Morphological Composition of the Skin
Histologically, the skin structure includes primary tissue layers, along with a complex of glands and appendages.
1. Primary Layers
- Epidermis. The most superficial layer. It is a keratinized stratified squamous epithelium.
- Dermis. The connective tissue foundation located directly beneath the epithelium.
Note: The subcutaneous adipose tissue (hypodermis) lies beneath the dermis. Although anatomically closely related to the skin, histologically it is a separate structure.
2. Glandular Apparatus
- Sweat glands (glandulae sudoriferae).
- Sebaceous glands (glandulae sebaceae).
- Mammary glands. Developmentally, these are modified sweat glands.
3. Skin Appendages These include keratinized structures: hair and nails.
Barrier and Protective Function
This is the primary function of the skin, essential for maintaining internal environment constancy (homeostasis).
- Isolation. The skin maintains a unique chemical composition of the body. Intercellular tight junctions in the epithelium prevent substance leakage. Additionally, the stratum corneum, together with sebum, forms a water-repellent hydrophobic film. This is why topical medications are formulated on hydrophobic bases (ointments, oily creams).
- Mechanical and Physical Protection. Skin tissues are resistant to pressure and tearing. The pigment melanin, contained in cells, absorbs ultraviolet radiation, protecting deeper structures.
- Biological and Chemical Barrier. A healthy epidermis is impermeable to bacteria and toxins. Sebaceous gland secretions create an acidic environment (pH 4.2–5.6), which is lethal to many microbes. Intraepithelial macrophages (Langerhans cells), along with lymphocytes and plasma cells located in the dermis, mediate the immune response.
Thermoregulation and Metabolic Participation
The skin functions as a sophisticated thermostat thanks to the coordinated action of blood vessels, glands, and receptors.
- Heat Dissipation (Cooling). Sweat evaporation and heat radiation through dilated blood vessels account for up to 80% of all body heat loss.
- Heat Conservation. Lipid secretions from sebaceous glands reduce heat loss, and cornified cells act as thermal insulators due to low thermal conductivity.
Additional Systemic Functions:
- Receptor: perception of pain, temperature, and tactile stimuli (pressure, touch).
- Excretory: elimination of water (about 500 mL per day), nitrogenous waste, and salts in sweat.
- Synthetic: production of vitamin D in the epidermis under the influence of ultraviolet rays.
- Reservoir (Depot): dermal blood vessels can store up to 1 liter of blood.
- Metabolic: participation in hormone metabolism and toxin breakdown.