The Structure of Skin
Skin varies in thickness from less than one millimeter in the eyelids to greater than four millimeters on the soles of the feet, but everywhere, Structure of skin is composed of two layers, the epidermis and the dermis, underlain by a sheet of subcutaneous tissue
The outer layer of the skin is the epidermis. The deepest part of the epidermis is a row of germinative cells. Germinative cells are specialized stem cells that continually divide to give off keratinocytes, the main cells in the remainder of the epidermis. As they age, the new keratinocytes fill with keratin (a tough fibrous protein) and are pushed to the surface, where they die; thus, the outermost layer of the epidermis is made of flat, dead keratinocytes. The epidermis also contains melanocytes (pigment-containing cells) and immune system cells. The epidermis, a protective layer that is normally impermeable to water, does not have sufficient strength to hold sutures or staples.
There are no blood vessels in the epidermis, and it receives its oxygen and nutrients by diffusion from blood circulating in the underlying dermis. Hair, nails, sweat glands, and sebaceous glands are sunken epidermal appendages that lie in deep valleys in the dermis surrounded by a row of germinative epidermal cells.
During normal healing, the epidermis re-grows from germinative cells left in the skin at the edges of the wound. The growing cells are called epithelial cells, and the regrowth of the epidermis is called re-epithelialization.
The layer of skin directly beneath the epidermis is the dermis. A basement membrane separates these two layers. The dermis is mainly connective tissue and is therefore much stronger than the epidermis. The dermis varies in thickness across the surface of the body, but everywhere it is significantly thicker than the overlying epidermis.
The connective tissue of the dermis contains small blood vessels, lymph vessels, nerves, nerve endings, and in a few places, muscles. The dermis is also populated by a variety of individual cells including macrophages, fibroblasts (which synthesize the extracellular connective tissue components such as collagen), and mast cells (which release histamine and other molecules that increase inflammation).
The dermis is loosely stratified. The upper (most superficial) layer contains capillaries and sensory endings of nerves. The deepest layer has thick interlacing collagen and elastic fibers arranged in parallel rows. The extracellular fibers in the deep dermis are responsible for the strength and toughness of the skin. When closing a wound with sutures, they must be anchored in the strong connective tissue of the lower layer of the dermis.
Beneath the dermis is a layer of subcutaneous tissue containing fat. The thickness of the subcutaneous layer varies throughout the body. It is thickest along the anterior thigh and thinnest on the back of the hands.
Besides fat cells, subcutaneous tissue contains blood vessels, lymph vessels, and nerves. The subcutaneous layer is held together by a continuous sheet of fibrous membrane that runs parallel to the surface of the skin. This membrane is called the superficial fascia.
Beneath the subcutaneous tissue layer, structures (such as muscles and organs) are enclosed in their own separate connective tissue sheaths. The generic name for these sheaths is deep fasciae. Deep fasciae generally look off-white in fresh wounds. When treating a wound, tears in the deep fasciae are repaired whenever possible.