The integument system
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The skin is the largest organ of the body, covering about 2 square meters and weighing around 5 kg. It has two main layers - the epidermis and dermis. The epidermis is composed of keratinized stratified squamous epithelium consisting of keratinocytes, melanocytes, Langerhans cells, and Merkel cells. The dermis lies below the epidermis and contains blood vessels, nerves, hair follicles, and glands. The skin acts as a protective barrier and plays roles in temperature regulation, sensation, and vitamin D synthesis.
The integument system
- 2. Introduction ▪ The skin (also known as the cutaneous membrane or integument) covers the external surface of the body and is the largest organ of the body in both surface area and weight. ▪ In adults, the skin covers an area of about 2 square meters (22 square feet) and weighs 4.5–5 kg (10–11 lb), about 16% of total body weight. ▪ It ranges in thickness from 0.5 mm (0.02 in.) on the eyelids to 4.0 mm (0.16 in.) on the heels. ▪ However, over most of the body it is 1–2 mm (0.04–0.08 in.) thick. Structurally, the skin consists of two main parts The superficial, thinner portion, which is composed of epithelial tissue, is the epidermis. The deeper, thicker connective tissue portion is the dermis. ▪ Deep to the dermis, but not part of the skin, is the subcutaneous layer. Also called the hypodermis , this layer consists of areolar and adipose tissues.
- 4. Epidermis ▪ The epidermis is composed of keratinized stratified squamous epithelium. It contains four principal types of cells: keratinocytes, melanocytes, Langerhans cells, and Merkel cells. ▪ About 90% of epidermal cells are keratinocytes, which are arranged in four or five layers and produce the protein keratin. ▪ Keratin is a tough, fibrous protein that helps protect the skin and underlying tissues from heat, microbes, and chemicals. ▪ Keratinocytes also produce lamellar granules, which release a water-repellent sealant that decreases water entry and loss and inhibits the entry of foreign materials.
- 5. ▪ About 8% of the epidermal cells are melanocytes , which develop from the ectoderm of a developing embryo and produce the pigment melanin. ▪ Their long, slender projections extend between the keratinocytes and transfer melanin granules to them. ▪ Melanin is a yellow-red or brown-black pigment that contributes to skin color and absorbs damaging ultraviolet (UV) light. ▪ Once inside keratinocytes, the melanin granules cluster to form a protective veil over the nucleus, on the side toward the skin surface. In this way, they shield the nuclear DNA from damage by UV light. ▪ Although their melanin granules effectively protect keratinocytes, melanocytes themselves are particularly susceptible to damage by UV light.
- 6. Contd……………… ▪ Langerhans cells arise from red bone marrow and migrate to the epidermis, where they constitute a small fraction of the epidermal cells. ▪ They participate in immune responses mounted against microbes that invade the skin, and are easily damaged by UV light. ▪ Their role in the immune response is to help other cells of the immune system recognize an invading microbe and destroy it.
- 7. ▪ Merkel cells are the least numerous of the epidermal cells. They are located in the deepest layer of the epidermis, where they contact the flattened process of a sensory neuron (nerve cell), a structure called a Merkel (tactile) disc. ▪ Merkel cells and their associated Merkel discs detect touch sensations.
- 10. Layers of skin In most regions of the body the epidermis has four strata or layers— ▪ stratum basale ▪ stratum spinosum ▪ stratum granulosum ▪ thin stratum corneum. This is called thin skin. Where exposure to friction is greatest, such as in the fingertips, palms, and soles, the epidermis has five layers— ▪ stratum basale ▪ stratum spinosum ▪ Stratum granulosum ▪ stratum lucidum ▪ thick stratum corneum. This is called thick skin.
- 11. Stratum Basale ▪ The deepest layer of the epidermis is the stratum basale, composed of a single row of cuboidal or columnar keratinocytes. ▪ Some cells in this layer are stem cells that undergo cell division to continually produce new keratinocytes. ▪ The nuclei of keratinocytes in the stratum basale are large, and their cytoplasm contains many ribosomes, a small Golgi complex, a few mitochondria, and some rough endoplasmic reticulum.
- 12. ▪ The cytoskeleton within keratinocytes of the stratum basale includes scattered intermediate filaments, called tonofilaments. ▪ The tonofilaments are composed of a protein that will form keratin in more superficial epidermal layers. ▪ Tonofilaments attach to desmosomes, which bind cells of the stratum basale to each other and to the cells of the adjacent stratum spinosum, and to hemidesmosomes, which bind the keratinocytes to the basement membrane positioned between the epidermis and the dermis. ▪ Melanocytes and Merkel cells with their associated Merkel discs are scattered among the keratinocytes of the basal layer. The stratum basale is also known as the stratum germinativum to indicate its role in forming new cells.
- 13. Stratum Spinosum ▪ Superficial to the stratum basale is the stratum spinosum arranged in 8 to 10 layers of many-sided keratinocytes fitting closely together. ▪ These keratinocytes have the same organelles as cells of the stratum basale. ▪ When cells of the stratum spinosum are prepared for microscopic examination, they shrink and pull apart so that they seem to be covered with thornlike spines, although they appear rounded and larger in living tissue. ▪ spiny projection in a prepared tissue section is a point where bundles of tonofilaments are inserting into a desmosome, tightly joining the cells to one another. ▪ This arrangement provides both strength and flexibility to the skin. ▪ Langerhans cells and projections of melanocytes are also present in this layer.
- 14. Stratum Granulosum ▪ At about the middle of the epidermis, the stratum granulosum consists of three to five layers of flattened keratinocytes that are undergoing apoptosis. ▪ Apoptosis is an orderly, genetically programmed cell death in which the nucleus fragments before the cells die.) ▪ The nuclei and other organelles of these cells begin to degenerate, and tonofilaments become more apparent. ▪ A distinctive feature of cells in this layer is the presence of darkly staining granules of a protein called keratohyalin, which converts the tonofilaments into keratin. Also present in the keratinocytes are membrane enclosed lamellar granules, which release a lipid-rich secretion.
- 15. ▪ This secretion fills the spaces between cells of the stratum granulosum, stratum lucidum, and stratum corneum. The lipidrich secretion acts as a water-repellent sealant, retarding loss and entry of water and entry of foreign materials. ▪ As their nuclei break down during apoptosis, the keratinocytes of the stratum granulosum can no longer carry on vital metabolic reactions, and they die. ▪ Thus, the stratum granulosum marks the transition between the deeper, metabolically active strata and the dead cells of the more superficial strata.
- 16. Stratum Lucidum ▪ The stratum lucidum is present only in the thick skin of areas such as the fingertips, palms, and soles. ▪ It consists of three to five layers of flattened clear, dead keratinocytes that contain large amounts of keratin and thickened plasma membranes.
- 17. Stratum Corneum ▪ The stratum corneum consists on average of 25 to 30 layers of flattened dead keratinocytes. ▪ These cells are continuously shed and replaced by cells from the deeper strata. The interior of the cells contains mostly keratin. ▪ Between the cells are lipids from lamellar granules that help make this layer an ffective water-repellent barrier. ▪ Its multiple layers of dead cells also help to protect deeper layers from injury and microbial invasion. Constant exposure of skin to friction stimulates the formation of a callus, an abnormal thickening of the stratum corneum.
- 18. Dermis ▪ The second, deeper part of the skin, is composed of connective tissues. Blood vessels, nerves, glands, and hair follicles are embedded in dermal tissues. Dermis can be divided into : ▪ Papillary region ▪ reticular region.
- 19. Papillary region ▪ makes up about one-fifth of the thickness of the total layer. It consists of areolar connective tissue containing thin collagen and fine elastic fibers. Its surface area is greatly increased by dermal papillae, small, fingerlike structures that project into the undersurface of the epidermis. ▪ Some of these nipple-shaped structures contain capillary loops (blood vessels). Some dermal papillae also contain tactile receptors called Meissner corpuscles or corpuscles of touch, nerve endings that are sensitive to touch, and free nerve endings, dendrites that lack any apparent structural specialization. Different free nerve endings initiate signals that give rise to sensations of warmth, coolness, pain, tickling, and itching.
- 20. Reticular region ▪ The, which is attached to the subcutaneous layer, consists of dense irregular connective tissue containing fibroblasts, bundles of collagen, and some coarse elastic fibers. ▪ The collagen fibers in the reticular region interlace in a netlike manner. A few adipose cells, hair follicles, nerves, sebaceous (oil) glands, and sudoriferous (sweat) glands occupy the spaces between fibers. ▪ The combination of collagen and elastic fibers in the reticular region provides the skin with strength, extensibility (ability to stretch), and elasticity (ability to return to original shape after stretching). ▪ The extensibility of skin can be readily seen around joints and in pregnancy and obesity. Extreme stretching may produce small tears in the dermis, causing striae or streaks, or stretch marks, visible as red or silvery white streaks on the skin surface.
- 21. FUNCTIONS OF THE SKIN ▪ Protection ▪ Preventions of water loss ▪ Barrier to germs and chemicals ▪ Safety against sunburn ▪ Chemical and immunological defence ▪ Maintenance of body form ▪ Thermoregulation ▪ Excretion and hemostatsis ▪ Secretions and sensation
- 22. Contd……… ▪ Synthesis of vitamin D ▪ Formation of bones and teeths ▪ Aids in diagnosis ▪ Storage of food ▪ Colour ▪ Healing of wounds ▪ Absorption ▪ Blood reservior
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