three to six years. Each hair grows in a hair cycle that can be divided into three stages: the growing stage (Anacin) lasting from two to six years, the partial degeneration stage (Catalan) lasting between 10 and 14 days, and finally the resting phase (Telogen) during which time the hair is shed and there is an involution of the hair follicles lasting about one hundred days.
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