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• Integumentary system


  The skin
• Integumentary system


  • Describe in detail the histology of each layer of the integumentary system (skin) and subcutaneous layer
  • Explain how the structure of the skin is related to its functions
• Formation of the integumentary system (skin)


  1. Skin is formed during embryonic development when the blastocyst transforms into a gastrula by gastrulation

  2. The skin forms concurrently with the nervous systems during gastrulation

  3. The skin (epidermis) and NS are both formed from the ectoderm germ layer
• Gastrulation


  formation of grastula from blastula
• Integumentary system (skin)


  Composed of two distinct layers: Epidermis, Dermis
• Epidermis


  The outermost protective covering of the skin is composed of layers of epithelial cells
• Stratified squamous epithelium


  Function: Provides protection
• Epidermis is avascular
• Nutrients and oxygen for epidermal cells


  Diffuse from blood vessels in the dermis through tissue fluid
• Most metabolically active epidermal cells must be located close to the basement membrane of the epidermal layer
• Cells in strata beyond the diffusion limits of nutrients and O2 will die
• Keratinocytes


  Approximately 90% of the cells in the epidermis, dominate the 4 or 5 strata (or layers) of the epidermis
• Keratin


  Intermediate filament protein that eventually constitutes over 50% of the total protein content of keratinocytes, conferring most of the protective properties of skin such as its mechanical strength
• Other cell types in the epidermis


  • Melanocytes
  • Langerhans cells
  • Merkel cells
• Layers of the epidermis


  • Stratum basale
  • Stratum spinosum
  • Stratum granulosum
  • Stratum lucidum
  • Stratum corneum
• Stratum basale


  Layer of dividing stem cells that produce a steady stream of keratinocytes to replace those lost in the superficial stratum to maintain the stratum corneum
• Terminal differentiation of keratinocytes begins once they detach from the basement membrane of the stratum basale and enter the stratum spinosum
• Terminal differentiation is regulated by retinoic acid
• 10-20% of cells in the stratum basale are melanocytes
• A fewer proportion of Merkel cells are also present in the stratum basale in non-hairy skin
• Stratum spinosum


  Most of the keratinocytes entering this stratum are post-mitotic, they then undergo terminal differentiation
• Keratinocytes in the stratum spinosum are large and irregular in shape, cuboidal or polygonal and some are slightly flattened
• Stratum spinosum contains an abundance of Langerhans cells interspersed amongst keratinocytes
• Stratum spinosum also contains melanin granules
• Stratum granulosum


  Terminal differentiation continues, keratinocytes have permanently exited the cell cycle, begin to die due to lack of nutrients and oxygen because they are too far from the blood supply in the dermis
• Keratohyaline granules and lamellar bodies


  Produced by keratinocytes in the stratum granulosum, contain a water repellant glycolipid released into the extracellular compartment, significantly reducing water loss from the epidermis
• Filaggrin is expressed by keratinocytes in the stratum granulosum
• Stratum lucidum


  Observed most clearly in thick skin, comprised of 3-5 rows of clear, flat, dead keratinocytes filled with large quantities of keratin
• Stratum corneum


  Consists of 25-30 cell layers of dead, dehydrated flattened keratinocytes still attached by desmosomes, millions are lost daily and must be replaced
• Terminal keratinocyte differentiation


  Self-renewing property of the epidermis, a complex multistep process regulated by retinoic acid, leading to keratinisation of keratinocytes
• Functions of the stratum corneum


  • High keratin content protects skin against abrasion and bacterial infection
  • Glycolipids between dead cells confer water-repellent property, preventing water loss and dehydration
  • Helps protect skin from heat loss and maintains structural integrity
• Langerhans cells


  Represent 3-6% of cells in the epidermis, originate from bone marrow, migrate to and reside in the epidermis, have a rounded cell body with network of dendritic processes that extend between keratinocytes to act as the skin's microorganism surveillance system
• Merkel cells and melanocytes are formed during embryogenesis from the neural crest
• Merkel cells


  Specialised epithelial cells that are sensitive to touch - mechanoreceptors, form synaptic contact with sensory nerve endings in skin, release chemical messengers that stimulate the sensory nerve endings
• Melanocytes


  Have long slender dendritic projections that extend between keratinocytes, produce the pigment melanin from the amino acid tyrosine by the enzymatic action of tyrosinase, concentrated in intracellular organelles termed melanosomes
• Melanin transfer


  1. Melanosomes are transported along the dendritic processes and transferred to adjacent keratinocytes

  2. Inside keratinocytes, melanosomes form an umbrella-like cap over the nucleus protecting it from harmful effects of UV radiation
• DNA damage caused by UV radiation in skin cells can result in skin cancers