Hair Anatomy

The scalp is divided into five layers, which are the skin, connective subcutaneous tissue, galea aponeurosis, loose connective tissue, and periosteum over the cranium.

The skin contains all the epidermal appendages, including the hair follicles which extend into the underlying connective subcutaneous layer. How deep the hair follicle is also depends on the stage of hair cycle. Just before fall the hair follicle is quiet superficial in the scalp. In areas which have undergone hair loss, there is thinning of the outer two layers. This can be better appreciated on comparing the skin punch biopsy from the scalp in recipient areas with that in donor areas.

Blood supply to the hair follicles come through the vascular channels in the subcutaneous layer. This has importance while harvesting hair from the donor areas where one has to stay superficial along the connective subcutaneous tissue layer to avoid compromising hair density or hair loss at the donor area. One must not try to reach galea during graft harvesting to avoid compromising scalp blood supply and future neuroma formation (sensory innervation of the scalp closely follows the vascular supply). Deeper penetration might on occasions can lead to reduced scalp sensation over few weeks when a large number of graft recipient slits are made along the hairline.

Scalp hairs mostly do not grow individually; they are often found to grow in tiny follicular-unit bundles, which usually contain 2-3 hairs and occasionally 1 or 4 hairs. An individual follicular unit contains on an average 2-3 hairs, a sebaceous gland element, and insertions of the arrector pili muscles wrapped in a tissue sheath. There are few areas having stem cells in the follicular unit and this is the area of great research and hope for hair transplant patients. While separating individual hair follicles one should try to inflict minimal damage to the tissue adhered to the hair bud so that maximum number of live stem cells can be transferred to the bald areas of scalp.

Hair is primarily made of a protein called keratin that also makes nails and forms the outer protective layer of skin. Each hair consists of three layers:

1. The cuticle – the outer layer, thin and colourless. It acts as the protective layer.
   2.The cortex which contains melanin, which is responsible for hair colour.
   3. The medulla, the innermost layer which reflects light.

Hair follicle has a very complex biologic structure and hairs undergo growth cycle which is controlled by many factors including various growth factors. Hair undergoes three stages during its lifecycle:

• Anagen phase during which there is active hair shaft production meaning increasing length of hair.
• Catagen phase during which the hair follicle undergoes regression and this process in apoptosis driven.
• Telogen phase which is the resting phase of the hair cycle with the involution of hair follicle.

Many growth factors play an important role in the life-long cyclic transformation of the hair follicles by functioning as biologic switches controlling various phases of hair growth. The main growth factors involved in the growth of hair follicle are vascular endothelial growth factor (VEGF), epidermal growth factor (EGF), insulin 1-like growth factor and fibroblast growth factor (FGF).