5. Skin Histology Flashcards
- Identify and describe the characteristic histological and cytological features of cells from skin (objective)
Answer later
- Describe the physiological roles of different cell types from the skin (objective)
Answer later
Protective Function of Skin
- Physical barrier against thermal and mechanical insults and most potential pathogens
- Dark pigment melanin in epidermis protects cell nuclei from UV radiation
- Permeability barrier against excessive loss or uptake of water
- Selective permeability of some lipophillic drugs
Sensory Function of Skin
- Sensory receptors allow skin to constantly monitor the environment
- Various skin mechanoreceptors help regular body’s interactions with physical objects
Thermoregulatory Function of Skin
- Constant body temperature is normally easily maintained thanks to skin’s insulating components (ie fatty layer and hair on the head)
- Mechanisms for accelerating heat loss (sweat production and a dense superficial microvasculature), especially with fatty tissue
Metabolic Function of Skin
- Cells of skin synthesize vitamin D3, needed in calcium metabolism and proper bone formation, through local action of UV light on the vitamin’s precursor
- Excess electrolytes can be removed in sweat, and the subcutaneous layer stores a significant amount of energy in the form of fat
Other General Properties of Skin
- Elasticity
- Self-renewing throughout life (injured skin is repaired rapidly)
General Layers of Skin
Epidermis, Dermis (papillary and reticular layer), Subcutaneous Layer
Epidermis (general)
- Forms major distinction between thick skin (palms and soles, 400-1400um) and thin skin (75-150um)
- Lacks microvasculature but receives nutrients and oxygen by diffusion from dermis
Epidermis is a stratified squamous keratinized epithelium
Cellular component:
-epithelial cells are called keratinocytes (squamous cells)
-pigment producing melanocytes
-antigen-presenting Langerhans cells
-Tactile Merkel cells
Layers:
-stratum basale, spinosum, granulosum, lucidum, corneum
Stratum Basale
- Intense mitotic activity and contains (along with deepest part of next layer) all progenitor cells for all epidermal layers
- Single layer of basophilic cuboidal or columnar cells on the basement membrane at dermal-epidermal junction
- Hemidesmosomes in basal cell membranes join these cells to the basal lamina
- Desmosomes bind cells together in their lateral and upper surfaces
- During differentiation cells move upward
Stratum Spinosum
- Just above basal layer
- Thickest layer, especially in epidermal ridges
- Consists of generally polyhedral cells with central nuclei
- Desmosomes holding cell layers together
- Thick skin subject to continuous friction and pressure (such as foot soles) has a thicker stratum spinosum
- Most basal layer can repair
Stratum Granulosum
- 3-5 layers of flattened cells, undergoing terminal differentiation process of keratinization
- Cytoplasm is filled with intensely basophilic masses (called keratohyaline granules and lamellar granules)
- Lamellar granules undergo exocytosis, producing lipid-rich impermeable layer around the cells
Stratum Lucidum
- Only in thick skin
- Consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes
- Nuclei and organelles have been lost
- Cytoplasm consists almost exclusively of packed keratin filaments
Stratum Corneum
- 15-20 layers of squamous, keratinized cells
- By end of keratinization, the cells only contain amorphous fibrillar proteins with plasma membranes surrounded by lipid-rich layer
- These fully keratinized or cornified cells called squames are continuously shed at epidermal surface as desmosomes and lipid-rich envelopes break down
Squamous Cell Carcinoma
Stratum Spinosum (keratinocytes)
Basal Cell Carcinoma
Stratum Basale (basal cells)
Melanoma
Stratum Basale (melanocytes)
Merkel Cell Carcinoma
Stratum Basale (Merkel Cells)
Friction Blisters
Fluid build-up between epidermis and dermis
Hypopigmentation (Albinism)
Congenital defect in melanin-producing pathway (defect in tyrosinase; tyrosine to dopa blocked)
Pathway ends in two pigments (pheomelanins and eumelanins)
Skin Pigments
- Eumelanins are brown or black pigments produced by melanocyte
- Pheomelanin in red hair
Depigmentation (Vitiligo)
Acquired condition with loss or decreased activity of melanocytes
Dermis
Connective tissue that supports epidermis and binds subcutaneous tissue
Dermis (cellular component)
- Fibroblasts
- Macrophages
- Mast cells
- Other leukocytes
Dermis (non-cellular component)
- Collagen (Type 1 and 3)
- Elastic fibers (elastin)
Dermis (layers)
- Papillary layer (loose connective tissue)
- Reticular layer (dense irregular connective tissue)
Dermatofibroma
Fibroblasts/dendritic cells
-Non-malignant version
Dermatofibrosarcoma Proteburans
Fibroblasts
-Malignant version
Hemangioma
Endothelial cells (vessels) -Non-malignant version
Hemangiosarcoma
Endothelial cells (vessels) -Malignant version
Wound Healing
Time depends on size, medical condition, need vitamin C, different responses depending how deep
- Cut blood vessels bleed into the wound
- Blood clot forms and leukocytes clean wound
- Blood vessels regrow and granulation tissue forms
- Epithelium regenerates and connective tissue fibrosis occurs (scar tissue)
Skin Summary 1/3
Functional tissue is stratified squamous keratinized epithelium (epidermis)
Composed of highly specialized squamous cells called keratinocytes
-Tightly bound to other cells (tight and adherent junctions, desmosomes) and to basement membrane (hemidesmosomes)
-Specialized functions: lamellar granules in stratum granulosum extrude lipid-rich material to form lipid barrier; stacks of flattened plates form protective acellular layer (stratum corneum)
Skin Summary 2/3
Stroma of skin is composed of dermis (supports and nourishes epithelial layer) and hypodermis (loose connective tissue that attaches dermis to underlying tissues)
Skin Summary 3/3
Skin epithelium also shows folds, analogous to GI epithelium of small bowel, but in this case the folds are at basal surface and are to increase adhesive area rather than absorptive area
-Epidermal (rete) ridges and dermal papillae
