Part 2

• Types of cells in the epidermis

  • Keratinocytes
  • Melanocytes
  • Langerhans cells
  • Merkel cells

  
• Keratinocytes


  Predominant cell type of the epidermis
• Keratinocytes

  • Participate in the formation of the epidermal water barrier
  • Immature keratinocytes are highly basophilic due to large number of free ribosomes engaged in keratin synthesis
  • Produce keratin, the major structural protein of the epidermis
  • Keratin assembled into keratin filaments
  • In the stratum spinosum, keratin filaments are grouped into bundles called tonofibrils
  • Keratohyaline granules contain profillagrin and trichohyalin proteins which function as promoters in the aggregation of keratin filaments into tonofibrils
  • Tonofibrils formed is called soft keratin in contrast to the hard keratin of the hair and nails
  • Keratinization involves breakdown of nucleus and organelles, thickening of plasma membrane, and change in pH from neutral in stratum granulosum to acidic in stratum corneum
  • Desquamation of surface keratinocytes from stratum corneum is carried out by proteolytic degradation of cell desmosomes by human kallikrein related serine peptidases in acidic pH environment
  • Lymphoepithelial kazal type inhibitor (LEKTI) is a physiological serine protease inhibitor in neutral pH environment in stratum granulosum which prevents desmosomal cleavage

  
• Epidermal water barrier

  • Established by 1) deposition of insoluble proteins on inner surface of plasma membrane, and 2) deposition of lipid layer on outer surface of plasma membrane
  • Spinous and granular cells synthesize probarrier lipids and lipid processing enzymes, assembled into lamellar bodies in Golgi and secreted by exocytosis into intercellular space
  • Organization of these intercellular lipid lamellae is responsible for formation of epidermal water barrier

  
• Cell envelope


  • 15nm thick layer of insoluble proteins deposited on inner surface of plasma membrane
  • Contributes to strong mechanical properties of epidermal water barrier
  • Formed by cross-linking small proline rich proteins with large structural proteins like cystatin, desmoplakin, elafin, envoplakin, fillagrin, involucrin, keratin chains and loricrin
  • Loricrin is the major structural protein, accounting for 80% of total cell envelope mass

  
• Lipid envelope


  • 5nm thick layer of lipids attached to cell surface by ester bonds
  • Major lipid components include ceramides, cholesterol, and free fatty acids
  • Monomolecular layer of acylglucosyl ceramide is the most important component, providing a teflon-like coating on the cell surface
  • Ceramides play important roles in cell signaling, differentiation, apoptosis, and reducing proliferation

  
• Melanocytes


  • Pigment producing cells
  • Produce melanin pigment
  • 7-10% of epidermal cell population
  • Dispersed among cells of stratum germinativum, spinosum and superficial dermis
  • Small cells with long thin processes in contact with keratinocytes
  • Cytoplasm contains membrane-bound melanosomes where melanin is produced
  • Pigment donation process where melanosomes are phagocytosed by neighboring keratinocytes to protect them from solar radiation
  • 1 melanocyte supplies up to 30 keratinocytes with melanin
  • Epidermal melanin unit composed of 1 melanocyte and all the keratinocytes it supplies

  
• Skin color


  • Dependent on amount of melanin produced and rate of melanin degradation
  • Also influenced by carotene (yellowish hue) and hemoglobin (red color) in the epidermis and dermis

  
• Langerhans cells


  • Antigen presenting cells
  • Comprise 3-8% of epidermal cells, most numerous in stratum spinosum
  • Possess dendritic processes like melanocytes
  • Contain Birbeck or vermiform granules, have indented nucleus
  • Phagocytose antigens, process and migrate to regional lymph nodes to interact with T lymphocytes
  • Initiate cutaneous hypersensitivity reactions, involved in contact or allergic dermatitis

  
• Merkel cells


  • Associated with sensory nerve endings
  • Least numerous among epidermal cell types
  • Disc-shaped cells with short cytoplasmic processes and lobed nucleus in basal region of epidermis
  • Merkel disc composed of Merkel cell and terminal disk of afferent axon, responds to pressure and touch
  • Characterized by presence of 80nm dense cored neurosecretory granules containing catecholamine-like hormones