Skin is the largest organ of the body which provides a protective cover to the internal organs. The surface of skin measures 1.62 sq metres in an average adult male and 1.43 sq metres in an average adult female. Although the most superficial layer of skin is made up of dead cells, rather than being a dead covering for the body, it has several inbuilt mechanisms for interacting with the environment. Skin consists of three main layers, the outer-most epidermis, the deeper dermis and the deepest subcutaneous tissue. In addition, the skin has two appendages namely the hair and the nails, and three glands namely the sweat glands, the sebaceous glands and the apocrine glands.
The epidermis is made up entirely of cells. The main cell of the epidermis is the keratinocyte which produces keratin. In addition, melanocytes produce the black pigment of the skin, the melanin, while Langerhans cells are responsible for phagocytosing the agents which invade the skin. The keratinocytes lie side by side in several layers, the number of such layers varying from region to region. The deeper-most layer of the epidermis is called basal cell layer or stratum germinativum and consists of a single layer of columnar cells. The cells in this layer keep dividing at a fixed rate and progressively move towards the surface. The layer superficial to the basal cell layer is called the prickle cell layer or the malpighian cell layer. This is made up of several layers of cells which are polygonal in shape and attached to each other as well as to the cells of the basal cell layer by means of specialised cell-wall structures called desmosomes. In addition, these cells are covered with a proteinous substance called the intercellular cement. The cytoplasm of the basal cells as well as the prickle cells contains very fine filamentous structures called tonofilaments which consist of long chains of aminoacids.
These filaments are attached to the desmosomes and form a criss-cross pattern in the cytoplasm of the cells. Tonofilaments do not cross from one cell to the other but constitute a sort of skeleton for the cell. The cells in the most superficial layer of the prickle cell layer develop small circular organelle called membrane coating granules or Odland bodies which progressively move towards and fuse with the cell membrane and discharge their content in the intercellular space. Superficial to the prickle cell layer lies the granular cell layer which consists of two layers of rhomboid shaped cells which are flatter than the cells of the prickle cell layer. These cells characteristically contain irregular shaped masses of dark-staining material lying in between the tonofilaments. The tonofilaments progressively become parallel to each other, develop interfilamentous bonds and together with the cement substance constitute larger fibrils called tonofibrils.
The most superficial layer of the epidermis is called the corneal cell layer or the stratum corneum. It consists of several layers of flat cells lying over each other. These cells have lost their nuclei and other cellular inclusions but contain a protein called keratin which consists of bundles of tonofibrils embedded in the interfibrillar cement substance. The deeper layers of stratum corneum are more compact while the superficial layers are loose. The cells in the superficial layer become fragmented and fall off from the surface. Thus there is a continuous outward movement of the epidermal cells, the new cells being generated in the basal cell layer and the old cells falling off from the surface. This process is called epidermopoiesis which continues at a fixed rate. The time taken for an epidermal cell to move from the basal cell layer and fall off from the surface is approximately 28 days. This rate however gets modified if the skin gets injured and also in some disease processes. The entire process is under a local control.
The continuous outward movement of the epidermal cells constitutes a barrier for the agents which tend to penetrate the skin from outside. The stratum corneum also constitutes a mechanical barrier against the invading organisms unless it has been destroyed by injury or disease. Stratum corneum in addition is a highly selective barrier for chemicals in as much as it does not allow even water to penetrate the skin from outside while it can allow certain larger molecules to penetrate the intact skin.
The dermis can be broadly divided into two layers, the more superficial papillary dermis, and the deeper layer called reticular dermis. The major component of dermis is the collagen fibres which lie singly or in smaller bundles in the papillary dermis, while the lower part of the dermis contains thick and larger bundles lying in a criss-cross pattern. Another important component of the dermis is the elastic fibres which form a horizontal network at the junction of the papillary dermis with the reticular dermis, from where smaller elastic fibres rise vertically towards the epidermis. The collagen and the elastic fibres are embedded in a ground substance made-up of water, electrolytes and proteoglycans (mucopolysaccharides). The proteoglycans in dermis consist of hyaluronic acid, dermatan sulphate (chondroitin sulphate B), chondroitin 6-sulphate (chondroitin sulphate C) and heparan sulphate. In addition, the dermis contains the cellular elements which are mainly of three types:
- Fibroblasts which are spindle shaped cells with a central oval nucleus. These cells are concerned with the formation of collagen and elastic fibres.
- Histiocytes which are amoeboid in shape and concerned with phagocytosis and immunologic functions.
- Mast cells which contain the mast cell granules and can release histamine and several other substances with potent biologic functions. These cells have a role in inflammatory, immunologic and repair mechanisms.
The dermis is also richly supplied with blood vessels which form one dense network at the level of the reticular dermis, and a second network at the junction of the papillary dermis with the reticular dermis. These two networks are inter-connected by means of vertical blood vessels. The papillary portion of the dermis is nourished by the vertical vessels which arise from the superficial plexus and form a loop in each of the dermal papillae. The venous blood is returned by the corresponding veins which form venous plexuses around the arterial plexuses. There 3are no blood vessels in the epidermis which is nourished by the seepage of nutrients from the dermis through the dermo-epidermal junction.
The skin is also supplied with sensory nerves which are present in between the epidermal cells as free nerve endings and also as specialised nerve endings in the dermis and around the cutaneous appendages. These nerve endings subserve the sensations of touch, pain, temperature, wetness and itch. The autonomic nerves innervate the blood vessels and the sweat glands.
The subcutaneous tissue consists mainly of fat cells and forms a padding between the dermis and the underlying structures. The fat cells are circular cells, which in histopathological sections appear as closely packed empty sacs. The nucleus of the cell lies alongside the cell wall.
The interphase between the dermis and the epidermis, the dermo-epidermal junction is a specialised structure. The upper surface of the dermis has numerous projections called dermal papillae which project into the under surface of the epidermis. A vertical section through the skin therefore shows the dermo-epidermal junction to be wavy. The corresponding epidermal projections into the dermis are called rete pegs or rete ridges. This arrangement helps to strengthen the dermo-epidermal junction. This interphase is further strengthened by a basement membrane zone which is constituted by the plasma membrane of the cells of the basal cell layer. This membrane has structures like desmosomes which serve to attach the lower surfaces of the basal cells to the dermis. These structures are called hemi-desmosomes because they do not have the second component from the adjoining cell. The basement membrane is a homogenous layer rich in collagen III. This membrane has an upper electron translucent zone called lamina lucida and a lower electron dense layer called the lamina densa. The fourth component of the basement membrane zone is the sub-lamina densa fibrillar zone which contains the anchoring fibrils, microfibrils and single collagen fibres. These fibrils traverse through the basement membrane zone and provide attachment between the elastic fibres in the dermis and the hemi-desmosomes on the plasma membrane on the lower surface of the basal cells.