Skin Epidermis PDF

 Skin 1- Epidermis: structure & types of cells. 2- Non keratinocytes: types, structure & function.  The integumentary system It consists of: 1- The skin. 2- The skin appendages: Hair. Nail. Sweat, sebaceous & mammary glands. ...

 Skin 1- Epidermis: structure & types of cells. 2- Non keratinocytes: types, structure & function.  The integumentary system It consists of: 1- The skin. 2- The skin appendages: Hair. Nail. Sweat, sebaceous & mammary glands.  Skin It consists of two layers: 1- The epidermis: the superficial epithelium derived from ectoderm. It is avascular but rich in sensory nerve endings especially for pain. 2- The dermis: the deeper vascular connective tissue layer derived from mesoderm.  The epidermis It is a keratinized stratified squamous epithelium. It consists of four types of cells: 1-Keratinocytes. 2-Melanocytes. 3- Langerhans cells. 4- Merkel’s cells.  Keratinocytes They are the most abundant cells in the epidermis. Keratinization (15-30 days): The superficial epithelial cells are shed off at the surface and are replaced by cells arise from stem cells in the basal layer. As the cells move upwards: 1- The keratin filaments (cytokeratin filaments) accumulate inside the cells. 2- The nucleus and organelles are disappearing. 3- The the cells die.  Keratinocytes If the process of keratinization occurs more rapidly ( one week): The superficial keratinized cells will accumulate and desquamate forming thick scales (dandruff ). The keratinocytes are arranged in 5 layers 1-Stratum germinativum =stratum basale Number of layers: one layer. Structure: 1- Shape of cells: low columnar. 2- Nucleus: large oval. 3- Cytoplasm: deep basophilic (numerous polysomes). 4- The cells are attached together by desmosomes and to the basement membrane by hemidesmosomes. 5- Frequent mitotic figures. Function: renewal of keratinocytes 2-Stratum spinosum = prickle cell layer Number of layers: 5-10. Structure: 1- Shape of cells: polyhedral. 2- Multiple spinous processes which interdigitate the similar processes of adjacent cells by numerous desmosomes. Function: resisting the effects of friction and pressure.  3-Stratum granulosum = granular layer Number of layers: 3-5. Structure: 1- Shape of cells: diamond shaped. 2- Nucleus: flattened. 3- Cytoplasm: deeply basophilic& granular, contains: a. Keratohyaline granules: non-membrane-bounded granules ( packing keratin filaments together). b. Membrane-bounded lamellated granules: Its synthesis starts in stratum spinosum and they increase in number in stratum granulosum, collect near the cell membrane then discharge their contents in the intercellular spaces. Function: waterproof-barrier of the skin. 4-Stratum lucidum = clear zone Structure: 2-3 layers. Structure: 1- Shape of cells: flattened & pale. 2- The nuclei and the organelles: are degenerating. 3-The cytoplasm: contains densely packed keratin filaments.  5-Stratum corneum = horny layer Number of layers: 20-30. Structure: 1- Shape of cells: flat cells in the form of eosinophilic horny scales. 2- The nuclei and organelles: degenerated. 3- Cytoplasm: contains densely packed keratin filaments. 4- The cells adhere to each another by remnants of desmosomes. Non keratinocytes  Melanocytes The color of the skin depends on the interaction of three factors: 1- The content of carotene ( yellow). 2- The oxygenated hemoglobin in the capillaries(red). 3- The melanin pigments (brown to black ).  I-Melanocytes Features: 1- Site: in the epidermis, between the cells of the stratum basale (also found in the hair follicle). 2- Structure: branching cell, its cytoplasm contain melanosomes. Function: Melanin formation which protect against the harmful effect of ultraviolet rays, thus exposure to sun light accelerates the rate of melanin production.  Melanocytes Cytocrine secretion: unusual mode of secretion, in which the cytoplasmic processes of melanocytes transfer the melanosomes into the neighboring epidermal cells.  Melanocytes The number of melanocytes: Nearly the same in all races. Although, hereditary and racial factors control the amount and size of melanosomes and the rate of transfer by melanocytes.  Melanocytes 1- Albinism: Lack of pigmentation, in which the melanocytes fail to form melanin. 2- Vitiligo : a depigmentation disorder occurs due to degeneration of melanocytes.  II-Langerhans cells Origin: blood monocytes ( mononuclear phagocytic system ) features: 1- LM: Shape: branching cells with multiple cytoplasmic processes. Nucleus: folded & irregular. Cytoplasm: pale. Site: between the cells of stratum spinosum. 2- EM: numerous lysosomes. 2 Nos: No keratin & no desmosomes between the Langerhans cells and keratinocytes.  Langerhans cells Function: Immune function Antigen presenting cells (binding, processing and presenting the antigen to T lymphocytes. They are involved in the cutaneous contact hypersensitivity reaction.  III-Merkel, s cells Features: Shape: large cells with short processes. Site: the basal layer of the epidermis.  They are attached to the keratinocytes by desmosomes. The sensory nerve fibers pierce the basement membrane, end as expanded discs near the Merkel’s cells Function: Receptors for fine touch sensation.  Melanocyte Langerhan , s cell Merkel , s cell 1- Site Basal layer. Stratum spinosum. Basal layer. 2- Function Melanin secretion which acts Antigen presenting cell Receptors for light touch. as a screen against UV responsible for cutaneous radiation. hypersensitivity. 3- Structure Branching cells. Branching cells with irregular Large cell with short processes. Cytoplasm contains nucleus. Connected to keratinocytes by melanosomes. Cytoplasm contains lysosomes. numerous desmosomes. Which structure is responsible for water proof barrier of the skin? 1- Cytokeratin in the stratum corneum. 2- Keratohyaline granules in the stratum granulosum. 3- Desmosomes in the stratum spiosum. 4- Lamellated granules in the stratum granulosum. Which structure is responsible for water proof barrier of the skin? 1- Cytokeratin in the stratum corneum. 2- Keratohyaline granules in the stratum granulosum. 3- Desmosomes in the stratum spiosum. 4- Lamellated granules in the stratum granulosum.