The skin constantly renews itself by shedding dead cells from the surface and generating new cells below. As a result, surface cells that are lost through wear, damage, or disease are quickly replaced. New cells are made in the epidermis, the skin’s upper layer, which serves as a tough, protective covering.
In most areas of the body, the epidermis has four layers. In the lowest, the basal layer, new cells are produced. As new cells move to the surface, they change to form intermediate layers of prickle cells and granular cells. The cells reach the surface in 1–2 months. The layer at the surface consists of dead, flat cells, which are continually shed.
When the skin is injured, it responds by repairing the damaged tissue and replacing lost tissue with new cells. During the process of repair, dead or damaged tissue is initially supplanted by scar tissue and eventually by healthy new cells. In some cases, a scar remains. Skin repair takes place over a series of stages, shown here.
Any injury in which the skin is broken, no matter how superficial the injury, may damage the blood vessels in the dermis and cause bleeding.
Blood seeps from the blood vessels and forms a clot. Fibroblasts and other specialized repair cells multiply and migrate to the damaged area.
Fibroblasts produce a plug of fibrous tissue within the clot. As strands of fibrin in the plug contract, the plug shrinks. New skin tissue forms beneath.
The fibrous plug hardens to become a scab on the skin’s surface, which falls off when new skin growth is complete. However, a scar may remain.
Hair grows from a follicle, a specialized area of epidermis that grows down into the dermis. A hair is generated from rapidly dividing cells in the bulb at the base of the follicle. The papilla under the bulb contains blood vessels that carry nutrients. Each follicle has growth periods followed by rest phases. However, the phases of different follicles are not synchronized. Every day some hairs are growing while others are shed. A single hair grows between 6 mm (
During the rest phase of a hair follicle, cell activity in the papilla and hair bulb slows down and eventually stops. When cell activity stops, the hair dies.
During the growth phase, cells divide rapidly in the bulb and initiate the growth of a new hair. The new growing hair slowly pushes the dead hair out of the hair follicle.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.