Integumentary System Lecture

(Integumentary System)  Skin is the largest organ in our body(composed of many layers and tissues ); it weighs about 5 Kg and covers an area up to 2 m²  Integument (in Latin): to cover  The skin covers the outer surface of our bodies and it is continuous with the mucus membranes (the oral cavity or nasal cavity for example)  The skin (known as “cutaneous membrane”) is composed of two layers: 1- Epidermis: keratinized stratified squamous epithelium 2- Dermis: (Derma = skin) is a connective tissue (loose+dense)  Below the skin is the subcutaneous layer which is also called hypodermis because it is located under the dermis  Hypodermis is adipose tissue and in anatomy it is the superficial fascia and it is NOT part of the skin  Under the hypodermis, you can find the muscles under the deep fascia. Skin appendages (accessory structures associated with the skin): 1- hair follicles: Invaginations from epidermis into dermis where we can find the hair shaft 2- Sweat glands 1|Page 3- Sebaceous glands which open on the hair follicles. 4- Nails  Major functions of the skin: 1- Protection from the outer environment and microorganisms 2- Protection of our DNA from the ultraviolet light 3- Prevention of water loss by forming impermeable layer and conservation of water inside our body (prevent dehydration ) 4- 4- Thermoregulation: a- Hot weather >> sweating >> cooling body’s temperature when it evaporates b- b- Cold weather >> hair terminals will stand up to close pores and prevent heat loss >> keeping the body warm c- furthermore, in hot weather cutaneous vessels dilate increasing blood flow , increase heat loss and in cold weather the cutaneous vessels constrict reducing blood flow and reducing heat loss . 5- Sensation: a lot of nerve endings that sense pain, touch, pressure, and temperature are located in the skin (sensory receptors) 6- Excretion: sweat (water and other electrolytes) 7- 7- Formation of vitamin D Our skin produces a steroid hormone calciferol which is synthesized from cholesterol with the use of sunlight. Calciferol then enters the blood stream goes to the small intestine and increases absorption of calcium ,nowadays due to our lifestyle we are less exposed to the sun hence calciferol (vitamin D )is added to the milk so you are simultaneously taking in calcium and vitamin D which increases its absorption 2|Page  Skin is an organ, it is NOT a tissue, it is composed of different tissues (epithelium and connective tissues for example) and each tissue has a different origin Epidermis: originates from ectoderm Dermis: originates from mesoderm  Epidermaldermal junction is the junction between epidermis and dermis, we mentioned that the epidermis is epithelial tissue and the dermis is connective tissue, so the epidermal-dermal junction represents the basement membrane of the epithelial tissue  This basement membrane is a wavy line, not a straight line, because there are invaginations from epidermis toward the dermis and we call these epidermal ridges.  And there are invaginations in the opposite direction (toward the epidermis), the part of the dermis that invaginates inside the epidermis is called dermal papilla (papilla means nipple like structure, pl. papillae).  This wavy junction is important to increase surface area for adhesion, and to increase the surface for diffusion; because epidermis is avascular so it gets its nutrition from vessels in the dermis.  Also to hold epidermis on dermis to prevent sliding and separation 3|Page  These invaginations are more prominent in the palms of hands and in soles of feet. Also, these invaginations create a unique pattern for each individual called fingerprint and footprint; these are so unique and can’t be identical even between identical twins!  The interdigitations are called frictional ridges ,these frictional ridges are important for grasping with our hands and for walking barefoot  Repeated rubbing of skin separates both layers from each other producing or developing Blisters  Blister is a pocket that is filled with fluid . Epidermis: • The outermost layer of skin composed of many layers (4 or 5) Keratin is a tough protein and the epidermis is rich in keratin Avascular Derived from ectoderm and contains 4 cell types a- Keratinocytes b- Melanocytes c- Langerhans cells d- Merkel cells Layers of epidermis: • The lowermost layer (right above the basement membrane) is a single layer of basophilic cuboidal cells (or columnar cells), this layer is called Stratum Basale, and its cells are characterized by mitotic activity responsible for replenishing the epidermis 4|Page • Note that the outermost layer (stratum corneum) is always shedding • (here the cells divide into 2 one goes up and the other stays to continue multiplying) • Cells of Stratum Basale are connected To each other by desmosomes To the basement membrane by hemidesmosomes • The second layer (above Stratum Basale) is called Stratum Spinosum and it is composed of more than one layer of cells(8-10), the shape of these cells is polyhedral, but it appears spinous in shape in histological preparation. The spinous appearance in this layer is due to desmosomes (during preparation, the cells shrink and the spines represent the areas of desmosomal attachments between the cells) - Note that the skin cells have high amount of desmosomes in order to hold the cells together In the lower part of this layer we can find some mitotically active cells (as in Stratum Basale) and we refer to these two 5|Page layers together [Basale and lower part of spinosum] as Stratum Germinativum. • The third layer is Stratum Granulosum, it is named “granulosum” because we can find basophilic granules in its cells’ cytoplasm, has 3-5 layers and Its cells are flat and connected to each other by desmosomes. The basophilic granules are keratin granules called Keratohyalin granules(keratin is stored)  There is another type of granules in these cells, lamellar granules, they are called lamellar because they show lamellar pattern (composed of layers) when viewed under the electron microscope, lamellar granules contain lipids, and by exocytosis the lipids will be discharged from the cells, and form a lipid rich layer around the keratinocytes (impermeable layer that prevents water loss or gain) 6|Page Keratohyalin granules Lamellar granules Basophilic (visible under light microscope) Not visible under light microscope (only under E.M.) Not surrounded by a membrane Surrounded by a membrane ** Granules start appearing at the upper part of stratum spinosum ** • The fourth layer is the Stratum Lucidum,it is a thin layer and not found in all skin it has flat cells which are dead and full of keratin, and it is 3-4 layers of cells. • Cells lack nucleus and organelles and keratin is less compacted in comparison to the keratin in the stratum corneum layer and thus the layer appears translucent • The fifth layer is Stratum Corneum, its cells are dead, filled with keratin, and surrounded by lipid layer (from lamellar granules) Why do the cells die as we go upward?? Because they are pushed farther away from blood supply (the shedding of the dead cells is called desquamation)(desquamation from the surface), and the cytoplasm is filled gradually with keratin. Continuous pressure will cause stratum corneum to become thicker forming calluses in the hand and corns in the feet 7|Page Again, epidermal cells are connected to each other by desmosomes, and the desmosomes – as you know – are connected to intermediate filaments in the cytoplasm, these filaments are keratin in the skin. As we go up in epidermis, synthesis of keratin intermediate filaments increases. Keratin filaments are called tonofilaments, these tonofilaments are packed gradually as we go up and cross-linked with each other by linker proteins like Filaggrin protein to create thick bundles called tonofibrils. The process of keratin synthesis and accumulation is called keratinization (or maturation), and it occurs in all cell stages of epidermis until the cell is dead; and that is why cells of skin are called keratinocytes. Keratinization takes 2-4 weeks to be completed so your skin renews it self every 2-4 weeks Clinical case Psoriasis(‫الصدفية‬ Keratinization process is fast ,cells build up very fast on the surface ,the extra skin will form scales and red patches that are itchy and painful Keratinocytes count for 90% of the cells of epidermis, and the epidermis contains other types of cells (less in number) like: 8|Page - Melanocytes: this type produces melanin, and they are located in Stratum Basale and connected to the basement membrane (only to it) by hemidesmosomes, but they are NOT connected to keratinocytes.  Melanocytes have many processes that run between keratinocytes. These processes come into contact with up to 30 cells  Melanocytes synthesize melanin (melanosomes,vesicles containing melanin), but they don’t store it in their cytoplasm.  Melanosomes are transported to the tips of the cytoplasmic processes of melanocytes to enter the keratinocytes by phagocytosis. WHAT?! Yes, keratinocytes phagocytose the tips of melanocytes’ processes to get the melanin.  When melanin enters the keratinocyte, it will form a structure like an umbrella above the nucleus to protect it from ultraviolet light. 9|Page Melanin is only found in stratum germinativum to protect the mitotically active cells .  Lighter colored people (less melanin) are more prone to develop skin cancer than darker colored people. Melanoma because of excessive sunlight  Sun exposure stimulates melanocytes to synthesize more melanin (sun tanning)  The number of melanocytes is equal in all humans but their activity may differ.  The number of melanocytes is 1 cell for each 10 keratinocytes of Stratum Basale.  Under the light microscope, you can see these cells rounded in shape and have a clear cytoplasm. They have fine processes passing between the keratinocytes but you can’t see them (under LM).  Clinical case :  Albinism :a genetic disease that results in a deficiency in production of melanin  The patient looks pale  Melanin produced from tyrosin Light color people contain less melanin ,why because melanin gives color to the hair ,skin and iris of the eye. 10 | P a g e - Langerhans cells: immune star shaped cells which are macrophages in the skin These cells are found mainly in stratum spinosum but you can also find them in other layers. They form 2-8% of cells in the skin. They originate from bone marrow ,monocyte Function: recognize ,phagocytose and process foreign antigens Merkel Cells: - Found in stratum basale and have many processes They are attached to basement membrane by hemidesmosomes and to other keratinocytes by desmosomes. They have the same origin of the keratinocytes (ectoderm). They are involved in tactile sensation (our sense of touch). They are associated with sensory neurons, this structure is called Merkel’s disk (which is a type of sensory receptors, and it belongs to mechanoreceptors). They are more prominent in finger tips. 11 | P a g e Dermis:  Made up of two layers  Responsible for elasticity and strength of the skin  The upper part of it is loose connective tissue, contain lots of spaces it is called papillary layer of dermis., and is important for diffusion of nutrients  And the lower part is dense irregular connective tissue, it is called reticular layer (but this doesn’t mean it contains mainly reticular fibers!) it is named reticular because collagen fibers and elastic fibers form a network structure. It is thicker than loose connective tissue.  The importance of dense irregular connective tissue is that the skin is exposed to forces in all different directions .  There are blood vessels exactly under the papillary layer (subpapillary plexus).  And blood vessels between the dermis and the hypodermis (Subdermal plexus) >> these give branches to form subpapillary plexus and they are important in controlling body heat (When the weather is cold the diameter of these vessels decreases (vasoconstriction), decreasing blood flow toward the skin, preventing heat loss). In hot weather, vasodilation of 12 | P a g e these blood vessels takes place resulting in heat loss through the surface of the skin. Ecchymosis o Haemorrhage from the cutaneous blood vessels in the dermis o Color change from red -> orange-> yellow (due to RBC’s being broken down)  There are two types of skin: thin skin and thick skin  These two types refer to the thickness of the EPIDERMIS not the whole thickness of the skin.  Thick skin refers only to the skin of the palms of hands and soles of feet, and skin of the rest of the body is thin skin.  Thick skin is always non-hairy and has no sebaceous glands (but of course it has eccrine sweat glands) but no apocrine  Remember apocrine sweat glands open into hair follicles, while eccrine sweat glands open into the surface of the skin  Epidermis of thick skin has 5 layers (mentioned earlier). whereas in thin skin, the epidermis lacks stratum lucidum (so it is only 4 layers)  Generally, thin skin has thicker dermis than thick skin Last layer of the skin is called Acid mantle its very fine, slightly acidic and fluid ,it is an oily material (from sebaceous gland)with sweat and dead cells Function: protection from harmful contaminants (alkaline) , protection of bacteria and keep the surface moist 13 | P a g e