Abstract
The skin is a large organ, weighing an average of 4 kg and covering an area of 2 m 2 . Its major function is to act as a barrier against an inhospitable environment – to protect the body from the influences of the outside world. The importance of the skin is well illustrated by the high mortality rate associated with extensive loss of skin from burns.
The major barrier is provided by the epidermis. Underlying the epidermis is a vascularized dermis that provides support and nutrition for the dividing cells in the epidermis. The dermis also contains nerves and appendages: sweat glands, hair follicles, and sebaceous glands. Nails are also considered skin appendages. The third and deepest layer of the skin is the subcutaneous fat. The functions of all these components are listed in Table 2.1 .
- 1.
The major function of the skin is as a barrier to maintain internal homeostasis
- 2.
The epidermis is the major barrier of the skin
Components of skin:
- 1.
Epidermis
- 2.
Dermis
- 3.
Skin appendages
- 4.
Subcutaneous fat
Skin disease illustrates structure and function. Loss of or defects in skin structure impair skin function. Skin disease is discussed in more detail in the other chapters.
Epidermis
- 1.
Keratinocytes are the principal cell of the epidermis
- 2.
Layers in ascending order: basal cell, stratum spinosum, stratum granulosum, stratum corneum
- 3.
Basal cells are undifferentiated, proliferating cells
- 4.
Stratum spinosum contains keratinocytes connected by desmosomes
- 5.
Keratohyalin granules are seen in the stratum granulosum
- 6.
Stratum corneum is the major physical barrier
- 7.
The number and size of melanosomes, not melanocytes, determine skin color
- 8.
Langerhans cells are derived from bone marrow and are the skin’s first line of immunologic defense
- 9.
The basement membrane zone is the substrate for attachment of the epidermis to the dermis
- 10.
The four major ultrastructural regions of the basement membrane zone include the hemidesmosomal plaque of the basal keratinocyte, lamina lucida, lamina densa, and anchoring fibrils located in the sublamina densa region of the papillary dermis
| Function | Responsible Structure |
|---|---|
| Barrier | Epidermis |
|
|
|
|
|
|
| Tough flexible foundation | Dermis |
| Temperature regulation | Blood vessels |
| Eccrine sweat glands | |
| Sensation | Nerves |
| Grasp | Nails |
| Decorative | Hair |
| Unknown | Sebaceous glands |
| Insulation from cold and trauma | Subcutaneous fat |
| Calorie reservoir | Subcutaneous fat |
The epidermis is divided into four layers, starting at the dermal junction with the basal cell layer and eventuating at the outer surface in the stratum corneum. The dermal side of the epidermis has an irregular contour. The downward projections are called rete ridges , which appear three-dimensionally as a Swiss cheese-like matrix with the holes filled by dome-shaped dermal papillae. This configuration helps to anchor the epidermis physically to the dermis. The pattern is most pronounced in areas subject to maximum friction, such as the palms and soles.
The cells in the epidermis undergo division and differentiation. Cell division occurs in the basal cell layer, and differentiation in the layers above it.
Cell division occurs in the basal cell layer.
Structure
Basal Cell Layer
The basal cells are the undifferentiated, proliferating cells. Skin stem cells are located in the basal layer in the interfollicular epidermis, and they give rise to keratinocytes. For normal skin homeostasis, daughter cells from the basal cell layer migrate upward and begin the process of differentiation. In normal skin, cell division does not take place above the basal cell layer. It takes about 2 weeks for the cells to migrate from the basal cell layer to the top of the granular cell layer, and a further 2 weeks for the cells to cross the stratum corneum to the surface, where they finally are shed. Injury and inflammation increase the rate of proliferation and maturation ( Fig. 2.1 ).
Stratum Spinosum
The stratum spinosum lies above the basal layer and is composed of keratinocytes , which differentiate from the basal cells beneath them. The keratinocytes produce keratin, a fibrous protein that is the major component of the horny stratum corneum. The stratum spinosum derives its name from the “spines,” or intercellular bridges, that extend between keratinocytes and which are visible with light microscopy. Ultrastructurally, these are composed of desmosomes, which are extensions from keratin within the keratinocyte; functionally, they hold the cells together ( Fig. 2.2 ).
Keratinization begins in the stratum spinosum.
Stratum Granulosum
The process of differentiation continues in the stratum granulosum, or granular cell layer, in which the cells acquire additional keratin and become more flattened. In addition, they contain distinctive dark granules, seen easily on light microscopy, that are composed of keratohyalin. Keratohyalin contains two proteins, one of which is called profilaggrin , the precursor to filaggrin. As its name suggests, filaggrin plays an important role in the aggregation of keratin filaments in the stratum corneum. The other protein is called involucrin (from the Latin for “envelope”), and plays a role in the formation of the cell envelope of cells in the stratum corneum. Ichthyosis vulgaris ( ichthys , Greek for “fish”) is an inherited dry skin condition secondary to deficient filaggrin production, as noted on light microscopy of a skin biopsy by a reduced or absent granular layer ( Fig. 2.3 ).
Granular cells also contain lamellar granules, which are visualized with electron microscopy. Lamellar granules contain polysaccharides, glycoproteins, and lipids that extrude into the intercellular space and ultimately are thought to help form the “cement” that holds together the stratum corneum cells. Degradative enzymes also are found within the granular cells; these are responsible for the eventual destruction of cell nuclei and intracytoplasmic organelles.
Granular cells contain keratohyalin and lamellar granules.
Stratum Corneum
A remarkably abrupt transition occurs between the viable, nucleated cells at the top of the granular cell layer and the dead cells of the stratum corneum ( Fig. 2.4 ). The cells in the stratum corneum are large, flat, polyhedral, plate-like envelopes filled with keratin. They are stacked in vertical layers that range in thickness from 15 to 25 layers on most body surfaces to as many as 100 layers on the palms and soles. The cells are held together by a lipid-rich cement in a fashion similar to “bricks and mortar.” The tightly packed, keratinized envelopes in the stratum corneum provide a semi-impenetrable layer that constitutes the major physical barrier of the skin.
The stratum corneum is the major physical barrier.
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