Skin Structure and Layers PDF

## Skin ### Structure 1. **Epidermis:** - Outer epithelial layer (keratinized stratified squamous epithelium). - Derived from ectoderm. 2. **Dermis:** - Thicker deep CT layer. - Derived from mesoderm. **N.B.** **Hypodermis:** Is not a part of the skin (Greek: hypo = under, dermis = skin). ###...

## Skin ### Structure 1. **Epidermis:** - Outer epithelial layer (keratinized stratified squamous epithelium). - Derived from ectoderm. 2. **Dermis:** - Thicker deep CT layer. - Derived from mesoderm. **N.B.** **Hypodermis:** Is not a part of the skin (Greek: hypo = under, dermis = skin). ### Types of skin According to the thickness of the epidermis, skin is classified into **thick** and **thin**. #### Thick (Non-Hairy) Skin - It has a thick epidermis (400-1400 µm) and a thick horny layer. - Present in palms and soles. - It is formed of epidermis and dermis. ## The Epidermis - It is a keratinized stratified squamous epithelium. - Thicker over soles than palms. - Avascular layer receiving its nutrition by diffusion. - Rich in free nerve endings. - The epidermis is formed of **keratinocytes** and **non-keratinocytes**. #### Keratinocytes - 85% of the cells in epidermis. - Deeper layers are continuously dividing, differentiating, and accumulating keratin filaments (keratin formation) while progressing upwards. - Superficial layers are continuously shed off. - According to keratinocytes maturation, epidermis consists of 5 layers: 1. **Stratum Basale or (Basal Cell Layer)**: - **LM:** Single layer of low columnar or cuboidal cells, resting on a clear wavy basement membrane. Basophilic cytoplasm with large basal oval nucleus. Intense mitotic figures (responsible for renewal). Melanocytes and Merkel's cells are found in this layer. - **EM:** Attached to each other and to the following layer by desmosomes and to basement membrane by hemi-desmosomes. Rich in free ribosomes and polysomes with few other organelles (G.A, mitochondria and rER). Keratin intermediate filaments ending in desmosomes. (Tona filamenii) 2. **Stratum Spinosum (Prickle Cell Layer):** - **LM:** 4-8 layers of polyhedral cells are present above the basal cell layer. Less basophilic cytoplasm than cells of stratum Basale. Cells have central rounded nuclei. Borders of cells appear to be separated from one another by small spaces that are traversed by fine spine-like processes (desmosomes), giving prickly appearance. Langerhans cells are present in this layer. - **EM:** bundles of intermediate filaments (Tono-filaments) that end into the dense plaques of numerous desmosomes along highly inter- digitating cell boundaries. 3. **Malpighian layer:** Consists of both stratum Basale and stratum spinosum. 4. **Stratum Granulosum (Granular Cell Layer):** - **LM:** layers of spindle-shaped cells above the spinous cell layer. Deep basophilic and granular cytoplasm with flat pale nuclei. - **EM:** The cytoplasm shows 2 types of granules. - **Keratohyalin granules:** Aggregate to form keratin filaments (tonofilaments). Non membranous. - **Membrane-coated lamellar granules:** Release a lipid-rich secretion that fills spaces between cells. Membranous granules. 5. **Stratum Lucidum (Clear Layer):** - **LM:** Thin, lightly stained, clear, homogeneous layer. Formed of much flattened cells. Nuclei are on their way to disappear by karyolysis. - **EM:** - Thickened cell membranes. - Few remnants of desmosomes. - Organelles disappear (by lysosomal activity). - Nuclei appear as ghosts or completely absent - Densely packed keratin filaments (Tono- fibrils) embedded in an electron-dense matrix formed by keratohyalin granules. 6. **Stratum Corneum (Horny Layer):** - **LM:** Thick eosinophilic layers formed of heavily keratinized dead cells, called scales. - **EM:** - Thickened cell membranes attached together by remnants of desmosomes. - Filled with mature keratin filaments (Tono-fibrils) embedded in amorphous matrix. - No nuclei nor organelles. ***Diagram***: A diagram showing layers of skin.