Normal healthy hair grow an average half an inch per month. Scalp has five layers, Healthy hair root is in the fat tissue of 2nd layer, an average 4 mm deep from the surface of skin. Normal hair diameter is more than 0.06 mm.
Hair root is called Papilla. Papilla is an oval mass, contain fibroblasts. Size of the papilla controls the size of the hair. Inferiorly papilla through a stalk is connected to the fibrous root sheath, which surrounds the entire hair and blend to the overlying skin. Papilla is surrounded above by the Matrix cells and melanocytes. Matrix cells are germinative cells. Just above the matrix, various layers of the hair begin to differentiate. Starting from the center of hair and moving outward, the layers are the hair shaft medulla, hair shaft cortex, hair shaft cuticle, and these three layers make the hair shaft which exit from the surface of the skin as hair. In the scalp hair shaft is surrounded by the inner root sheath, outer root sheath, vitreous layer and fibrous root sheath.
Hair has a muscle, Arrector pili, which start from underneath the skin and attached to the hair most outer covering, the fibrous root sheath, almost at a level of 45% of the complete length of the hair, from the surface of the skin. 55% of the hair lies below the insertion of arrector pili.
Just above the arrector pili muscle lies the sebaceous gland, sebaceous gland through it’s duct opens up into the hair canal, sebaceous duct opens almost at the level of 20% of the length of the hair from the surface of the skin.
Hair can be divided into three vertical zones, hair below the insertion of arrector pili is called the lower portion of hair, Hair above the insertion of arrector pili and below the opening of sebaceous duct is called middle portion or Isthmus. Hair above the opening of sebaceous duct to the level of skin where the hair exit the surface of the skin is called the upper portion or Infundibulum. Infundibulum and the isthmus, constitute the permanent portion of the hair because it remains intact during the entire hair cycle. Lower portion of the hair go through regression and regeneration during the hair cycle.
Inner Root Sheath is a strong layer serve as a mold to the growing shaft. Inner root sheath start at matrix cell level and end just above the insertion of arrector pili muscle.
Outer Root sheath, start at Matrix cell level ,cover the inner root sheath as inner root sheath ends just above the insertion of arrector pili , outer root sheath continue up and merge into skin. Hair stem cells are widely spread along the Outer Root Sheath especially at the level of arrector pili insertion. These stem cells play role in the regeneration of hair during the entire lifetime of a hair.
Fibrous Root Sheath, it surrounds the entire hair and blends into the skin. Mesenchymal stem cells are present in the fibrous root sheath, these stem cells play role in dermal wound healing.
Hair grows as follicular units. On an average 10-20 % of the hair grows as single follicular unit, 50-60 % grows as two hair follicular unit , 10-20 are three hair follicular units and very few grow as 4 or more hair follicular units . The Follicular units are composed of hairs, their sebaceous glands and common arrector pili muscle, surrounded by collagen band called perifolliculum. This grouping of hair in follicular units is more apparent above the insertion of aarector pili muscle.