The skin is divided into three layers:
1 The epidermis
2 The dermis
3 The subcutaneous layer
The epidermis consists of stratified epithelium. This is formed by cells (keratinocytes) from the germinal basal layer, which produces successive layers of cells that lose their nuclei and die as they reach the surface.
The epidermis is divided into two layers:
1 The inner malpighian layer contains the germinal basal layer (stratum germinativum), above which lies the stratum spino sum (spinous or prickle cell layer) ; above this is the stratum granulosum (granular layer).
2 The outer layer consists of anucleate cornified cells (stratum corneum). The stratum lucidum lies between this and the granular layer.
STRATUM CORNEUM. Despite consistmg of cornified cells this layer forms a dynamic surface barrier to the skin. Experiments have shown that there is secretion of lipids altered epidermal DNA synthesis and synthesis of various cytokines. Diseases of this layer can occur; for example in X-linked recessive ichthyosis there is an impairment of desquamation due to an alteration in the constituents in the lipid layer, i.e. an accumulation of cholesterol sulphate with steroid sulphatase enzyme deficiency. Furthermore an alteration of this skin surface-environment barrier by infection, for example in atopic eczema, may trigger inflammation below in the dermis. This is in contradiction to the generally accepted view that epidermal changes are secondary to dermal inflammation. Cells in the epidermis
KERATlNOCYTES form the major cell in the epidermis. They move peripherally from the basal layer where they are continually formed by mitosis. Keratins are major structural proteins produced by keratinocytes in the epidermis. Human epithelial cells contain more than 30 different keratin polypeptides in two families: type I (acidic) and type II (neutral or basic) keratins. Keratin filaments consist of at least one basic and acidic type. They form the major component of the cytoskeleton of keratinocytes and connect one cell to another through desmosomes and basal cells to the basal lamina via hemidesmosomes .
Genetic abnormalities of keratin have been demonstrated in epidermolysis bullosa complex, where clumping of abnormal keratin filaments is followed by cytolysis of basal cells which follows minor trauma to the skin. Similar microscopic changes occur in another genetic disease of keratin, epidermolytic hyperkeratosis, where clumping is seen in suprabasal cells.
MELANOCYTES are dendritic cells that arise from the neural crest. They synthesize melanin and transfer it as pigment granules to the epidermal keratinocytes.
LANGERHANS’ CELLS. These are also dendritic cells that originate in bone marrow. They are in the suprabasal layers of the epidermis and their surfaces express HLA-DR and the antigens CDl and CD4. They also have surface receptors for C3 and the Fc fragment of IgG and can
1. They therefore function as antigenpresenting and antigen-trapping cells and also cooperate with lymphocytes in eliciting an immune response. Epidermotrophic T cells, keratinocytes and draining lymph nodes form the skin-associated lymphoid tissue (SALT) that maintains immunosurveillance.
MERKEL CELLS migrate from the neural crest or arise from epithelial cells and are most numerous on digital pads, lips, the oral cavity and hair follicles where they are involved in sensation. They act as mechanoreceptors and contain neurotransmitter materials.
The epidermodermal junction
The structure. Anchoring fibrils extend from the lamina densa into the dermis. This junctional area is highly antigenic and is the site of a number of disorders.
The sweat glands
The sweat glands are of two varieties:
1 The eccrine glands are present all over the skin, particularly the palms, but not in mucous membranes. The glands are situated in the dermis and secrete a watery fluid containing chlo ride, lactic acid, fatty acids, urea, glycoproteins and mucopolysaccharides.
2 The apocrine glands are large sweat glands whose ducts open into the hair follicles. They are found in the axillae, anogenital areas, nipples, areolae and scalp. They do not function until puberty. These glands produce wax in the ears.
The sebaceous glands
Sebaceous glands occur all over the skin (except on the palms and soles) but are most numerous on the scalp and face. They have no lumen and their secretion is due to decomposition of the cells. Meibomian glands on the eyelids are modified sebaceous glands. They produce a whitesecretion containing proteins and complex carbohydrates. The s ecretion, called sebum, consists mainly of fatty acids and cholesterol and is discharged into the pilosebaceous follicle.
Each hair consists of a shaft of keratinized cells that isproduced by the hair bulbs deep in the dermis and projects through the skin from the hair follicle. The hair follicle is richly supplied by nerves and blood vessels and contains melanin.
There are three types of hair:
TERMINAL-medullated coarse hair, e.g. scalp, mole, beard, eyebrows, pubic
Ver.i.us=-non-medullated short, fine, downy hair, e.g. on the face of women and prepubertal boys LANUGo-hair covering the fetus Every hair follicle has a phase of involution (catagen phase), a shedding phase (telogen phase) and a growth phase (anagen phase) which may last up to 3 years in scalp hair. The length to which hair can grow dependson the rate of growth and the length of the anagen phase, which is cyclical. The anagen phase can vary depending on the site of the hair, e.g. 3-5 years on the scalp. Shed telogen or anagen hair can be distinguished as the club of the telogen hair end is depigmented.
Nails are hard, translucent plates of keratin that growfrom beneath the nail fold. A finger-nail takes up to 6 months to replace itself and its growth is affected by disease and malnutrition.
The subcutaneous layer
This contains fat, sweat glands and blood vessels.