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.
Understand the Problem
The text provides detailed information about the structure and functions of the epidermis and dermis layers of skin, along with the different types of cells found within these layers, their origins, and their roles in skin biology.
Answer
The epidermis is a keratinized, avascular layer composed mainly of keratinocytes.
The epidermis is a keratinized stratified squamous epithelium that is avascular, nourished by diffusion, and contains both keratinocytes and non-keratinocytes.
Answer for screen readers
The epidermis is a keratinized stratified squamous epithelium that is avascular, nourished by diffusion, and contains both keratinocytes and non-keratinocytes.
More Information
The epidermis plays a crucial role in protecting the underlying layers of the skin and includes distinct layers corresponding to keratinocyte maturation. These involve continual cell division in deeper layers and shedding in superficial layers.
Tips
It's important not to confuse the layers of the epidermis with those of the dermis, as they have different structures and functions.