Morphology: Skin structure The section "Introduction to morphology" was written by Michael Heidenheim.
Michael Heidenheim is chief physician and specialist in dermato-venerology at Roskilde Hospital, Sjælland Region. His areas of interest include propagation of medical-dermatological knowledge, practical clinical procedures and teledermatology.
The skin is the body's largest organ, with an area of 1.8 m2 and comprising 16% of body weight.
The skin includes epithelial, mesenchymal, glandular and neurovascular components.
The skin can be divided into three functional entities
- Epidermis derived from ectoderm
- Dermis derived from mesoderm
- Adnexal structure derived from ectoderm and mesoderm
- Eccrine sweat glands
- Apocrine sweat glands
- Sebaceous glands
- Hair follicles
- Nails
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The skin's function
- Physical barrier
- Mechanical barrier
- Regulation of fluid and electrolyte balance
- Reduction of ultraviolet light effects
- Regulation of body temperature
- Sensory organ
- Immune monitoring
- Vitamin D production
- Cosmetic organ
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Epidermis is made up of keratinocytes in a multilayer structure with these layers:
1. Stratum basale (germinativum)
2. Stratum spinosum
3. Stratum granulosum
4. Stratum corneum
The basal cell layer consists of columns of epithelial cells with inserted melanocytes, dendrite cells (Langerhan's cells) with offshoots to the other layers. Langerhan's cells are derived from bone marrow and have macrophagic and antigen-presenting functions.
Stratum spinosum keratinocytes are reciprocally connected to desmosomes.
The stratum corneum consists of acaryote (without nucleus) keratinocytes separated by lipids, which structurally can be compared to "brick and mortar" construction, with an inherent barrier function against external influences.
The thickness of the stratum corneum varies depending on the region. The thinnest area is on the eyelids, the thickest on the palms and soles.
Epidermal turnover time is 60 days. The epidermis' functioning and growth is stimulated by several factors controlled by immunological cytokines and growth active keratinocytes. Activate epidermal keratinocytes secrete a great quantity of cytokines, which modulate lymphocyte and granulocyte functions in a complex immunological pattern with pathogenetic significance for a long list of dermatoses.
Keratin is the chief structural protein in the epidermis. At least 20 keratin proteins have been identified and subdivided into two main groups according to molecular weight. All keratin proteins are genetically assigned to various anatomical regions of the skin, glands and mucosa.
The epidermis is separated from the dermis by the basal membrane located in the epidermo-dermal junction.
Ultrastructurally, the basal keratinocytes are connected to the basal membrane at hemidesmosomes associated with tonofilaments and extracellular components in the lamina lucida with adhesive properties. Under the lamina lucida are the lamina densa and lamina fibroreticularis with vertically oriented anchoring filaments.
In relation to the basal membrane and dermis are found various collagen types (collagens IV-VII) which are important for adhesion.
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