Skin

• Skin is the biggest organ in the human body, making up 16% of the body weight and covering 1,5 - 1,7 m2 of surface.
• Skin has a protective function against bacteria and chemical substances.
• Skin contains the pigment melanin which protects the underlying structures against UV rays.
• Skin takes part in the immune defense system.
• There are two types of skin recognized: hairless (thick) on the palm of the hand and feet, and skin with hairs (thin) on the rest of the body.
• Hairless skin is characterized by a lack of hairs, a lot of sweat glands and a well-developed epidermis.
• Skin with hairs (thin) is characterized by hair follicles, very thin epidermis, and thin but hairless skin is present in pars intermedia of the lips, glans penis, preputium, labia minora pudenda.
• Skin consists of three parts: epidermis and its structures, dermis (corium), and hypodermis (tela subcutanea, subcutis).
• Epidermis is a multistratified squamous keratinized epithelium consisting of five cellular layers: Stratum basale (germinativum), Stratum spinosum, Stratum granulosum, Stratum lucidum, Stratum corneum.
• Stratum basale (germinativum) is one layer of cylindrical in shape cells resting upon the basal membrane (hemi-desmosomes), connected through desmosomes, possessing a lot of ribosomes (basophilic cytoplasm), mitochondria, and numerous mitosis, indicating a great regeneration ability.
• Spinosum cells are very light stained on routine sections.
• Spinosum cells contain specific rod-shaped granules known as Birbeck granules, which are the material of endocytosis.
• Spinosum cells are without tonofibrils, abundant lysosomes, GER, Mi, AG.
• Stratum spinosum consists of 5-10 layers of polygonal in shape cells, flattened in the upper part, possess specific processes (spinae), acidophilic cytoplasm, a lot of desmosomes between the cells, and they are very rich of tonofibrils.
• Sensory epithelial cells of Merkel are receptors of touch, single or in groups (discs of Merkel), and they have synaptic connections with sensory nerve fibers.
• Spinosum cells possess specific markers CD1.
• Stratum basale (germinativum) consists of 3 layers of cells: stratum spinosum, stratum granulosum, and stratum basale (germinativum).
• Merkel cells resemble the melanocytes, with shorter processes and secretory granules of neuroendocrine type (core dense granule).
• Spinosum cells are a type of antigen-presenting cells and belong to the defense immune system.
• Cells of Langerhans are positive for CD-1 marker.
• Cells of Langerhans with Birbeck granules are found in the stratum spinosum.
• In the lower part of stratum spinosum, there is mitosis, and together with the mitotic active cells of the basal layer, they form an additional layer - stratum germinativum.
• Stratum lucidum has 3-4 layers of strong flattened cells without cellular organells and nuclei, their cytoplasm is filled with specific substance elaidin.
• Sweat glands are formations of the epidermis, their function is under the control of the vegetative nervous system, they are exocrine glands, take part in the thermoregulation and water-salt balance, and are missing in the lips and glans penis.
• Odland granules (with glycophospholipids) are visible on Stratum lucidum.
• The structure of the epidermis includes Dermis (corium), which is under the epidermis with mesoblatic origin, consists of two main layers: Stratum papillare under the basal membrane of the epidermis and Stratum reticulare, the main part of the dermis, build from collagenous and elastic fibers, contains hair follicles, fat glands and sweat glands, and deep localized blood capillaries network.
• The cells of Stratum corneum are constantly removed and their replacement is through dividing of the basal cells.
• The regeneration of Stratum corneum is from lower to the upper parts of the epidermis and takes about 25-30 days.
• The dermo-epidermal connecting complex consists of elements: 3 layers of the basal membrane, lamina lucida (rarata), lamina densa, lamina fibroreticularis, and connecting fibers of the dermis from collagen type VI.
• The most superficial layer of the epidermis is Stratum corneum, which consists of keratinized cells containing the protein substance keratin and air, and is without organelles and nuclei.
• Sweat glands are distributed especially in hair skin of the head, hands and feet.
• Stratum granulosum includes 2-3 layers of strong flattened cells with specific basophilic granules (keratohyaline substance) and a lot of tonofibrils.
• Stratum basale (germinativum) also includes three additional cellular types: melanocytes with neuroectodermal origin, producing melanin - brown to black in color pigment, protecting the tissues against UV rays, and are very light stained on routine section, and are situated between the basal cylindrical cells.
• Langerhans cells, or epidermal macrophages, are also recognized in the basal layer of stratum basale (germinativum).
• The secretory portions of sweat glands are located in the hypodermis, which is build from adipose connective tissue and is rich of blood capillaries and nerve endings.
• Modified apocrine glands include moll glands (eyelid margin), tubercula Montgomery (areola mammae), and glandulceruminosae (external auditive meatus).
• The excretory ducts of merocrine sweat glands pass through the epidermis and open on the surface.
• Sweat glands are of two types: merocrine, which are numerous and found everywhere on the skin surface, and apocrine, which are localized in specific regions of the skin surface and are large in size.
• Apocrine sweat glands are localized in specific regions of the skin surface, are large in size, and their excretory ducts open in the hair follicle.
• Sweat glands consist of secretory portions and excretory ducts.