Epidermis and Dermis of the Skin
The skin is a highly underestimated organ. It performs many vital functions and has a complex structure which most people are unaware of. This page provides a basic overview of the anatomy and physiology (structure and function) of the human skin. In this part II, we will take a closer look at the Epidermis and Dermis of the skin.
Basic Anatomy & Physiology of the Skin
The epidermis is composed of stratified squamous epithelium and contains four principal types of cells. About 90% of the epidermal cells are keratinocytes (ker-aTIN-b-sitq kerato = horny). They produce the protein keratin that helps waterproof and protect the skin and underlying tissues. Anchoring junctions called desmosomes weld keratinocytes to one another.
Melanocytes (MEL-a-no-sits), which produce the pigment melanin, comprise about 8% of the epidermal cells. Their long, slender projections extend between and transfer granules of melanin to keratinocytes. Melanin (melan = black) is a brown-black pigment that contributes to skin colour and absorbs ultraviolet (UV) light. Once inside keratinocytes, the melanin granules cluster to form a protective veil over the nucleus, on the side toward the skin surface. In this way they shield the genetic material from damaging UV light.
A fourth type of cell found in the epidermis is called a Merkel cell. These cells are located in the deepest layer (stratum basale) of the epidermis of hairless skin, where they are attached to keratinocytes by desmosomes. Merkel cells make contact with the flattened portion of the ending of a sensory neuron (nerve cell), called a tactile (Merkel) disc, and are thought to function in the sensation of touch.
Four or five distinct layers of cells form the epidermis. In most regions of the body the epidermis is about O. 1 mm thick and has four layers. Where exposure to friction is greatest, such as in the palms and soles, the epidermis is thicker (l to 2 mm) and has five layers. Constant exposure of thin or thick skin to friction or pressure stimulates formation of a callus, an abnormal thickening of the epidermis. (See Diagram)
collagen and elastic fibres. The few cells in the dermis include fibroblasts, macrophages, and adipocytes. The dermis is very thick in the palms and soles and very thin in the eyelids, penis, and scrotum. It also tends to be thicker on the dorsal than the ventral aspects of the body and thicker on the lateral than the medial aspects of the extremities. Blood vessels, nerves, glands, and hair follicles are embedded in the dermis.
The outer portion of the dermis, about one-fifth of the thickness of the total layer, is named the papillary region (layer). It consists of areolar connective tissue containing fine elastic fibers. Its surface area is greatly increased by small, finger like projections called dermal papillae (pa-PIL-e; papilla = nipple). These nipple-shaped structures indent the epidermis, and many contain loops of capillaries. Some dermal papillae also contain tactile receptors called corpuscles of touch (Meissner's corpuscles), nerve endings that are sensitive to touch. Dermal papillae cause ridges in the overlying epidermis. It is these ridges that leave fingerprints on objects that are handled.
The deeper portion of the dermis is called the reticular (rete = net) region (layer). It consists of dense, irregular connective tissue containing interlacing bundles of collagen and coarse elastic fibres. Within the reticular region, bundles of collagen fibres interlace in a netlike manner. Spaces between the fibres are occupied by a small quantity of adipose tissue, hair follicles, nerves, oil glands, and the ducts of sweat glands. Varying thicknesses of the reticular region contribute to differences in the thickness of skin.
The combination of collagen and elastic fibres in the reticular region provides the skin with strength, extensibility, and elasticity. (Extensibility is the ability to stretch; elasticity is the ability to return to original shape after stretching.) The ability of the skin to stretch can readily be seen in pregnancy, obesity, and oedema. Small tears that occur in the dermis during extreme stretching are initially red and remain visible afterward as silvery white streaks called striae (STRI-e) or stretch marks.
The reticular region is attached to underlying organs, such as bone and muscle, by the subcutaneous layer, also called the hypodermis or superficial fascia. The subcutaneous layer also contains nerve endings called lamellated or Pacinian (pa-SIN-e-an) corpuscles that are sensitive to pressure. Nerve endings sensitive to cold are found in and just below the dermis, while those sensitive to heat are located in the middle and outer dermis.
This information is provided from the following source:
Tortora, G.J. & Grabowski, S.R. (1993) Principles of Anatomy and Physiology (7th Edition). HarperCollins College Publisher, New York. [ISBN0-06-046702-9]
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