Why does hair not grow on scars?

Skin consists of two principal layers — epidermis and dermis. The epidermis is a continuously self renewing layer. Keratinocyte (a cell producing a protein found in tissues like hair, nails, scales and skin.) is the chief cell in this layer. It also consists of melanocyte — the pigment forming cell.

Deeper to the epidermis, is the dermis, which is composed of collagenous (Greek; Kolla: glue) and elastic fibre network in an amorphous ground substance. It accommodates nerves, blood vessels and pilo-sebaceous unit.

A pilo-sebaceous unit consists of hair and its follicle, associated muscle and a secretory gland called sebaceous gland. An injury to the skin results in a scar.

A scar (Greek; eschara: a hearth, fire place, scab) is defined as a mark consisting of fibrous material formed in the healing process of a wound.

The scar forms in three overlapping phases — an immediate response to the injury, when the hemostasis (arrest of bleeding) is achieved and the germs are engulfed, followed by formation of new blood vessels and new cells required for the regeneration of epidermis and finally replacement of these newly formed blood vessels and cells by fibres, i.e., the scar.

The scar contains few cells and blood vessels and no pilo-sebaceous unit and nerves as they never regenerate, hence no hair growth.

Experimentally, addition of dermal papillae fibroblasts to a wound can induce hair follicle formation. Scars bleed despite a few blood vessels, because they lack the elastic and contracting properties of normal skin.

Source: The Hindu