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 en... 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. a- 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: - 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. Diagram showing layers of skin Page | 19 - 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. ❖ Malpighian layer: Consists of both stratum Basale and stratum spinosum. ➢ EM: bundles of intermediate filaments (Tono-filaments) that end into the dense plaques of numerous desmosomes along highly inter-digitating cell boundaries. - 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. A. Keratohyalin granules: - Non membranous. - Aggregate to form keratin filaments (tonofilaments) B. Membrane-coated lamellar granules: - Membranous granules. - Release a lipid-rich secretion that fills spaces between cells. 4- 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. 5- 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. Page | 20 Non-Keratinocytes 1- Langerhans Cells: • Origin: Bone marrow precursors migrate via blood to the dermis then epidermis. • LM: ▪ Represent 3-8% of epidermal cells. ▪ They are stellate-shaped cells found mainly between cells of the stratum spinosum of epidermis. ▪ In H.&E. skin sections; cell appears with a dark-staining nucleus and a pale clear cytoplasm. ▪ They can be identified using vital stains. • EM: ▪ A prominent Golgi complex and numerous lysosomes ▪ Special tennis-racquet-shaped granules (Birbeck’s granules). Some may contain hydrolytic enzymes. ▪ The nucleus is dark and highly irregular. ▪ Absence of keratin filaments and desmosomes. ▪ Absence of melanin granules. ▪ Absence of cell junctions between them and keratinocytes. • Function: Acts as antigen presenting cell; capable of binding antigen that contacts skin and then presenting it to T-lymphocytes. So, they have a significant role in skin immunological reactions (allergic dermatitis). 2- Merkel’s Cells: • Origin: Ectodermal in origin. They are modified epithelial cells. • LM: ▪ They resemble epidermal cells. ▪ Present in-between cells of the basal layer. Abundant in highly sensitive skin like that of fingertips and at the bases of some hair follicles. ▪ Free nerve fiber (sensory) traverses basal lamina to terminate as disc-shaped expansions beneath Merkel’s cell forming Merkel cell-neurite complex. • EM: ▪ Cells are attached to neighboring keratinocytes by desmosomes. ▪ Cytoplasm contains electron-dense granules resembling those of neuroendocrine cells elsewhere (APUD). ▪ Deeply invaginated nucleus. • Function: - Mechanoreceptors for light touch sensation. - Neurosecretory function (granules). 3- Melanocytes: • Origin: Precursors arise from neural crest (ectoderm) and migrate to the skin early in development and differentiate to melanocytes. • LM: ▪ Cell bodies of melanocytes are present in-between and just below the cells of stratum Basale. ▪ They have rounded cell bodies from which long irregular cytoplasmic processes extend between keratinocytes. Read more